Modesto City Council
July 9, 2024

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Appendix G ‐ Water Quality Monitoring Network
Well ID Latitude Longitude Well Type Well Depth (ft bgs) Top of Screen (ft bgs) Screen Length (ft) Dataset Name Alternative Well ID Alternative Well ID 2 Nitrate Uranium PCE TCP DBCP TDS Arsenic Water Quality Parameters
AGW080012860‐HOME 37.67647 ‐120.71800 Domestic 0 0 0 AGLAND HOME HOME x AGW080012178‐AGR 37.74813 ‐120.91754 Domestic 0 0 0 AGLAND AGR AGR x AGW080012936‐5937 37.72460 ‐120.79350 Domestic 0 0 0 AGLAND 5937 5937 x AGW080013323‐4718 37.67395 ‐121.08119 Domestic 0 0 0 AGLAND 4718 4718 x AGW080010976‐JKSN SOUTH 37.70816 ‐120.67605 Domestic 0 0 0 AGLAND JKSN SOUTH JKSN SOUTH x AGW080012609‐PRICE 37.65470 ‐120.75050 Domestic 0 0 0 AGLAND PRICE PRICE x AGW080012664‐4912 37.60724 ‐121.08406 Domestic 0 0 0 AGLAND 4912 4912 xAGW080011033‐GIL2 37.75067 ‐120.79034 Domestic 0 0 0 AGLAND GIL2 GIL2 x AGW080011480‐DW1 37.71468 ‐120.78850 Domestic 0 0 0 AGLAND DW1 DW1 x AGW080011023‐DW2 37.70045 ‐120.77700 Domestic 0 0 0 AGLAND DW2 DW2 x AGW080012327‐HOME 37.71006 ‐120.78962 Domestic 0 0 0 AGLAND HOME HOME xAGW080010965‐HOUSE 37.70330 ‐120.64263 Domestic 0 0 0 AGLAND HOUSE HOUSE x AGW080012603‐BT WARD 37.67504 ‐120.69885 Domestic 0 0 0 AGLAND BT WARD BT WARD x AGW080011224‐1131 37.62612 ‐121.08638 Domestic 0 0 0 AGLAND 1131 1131 xAGW080012103‐HOUSE 37.78000 ‐120.75480 Domestic 0 0 0 AGLAND HOUSE HOUSE xAGW080013770‐6725 37.69784 ‐121.11962 Domestic 0 0 0 AGLAND 6725 6725 xAGW080012942‐DW1 37.65250 ‐120.53320 Domestic 0 0 0 AGLAND DW1 DW1 x AGW080012673‐1 37.65303 ‐120.90810 Domestic 0 0 0 AGLAND 1 1 xAGW080011035‐HALL 37.71903 ‐121.12773 Domestic 0 0 0 AGLAND HALL HALL x AGW080012014‐6401 37.73283 ‐121.07351 Domestic 0 0 0 AGLAND 6401 6401 xAGW080012937‐5737 37.72200 ‐120.79350 Domestic 0 0 0 AGLAND 5737 5737 xAGW080011021‐918 37.65504 ‐120.90046 Domestic 0 0 0 AGLAND 918 918 x AGW080013782‐454 37.64352 ‐120.81778 Domestic 0 0 0 AGLAND 454 454 x AGW080012604‐HARRIS 37.67504 ‐120.69885 Domestic 0 0 0 AGLAND HARRIS HARRIS x AGW080012011‐6373 37.73759 ‐121.07469 Domestic 0 0 0 AGLAND 6373 6373 xAGW080011029‐GIL1 37.74882 ‐120.77300 Domestic 0 0 0 AGLAND GIL1 GIL1 x AGW080011760‐OWD1 37.73642 ‐120.83138 Domestic 0 0 0 AGLAND OWD1 OWD1 x AGW080010971‐HQ 37.69691 ‐120.77239 Domestic 0 0 0 AGLAND HQ HQ x AGW080012606‐TA WARD 37.65216 ‐120.74834 Domestic 0 0 0 AGLAND TA WARD TA WARD x AGW080013065‐COFF 37.73042 ‐120.97544 Domestic 0 0 0 AGLAND COFF COFF x AGW080012667‐1313 37.61906 ‐121.08775 Domestic 0 0 0 AGLAND 1313 1313 xAGW080011024‐DW1 37.70099 ‐120.78019 Domestic 0 0 0 AGLAND DW1 DW1 x AGW080013900‐237 37.63519 ‐120.81686 Domestic 0 0 0 AGLAND 237 237 x AGW080014842‐HOME 37.66093 ‐120.77381 Domestic 0 0 0 AGLAND HOME HOME x AGW080013120‐2901 37.73540 ‐121.04881 Domestic 0 0 0 AGLAND 2901 2901 x5000433‐004 37.78037 ‐120.80252 Municipal 0 0 100 DHS 5000433‐004 HILLSBOROUGH ESTATES WELL NO. 01x x x x x 5000141‐004 37.70900 ‐121.00577 Municipal 0 50 180 DHS 5000141‐004 WELL #3 (COLD STORAGE) x 5000433‐006 37.77968 ‐120.77772 Municipal 0 0 0 DHS 5000433‐006 COUNTRY OAK MANOR WELL NO. 01 x x x x x 5000015‐002 37.77225 ‐120.82033 Municipal 0 350 125 DHS 5000015‐002 WELL #1 ‐ SOUTH x xxx 5000099‐003 37.74545 ‐121.00378 Municipal 0 50 40 DHS 5000099‐003 NEW NORTH GATE x x xxx 5000048‐002 37.74658 ‐120.90888 Municipal 0 164 20 DHS 5000048‐002 NORTH EAST WELL #1 x x 5000014‐002 37.74884 ‐120.88009 Municipal 0 60 35 DHS 5000014‐002 WELL#2 x x x x 5000067‐001 37.71702 ‐121.01164 Municipal 0 330 20 DHS 5000067‐001 WELL 03 x x x 5000411‐003 37.71786 ‐121.00124 Municipal 0 310 38 DHS 5000411‐003 WELL #3 WEST PARK x x 5010018‐008 37.72194 ‐120.95380 Municipal 0 210 40 DHS 5010018‐008 WELL 08 xxx xxx 5010006‐003 37.64117 ‐120.74547 Municipal 0 124 50 DHS 5010006‐003 WELL NO. 245 x 5010042‐002 37.63917 ‐120.75000 Municipal 0 0 0 DHS 5010042‐002 WELL NO. 02 ‐ RAW ‐ GRNSD ‐ FE&MN x x 5010010‐049 37.64931 ‐120.93879 Municipal 0 0 110 DHS 5010010‐049 WELL 047 xx xx 5010010‐047 37.66340 ‐120.91952 Municipal 0 0 153 DHS 5010010‐047 WELL 045 xxx xx 5000066‐001 37.69706 ‐120.99203 Municipal 0 200 97 DHS 5000066‐001 NORTH EAST NEW WELL (MAIN WELL) x 5000189‐004 37.70716 ‐121.00371 Municipal 0 280 90 DHS 5000189‐004 W.WELL#3 (BEHIND 4719 N. STAR WAY) xxxx 5000133‐003 37.66597 ‐121.06601 Municipal 0 0 0 DHS 5000133‐003 2011 WELL x x 5010010‐241 37.70767 ‐121.05488 Municipal 0 0 0 DHS 5010010‐241 WELL 61 xx xx

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Appendix G ‐ Water Quality Monitoring Network
Well ID Latitude Longitude Well Type Well Depth (ft bgs) Top of Screen (ft bgs) Screen Length (ft) Dataset Name Alternative Well ID Alternative Well ID 2 Nitrate Uranium PCE TCP DBCP TDS Arsenic Water Quality Parameters
5010014‐011 37.76502 ‐120.83228 Municipal 0 240 140 DHS 5010014‐011 WELL 08 x 5000155‐001 37.63823 ‐120.61884 Municipal 0 50 10 DHS 5000155‐001 WELL 01 x 5010010‐243 37.69540 ‐121.05603 Municipal 0 0 0 DHS 5010010‐243 WELL 63 x x x 5010005‐008 37.71553 ‐121.08905 Municipal 0 240 120 DHS 5010005‐008 WELL 298 x x 5000372‐003 37.66461 ‐121.06086 Municipal 0 0 0 DHS 5000372‐003 SW NEW WELL x 5000317‐001 37.68982 ‐121.07024 Municipal 0 312 73 DHS 5000317‐001 WELL#1 x x x x 5000592‐001 37.71245 ‐120.82519 Municipal 0 0 0 DHS 5000592‐001 2014 WELL x 5000189‐005 37.70721 ‐121.00081 Municipal 0 320 20 DHS 5000189‐005 E.WELL, #4 622 GALAXY WAYx x x x 5010018‐012 37.73216 ‐120.92441 Municipal 0 0 0 DHS 5010018‐012 WELL NO. 12 x x xxx 5000433‐003 37.77747 ‐120.79795 Municipal 0 264 100 DHS 5000433‐003 HUNTER RANCH ESTATES WELL NO. 01x x x x x 5000013‐001 37.78530 ‐120.81297 Municipal 0 0 35 DHS 5000013‐001 WELL 01 x x xxx 5000014‐001 37.78058 ‐120.79294 Municipal 0 50 45 DHS 5000014‐001 WELL#1 x x x x 5010014‐008 37.76212 ‐120.84250 Municipal 0 195 250 DHS 5010014‐008 WELL 05‐A ‐ SIERRA & Jx 5000404‐002 37.67000 ‐121.08000 Municipal 0 245 20 DHS 5000404‐002 02 NEW SCHOOL xxxx 5010010‐245 37.68948 ‐120.93022 Municipal 0 0 0 DHS 5010010‐245 WELL NO. 67 xxxxxxx 5000433‐002 37.77809 ‐120.80597 Municipal 0 325 42 DHS 5000433‐002 COUNTRY CLUB ESTATES WELL NO. 02 x x x x x 5010014‐010 37.76164 ‐120.87669 Municipal 0 274 204 DHS 5010014‐010 WELL 07 x 5010018‐002 37.73336 ‐120.92734 Municipal 0 0 68 DHS 5010018‐002 WELL 02 x x xxx 5000048‐003 37.74622 ‐120.91000 Municipal 0 50 10 DHS 5000048‐003 WEST #02 x x 5010006‐004 37.64558 ‐120.77354 Municipal 0 200 92 DHS 5010006‐004 WELL NO. 286 x 5010010‐130 37.68534 ‐120.99272 Municipal 0 0 55 DHS 5010010‐130 WELL 264 ‐ SHERWOOD FOREST x x x x 5010006‐005 37.63711 ‐120.77367 Municipal 0 152 85 DHS 5010006‐005 WELL NO. 302 x x x x 5000249‐004 37.71283 ‐121.02746 Municipal 0 285 70 DHS 5000249‐004 WELL 02 RAW x xxxxx5000563‐001 37.71561 ‐121.00339 Municipal 0 270 20 DHS 5000563‐001 WELL x x5000565‐001 37.71575 ‐121.00392 Municipal 0 0 0 DHS 5000565‐001 NEW WELL x xxx x 5000110‐002 37.64922 ‐120.97849 Municipal 0 50 10 DHS 5000110‐002 NORTH/BACK UP WELL x 5010014‐012 37.75455 ‐120.87014 Municipal 0 0 0 DHS 5010014‐012 WELL 09 x xxxxx 5000481‐002 37.66285 ‐120.78124 Municipal 0 50 20 DHS 5000481‐002 OLD WELL (WESTERN BY PLANT) x 5010010‐131 37.68089 ‐120.99341 Municipal 0 0 45 DHS 5010010‐131 WELL 262 ‐ HART WELL 02 x 5010010‐068 37.69341 ‐120.94873 Municipal 0 162 58 DHS 5010010‐068 WELL 054 x x x5010010‐053 37.70363 ‐121.04910 Municipal 0 0 225 DHS 5010010‐053 WELL 051 xx xxxx 5000584‐001 37.73803 ‐120.99481 Municipal 0 0 0 DHS 5000584‐001 NEW WELL 2009 x 5000179‐004 37.66001 ‐120.65574 Municipal 0 350 130 DHS 5000179‐004 #4 WELL NORTH WEST x 5010029‐001 37.74016 ‐121.01405 Municipal 0 380 40 DHS 5010029‐001 WELL 271 ‐ HILLCREST ESTATES x x x x5010010‐097 37.66944 ‐120.95000 Municipal 0 0 0 DHS 5010010‐097 WELL 65 ‐ RAW xxx 5010005‐005 37.70691 ‐121.09319 Municipal 0 224 62 DHS 5010005‐005 WELL 288 ‐ SUNNYBROOK x x 5010006‐001 37.64277 ‐120.76405 Municipal 0 290 55 DHS 5010006‐001 WELL NO. 242 x x x 5010010‐062 37.68394 ‐120.94584 Municipal 0 0 190 DHS 5010010‐062 WELL 052 xxx xxx 5000433‐005 37.78032 ‐120.79170 Municipal 0 0 0 DHS 5000433‐005 SIERRA SUNSET ESTATES WELL NO. 01 x x x x x 5010018‐004 37.73973 ‐120.93995 Municipal 0 132 154 DHS 5010018‐004 WELL 04 xxx xxx 5010005‐001 37.70083 ‐121.08642 Municipal 0 225 85 DHS 5010005‐001 WELL 250 ‐ SALIDA GAS x x 5010010‐129 37.68533 ‐120.97581 Municipal 0 0 45 DHS 5010010‐129 WELL 259 ‐ COFFEE VILLAGE 01 x x x 5000573‐002 37.71230 ‐121.00251 Municipal 0 0 0 DHS 5000573‐002 SCS 2007 WELL x x 5000384‐003 37.65604 ‐121.02473 Municipal 0 390 40 DHS 5000384‐003 NEW LONE PALM x x xxx 5000055‐003 37.70586 ‐120.92032 Municipal 0 104 10 DHS 5000055‐003 EAST FIELD x x xxx5000016‐001 37.74986 ‐120.87875 Municipal 0 160 24 DHS 5000016‐001 WELL#2 x x x x 5000517‐001 37.71001 ‐120.99702 Municipal 0 330 40 DHS 5000517‐001 WELL x x 5000317‐002 37.78055 ‐120.78424 Municipal 0 418 54 DHS 5000317‐002 WELL#2 x x x x 5010018‐006 37.72784 ‐120.93318 Municipal 0 195 360 DHS 5010018‐006 WELL 06 x xxxxx 5000499‐004 37.68138 ‐121.10948 Municipal 0 0 0 DHS 5000499‐004 2018 WELL xx xx x 5010014‐005 37.77968 ‐120.83856 Municipal 0 137 160 DHS 5010014‐005 WELL 03 ‐ ON THE HILL x x 5000568‐001 37.72180 ‐121.05999 Municipal 0 0 0 DHS 5000568‐001 WELL #1 2007 x x 5010005‐017 37.70294 ‐121.07842 Municipal 0 0 0 DHS 5010005‐017 WELL 313 ‐ RAW x x

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Appendix G ‐ Water Quality Monitoring Network
Well ID Latitude Longitude Well Type Well Depth (ft bgs) Top of Screen (ft bgs) Screen Length (ft) Dataset Name Alternative Well ID Alternative Well ID 2 Nitrate Uranium PCE TCP DBCP TDS Arsenic Water Quality Parameters
5010010‐043 37.66040 ‐120.93046 Municipal 0 124 92 DHS 5010010‐043 WELL 041 x x x 5000552‐001 37.71237 ‐121.00386 Municipal 0 0 0 DHS 5000552‐001 WELL x xxxxx 5000017‐002 37.73936 ‐120.96136 Municipal 0 0 10 DHS 5000017‐002 PARK RIDGE WEST x x 5000049‐001 37.77481 ‐120.82256 Municipal 0 117 40 DHS 5000049‐001 NORTH WELL x x xxx 5000263‐002 37.71179 ‐120.99603 Municipal 0 320 40 DHS 5000263‐002 NEW 2006 x x 5000179‐003 37.74886 ‐120.84306 Municipal 0 280 300 DHS 5000179‐003 #3 WELL SOUTH x x 5010029‐004 37.74423 ‐121.00330 Municipal 0 360 109 DHS 5010029‐004 WELL 289 ‐ KRISTINA x x 5000335‐001 37.68982 ‐121.07024 Municipal 0 240 20 DHS 5000335‐001 WELL, PUBLIC/SOUTH x x 5010014‐013 37.75502 ‐120.85043 Municipal 0 0 0 DHS 5010014‐013 WELL 10 x 5000580‐001 37.73025 ‐121.06814 Municipal 0 0 0 DHS 5000580‐001 WELL x x x 5000211‐003 37.71228 ‐120.91821 Municipal 0 50 150 DHS 5000211‐003 WELL NO. 06 x 5010029‐002 37.74611 ‐121.01690 Municipal 0 139 132 DHS 5010029‐002 WELL 282 ‐ DEL RIO x x 5010029‐010 37.73200 ‐121.00397 Municipal 0 0 0 DHS 5010029‐010 WELL NO. 68 xxxxxxx 5000433‐007 37.77693 ‐120.78556 Municipal 0 0 0 DHS 5000433‐007 OLIVE RANCH ESTATES WELL NO. 01 x x x x x 5000433‐001 37.77810 ‐120.80610 Municipal 0 323 42 DHS 5000433‐001 COUNTRY CLUB ESTATES WELL NO. 01 x x x x x 5010006‐006 37.64727 ‐120.76391 Municipal 0 224 80 DHS 5010006‐006 WELL NO. 303 ‐ RAW TO GAC x xxx 5000099‐001 37.74617 ‐121.00344 Municipal 0 115 120 DHS 5000099‐001 NORTH WELL LAKE WELL x x x x x 5000189‐006 37.70981 ‐121.00082 Municipal 0 195 40 DHS 5000189‐006 N.WELL, #5, 4825 STRATOS xxx x x 5010014‐009 37.75773 ‐120.84036 Municipal 0 240 160 DHS 5010014‐009 WELL 06 x 5010010‐048 37.67571 ‐120.94764 Municipal 0 149 145 DHS 5010010‐048 WELL 046 xxx xx 5010010‐221 37.68369 ‐120.98493 Municipal 0 0 185 DHS 5010010‐221 WELL 058 xx xxx 5010010‐050 37.70231 ‐120.99673 Municipal 0 0 165 DHS 5010010‐050 WELL 048 x x 5010005‐007 37.69834 ‐121.07377 Municipal 0 292 120 DHS 5010005‐007 WELL 297 x x 5000530‐004 37.63466 ‐120.79356 Municipal 0 0 0 DHS 5000530‐004 2011 WELL x 5010010‐170 37.62793 ‐120.93048 Municipal 0 0 75 DHS 5010010‐170 WELL 308 x x x x 5010014‐007 37.76531 ‐120.86258 Municipal 0 90 156 DHS 5010014‐007 WELL 04 OAK STREET x 5010010‐226 37.64198 ‐120.91903 Municipal 0 0 120 DHS 5010010‐226 WELL 059 x x x 5010010‐044 37.68880 ‐121.05788 Municipal 0 144 55 DHS 5010010‐044 WELL 042 xxx xx 5010010‐180 37.63785 ‐120.93172 Municipal 0 0 55 DHS 5010010‐180 WELL 291 ‐ MARIPOSA EAST x x x x x x x 5000529‐001 37.70417 ‐120.95640 Municipal 0 0 0 DHS 5000529‐001 WELL x 5010010‐124 37.65796 ‐121.03818 Municipal 0 0 36 DHS 5010010‐124 WELL 241 ‐ HAMMET x x 5000117‐001 37.77475 ‐120.82256 Municipal 0 210 20 DHS 5000117‐001 DOMESTIC WELL xxxx5010005‐006 37.71402 ‐121.08200 Municipal 0 164 112 DHS 5010005‐006 WELL 290 ‐ CLARENDON x x x x 5010010‐127 37.65759 ‐120.93726 Municipal 0 0 53 DHS 5010010‐127 WELL 265 ‐ LINCOLN ESTATES x x 5000154‐002 37.63783 ‐120.84967 Municipal 0 0 0 DHS 5000154‐002 WELL 02 OLD EASTERN xxxxx x 5010018‐009 37.71361 ‐120.94250 Municipal 0 0 0 DHS 5010018‐009 WELL 09 xxx xxx 5010010‐061 37.65147 ‐121.02083 Municipal 0 0 70 DHS 5010010‐061 WELL 056 x x x x 5010010‐041 37.69001 ‐120.97187 Municipal 0 116 100 DHS 5010010‐041 WELL 039 x x 5010010‐191 37.64560 ‐120.90525 Municipal 0 0 50 DHS 5010010‐191 WELL 247 ‐ NORTH EMPIRE x x x x x x 5010010‐052 37.69679 ‐121.01066 Municipal 0 200 90 DHS 5010010‐052 WELL 050 x x x x x 5000372‐001 37.66433 ‐121.05939 Municipal 0 245 20 DHS 5000372‐001 WELL 01 x x 5010010‐184 37.63483 ‐120.93568 Municipal 0 0 100 DHS 5010010‐184 WELL 279 ‐ FARRAR (OLD 06) x x x x x x x 5010018‐007 37.72726 ‐120.95580 Municipal 0 209 132 DHS 5010018‐007 WELL 07 x x xxx 5000058‐002 37.74658 ‐120.90888 Municipal 0 80 20 DHS 5000058‐002 WEST‐ MHP WELL x x 5010018‐010 37.71508 ‐120.95810 Municipal 0 0 0 DHS 5010018‐010 WELL 10 xxx xxx 5000017‐001 37.73708 ‐120.95675 Municipal 0 150 100 DHS 5000017‐001 ARROWOOD (EAST) WELL x 5000211‐004 37.71232 ‐120.91980 Municipal 0 104 110 DHS 5000211‐004 WELL NO. 05 x 5010010‐045 37.69369 ‐121.02357 Municipal 0 151 152 DHS 5010010‐045 WELL 043 ‐STANDBY x x 5010010‐027 37.68571 ‐121.00140 Municipal 0 91 275 DHS 5010010‐027 WELL 025 x x x 5000110‐001 37.64850 ‐120.97817 Municipal 0 20 10 DHS 5000110‐001 SOUTH/ MAIN WELL x5000013‐002 37.78609 ‐120.81264 Municipal 0 0 0 DHS 5000013‐002 WELL 02‐ 2709 OAKHURST x x x x 5010018‐003 37.73033 ‐120.94992 Municipal 0 0 80 DHS 5010018‐003 WELL 03 x xxxxx 5000213‐001 37.66593 ‐121.06596 Municipal 0 0 0 DHS 5000213‐001 LPA REPORTED PRIMARY SOURCE x

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Appendix G ‐ Water Quality Monitoring Network
Well ID Latitude Longitude Well Type Well Depth (ft bgs) Top of Screen (ft bgs) Screen Length (ft) Dataset Name Alternative Well ID Alternative Well ID 2 Nitrate Uranium PCE TCP DBCP TDS Arsenic Water Quality Parameters
5000041‐001 37.63766 ‐121.15292 Municipal 0 210 20 DHS 5000041‐001 EAST WELL NEW #02 x 5010010‐148 37.63222 ‐121.01908 Municipal 0 0 75 DHS 5010010‐148 WELL 283 ‐ ANWAR MANOR x x xxx 5000316‐001 37.62464 ‐121.05458 Municipal 0 50 10 DHS 5000316‐001 WELL 01 x x x x 5010010‐070 37.63391 ‐120.99295 Municipal 0 0 148 DHS 5010010‐070 WELL 057 xxx xxx 5010010‐009 37.65093 ‐120.99944 Municipal 0 160 50 DHS 5010010‐009 WELL 007 x x x x5010010‐146 37.62581 ‐121.03147 Municipal 0 0 38 DHS 5010010‐146 WELL 304 xxx xxx 5010010‐187 37.66055 ‐120.96670 Municipal 0 0 75 DHS 5010010‐187 WELL 269 ‐ ROSE AVENUE x x x 5010010‐149 37.64199 ‐121.03415 Municipal 0 0 10 DHS 5010010‐149 WELL 237 ‐ ELM x x x 5000189‐003 37.70452 ‐121.00170 Municipal 0 295 20 DHS 5000189‐003 S. WELL #1 (BY 4500 N. STAR) xxx x x 5000295‐001 37.60964 ‐121.11564 Municipal 0 110 20 DHS 5000295‐001 WELL 01 x 5000411‐001 37.72012 ‐120.99655 Municipal 0 185 20 DHS 5000411‐001 WELL 4 EAST MAIN WELL x 5010010‐008 37.65071 ‐120.98702 Municipal 0 0 100 DHS 5010010‐008 WELL 006 xxxx xx 5010010‐042 37.64458 ‐120.94783 Municipal 0 97 132 DHS 5010010‐042 WELL 040 xx xxx 5010010‐196 37.64526 ‐120.97845 Municipal 0 0 95 DHS 5010010‐196 WELL 211 ‐ THOUSAND OAKSx x x 5010010‐194 37.63565 ‐120.94334 Municipal 0 0 95 DHS 5010010‐194 WELL 212 ‐ BEARD AVENUE x x x x x x x 5010010‐172 37.66808 ‐120.98508 Municipal 0 0 68 DHS 5010010‐172 WELL 300 x x x x 5010010‐178 37.63784 ‐120.93285 Municipal 0 0 60 DHS 5010010‐178 WELL 292 ‐ MARIPOSA WEST x x x x x x x 5010010‐003 37.64277 ‐120.99117 Municipal 0 0 100 DHS 5010010‐003 WELL 001 x x x 5010010‐147 37.62531 ‐121.03148 Municipal 0 0 38 DHS 5010010‐147 WELL 301 x x x x 5000274‐001 37.62464 ‐121.05458 Municipal 0 72 73 DHS 5000274‐001 NEW WELL xxxx 5010010‐035 37.67377 ‐121.03156 Municipal 0 96 188 DHS 5010010‐035 WELL 033 x x x 5000346‐001 37.71408 ‐120.99550 Municipal 0 310 20 DHS 5000346‐001 WELL 01 x 5000435‐002 37.77464 ‐120.80089 Municipal 0 264 25 DHS 5000435‐002 NEW WELL 01 x 5000049‐002 37.77475 ‐120.82256 Municipal 0 50 10 DHS 5000049‐002 SOUTH WELL x x xxx5000090‐002 37.62556 ‐120.84303 Municipal 0 50 20 DHS 5000090‐002 SOUTH WELL xx xx5000090‐013 37.62557 ‐120.84319 Municipal 0 110 30 DHS 5000090‐013 SOUTH WEST NEW WELL x x xx 5000320‐001 37.71000 ‐121.03000 Municipal 0 0 0 DHS 5000320‐001 WELL 01 ‐ INACTIVE x 5000164‐001 37.65733 ‐120.66006 Municipal 0 0 10 DHS 5000164‐001 WELL #1 x 5000164‐004 37.66001 ‐120.65574 Municipal 0 235 40 DHS 5000164‐004 WELL #4 x 5000164‐003 37.65726 ‐120.66549 Municipal 0 300 25 DHS 5000164‐003 WELL #3 x5000054‐002 37.71066 ‐120.96966 Municipal 0 20 18 DHS 5000054‐002 SOUTH WELL x x x x 5000284‐001 37.60964 ‐121.11564 Municipal 0 50 24 DHS 5000284‐001 WELL 01 x 5010010‐189 37.66316 ‐120.97808 Municipal 0 0 75 DHS 5010010‐189 WELL 267 ‐ ORANGEBURGx x x 5010010‐151 37.64091 ‐121.01933 Municipal 0 0 55 DHS 5010010‐151 WELL 236 ‐ EMERALD x x x x 5000261‐003 37.72249 ‐120.99584 Municipal 0 0 0 DHS 5000261‐003 2007 WELL x 5000535‐001 37.71417 ‐121.00101 Municipal 0 0 0 DHS 5000535‐001 2003 WELL 01 x x 5000562‐002 37.71516 ‐120.99481 Municipal 0 0 0 DHS 5000562‐002 NEW 2006 WELL x 5000571‐001 37.66536 ‐120.74831 Municipal 0 0 0 DHS 5000571‐001 WELL x 5000493‐002 37.70913 ‐120.92022 Municipal 0 0 0 DHS 5000493‐002 2016 WELL x 5000509‐001 37.77256 ‐120.77358 Municipal 0 330 40 DHS 5000509‐001 MAIN 2/96 WELL OLD OFFICE x 5000457‐002 37.72415 ‐120.99566 Municipal 0 0 0 DHS 5000457‐002 WELL 01 x 5000516‐001 37.70967 ‐120.94115 Municipal 0 205 20 DHS 5000516‐001 WELL x5010010‐192 37.63757 ‐120.95876 Municipal 0 0 158 DHS 5010010‐192 WELL 225 ‐ BUDGET PACK x x x 5000538‐001 37.66759 ‐120.90568 Municipal 0 0 0 DHS 5000538‐001 2003 WELL x 5000462‐001 37.68692 ‐120.92228 Municipal 0 333 30 DHS 5000462‐001 MOTEL WELL x5000467‐001 37.68692 ‐120.92228 Municipal 0 130 20 DHS 5000467‐001 LPA REPORTED PRIMARY SOURCE x5000426‐001 37.70085 ‐120.98959 Municipal 0 50 10 DHS 5000426‐001 WELL 01 x 5000585‐001 37.63855 ‐121.12369 Municipal 0 0 0 DHS 5000585‐001 1999 DOMESTIC WELL x 5000409‐001 37.60867 ‐121.11690 Municipal 0 50 10 DHS 5000409‐001 LPA REPORTED PRIMARY SOURCE x 5000164‐002 37.66297 ‐120.67831 Municipal 0 0 14 DHS 5000164‐002 WELL #2 x 5000561‐001 37.71313 ‐120.99368 Municipal 0 0 0 DHS 5000561‐001 2005 DOMESTIC WATER WELL x 5000368‐001 37.69661 ‐120.97175 Municipal 0 92 110 DHS 5000368‐001 WELL 01 x 5000388‐001 37.65169 ‐121.02475 Municipal 0 50 10 DHS 5000388‐001 WELL 01 x

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Appendix G ‐ Water Quality Monitoring Network
Well ID Latitude Longitude Well Type Well Depth (ft bgs) Top of Screen (ft bgs) Screen Length (ft) Dataset Name Alternative Well ID Alternative Well ID 2 Nitrate Uranium PCE TCP DBCP TDS Arsenic Water Quality Parameters
5000401‐001 37.60867 ‐121.11690 Municipal 0 100 200 DHS 5000401‐001 LPA REPORTED PRIMARY SOURCE x 5000091‐001 37.77980 ‐120.81679 Municipal 0 80 10 DHS 5000091‐001 SOUTH WELL x 5000506‐001 37.69836 ‐120.88367 Municipal 0 0 0 DHS 5000506‐001 WELL 01 x 5000551‐001 37.70059 ‐120.93784 Municipal 0 0 0 DHS 5000551‐001 WELL x 5000290‐001 37.63844 ‐121.12181 Municipal 0 50 10 DHS 5000290‐001 LPA REPORTED PRIMARY SOURCE x5000583‐001 37.64193 ‐121.06593 Municipal 0 0 0 DHS 5000583‐001 WELL 1x 5000486‐001 37.70914 ‐120.92019 Municipal 0 0 10 DHS 5000486‐001 LPA REPORTED PRIMARY SOURCE x L10005824413‐MW‐12S 37.62429 ‐120.84759 Monitoring 60.35 43 20 EDF MW‐12S MW‐12S x xxxxx L10005824413‐MW‐18D 37.63122 ‐120.84827 Monitoring 128.82 108 20 EDF MW‐18D MW‐18D x xxxxx L10005824413‐MW‐15S 37.61763 ‐120.85804 Monitoring 42.63 0 0 EDF MW‐15S MW‐15S x xxxxx L10005824413‐MW‐1S 37.62139 ‐120.84983 Monitoring 62.94 48 63 EDF MW‐1S MW‐1S x xxxxx L10005824413‐MW‐22D 37.62909 ‐120.84804 Monitoring 116.89 100 20 EDF MW‐22D MW‐22D x xxxxx L10005824413‐MW‐2D 37.61980 ‐120.85249 Monitoring 97.18 75 20 EDF MW‐2D MW‐2D x xxxxx L10005824413‐MW‐23S 37.62277 ‐120.85776 Monitoring 37.09 0 0 EDF MW‐23S MW‐23S x xxxxx L10005824413‐MW‐25D3 37.62267 ‐120.85618 Monitoring 132.34 132.25 15 EDF MW‐25D3 MW‐25D3 x xxxxx L10005824413‐MW‐26S 37.62829 ‐120.85277 Monitoring 87.34 87.2 20 EDF MW‐26S MW‐26S x xxxxxL10005824413‐MW‐13S 37.62747 ‐120.84811 Monitoring 81.18 60 20 EDF MW‐13S MW‐13S x xxxxx L10005824413‐MW‐16S 37.62618 ‐120.84678 Monitoring 87.15 64 20 EDF MW‐16S MW‐16S x xxxxx L10005824413‐MW‐17D 37.63090 ‐120.85130 Monitoring 118.74 98 20 EDF MW‐17D MW‐17D x xxxxx L10005824413‐MW‐19D 37.62471 ‐120.84766 Monitoring 98.15 84 20 EDF MW‐19D MW‐19D x xxxxx L10005824413‐MW‐8S 37.62040 ‐120.85687 Monitoring 29.95 0 0 EDF MW‐8S MW‐8S x xxxxx L10005824413‐MW‐21D 37.63065 ‐120.84806 Monitoring 116.09 109 10 EDF MW‐21D MW‐21D x xxxxx L10005824413‐MW‐23D 37.62281 ‐120.85772 Monitoring 80.24 0 0 EDF MW‐23D MW‐23D x xxxxx L10005824413‐MW‐24S 37.62620 ‐120.84461 Monitoring 93.04 93 20 EDF MW‐24S MW‐24S x xxxxxL10005824413‐MW‐3D 37.62532 ‐120.85532 Monitoring 85.53 0 0 EDF MW‐3D MW‐3D x xxxxx L10005824413‐MW‐1D 37.62137 ‐120.84984 Monitoring 90.29 80 10 EDF MW‐1D MW‐1D x xxxxx L10005824413‐MW‐21S 37.63065 ‐120.84806 Monitoring 80.74 65 20 EDF MW‐21S MW‐21S x xxxxxL10005824413‐MW‐14SR 37.62154 ‐120.85382 Monitoring 65.96 66 20 EDF MW‐14SR MW‐14SR x xxxxx L10005824413‐MW‐24D 37.62620 ‐120.84469 Monitoring 132.81 133 20 EDF MW‐24D MW‐24D x xxxxx L10005824413‐MW‐27D 37.62883 ‐120.86088 Monitoring 72.25 72.3 20 EDF MW‐27D MW‐27D x xxxxx L10005824413‐MW‐5S 37.61952 ‐120.85203 Monitoring 63.91 0 0 EDF MW‐5S MW‐5S x xxxxxL10005824413‐PZ‐3 37.62822 ‐120.85672 Monitoring 25.88 0 0 EDF PZ‐3PZ‐3 x xxxxx L10005824413‐PZ‐6 37.62959 ‐120.86088 Monitoring 25.29 0 0 EDF PZ‐6PZ‐6 x xxxxx L10005824413‐MW‐9S 37.61878 ‐120.85437 Monitoring 29.66 12 20 EDF MW‐9S MW‐9S x xxxxxL10005824413‐MW‐17S 37.63090 ‐120.85130 Monitoring 88.58 68 20 EDF MW‐17S MW‐17S x xxxxxL10005824413‐MW‐18S 37.63122 ‐120.84827 Monitoring 88.17 68 20 EDF MW‐18S MW‐18S x xxxxxL10005824413‐MW‐22S 37.62909 ‐120.84804 Monitoring 77.89 62 20 EDF MW‐22S MW‐22S x xxxxxL10005824413‐MW‐26D 37.62830 ‐120.85280 Monitoring 127.11 127 20 EDF MW‐26D MW‐26D x xxxxx L10005824413‐MW‐7D 37.62611 ‐120.84943 Monitoring 126.34 104 20 EDF MW‐7D MW‐7D x xxxxx L10005824413‐PZ‐2 37.63084 ‐120.85678 Monitoring 24.96 0 0 EDF PZ‐2PZ‐2 x xxxxx L10005824413‐PZ‐4 37.62958 ‐120.85914 Monitoring 26.93 0 0 EDF PZ‐4PZ‐4 x xxxxx L10005824413‐MW‐11S 37.62294 ‐120.84817 Monitoring 80.24 55 20 EDF MW‐11S MW‐11S x xxxxxL10005824413‐MW‐7S 37.62610 ‐120.84943 Monitoring 84.35 63 20 EDF MW‐7S MW‐7S x xxxxxL10005824413‐MW‐25D2 37.62269 ‐120.85618 Monitoring 82.25 82.2 10 EDF MW‐25D2 MW‐25D2 x xxxxx L10005824413‐PZ‐1 37.62960 ‐120.85449 Monitoring 25.36 0 0 EDF PZ‐1PZ‐1 x xxxxx L10005824413‐MW‐4S 37.62283 ‐120.85614 Monitoring 34.93 0 0 EDF MW‐4S MW‐4S x xxxxxL10005824413‐MW‐15D 37.61766 ‐120.85800 Monitoring 76.72 63 10 EDF MW‐15D MW‐15D x xxxxx L10005824413‐MW‐3S 37.62534 ‐120.85531 Monitoring 25.05 0 0 EDF MW‐3S MW‐3S x xxxxxL10005824413‐MW‐4D 37.62277 ‐120.85618 Monitoring 60.29 0 0 EDF MW‐4D MW‐4D x xxxxx L10005824413‐MW‐19S 37.62471 ‐120.84767 Monitoring 66.72 49 20 EDF MW‐19S MW‐19S x xxxxxL10005824413‐MW‐2S 37.61982 ‐120.85246 Monitoring 57.45 0 0 EDF MW‐2S MW‐2S x xxxxxL10005824413‐MW‐10S 37.62024 ‐120.85017 Monitoring 68.06 50 20 EDF MW‐10S MW‐10S x xxxxxL10005824413‐MW‐27S 37.62885 ‐120.86090 Monitoring 39.28 39.4 20 EDF MW‐27S MW‐27S x xxxxx

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Appendix G ‐ Water Quality Monitoring Network
Well ID Latitude Longitude Well Type Well Depth (ft bgs) Top of Screen (ft bgs) Screen Length (ft) Dataset Name Alternative Well ID Alternative Well ID 2 Nitrate Uranium PCE TCP DBCP TDS Arsenic Water Quality Parameters
100834 37.63130 ‐120.99850 Municipal 0 0 0 LLNL 100834 03S/09E‐32G01 M xxxxxxx 100830 37.68420 ‐120.96730 Municipal 0 0 0 LLNL 100830 03S/09E‐10P01 M xxxxxxx 100833 37.67570 ‐120.94760 Municipal 0 0 0 LLNL 100833 03S/09E‐14G01 M xxxxxxx 100832 37.64210 ‐120.91890 Monitoring 0 35 75 LLNL 100832 03S/10E‐30M01 M xxxxxxx 100829 37.69680 ‐121.01070 Monitoring 0 70 62 LLNL 100829 03S/09E‐05N02 M xxxxxxx 5000055‐002 37.70583 ‐120.92042 Municipal 0 100 40 DHS 5000055‐002 WEST FIELD x xSL205012989‐M‐19C1 37.73000 ‐121.11000 Monitoring 0 137 20 EDF M‐19C1 M‐19C1 x SL205012989‐M‐31C2D 37.72000 ‐121.12000 Monitoring 0 196 15 EDF M‐31C2D M‐31C2D x SL205833043‐MMW‐01A 37.68713 ‐120.92128 Monitoring 0 70 90 EDF MMW‐01A MMW‐01A x x x SL205833043‐MMW‐24A 37.68665 ‐120.92103 Monitoring 0 70 90 EDF MMW‐24A MMW‐24A x x x SL205012989‐M‐20C1 37.72000 ‐121.12000 Monitoring 0 140 10 EDF M‐20C1 M‐20C1 x SL205012989‐M‐21C1 37.72000 ‐121.13000 Monitoring 0 125 20 EDF M‐21C1 M‐21C1 x SL205012989‐M‐21D 37.72000 ‐121.13000 Monitoring 0 215 20 EDF M‐21D M‐21D x SL205012989‐M‐23C1 37.72000 ‐121.12000 Monitoring 0 110.8 20 EDF M‐23C1 M‐23C1 x SL205012989‐M‐31C1 37.72000 ‐121.12000 Monitoring 0 120 5 EDF M‐31C1 M‐31C1 xSL205012989‐M‐5C1 37.73000 ‐121.11000 Monitoring 0 149 15 EDF M‐5C1 M‐5C1 x SL205012989‐M‐35A 37.72030 ‐121.13850 Monitoring 0 115 5 EDF M‐35A M‐35A x SL205012989‐MW‐11 37.72000 ‐121.14000 Monitoring 0 125 30 EDF MW‐11 MW‐11 xSL205012989‐M‐7A 37.73000 ‐121.11000 Monitoring 0 94 20 EDF M‐7A M‐7A x SL205012989‐M‐32D 37.72050 ‐121.13170 Monitoring 0 217 15 EDF M‐32D M‐32D x SL205012989‐M‐30C2 37.72000 ‐121.12000 Monitoring 0 150 5 EDF M‐30C2 M‐30C2 x SL205012989‐TH‐9 37.72000 ‐121.12000 Monitoring 0 80 20 EDF TH‐9TH‐9x SLT5S1883227‐DD‐4 37.66904 ‐120.99180 Monitoring 119.22 109.22 10 EDF DD‐4DD‐4 xxx SL205012989‐M‐30C1 37.72000 ‐121.12000 Monitoring 0 120 15 EDF M‐30C1 M‐30C1 x SL205012989‐M‐34D 37.72050 ‐121.13240 Monitoring 0 224 10 EDF M‐34D M‐34D x SL205012989‐M‐35D 37.72030 ‐121.13850 Monitoring 0 244 7 EDF M‐35D M‐35D x SL205833043‐MMW‐27A 37.68517 ‐120.91972 Monitoring 0 70 90 EDF MMW‐27A MMW‐27A x x x SL205012989‐TH‐10 37.72000 ‐121.12000 Monitoring 0 120 10 EDF TH‐10 TH‐10 x SL205833043‐MMW‐28A 37.68629 ‐120.92163 Monitoring 0 70 90 EDF MMW‐28A MMW‐28A x x x SL205012989‐M‐23D 37.72000 ‐121.12000 Monitoring 0 221.2 10 EDF M‐23D M‐23D x SL205012989‐M‐36C 37.72130 ‐121.12380 Monitoring 0 134 5 EDF M‐36C M‐36C x SL205833043‐MMW‐18A 37.68647 ‐120.92049 Monitoring 0 70 90 EDF MMW‐18A MMW‐18A x x xSL205012989‐TH‐1 37.73000 ‐121.11000 Monitoring 0 250 60 EDF TH‐1TH‐1x SL205012989‐MW‐7 37.73000 ‐121.11000 Monitoring 0 80 40 EDF MW‐7MW‐7x SL205012989‐M‐20D 37.72000 ‐121.12000 Monitoring 0 205 20 EDF M‐20D M‐20D x SL205012989‐M‐23A 37.72000 ‐121.12000 Monitoring 0 74.8 20 EDF M‐23A M‐23A x SL205012989‐M‐34A 37.72050 ‐121.13240 Monitoring 0 79 10 EDF M‐34A M‐34A x SL205012989‐M‐35B 37.72030 ‐121.13850 Monitoring 0 60 10 EDF M‐35B M‐35B xSL205012989‐M‐5C2 37.73000 ‐121.11000 Monitoring 0 180 10 EDF M‐5C2 M‐5C2 x SL205012989‐M‐5A 37.73000 ‐121.11000 Monitoring 0 95 20 EDF M‐5A M‐5A x SL205012989‐M‐34C 37.72050 ‐121.13240 Monitoring 0 135 10 EDF M‐34C M‐34C x SLT5S1883227‐DD‐1 37.66953 ‐120.99252 Monitoring 118.67 108.67 10 EDF DD‐1DD‐1 xxx SL205012989‐M‐26C2 37.73000 ‐121.11000 Monitoring 0 180 15 EDF M‐26C2 M‐26C2 x SL205833043‐MMW‐14A 37.68550 ‐120.92110 Monitoring 0 70 90 EDF MMW‐14A MMW‐14A x SL205833043‐MMW‐21A 37.68613 ‐120.92034 Monitoring 0 70 90 EDF MMW‐21A MMW‐21A x SL205833043‐MMW‐02A 37.68549 ‐120.92007 Monitoring 0 70 90 EDF MMW‐02A MMW‐02A x x SL205833043‐MMW‐25A 37.68758 ‐120.92127 Monitoring 0 70 90 EDF MMW‐25A MMW‐25A x 5000588‐001 37.65809 ‐121.03037 Municipal 0 0 0 DHS 5000588‐001 WELL 01 xSL185742938‐M‐151 37.64856 ‐121.01341 Monitoring 88 68 20 EDF M‐151 M‐151 xx SL185742938‐M‐101 37.64664 ‐121.01610 Monitoring 75 55 20 EDF M‐101 M‐101 xx SL185742938‐M‐103 37.65059 ‐121.01623 Monitoring 75 55 20 EDF M‐103 M‐103 xx SL185742938‐M‐107 37.65057 ‐121.01623 Monitoring 145 134 11 EDF M‐107 M‐107 xx SL185742938‐M‐113 37.64365 ‐121.01084 Monitoring 80 55 20 EDF M‐113 M‐113 xx

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Appendix G ‐ Water Quality Monitoring Network
Well ID Latitude Longitude Well Type Well Depth (ft bgs) Top of Screen (ft bgs) Screen Length (ft) Dataset Name Alternative Well ID Alternative Well ID 2 Nitrate Uranium PCE TCP DBCP TDS Arsenic Water Quality Parameters
SL185742938‐M‐121 37.64566 ‐121.00876 Monitoring 71 60 25 EDF M‐121 M‐121 xx SL185742938‐M‐150 37.64871 ‐121.01612 Monitoring 175 155 20 EDF M‐150 M‐150 xx SL185742938‐M‐154 37.64725 ‐121.02637 Monitoring 65 45 20 EDF M‐154 M‐154 xx SL185742938‐M‐157 37.64161 ‐121.02370 Monitoring 65 45 20 EDF M‐157 M‐157 xx SL185742938‐M‐159 37.63559 ‐121.00900 Monitoring 65 45 20 EDF M‐159 M‐159 xx SL185742938‐M‐9R 37.65204 ‐121.02030 Monitoring 75 55 20 EDF M‐9R M‐9R xx SL185742938‐M‐105 37.65301 ‐121.01874 Monitoring 75 55 20 EDF M‐105 M‐105 xx SL185742938‐M‐111 37.64751 ‐121.01610 Monitoring 125.5 96 24 EDF M‐111 M‐111 xx SL185742938‐M‐152 37.64703 ‐121.01359 Monitoring 95 75 20 EDF M‐152 M‐152 xx SL185742938‐M‐156 37.64161 ‐121.02377 Monitoring 168 148 20 EDF M‐156 M‐156 xx SL185742938‐M‐161 37.64677 ‐121.01631 Monitoring 172 152 20 EDF M‐161 M‐161 xx SL185742938‐M‐2R 37.65010 ‐121.02073 Monitoring 75 55 20 EDF M‐2R M‐2R xx SL185742938‐M‐102 37.64854 ‐121.01611 Monitoring 75 55 20 EDF M‐102 M‐102 xx SL185742938‐M‐118 37.65303 ‐121.01877 Monitoring 170 146 19 EDF M‐118 M‐118 xx SL185742938‐M‐153 37.64867 ‐120.99769 Monitoring 65 45 20 EDF M‐153 M‐153 xx SL185742938‐M‐158 37.63557 ‐121.00898 Monitoring 150 130 20 EDF M‐158 M‐158 xx SL185742938‐M‐162 37.64693 ‐121.01441 Monitoring 175 155 20 EDF M‐162 M‐162 xx SL185742938‐M‐112 37.64369 ‐121.01082 Monitoring 180 145 30 EDF M‐112 M‐112 xx SL185742938‐M‐104 37.64899 ‐121.01712 Monitoring 75 55 20 EDF M‐104 M‐104 xx SL185742938‐M‐120 37.65110 ‐121.01524 Monitoring 190 155 30 EDF M‐120 M‐120 xx SL185742938‐M‐155 37.64736 ‐121.03298 Monitoring 147 125 20 EDF M‐155 M‐155 xx SL185742938‐M‐108 37.65060 ‐121.01623 Monitoring 105 95 10 EDF M‐108 M‐108 xx SL185742938‐M‐160 37.64939 ‐121.01989 Monitoring 170 150 20 EDF M‐160 M‐160 xx SL185742938‐M‐109 37.64763 ‐121.01610 Monitoring 93.5 60 28 EDF M‐109 M‐109 xx SL185742938‐M‐163 37.64860 ‐121.01338 Monitoring 165 145 20 EDF M‐163 M‐163 xx SL185742938‐M‐119 37.65112 ‐121.01527 Monitoring 80 56 19 EDF M‐119 M‐119 xx SL185742938‐M‐6R 37.64782 ‐121.01803 Monitoring 75 55 20 EDF M‐6R M‐6R xx SL185742938‐M‐106 37.64871 ‐121.01911 Monitoring 75 55 20 EDF M‐106 M‐106 xx T10000009029‐MW‐12C 37.72915 ‐120.93208 Monitoring 0 139 10 EDF MW‐12C MW‐12C x T10000009029‐MW‐3R 37.73055 ‐120.93464 Monitoring 0 75 20 EDF MW‐3R MW‐3R x T10000009029‐MW‐12A 37.72915 ‐120.93213 Monitoring 0 93.5 10 EDF MW‐12A MW‐12A x T10000009029‐MW‐20 37.73093 ‐120.93474 Monitoring 0 95 15 EDF MW‐20 MW‐20 x T10000009029‐MW‐22 37.73061 ‐120.93465 Monitoring 0 0 0 EDF MW‐22 MW‐22 x T10000009029‐MW‐4R 37.73033 ‐120.93411 Monitoring 0 81.5 20 EDF MW‐4R MW‐4R xT10000009029‐MW‐1R 37.73084 ‐120.93463 Monitoring 0 75 20 EDF MW‐1R MW‐1R x T10000009029‐MW‐12B 37.72915 ‐120.93217 Monitoring 0 115 5 EDF MW‐12B MW‐12B x T10000009029‐MW‐21 37.73023 ‐120.93472 Monitoring 0 0 0 EDF MW‐21 MW‐21 x T10000009029‐MW‐4B 37.73037 ‐120.93411 Monitoring 0 103 5 EDF MW‐4B MW‐4B x T10000009029‐MW‐4C 37.73044 ‐120.93412 Monitoring 0 135 5 EDF MW‐4C MW‐4C x T10000009029‐MW‐7R 37.73093 ‐120.93470 Monitoring 0 81.5 25 EDF MW‐7R MW‐7R x Total Count 323 57 162 88 144 150 174
Abbreviationsft: feet
bgs: below ground surface
PCE: Tetrachloroethene
TCP: 1,2,3‐TrichloropropaneDBCP: Dibromochloropropane
TDS: total dissolved solids

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5. WATER BUDGETS
Water budgets are a critical component of understanding and evaluating a groundwater basin’s sustainability. This chapter discusses the:
• General background on water budgets, the basis of the selected water budgets (historical, current conditions, projected conditions), and their components • Average annual Subbasin- and area1-wide stream, land and water use, and groundwater budgets summarized in tabular format
• Results and insights from the water budget for the historical, current conditions, and projected conditions budgets with supporting figures • Projected water budget under climate change conditions, including climate change methodology and resulting impacts on the Subbasin
• Sustainable yield assumptions and resulting water budgets • Discussion of the importance of hydrologic variability on the water budgets and the range of change in groundwater storage for the Projected Conditions, Climate Change scenario, and Sustainable Yield scenario for each water year type.

5.1. WATER BUDGET INFORMATION
Comprehensive hydrologic water budgets were developed to provide a quantitative understanding of water entering (inflows) and leaving (outflows) the Modesto Subbasin and are a requirement of the GSP regulations. Water budgets are provided for the three interconnected systems that define the overall hydrologic balance in the Modesto Subbasin - the land surface system, the stream and river system, and the groundwater system. Water entering and leaving each one of the physical systems, and water movement among the systems are a combination of natural processes and anthropogenic conditions. Figure 5-1 highlights the main water budget components and interconnectivity of stream, surface, and groundwater components used in this analysis.

The values presented in the water budget provide hydrologic information on the historical, current, and projected conditions of the Modesto Subbasin relating to water demand, water supply, land use, population, climate change, groundwater and surface water interaction, and subsurface groundwater flow. An understanding of these impacts can assist in management of the Subbasin by identifying the scale of different water uses, highlighting potential risks presented by each condition, and identifying potential opportunities to improve water supply conditions and use of resources.
1 The term “area” herein represents the four main subdivisions of the Modesto Subbasin discussed in this report – Modesto Irrigation District, Oakdale Irrigation District, Non-District East, and Non-District West. The establishment of these zones as Management Areas is discussed in Section 6.2. #25..
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Figure 5-1: Generalized Water Budget Diagram
The water budgets presented below reflect the interconnected movement of water through the land surface system (the soil zone), the stream system, and the groundwater system.

Together, these systems and their interactions comprise the integrated water resources system which represents the comprehensive water cycle for the Subbasin. This comprehensive water budget is consistent with SGMA, GSP regulations, best management practices (BMPs), and recommendations in the Handbook for Water Budget Development published by the DWR (2020).

Water budgets can also be developed at different temporal scales. Daily water budgets can be used to demonstrate diurnal variation in the temperature and water use for agriculture and/or stream flows to assess implications on the fisheries and wildlife. Monthly water budgets are typically used to demonstrate variability in agricultural water demand during the irrigation season, or monthly and seasonal variability in surface water supply and/or groundwater pumping. The water budget for the Modesto Subbasin were developed on monthly intervals, though are presented on an annual basis in this report for presentation purposes and to facilitate their incorporation into policy decisions.

GSP regulations require that three sets of annual water budgets be developed, each reflecting the hydrology under historical, current, and projected levels of urban and agricultural development. Water budgets are developed to capture long-term conditions, which are assessed by averaging hydrologic conditions over several different timeframes.

The historical water budgets reflect the average hydrology over a 25-year period (1991- 2015), while current conditions are represented by a recent average year from the historical period (2010), and projected conditions are represented by the average of a 50-year hydrologic period. This provides opportunities to incorporate dry years and drought conditions, wet periods, and normal periods. By incorporating these varied conditions into #25..
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the water budgets, the system can be analyzed in the short- and long-terms, allowing for assessment of the system response to certain hydrologic conditions (e.g., drought) and for assessment of broader system averages. The following subsection provides additional detail on identification of hydrologic periods.

5.1.1. Identification of Hydrologic Periods
Hydrologic periods were selected to meet the needs of developing historical, current, and projected water budgets. The GSP regulations require that the projected conditions are assessed over a 50-year hydrologic period to represent long-term hydrologic conditions.

Precipitation data for the Modesto Subbasin were used to identify hydrologic periods that are representative of wet and dry periods and long-term average conditions needed for water budget analyses.

Rainfall data for the Subbasin is derived from the detailed database provided by the Precipitation-Elevation Regressions on Independent Slopes Model (PRISM) dataset. This data set is commonly used by DWR and other organizations for mapping the spatial and temporal distribution of precipitation throughout the state. DWR uses PRISM for the California Simulation of Evapotranspiration of Applied Water (CALSIMETAW) model, which is a major source of estimates of ET of applied water (ETAW) throughout the state. Periods with a balance of wet and dry intervals were identified by evaluating the cumulative departure from mean precipitation. Figure 5-2 shows the annual precipitation and cumulative departure from the mean for the Modesto Subbasin. While the annual rainfall and precipitation data provides information on annual variability of rainfall over the course of the planning period, the cumulative departure from mean is indicative of long-term trends in Subbasin precipitation. In this context, the rising limbs of the cumulative departure line indicate short-term and long-term wet periods (e.g., 1978-83 and 1992-98), while falling limbs indicate short and long dry periods (e.g., 1976-77 and 2011-15). For the Modesto Subbasin water budget analysis, rainfall and water supply and demand conditions are available for the period October 1968 to September 2018 (WY 1969-2018), with an average annual rainfall of 12.4 inches. For the historical water budget analysis, the period WY 1991- 2015 (average annual precipitation of 12.6 inches) is used, which coincides with the period for which the C2VSimTM model is calibrated, and for which the historical water demand and supplies have been confirmed. These periods of record meet the GSP regulatory requirement of at least 10 years for the historical water budget analysis. For the projected water budget purposes, the full period of WY 1969-2018 is used, which provides a 50-year record as required by GSP regulations.
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Figure 5-2: 50-Year Historical Precipitation and Cumulative Departure from Mean Precipitation, Modesto Subbasin, California

5.1.2. Usage of C2VSimTM and Associated Data in Water Budget Development Water budgets were developed utilizing C2VSimTM, a fully integrated surface and groundwater flow model covering the entire Central Valley. This version of C2VSim is based on the C2VSimFG-BETA2 model released by DWR. To support the GSP, C2VSimTM was developed and refined with a focus on land and water use operational data for both the Modesto and Turlock Subbasins. C2VSimTM, a quasi-three-dimensional finite element model, was developed using the Integrated Water Flow Model (IWFM) 2015 software package to simulate the relevant hydrologic processes prevailing in the model domain. The C2VSimTM integrates the groundwater aquifer with the surface hydrologic system and land surface processes and operations. Using data from federal, state, and local resources, the C2VSimTM was calibrated for the hydrologic period of October 1991 to September 2015 by comparing simulated evapotranspiration, groundwater levels, and streamflow records with historical observed records. Development of the model involved the study and analyses of hydrogeologic conditions, agricultural and urban water demands, agricultural and urban water supplies, and an evaluation of regional water quality conditions. Additional information on the data used to develop C2VSimTM is included in Appendix C.

All integrated hydrologic models contain assumptions and some level of uncertainty. They are decision support tools used to better understand complex interactive systems. Sources of model uncertainty include heterogeneity in hydrogeologic properties and stratigraphy, quality of historical data, projections of future land use, hydrology, operational data, and climatic conditions.
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C2VSimTM has been calibrated and validated. The data and assumptions for Modesto and Turlock Subbasins were developed in a collaborative manner with the respective districts and are based on best available data and science. Projections of future land use and water demands were based on the most recent planning documents prepared by agencies in the Subbasin. In its current form, the model represents the best available data for the Subbasin.

As additional information is collected during GSP implementation, the model will be updated to reflect the newly available resources. Efforts to address Subbasin data gaps will improve information available for the model.

With the C2VSimTM as the underlying framework, model simulations were developed to allow for the estimation of water budgets. Four model simulations were used to develop the water budgets for historical, current, projected, and climate change conditions, which are discussed in detail below:

The historical water budget is based on a simulation of historical conditions in the Modesto Subbasin (1991-2015).

The current water budget is based on an average year (2010) of the historical simulation that incorporates current irrigation and operational practices.

The projected water budget is based on a simulation of future land and water use over the historical hydrologic conditions.

The climate change water budget is based on the projected water budget under 2070 climate conditions and is discussed in Section 5.2.

The sustainable yield water budget is based on the projected water budget refined to meet SGMA sustainability criteria and is discussed in Section 5.3

5.1.3. Water Budget Definitions and Assumptions
Definitions and assumptions for the historical, current, and projected water budgets are provided below. These assumptions are summarized in Table 5-1.

5.1.3.1. Historical Water Budget
The historical water budget is intended to evaluate availability and reliability of past surface water supply deliveries, aquifer response to water supply, and demand trends relative to WY type. The historical calibration of the C2VSimTM reflects the historical conditions in the Modesto Subbasin through the 2015 water year. The hydrologic period of WY 1991 through 2015 is selected for the GSP historical water budget because it provides a period of representative hydrology while capturing recent operations within the Subbasin. The period WY 1991 through 2015 has an average annual precipitation of approximately 12.6 inches, slightly higher than the long-term average of 12.4 inches observed for the 50-year projected hydrologic period of WY 1969-2018. Both periods include the recent WY 2012-2015 drought, the wetter years of WY 1998 and 2010-2011, and periods of normal precipitation. #25..
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5.1.3.2. Current Water Budget
The current conditions water budget uses recent historical conditions. The 2010 water year was selected to represent current conditions because it was the last normal water year before the 2012-2015 drought. It represents the current level of development within the Subbasin and reflects current agricultural irrigation practices, land use patterns, surface water operations, and urban water usage under non-drought conditions.

5.1.3.3. Projected Water Budget
The projected water budget is intended to assess the hydrologic systems of the Subbasin under the projected agricultural and urban demand, water supply, and operational conditions over the next 50-years. The Projected Conditions Baseline scenario applies projected future land and water use conditions to the 50-year hydrologic period of WY 1969-2018. The Projected Condition Baseline assumes urban population and land use expansion based on each municipality’s 2015 Urban Water Management Plan. Under projected conditions, agricultural land is held constant at 2015 cropping patters except where urban expansion pulls acreage out of production. Furthermore, under projected conditions, the consumptive use factor (CUF), or the ratio of evapotranspiration per unit of applied water, was increased relative to the historical to simulate modernization of irrigation management and technologies within the Subbasin.

The Projected Conditions Baseline includes the following conditions:
• Hydrologic period:
o WY 1969-2018 (50-year hydrology)
• River flow is based on:
o Tuolumne River: Tuolumne River System (TRS) operations model o Stanislaus River: Average monthly values by water year type o San Joaquin River: CalSim II baseline operations
• Land use is based on:
o 2015 agricultural land use and cropping patterns held constant o Urban land use expansion based on 2015 UWMP
• Agricultural water demand is based on:
o IWFM estimates based on current land use and refined CUF • Surface water deliveries are based on data from:
o Modesto ID – Tuolumne River System (TRS) operations model o Oakdale ID – Historical monthly average by water year type o Subbasin Riparian Users – Historical monthly average by water year type • Urban water demand is based on:
o 2015 Urban Water Management Plans (UWMPs)
o Continuation of historical population trends, while meeting 2020 State of California GPCD goals.
• Urban water supply is based on:
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o Expanded surface water deliveries from MID to the City of Modesto o Projected urban groundwater production based on 2015 UWMPs distributed to existing wells
Table 5-1: Summary of Groundwater Budget Assumptions
Water Budget
Type Historical Current Projected
Tool C2VSimTM C2VSimTM C2VSimTM
Scenario Historical
Simulation
Current
Conditions
Baseline
Projected Conditions
Baseline
Hydrologic Years WY 1991-2015 WY 2010 WY 1969-2018
Level of
Development Historical Records WY 2010 General Plan buildout Agricultural
Demand Historical Records WY 2010
Projected based on refined
2015 land use and modern
irrigation practices
Urban Demand Historical Records WY 2010
Projected based on local
UWMP data and historical
population growth
Water Supplies Historical Records WY 2010
Projected based on local
operations modeling and
historical trends

5.1.4. Water Budget Estimates
The primary components of the stream system, presented at the Subbasin scale, are:
• Inflows:
o Stream inflows into the Tuolumne River and Stanislaus River at the boundary of the model and San Joaquin River inflows at upstream of the confluence of the Tuolumne and San Joaquin River (bounding the Modesto Subbasin) o Tributary inflows from surface water contributions from small watersheds o Total stream gain from the groundwater system
o Surface runoff from precipitation to the stream system o Return flow of applied water to the stream system
• Outflows:
o San Joaquin River flow downstream of the Stanislaus River confluence #25..
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o Surface water supplies diverted from the stream system to meet agricultural or urban demand downstream of La Grange Dam.
o Stream seepage to the groundwater system
o Uptake of river water from native or riparian vegetation along the stream bed The primary components of the land surface system, presented for each water budget zone, include:
• Supplies:
o Precipitation
o Surface water supplies
o Groundwater supplies
o Uptake of river water from native or riparian vegetation along the stream bed • Demands:
o Evapotranspiration
o Surface runoff of precipitation to the stream system
o Return flow of applied water to the stream system
o Percolation of water to the groundwater system
o Land surface system balance
The primary components of the groundwater system, presented at the Subbasin scale, are:
• Inflows:
o Percolation of water from the land surface system
o Groundwater gains from stream system
o Subsurface inflow from neighboring subbasins and the foothills • Outflows:
o Groundwater discharge to the stream system
o Groundwater production (pumping)
o Subsurface outflow to neighboring subbasins
o Change in groundwater in storage - negative values represent a depletion of storage
The estimated water budgets are provided below in Table 5-2 through Table 5-8 for the historical, current, and projected water budgets. The land surface water budgets are presented for the entire Subbasin and for each water budget zone (Modesto Irrigation District managed zone (Modesto), Oakdale South, NDE, and Non-District West). Each of these zones represent the geographic area shown in Figure 5-3 and include all sectors, including agricultural, industrial, municipal, and domestic water users. These zones have been used to develop Management Areas (as defined in the GSP regulations) based primarily on the availability of surface water sources. These Management Areas, along with #25..
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the justification and rationale for each, are presented in Section 6.2 on Sustainable Management Criteria.

Developing operational water budgets for the land surface system has allowed the GSAs to better quantify how varying anthropogenic processes have affected and will continue to affect the aquifer system. In contrast, the stream and groundwater system budgets are presented at the subbasin scale, to best target the GSA’s sustainability goals and metrics.

Figure 5-3: Water Budget Zones
Modesto
Oakdale
South Non-District
East
NonDistrict
West
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Table 5-2: Average Annual Water Budget – Stream Systems, Modesto Subbasin (AFY)
Component Historical Condition
Water Budget
Current Condition
Water Budget
Projected Condition
Water Budget
Hydrologic Period WY 1991- 2015 WY 2010 Hydrology from
WY 1969 - 2018
Stream Inflows 2,547,000 1,625,000 2,650,000
Stanislaus River 520,000 320,000 536,000
Tuolumne River 742,000 593,000 812,000
San Joaquin River 1,285,000 711,000 1,302,000
Tributary Inflow1 6,000 - 6,000
Stream Gain from Groundwater 207,000 167,000 104,000
Modesto Subbasin 100,000 80,000 50,000
Stanislaus River - South2 35,000 27,000 12,000
Tuolumne River - North 51,000 39,000 27,000
San Joaquin River - East 15,000 13,000 11,000
Other Subbasins 108,000 88,000 54,000
Stanislaus River – North 37,000 30,000 12,000
Tuolumne River - South 56,000 44,000 31,000
San Joaquin River - West 15,000 14,000 11,000
Surface Runoff to the Stream
System3
57,000 35,000 60,000
Return Flow to Stream System3 104,000 97,000 113,000
Total Inflow 2,922,000 1,923,000 2,934,000
San Joaquin River Outflows 2,770,000 1,745,000 2,717,000 Diverted Surface Water4 43,000 47,000 33,000
Stream Seepage to Groundwater 74,000 95,000 146,000
Modesto Subbasin 40,000 51,000 76,000
Stanislaus River - South 19,000 20,000 36,000
Tuolumne River - North 20,000 30,000 38,000
San Joaquin River - East 1,000 - 2,000
Other Subbasins 34,000 44,000 71,000
Stanislaus River - North 13,000 14,000 31,000
Tuolumne River - South 20,000 30,000 38,000
San Joaquin River - West 1,000 - 2,000
Native & Riparian Uptake from
Streams 35,000 37,000 37,000
Total Outflow 2,922,000 1,923,000 2,934,000
Note: sub-categories may not sum together due to rounding error 1 Tributary inflow includes surface water contributions from small watersheds 2 Represents the location of the Modesto Subbasin relative to the stream, i.e., “South” represents the gains/losses of that stream to the Modesto Subbasin where as “North” represents the gains/losses of that stream to the Eastern San Joaquin Subbasin. 3 Includes runoff/return flow from all subbasins adjacent to the stream system, not just the Modesto Subbasin. 4 Some surface water diversions are upstream of the Tuolumne River or Stanislaus River inflows and thus not included in this stream system (streams and canals) water budget.
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Table 5-3: Average Annual Water Budget – Land Surface System, Modesto Subbasin (AFY)
Component Historical Condition
Water Budget
Current Condition
Water Budget
Projected Condition
Water Budget
Hydrologic Period WY 1991- 2015 WY 2010 Hydrology from
WY 1969 - 2018
Agricultural Areas Precipitation 147,000 122,000 139,000 Agricultural Water Supply 513,000 611,000 497,000
Agency Surface Water 264,000 250,000 241,000
Agency Groundwater 26,000 15,000 25,000
Private Groundwater 222,000 345,000 229,000
Urban Areas Precipitation 32,000 26,000 38,000
Urban Water Supply 89,000 88,000 111,000
Groundwater 63,000 56,000 60,000
Surface Water 26,000 32,000 51,000
Native Areas Precipitation 92,000 78,000 92,000
Native Uptake from Stream 20,000 20,000 22,000
Total Supplies 892,000 945,000 900,000
Agricultural ET 368,000 416,000 402,000
Agricultural ET of Precipitation 80,000 73,000 82,000
Agricultural ET of Surface Water 149,000 143,000 159,000 Agricultural ET of Agency
Groundwater 14,000 8,000 16,000
Agricultural ET of Private
Groundwater 125,000 192,000 146,000
Agricultural Percolation 246,000 236,000 201,000
Agricultural Percolation of
Precipitation 57,000 39,000 45,000
Agricultural Percolation of Surface
Water 99,000 83,000 75,000
Agricultural Percolation of Agency
Groundwater 10,000 5,000 8,000
Agricultural Percolation of Private
Groundwater 81,000 110,000 73,000
Agricultural Runoff & Return Flow 35,000 31,000 31,000
Urban Runoff & Return Flow 74,000 68,000 91,000
Urban ET 28,000 27,000 38,000
Urban Percolation 18,000 17,000 20,000
Native Runoff 12,000 5,000 12,000
Native ET 91,000 88,000 95,000
Native Percolation 8,000 3,000 7,000
Total Demands 879,000 892,000 898,000
Land Surface System Balance 13,000 53,000 2,000
Land Surface System Balance
(% of supplies) 1.5% 5.6% 0.2%
Note: sub-categories may not sum together due to rounding error #25..
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Table 5-4: Average Annual Water Budget – Land Surface System, Modesto Area (AFY)
Component Historical Condition
Water Budget
Current Condition
Water Budget
Projected Condition
Water Budget
Hydrologic Period WY 1991- 2015 WY 2010 Hydrology from
WY 1969 - 2018
Agricultural Areas Precipitation 73,000 58,000 65,000
Agricultural Water Supply 281,000 315,000 244,000
Agency Surface Water 125,000 121,000 106,000
Agency Groundwater 22,000 11,000 21,000
Private Groundwater 135,000 183,000 117,000
Urban Areas Precipitation 26,000 21,000 32,000
Urban Water Supply 73,000 72,000 96,000
Groundwater 47,000 40,000 45,000
Surface Water 26,000 32,000 51,000
Native Areas Precipitation 11,000 9,000 11,000
Native Uptake from Stream 5,000 5,000 5,000
Total Supplies 468,000 481,000 453,000
Agricultural ET 193,000 210,000 195,000
Agricultural ET of Precipitation 38,000 34,000 38,000
Agricultural ET of Surface Water 69,000 68,000 68,000
Agricultural ET of Agency
Groundwater 12,000 6,000 14,000
Agricultural ET of Private
Groundwater 74,000 103,000 75,000
Agricultural Percolation 136,000 137,000 97,000
Agricultural Percolation of
Precipitation 29,000 21,000 21,000
Agricultural Percolation of Surface
Water 48,000 44,000 33,000
Agricultural Percolation of Agency
Groundwater 8,000 4,000 6,000
Agricultural Percolation of Private
Groundwater 51,000 67,000 36,000
Agricultural Runoff & Return Flow 20,000 18,000 16,000
Urban Runoff & Return Flow 61,000 56,000 78,000
Urban ET 22,000 21,000 31,000
Urban Percolation 16,000 16,000 19,000
Native Runoff 1,000 - 1,000
Native ET 14,000 13,000 14,000
Native Percolation 1,000 1,000 1,000
Total Demands 463,000 471,000 453,000
Land Surface System Balance 6,000 10,000 1,000
Land Surface System Balance
(% of supplies) 1.2% 2.1% 0.1%
Note: sub-categories may not sum together due to rounding error #25..
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Table 5-5: Average Annual Water Budget – Land Surface System, Oakdale South Area (AFY)
Component Historical Condition
Water Budget
Current Condition
Water Budget
Projected Condition
Water Budget
Hydrologic Period WY 1991- 2015 WY 2010 Hydrology from
WY 1969 - 2018
Agricultural Areas Precipitation 46,000 40,000 45,000
Agricultural Water Supply 150,000 174,000 143,000
Agency Surface Water 120,000 109,000 121,000
Agency Groundwater 4,000 4,000 4,000
Private Groundwater 26,000 61,000 18,000
Urban Areas Precipitation 4,000 3,000 4,000
Urban Water Supply 11,000 12,000 9,000
Groundwater 11,000 12,000 9,000
Surface Water - - -
Native Areas Precipitation 13,000 10,000 13,000
Native Uptake from Stream 2,000 2,000 2,000
Total Supplies 225,000 241,000 217,000
Agricultural ET 112,000 125,000 124,000
Agricultural ET of Precipitation 25,000 24,000 27,000
Agricultural ET of Surface Water 69,000 63,000 81,000
Agricultural ET of Agency
Groundwater 2,000 2,000 3,000
Agricultural ET of Private
Groundwater 15,000 36,000 12,000
Agricultural Percolation 72,000 59,000 57,000
Agricultural Percolation of
Precipitation 17,000 11,000 14,000
Agricultural Percolation of Surface
Water 45,000 30,000 37,000
Agricultural Percolation of Agency
Groundwater 1,000 1,000 1,000
Agricultural Percolation of Private
Groundwater 9,000 17,000 5,000
Agricultural Runoff & Return Flow 8,000 6,000 7,000
Urban Runoff & Return Flow 9,000 9,000 8,000
Urban ET 4,000 4,000 5,000
Urban Percolation 2,000 1,000 1,000
Native Runoff 2,000 1,000 2,000
Native ET 12,000 11,000 12,000
Native Percolation 1,000 1,000 1,000
Total Demands 221,000 217,000 217,000
Land Surface System Balance 4,000 24,000 -
Land Surface System Balance
(% of supplies) 1.7% 9.8% 0.0%
Note: sub-categories may not sum together due to rounding error #25..
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Table 5-6: Average Annual Water Budget – Land Surface System, Non-District East (AFY)
Component Historical Condition
Water Budget
Current Condition
Water Budget
Projected Condition
Water Budget
Hydrologic Period WY 1991- 2015 WY 2010 Hydrology from
WY 1969 - 2018
Agricultural Areas Precipitation 19,000 16,000 19,000
Agricultural Water Supply 48,000 84,000 81,000
Agency Surface Water - - -
Agency Groundwater - - -
Private Groundwater 48,000 84,000 81,000
Urban Areas Precipitation - - -
Urban Water Supply - - -
Groundwater - - -
Surface Water - - -
Native Areas Precipitation 65,000 57,000 65,000
Native Uptake from Stream 6,000 6,000 7,000
Total Supplies 137,000 163,000 173,000
Agricultural ET 37,000 54,000 60,000
Agricultural ET of Precipitation 11,000 11,000 10,000
Agricultural ET of Surface Water - - -
Agricultural ET of Agency
Groundwater - - -
Agricultural ET of Private
Groundwater 26,000 43,000 50,000
Agricultural Percolation 22,000 23,000 34,000
Agricultural Percolation of
Precipitation 7,000 4,000 7,000
Agricultural Percolation of Surface
Water - - -
Agricultural Percolation of Agency
Groundwater - - -
Agricultural Percolation of Private
Groundwater 16,000 19,000 27,000
Agricultural Runoff & Return Flow 5,000 5,000 6,000
Urban Runoff & Return Flow - - -
Urban ET - - -
Urban Percolation - - -
Native Runoff 9,000 4,000 9,000
Native ET 56,000 54,000 58,000
Native Percolation 5,000 2,000 5,000
Total Demands 134,000 142,000 171,000
Land Surface System Balance 4,000 21,000 1,000
Land Surface System Balance
(% of supplies) 2.6% 13.1% 0.8%
Note: sub-categories may not sum together due to rounding error #25..
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Table 5-7: Average Annual Water Budget – Land Surface System, Non-District West (AFY)
Component Historical Condition
Water Budget
Current Condition
Water Budget
Projected Condition
Water Budget
Hydrologic Period WY 1991- 2015 WY 2010 Hydrology from
WY 1969 - 2018
Agricultural Areas Precipitation 10,000 8,000 10,000
Agricultural Water Supply 35,000 38,000 29,000
Agency Surface Water 19,000 20,000 15,000
Agency Groundwater - - -
Private Groundwater 15,000 17,000 14,000
Urban Areas Precipitation 2,000 2,000 2,000
Urban Water Supply 5,000 4,000 6,000
Groundwater 5,000 4,000 6,000
Surface Water - - -
Native Areas Precipitation 3,000 2,000 3,000
Native Uptake from Stream 7,000 7,000 8,000
Total Supplies 61,000 61,000 57,000
Agricultural ET 26,000 27,000 24,000
Agricultural ET of Precipitation 6,000 5,000 6,000
Agricultural ET of Surface Water 11,000 12,000 9,000
Agricultural ET of Agency
Groundwater - - -
Agricultural ET of Private
Groundwater 9,000 10,000 9,000
Agricultural Percolation 16,000 18,000 13,000
Agricultural Percolation of
Precipitation 4,000 3,000 3,000
Agricultural Percolation of Surface
Water 7,000 8,000 5,000
Agricultural Percolation of Agency
Groundwater - - -
Agricultural Percolation of Private
Groundwater 5,000 7,000 4,000
Agricultural Runoff & Return Flow 3,000 2,000 2,000
Urban Runoff & Return Flow 4,000 3,000 5,000
Urban ET 2,000 2,000 3,000
Urban Percolation - - -
Native Runoff - - -
Native ET 10,000 10,000 11,000
Native Percolation - - -
Total Demands 61,000 62,000 57,000
Land Surface System Balance - (2,000) -
Land Surface System Balance
(% of supplies) 0.7% -2.5% -0.2%
Note: sub-categories may not sum together due to rounding error #25..
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Table 5-8: Average Annual Water Budget – Groundwater System, Modesto Subbasin (AFY)
Component
Historical
Condition
Water Budget
Current Condition
Water Budget
Projected Condition
Water Budget
Hydrologic Period WY 1991- 2015 WY 2010 Hydrology from
WY 1969 - 2018
Gain from Stream 40,000 51,000 76,000
Gain from Stanislaus River 19,000 20,000 36,000
Gain from Tuolumne River 20,000 30,000 38,000
Gain from San Joaquin River 1,000 - 2,000
Canal & Reservoir Recharge 49,000 47,000 47,000
Deep Percolation 272,000 257,000 228,000
Subsurface Inflow 80,000 79,000 77,000
Flow from the Sierra Nevada
Foothills 9,000 5,000 9,000
Eastern San Joaquin Subbasin
Inflows 8,000 9,000 28,000
Turlock Subbasin Inflows 30,000 34,000 33,000
Delta Mendota Subbasin Inflows 33,000 31,000 7,000
Total Inflow 440,000 434,000 428,000
Discharge to Stream 100,000 80,000 50,000
Discharge to Stanislaus River 35,000 27,000 12,000
Discharge to Tuolumne River 51,000 39,000 27,000
Discharge to San Joaquin River 15,000 13,000 11,000
Subsurface Outflow 73,000 63,000 75,000
Eastern San Joaquin Subbasin
Outflows 6,000 5,000 35,000
Turlock Subbasin Outflows 32,000 24,000 34,000
Delta Mendota Subbasin Outflows 36,000 35,000 6,000
Groundwater Production 311,000 416,000 314,000
Agency Ag. Groundwater
Production 26,000 15,000 25,000
Private Ag. Groundwater
Production 222,000 345,000 229,000
Urban Groundwater Production 63,000 56,000 60,000
Total Outflow 483,000 559,000 438,000
Change in Groundwater in
Storage (43,000) (125,000) (11,000)
Note: sub-categories may not sum together due to rounding error

5.1.4.1. Historical Water Budget
The historical water budget is a quantitative evaluation of the historical surface and groundwater supply covering the 25-year period from WY 1991 to 2015. This period was selected as the representative hydrologic period as it reflects the most recent basin operations and has similar average precipitation compared to a longer historical period (WY 1969-2018). The goal of the water budget analysis is to characterize the water supply and #25..
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demand, while summarizing the accounting of water demand and supply components and their changes within each area, and the Subbasin as a whole.

Figure 5-4 below shows the average annual water budget components for the entirety of the Modesto Subbasin and the interaction between the land surface, stream, and groundwater systems for the historical simulation.

Figure 5-4: Average Annual Historical Water Budget – Modesto Subbasin Note: sub-categories may not sum together due to rounding error The existing stream system supplies multiple water users and agencies in the Modesto Subbasin, including Modesto ID, Oakdale ID, and riparian diverter along each of the major rivers. Analysis of the stream system accounts for potentially significant effects related to both natural interactions and managed operations of adjacent subbasins. Therefore, the water budget in Table 5-2 above and Figure 5-5, shown below, provides average annual quantities of surface and canal system flows within the Modesto Subbasin, plus estimates of interactions with adjoining subbasins. Average annual surface water inflow to the streams adjacent to the Subbasin is estimated to be 2,921,000 AFY. Most of these flows enter the stream system through inflows from regulated reservoirs and river courses, with an average of 742,000 AFY from the Tuolumne, 520,000 AFY from the Stanislaus, and 1,285,000 AFY from the San Joaquin Rivers, respectively. Other stream system inflows include inflow from tributary watersheds (6,000 AFY), surface runoff from precipitation (57,000 AFY), return flow from applied water (104,000 AFY), and gain from groundwater (207,000 AFY).

Outflows from the Modesto Subbasin stream system total 2,922,000 AFY and include stream losses to the groundwater system (74,000 AFY), surface water diversions (43,000 AFY), and riparian uptake (35,000 AFY). Most outflows from the stream system are San Joaquin River flows, which discharge from the Modesto Subbasin downstream of its confluence with the Stanislaus River at an average of 2,770,000 AFY. Note that surface water diversions for Oakdale and Modesto Irrigation Districts occur from reservoirs upstream of the Subbasin boundaries and are not included in the stream-system budget. #25..
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Figure 5-5: Historical Average Annual Water Budget – Stream Systems, Modesto Subbasin
The land surface system of the Modesto Subbasin, shown in Table 5-3 and in Figure 5-6, represents the demand and supplies in the Modesto Subbasin and in each zone. During the historical period, total average annual water supplies to the Modesto Subbasin is estimated at 892,000 AFY, consisting of precipitation (271,000 AFY), surface water deliveries (290,000 AFY), and groundwater supplies (312,000 AFY), as well as water uptake by riparian vegetation along the river courses (20,000 AFY). Surface water supplies are provided primarily through Modesto ID’s and Oakdale ID’s canal networks to growers in the districts, with some riparian surface water diversions in the Non-District West. Each of these areas supplement their surface water with some groundwater production to meet their agricultural and urban demand, whereas the NDE areas rely primarily on groundwater production for its agricultural supplies.

Average annual water demand in the Modesto Subbasin totals 879,000 AFY, and is comprised of agricultural crops, urban landscaping, and native evapotranspiration (487,000 AFY), surface runoff and return flow to the stream system (121,000 AFY), and deep percolation (272,000 AFY). Figure 5-7 shows the annual volumes of major agricultural water demand and supply components throughout the historical water budget period. The surface water supply in this water budget is reflective of the applied water thus does not include operational return flow or canal seepage.

Figure 5-8 shows the annual supply and demand for municipal and private domestic water use in the Modesto Subbasin.
#25..
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Figure 5-6: Historical Average Annual Water Budget – Land Surface System, Modesto Subbasin
Figure 5-7: Historical Annual Water Budget – Agricultural Land Surface System, Modesto Subbasin
#25..
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Figure 5-8: Historical Annual Water Budget – Urban Land Surface System, Modesto Subbasin
Table 5-8 highlights the major flow components of the Modesto Subbasin’s groundwater system. As shown in this table, the aquifer receives approximately 440,000 AFY of inflows each year, which consist of recharge from streams (40,000 AFY), seepage from canals and reservoirs (49,000 AFY), deep percolation from precipitation and applied water (272,000 AFY), as well as subsurface inflows from the Sierra Nevada foothills and the neighboring subbasins of Eastern San Joaquin, Delta-Mendota, and Turlock (80,000 AFY combined).

Table 5-8 also shows the outflows from the Modesto Subbasin. On average, the outflows exceed the inflows in the Subbasin. The largest component of outflow from the groundwater system is groundwater pumping (311,000 AFY), followed by discharge to streams (100,000 AFY), and subsurface outflow to the neighboring subbasins (73,000 AFY).

In conjunction with the land surface budgets presented for each water budget area, a netrecharge analysis was preformed to better understand the relationship of water supply conditions and recharge to the groundwater system. This analysis is documented below, both at the Subbasin level and for each water budget area.

Figure 5-9 shows the total annual groundwater pumped from, and the subsequent recharge to the Modesto Subbasin. In this figure, groundwater pumping represents the combination of groundwater extracted for both agricultural and urban use for each year during the historical period. Recharge into the aquifer system includes both deep percolation from the land system and direct recharge from the canal and reservoir system. The deep percolation in this figure includes recharge from percolated precipitation, agricultural applied water, outdoor irrigation from municipal and rural domestic users. #25..
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Figure 5-10 shows the net-recharge in the Modesto Subbasin and is based on the annual balance from the previous figure. This figure indicates that during the historical period, the Subbasin has trended increasingly toward net extraction, but has on average experienced net recharge. This is both indicative of local hydrology and increasing demand on the aquifer system. Over the 25-year historical period, the Modesto Subbasin has seen a large increase in both urban demand and agricultural production. Over time, increases in groundwater production has further stressed the subbasin leading to more consistently negative values, or net extractions. Furthermore, through the 2012-2015 drought, the subbasin experienced a greater net-extraction from the aquifer system corresponding to reduced surface water supply, whereas in periods of wetter or normal operations, the Subbasin has historically been a net-contributor to the groundwater system.

Figure 5-11 through Figure 5-18 show similar trends conditions for each water budget area.

The Oakdale South water budget zone (Figure 5-14) has predominately experienced net recharge, while the NDE zone has predominately experienced net extraction (Figure 5-16).

The Modesto water budget zone and the Non-District West zone experience more variable conditions trending in near-balance (Figure 5-12 and Figure 5-18, respectively). Over the historical period, all zones have trended increasingly toward net extraction due to increased water demand from all sectors and drought conditions at the end of the period.

Overall, the Modesto Subbasin’s groundwater system has experienced long term (25-year) decline in storage averaging 43,000 AFY as shown in Figure 5-19. This decline is more heavily weighted to the end of the study period due to increased stresses relating to both local hydrology, and water demand as shown in Figure 20. Figure 20 also shows the temporal breakdown of the groundwater budget and highlights the intensifying decline of groundwater in storage in recent years, particularly under drought conditions where groundwater production has increased to a long-term high.

The historical inflows and outflows to the Modesto Subbasin change with hydrologic conditions. In wet years, precipitation and increased surface water availability reduces the need for groundwater use. However, in dry years, more groundwater is pumped to meet the demand not met by surface water or precipitation. This leads to an increase in groundwater in storage in wet years and a decrease in dry years. These trends are shown in Table 5-9, which provides average historical water supply and demand by water year type. #25..
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Figure 5-9: Groundwater Recharge and Extraction – Modesto Subbasin Figure 5-10: Net Recharge – Modesto Subbasin
#25..
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Figure 5-11: Groundwater Recharge and Extraction – Modesto Zone Figure 5-12: Net Recharge – Modesto Zone
#25..
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Figure 5-13: Groundwater Recharge and Extraction – Oakdale South Zone Figure 5-14: Net Recharge – Oakdale South Zone
#25..
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Figure 5-15: Groundwater Recharge and Extraction – Non-District East Zone Figure 5-16: Net Recharge – Non-District East Zone
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Figure 5-17: Groundwater Recharge and Extraction – Non-District West Area Figure 5-18: Net Recharge – Non-District West Area
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Figure 5-19: Historical Average Annual Water Budget – Groundwater System, Modesto Subbasin
Figure 5-20: Historical Annual Water Budget – Groundwater System, Modesto Subbasin
On Figure 20, positive numbers indicate inflows into the Subbasin aquifer, while negative numbers indicate outflows from the Subbasin aquifer.
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Table 5-9: Water Supply and Demand Budget by Year Type (AFY) Component
Water Year Type (San Joaquin River Index)
Wet Above
Normal
Below
Normal Dry Critical Average
Agricultural Demand 479,000 526,000 511,000 532,000 533,000 516,000 Urban Demand 84,000 89,000 101,000 100,000 85,000 92,000 Total Water Demand 563,000 615,000 612,000 632,000 618,000 608,000 Total Surface Water Supply 317,000 332,000 335,000 342,000 289,000 323,000 Agricultural 292,000 299,000 302,000 308,000 271,000 294,000 Urban 25,000 33,000 33,000 34,000 18,000 29,000
Total Groundwater Supply 246,000 283,000 277,000 290,000 329,000 285,000 Agricultural 187,000 227,000 209,000 225,000 262,000 222,000 Urban 59,000 56,000 68,000 65,000 67,000 63,000
Total Water Supply 563,000 615,000 612,000 632,000 618,000 608,000 Change in GW Storage 90,000 -59,000 -69,000 -96,000 -136,000 -43,000 Notes: sub-categories may not sum together due to rounding error All values in Table 5-9 are from WYs 1991-2015

5.1.4.2. Current Water Budget
The current water budget quantifies inflows to and outflows from the basin under existing conditions. The 2010 water year was selected to represent current conditions because it reflects an average, non-drought water supply with existing land use and water demand.

Table 5-2 and Figure 5-21 summarize the average annual inflows and outflows of the Current Conditions Baseline in the Modesto Subbasin stream system. Under current conditions, inflows to the stream system total 1,923,000 AFY with 1,625,000 AFY coming directly as inflow to the Stanislaus, Tuolumne, and San Joaquin Rivers, 35,000 AFY is the result of surface runoff from precipitation, 97,000 AFY of return flow from applied water, and 167,000 AFY of groundwater contributions. In contrast to stream inflow, stream system outflows under current conditions include an average of 47,000 AFY of surface water diversions for agricultural use, 95,000 AFY of discharge to the groundwater system, 37,000 AFY of direct uptake by riparian vegetation, and 1,745,000 AFY of downstream outflows in the San Joaquin River.
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Figure 5-21: Current Conditions Annual Water Budget – Stream Systems, Modesto Subbasin
The land surface system water supply under Current Conditions, shown in Table 5-3 and in Figure 5-22, is estimated using 2010 cropping patterns as the Subbasin experienced significant changes due to the 2012-2015 drought. Under the current Conditions Baseline the average annual water supply is estimated to be 945,000 AFY, including 226,000 AFY of precipitation, 699,000 AFY of surface and groundwater supply for irrigation and urban use (282,000 AFY of surface water and 417,000 AFY of groundwater), and 20,000 AFY of riparian uptake from the stream system.

The total water demand is estimated to be 892,000 AFY, which includes evapotranspiration (531,000 AFY), surface runoff and return flow to the stream system (105,000 AFY), and deep percolation (257,000 AFY). Figure 5-22 summarizes the average annual current condition supplies and demands in the land surface budget for the Modesto Subbasin.

Figure 5-22: Current Conditions Average Annual Water Budget – Land Surface System, Modesto Subbasin
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The groundwater system budget for current conditions baseline indicates an average annual inflow of 434,000 AFY, including 257,000 AFY of deep percolation, 47,000 AFY of canal and reservoir seepage, 51,000 AFY from stream seepage, and total subsurface inflows of 79,000 AFY.

Analysis of the groundwater system budget indicates that the system’s average annual outflows exceed its inflows under current conditions, resulting in a net reduction in groundwater in storage. As under historical conditions, groundwater production (416,000 AFY) remains the largest component of groundwater discharge, with subsurface outflows (63,000 AFY) and discharge to the stream system (80,000 AFY) bringing the total system outflows to 559,000 AFY annually. Operational water budgets and net-groundwater interaction under current conditions remain like those of the historical period, based on the 2010 water year. On a Subbasin-wide scale, the groundwater in storage deficit under the current conditions baseline is approximately 125,000 AFY.

Figure 5-23 and Table 5-8 summarize the average current conditions groundwater inflows and outflows in the Modesto Subbasin.

Figure 5-23: Current Conditions Average Annual Water Budget – Groundwater System, Modesto Subbasin

5.1.4.3. Projected Water Budget
The projected water budget provides an estimate of supplies and demands as defined under the projected conditions baseline listed above, including land use operations and their impact toon the aquifer system. The projected conditions baseline is a version of C2VSimTM and was used to evaluate the water budget using projected operations in conjunction with the 50-year hydrologic period, 1969 to 2018. This hydrologic period has an average precipitation similar to the long-term average over the Subbasin. Within this 50-year period, there is variability in hydrologic conditions which allows the model to simulate different stresses.
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Development of the projected water demand is based on the population growth trends reported in the 2015 UWMPs and the land use, evapotranspiration, and crop coefficient information from the Modesto ID and Oakdale ID 2015 AWMPs. Projected Tuolumne River inflows to the groundwater Subbasin and surface water supplies are determined through a combination of historical trends and the Tuolumne River System (TRS) operations model.

Additional information about model development and inputs are detailed in the C2VSimTM Model Development Technical Memo in Appendix C.

Figure 5-24 shows the water budget schematic for the Modesto Subbasin with average annual projected values for each component.

Figure 5-24: Average Annual Projected Conditions Water Budget – Modesto Subbasin
Note: sub-categories may not sum together due to rounding error As shown in Table 5-2, average annual surface water inflows to the Modesto Subbasin’s stream system total an average of 2,934,000 AFY. As with the historical and current conditions water budgets, stream inflows from the Stanislaus, Tuolumne, and San Joaquin Rivers comprise most of the inflows, averaging 2,650,000 AFY. Other inflows include contributions from tributaries (6,000 AY), gain from the aquifer (104,000 AFY), surface runoff from precipitation (60,000 AFY), and return flow from applied water to the stream system (113,000 AFY).

Under projected conditions, volumes of surface water diverted from Modesto Subbasin’s stream system are lower than under historical conditions, down to 33,000 AFY from 43,000 AFY. Reduced diversion volumes under projected conditions are due to reduced demand by riparian users resulting from projected increases in irrigation efficiency. Other stream system outflows include seepage to the aquifer system (146,000 AFY), direct uptake by native vegetation (37,000 AFY), and San Joaquin River outflows downstream of the Tuolumne River confluence (2,717,000 AFY).
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Groundwater levels are predicted to be further reduced under projected conditions than under historical conditions, and thus the 86,000 AFY reduction in net contribution from the aquifer2to the stream system matches the expected trend. Under such a decrease in aquifer contribution, streams in Modesto Subbasin transition from average net gaining streams to net losing streams. Therefore, under historical conditions, aquifers on average recharge streams, but under projected conditions, streams on average, recharge the aquifer. Figure 5-25 summarizes the average projected inflows and outflows in the Modesto Subbasin surface water network.

Figure 5-25: Projected Conditions Average Annual Water Budget – Stream Systems, Modesto Subbasin
The land surface water budget for the Projected Conditions Baseline is shown on Table 5-3 and has average annual supplies of 900,000 AFY. Supplies are comprised of precipitation (270,000 AFY), applied surface water (293,000 AFY), applied groundwater (315,000 AFY), and riparian uptake from streams (22,000 AFY). Demands total 898,000 AFY and are comprised of evapotranspiration (536,000 AFY), surface runoff and return flow (134,000 AFY) to the stream system, and deep percolation (228,000 AFY).

Urban supplies and demands increase relative to historical conditions due to forecasted population growth. Additionally, agricultural demand (evapotranspiration) is higher because agricultural land use is assumed to be at the historical high, reflecting more developed acres than average historical conditions. However, there is less percolation out of the root zone and agricultural return flow because of the projected improvements in irrigation efficiency (e.g., drip irrigation). The lower runoff in the projected conditions baseline compared to the historical scenario is driven by lower precipitation. There are no projected changes to soil 2 Net contribution from the aquifer includes stream gains and losses within and outside of the Modesto Subbasin – any region adjacent to the Stanislaus River, Tuolumne River, and San Joaquin River.
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characteristics (i.e., curve number or soil parameters) between the historical and projected conditions baseline scenarios.

A summary of these flows can be seen below in Figure 5-26 though Figure 5-28. Figure 5-27 and
Figure 5-28 show the annual change in the land surface water budget components through the simulation period.

Figure 5-26: Projected Conditions Average Annual Water Budget – Land Surface System, Modesto Subbasin
Figure 5-27: Projected Conditions Annual Water Budget – Agricultural Land Surface System, Modesto Subbasin
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Figure 5-28: Projected Conditions Annual Water Budget – Urban Land Surface System, Modesto Subbasin
Anticipated growth in the Projected Conditions Baseline slightly increases groundwater production (314,000 AFY), compared to historical pumping. Subsurface outflows to neighboring subbasins (75,000 AF) and stream gain from groundwater (50,000 AFY) bring the total Subbasin discharges to 438,000 AFY.

Under projected conditions, the groundwater system of the Modesto Subbasin experiences an average of 428,000 AFY of inflows each year, of which 228,000 AFY is from deep percolation of rainfall and applied water. As previously mentioned, deep percolation from applied water is lower than under historical conditions because of projected increases in irrigation efficiency. Other inflows to the groundwater system consist of recharge from stream seepage (76,000 AFY), seepage from conveyance canals and reservoirs (47,000 AFY), and subsurface inflows from the Sierra Nevada foothills and neighboring subbasins of Eastern San Joaquin, Delta-Mendota, and Turlock (77,000 AFY combined). A summary of annual averages of the Modesto Subbasin groundwater system is provided on Table 5-8.

Under the projected conditions the groundwater system outflows are greater than the system inflows, resulting in an average annual groundwater in storage deficit of 11,000 AFY.

While an average groundwater in storage decline of 11,000 AFY is significantly less than historical depletion (43,000 AFY), the decline is buffered by the net gain of 86,000 AFY of seepage from the stream system. This change in the projected groundwater conditions and stream-aquifer interactions are considered significant and unreasonable, which affects groundwater sustainability of the Subbasin.

An analysis of net recharge in the Projected Conditions model was performed for Modesto Subbasin and for each water budget area. Figure 5-29 shows the total groundwater #25..
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production and land-surface recharge each year under the projected conditions scenario.

Additionally, the net-groundwater under projected conditions, shown in Figure 5-30, is predominantly negative, meaning that on average, the subbasin is a net-extractor. This continuation of historical trends reflects the relationship between the Subbasin’s increased groundwater demand and declining storage.

Figure 5-31 through Figure 5-38 show similar surface-to-groundwater operations and netinteraction to the historical water budgets. Under the projected conditions baseline, the Oakdale South water budget area maintains a constant net-contribution to the aquifer system while the Non-District West continues to be variable conditions and the NDE continues to be a net-extractor. The Modesto water budget area shows the greatest variance from the historical water budget, being predominantly a net-extractor under projected conditions. This is due to both changes in agricultural operations, combined with growing populations in the urban centers. Figure 5-39 summarizes the average projected groundwater inflows and outflows in the Modesto Subbasin, while Figure 5-40 shows the annual change in each component of the groundwater budget plus cumulative change in storage throughout the simulation period. Based on this figure, Modesto Subbasin is projected to experience approximately 11,000 AFY of storage decline under projected conditions, leading to cumulative reduction of approximately 530,000 AFY of groundwater in storage over the 50-year planning horizon.

Table 5-10 shows the minimum, maximum and averages numbers by Water Year Type for the groundwater budget components in the Projected Conditions scenario. The net change in groundwater storage indicates a maximum increase in storage of 167,000 AF in a wet year and a worst-case scenario decrease in storage of 161,600 AF in a critically dry year. These ranges highlight the effect of hydrologic conditions over the Subbasin when analyzing individual years. Even within the same Water Year types there are significant ranges of values which reflects different starting conditions on which each individual year is analyzed. #25..
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Table 5-10. Average and Range of annual values for components of the Projected Conditions Groundwater Budget by Water Year Type (AFY)
Component Wet Above
Normal
Below
Normal Dry Critical
Net Stream Seepage
(+)
Min 13,700 800 -2,900 -16,300 -23,200
Avg 48,700 15,700 18,100 5,000 17,300
Max 107,200 38,700 37,500 53,500 49,700
Canal and Reservoir
Recharge (+)
Min 45,200 46,900 45,700 45,100 43,500
Avg 47,100 48,400 47,800 48,300 46,200
Max 48,600 49,600 50,100 50,000 48,800
Deep Percolation (+)
Min 224,200 201,800 191,000 177,500 160,500
Avg 280,600 234,600 204,200 204,500 181,300
Max 344,800 266,400 229,900 235,200 212,700
Net Subsurface Flows
(+)
Min -8,100 -18,800 -6,200 -13,300 -18,900
Avg 8,500 -5,300 3,400 -2,500 800
Max 30,000 8,500 20,900 25,500 24,600
Groundwater
Pumping (-)
Min 249,600 274,500 271,500 266,700 303,700
Avg 287,700 302,200 304,400 297,700 364,100
Max 327,700 332,100 345,500 346,900 439,100
Change in
Groundwater Storage
Min 12,800 -57,200 -46,300 -91,200 -161,600
Avg 97,300 -8,700 -30,900 -42,300 -118,500
Max 167,100 42,700 -600 37,200 -49,400
Figure 5-29: Projected Conditions Groundwater Recharge and Extraction – Modesto Subbasin
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Figure 5-30: Projected Conditions Net Recharge – Modesto Subbasin Figure 5-31: Projected Conditions Groundwater Recharge and Extraction – Modesto Zone
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Figure 5-32: Projected Conditions Net Recharge – Modesto Zone Figure 5-33: Projected Conditions Groundwater Recharge and Extraction – Oakdale South Zone
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Figure 5-34: Projected Conditions Net Recharge – Oakdale South Zone Figure 5-35: Groundwater Recharge and Extraction – Non-District East Area #25..
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Figure 5-36: Net Recharge – Non-District East Area
Figure 5-37: Groundwater Recharge and Extraction – Non-District West Zone #25..
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Figure 5-38: Net Recharge – Non-District West Zone
Figure 5-39: Projected Conditions Average Annual Water Budget – Groundwater System, Modesto Subbasin
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Figure 5-40: Projected Conditions Annual Water Budget – Groundwater System, Modesto Subbasin

5.2. CLIMATE CHANGE ANALYSIS

5.2.1. Regulatory Background
SGMA requires consideration of uncertainties associated with climate change in the development of GSPs. Consistent with §354.18(d)(3) and §354.18(e) of the SGMA Regulations, analyses for the Modesto GSP evaluated the projected water budget with and without climate change conditions.

5.2.2. DWR Guidance
Climate change analysis and the associated methods, tools, forecasted datasets, and the predictions of greenhouse gas concentrations in the atmosphere are continually evolving.

The approach developed for this GSP is based on the methodology in DWR’s guidance document (DWR, 2018b), which, in combination with Subbasin-specific modeling tools, was deemed to be the most appropriate information for evaluating climate change in the Modesto Subbasin GSP. The following resources from DWR were used in the climate change analysis:
• SGMA Data Viewer
• Guidance for Climate Change Data Use During Sustainability Plan Development and Appendices (Guidance Document)
• Water Budget BMP
• Desktop IWFM Tools
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SGMA Data Viewer provides the location for which the climate change forecasts datasets3 were downloaded for the Modesto Subbasin (DWR, 2019b). The guidance document details the approach, development, applications, and limitations of the datasets available from the SGMA Data Viewer (DWR, 2018b). The Water Budget BMP describes in greater detail how DWR recommends projected water budgets be computed (DWR, 2016a). The Desktop IWFM Tools (DWR, 2018c) are available to calculate the projected precipitation and evapotranspiration inputs under climate change conditions.

The methods suggested by DWR in the above resources were used, with modifications where appropriate, to ensure the resolution would be reasonable for the Modesto Subbasin and align with the assumptions of the C2VSimTM. Figure 5-41 shows the overall process developed for the Modesto GSP consistent with the Climate Change Resource Guide (DWR, 2018b) and describes workflow beginning with baseline projected conditions to perturbed 2070 conditions for the projected model run. For this analysis, it is assumed that the projected climate change conditions for 2070 central tendency is used.

Figure 5-41: Modesto GSP Climate Change Analysis Process Table 5-10 summarizes the forecasted variable datasets provided by DWR that were used to carry out the climate change analysis. The “VIC” model (Variable Infiltration Capacity) referred to in Table 5-10 is the hydrologic model used by DWR to estimate unimpaired flows in upper watersheds. “Unimpaired” streamflow refers to the natural streamflow produced by a watershed, without modifications to streamflow from reservoir regulations, diversions, and other operations. On the other hand, “impaired” streamflow referred to in Table 5-10 is DWR’s terminology for streams whose flow is impacted by ongoing water operations and 3 In the industry, climate change impacted variable forecasts are sometimes referred to as “data” and their collections are called “datasets.” Calling forecasted variable values “data” can be misleading, so this document tries to be explicit when referring to data (historical data) vs. forecasts or model outputs.

Projected
Conditions Baseline
Climate Change
Perturbation
Factors
Projected
Conditions Baseline
with Climate
Change
C2VSimTM
C2VSimTM Projected
Conditions with
Climate Change
Water Budget
Projected
Conditions Water
Budget
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upstream regulations, such as diversions, deliveries, and reservoir storage. Flows on these streams are simulated using the CalSim II model results from the DWR baseline model. For Modesto Subbasin GSP, stream inflow and surface water deliveries to MID and OID were utilized from the CalSim II baseline model results. The San Joaquin River flows were also based on the results of CalSim II baseline model from DWR. All timeseries shown in Table 5- 10 use a monthly timestep. Section 5.2.3 includes further description of the methodology, datasets, and results.

Table 5-11: DWR-Provided Climate Change Datasets
Input
Variable DWR Provided Dataset
Unimpaired
Streamflow
Combined VIC model runoff and baseflow to generate
change factors, provided by HUC 8 watershed geometry
Impaired
Streamflow
(Ongoing
Operations)
CalSim II time series outputs in .csv format
Precipitation VIC model-generated GIS grid with associated change factor time series for each cell
Reference
ET
VIC model-generated GIS grid with associated change
factor time series for each cell

5.2.3. Climate Change Methodology
Climate change affects precipitation, streamflow, evapotranspiration and, for coastal aquifers, sea level rise, which in turn have impacts on the aquifer system. For the Modesto Subbasin, sea level rise is not relevant and not considered in this analysis. The method for perturbing the streamflow, precipitation, and evapotranspiration input files is described in the following sections. The late-century, 2070 central tendency climate scenario was evaluated in this analysis, consistent with DWR guidance (DWR, 2018b).

DWR combined 10 global climate models (GCMs) for two different representative climate pathways (RCPs) to generate the central tendency scenarios in the datasets used in this analysis. The “local analogs” method (LOCA) was used to downscale these 20 different climate projections to a scale usable for California (DWR, 2018b). DWR provides datasets for two future climate periods: 2030 and 2070. For 2030, there is one set of central tendency datasets available. For 2070, DWR has provided one central tendency scenario and two extreme scenarios: one that is drier with extreme warming and one that is wetter with moderate warming.

The 2070 central tendency projection serves to assess impacts of climate change over the long-term planning and implementation period and was therefore selected as the most appropriate scenario under which to assess in the Modesto GSP. #25..
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5.2.3.1. Streamflow under Climate Change
Hydrological forecasts for streamflow under various climate change scenarios are available from DWR as either a flow-based timeseries or a series of perturbation factors applicable to local data. DWR simulated volumetric flow in most regional surface water bodies by utilizing the Water Resource Integrated Modeling System (WRIMS, formally named CalSim II). While river flows and surface water diversions in the Tuolumne, Stanislaus, and San Joaquin Rivers are simulated in CalSim II, there are significant variations when compared to local historical data. Due to the uncertainty in CalSim II-simulated reservoir operations, flows from CalSim II provided by the state are not used directly in the Modesto GSP climate change analysis.

Instead, relative perturbation factors were used to derive surface water inflows and diversions for analysis with the C2VSimTM.

The major streams entering the Modesto Subbasin are the Tuolumne River and Stanislaus River. All rivers are regulated and there are no unimpaired rivers or creeks that contribute significantly to the basin.

CalSim II estimated flows for point locations on the Tuolumne River and Stanislaus River were downloaded from DWR. The key flows obtained from CalSim II include:
• Tuolumne River: La Grange Outflow
• Stanislaus River: Goodwin Outflow
The San Joaquin River inflow was not adjusted in the climate change analysis because the Friant Dam is located far from the Modesto Subbasin and subbasins that are upstream of the Modesto Subbasin can have significant impacts on stream accretions/depletions, diversions, and operations. As these upstream impacts which are outside of the Modesto Subbasin cannot be captured without detailed analysis of projected flows under climate change conditions, the San Joaquin River flows are assumed to be same as the projected baseline conditions. This would not have a significant impact on the climate change analysis for the Modesto Subbasin, as majority of the surface water supplies, and interaction of surface and groundwater systems take place within Subbasins and along Tuolumne and Stanislaus Rivers.

The streamflow data extracted from CalSim II represent projected hydrology with climate change based on reservoir outflow, operational constraints, and diversions and deliveries of water for the State Water Project and the Central Valley Project. CalSim II data from WY 1965 to WY 2003 was available. For WY 2004 to WY 2018, streamflow data was synthesized based on similar year methodology, and used flows from WY 1965 to WY 2003 and the DWR San Joaquin Valley water year type (CDEC, 2018). (For example, the streamflow for October 2009 was calculated as the average of the October 1966 and October 1971 streamflow because these are all the Below Normal water years between WY 1965 and WY 2003.) CalSim II outputs are considered more appropriate for regulated streams than streamflow derived using the unimpaired flow adjustment factors because CalSim II accounts for reservoir operations. As expected, streamflow simulated in CalSim II and those derived using the unimpaired flow adjustment factors did not present similar trends, particularly in dry #25..
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years. DWR-provided unimpaired flow change factors do not account for variations in the operation of the reservoirs that would result from climate change conditions. The CalSim II flows, however, were also not considered completely appropriate for local conditions so a method was derived to compute change factors from CalSim II flows, as described below.

Using DWR’s method of deriving the precipitation and evapotranspiration factors as a guide, a hybrid approach was derived to improve upon the discrepancy between the CalSim II and local models while accounting for some change in reservoir operations. In this approach, change factors are generated from the difference between each simulated future climate change CalSim II scenario (i.e., 2070) and the “without climate change” baseline CalSim II run. This “without climate change” baseline run is the CalSim II 1995 Historical Detrended simulation run provided through personal communication from DWR. The change perturbation factors are bounded by a maximum of 5 and minimum 0.2. For the purposes of simplicity, this method is referred to throughout the rest of the document as CalSim II Generated Perturbation Factors (CGPF). The generated change factors are then used to perturb the regulated baseline river inflows:
• Tuolumne River – CGPF multiplied by the projected conditions baseline for the Tuolumne River which is based on Tuolumne River System (TRS) operations model • Stanislaus River – CGPF multiplied by the projected conditions baseline for the Stanislaus River which is based on historical trends and local hydrology As previously discussed, the San Joaquin River flows were not perturbed due to the much larger tributary areas of the San Joaquin River that are outside the Modesto Subbasin. The CGPF method presents limitations given that the resulting flows are not directly obtained from an operations model. The actual mass balance on the reservoirs is not tracked in the estimates of the flows and, instead, the method relies on CalSim II tracking that storage and managing the reservoir based on the appropriate rule curves.

Figure 5-42 through Figure 5-49 provide a comparison of projected conditions baseline and the CGPF method described above. Exceedance curves are included for each of the CGPF flows against the projected conditions baseline.
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Figure 5-42: Tuolumne River Hydrograph
Figure 5-43: Tuolumne River Exceedance Curve
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Figure 5-44: Stanislaus River Hydrograph
Figure 5-45: Stanislaus River Exceedance Curve

5.2.3.2. Precipitation and Evapotranspiration under Climate Change Projected precipitation and evapotranspiration (ET) change factors provided by DWR were calculated using a climate period analysis based on historical precipitation and ET from January 1915 to December 2011 (DWR, 2018b). The Variable Infiltration Capacity (VIC) #25..
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hydrologic model was used by DWR to simulate land-surface atmosphere exchanges of moisture and energy on a six-kilometer grid. Model output includes both precipitation and reference evapotranspiration change factors. The change factors provided by DWR were calculated as a ratio of a variable under a “future scenario” divided by a baseline. The baseline data is the 1995 Historical Template Detrended scenario by the VIC model through GCM downscaling. The “future scenario” corresponds to VIC outputs of the simulation of future conditions using GCM forecasted hydroclimatic variables as inputs. These change factors are thus a simple perturbation factor that corresponds to the ratio of a future with climate change divided by the past without it. Change factors are available on a monthly time step and spatially defined by the VIC model grid. Supplemental tables with the time series of perturbation factors are available by DWR for each grid cell. DWR has made accessible a Desktop GIS tool for both IWFM and MODFLOW to process these change factors (DWR, 2018c).

5.2.3.2.1. Applying Change Factors to Precipitation
DWR change factors were multiplied by projected conditions baseline precipitation to generate projected precipitation under the 2070 central tendency future scenario using the Desktop IWFM GIS tool (DWR, 2018c). The tool calculates an area weighted precipitation change factor for each model grid geometry. This model grid geometry was generated based on polygons built around the PRISM nodes that are within the model area.

However, the DWR tool only includes change factors through 2011. The remaining seven years of the time series were synthesized according to historically comparable water years (i.e., wet years were synthesized based on a wet year within the available time frame of the DWR tool). The perturbation factor from the corresponding month of the comparable year was applied to the baseline of the missing years (2012-2018) to generate projected values.

Months with no precipitation in the baseline were assumed a monthly precipitation of 1 mm under climate change to account for increased precipitation that cannot be calculated from a baseline of 0 mm for these synthesized years. The comparable years that were used can be found in Table 5-121.

Table 5-12: Comparable Water Years (Precipitation)
Missing Water Year Comparable Water Year
2012 1968
2013 2007
2014 2002
2015 1971
2016 1981
2017 1993
2018 1987
The resulting perturbed precipitation values and the baseline precipitation values for the representative historical period can be found in Figure 5-46 below. The exceedance plot for these two times series can be found in Figure 5-47.
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Figure 5-46: Perturbed Precipitation Under Climate Change #25..
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Figure 5-47: Perturbed Precipitation Exceedance Curve
Figure 5-48 shows the difference between the regional average under 2070 climate change conditions and the regional average under projected conditions baseline plotted against different amounts of projected monthly precipitation. The average was taken across the area of the Modesto Subbasin.
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Figure 5-48: Variation from Baseline of Perturbed Precipitation Figure 5-48 demonstrates that in 2070 with climate change added, in low precipitation months, there is approximately equal probability that the month will be wetter or drier than projected conditions baseline. However, under climate change, the 2070 conditions will be wetter in months with precipitation above approximately 50 mm, indicated by the vertical gray dashed line. Therefore, under climate change conditions (in the scenario selected for the GSP), we can see that the occurrence of low precipitation months will likely not change significantly, but the higher precipitation months are predicted to be wetter overall than the projected conditions baseline.

5.2.3.2.2. Applying Change Factors to Evapotranspiration Potential ET in the Modesto Subbasin is aggregated to one of twenty-five land use categories but does not vary spatially. DWR provides change factors for ET in the same spatially distributed manner as precipitation, as described above. However, to match the level of discretization with the C2VSimTM, an average ET change factor was calculated across all VIC grid cells within the Modesto Subbasin boundary. Therefore, the tool to process ET provided by DWR was not needed or used. Change factors provided by DWR for November 1, 1964, through December 1, 2011, were averaged. This average ET change factor was then applied to the baseline ET time series for each crop type. Because the same ET change factor was applied over the entire baseline, no synthesis was required in this analysis. Refinement to the simulated evapotranspiration of orchards under 2070 climate conditions is shown in Figure 5-49 below as an example. For 2070, the average change factor is 1.08.
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Figure 5-49: Monthly ET for Sample Crops

5.2.3.3. Modesto Subbasin Water Budget Under Climate Change A climate change scenario was developed for the C2VSimTM to evaluate the hydrological impacts under these conditions. The analysis was based on the projected conditions baseline with climate change perturbed inputs for streamflow, precipitation, and ET. Results are presented below in Table 5-12 though Table 5-14.

Under the climate change scenario, the average annual volume of evapotranspiration is over six percent higher than the projected conditions baseline, increasing from 536,000 AFY to 568,000 AFY. Due to changes to local hydrology, the average annual surface water availability is projected to decrease by 1.6 percent from 293,000 AFY to 288,000 AFY.4 As a result of less surface water and increased agricultural demands, private groundwater production is simulated to increase by approximately 14 percent, from 230,000 AFY to 262,000 AFY. Under climate change conditions, depletion in aquifer storage is expected to increase by more than half to an average annual rate of 17,000 AFY, from 11,000 AFY in the projected conditions baseline. This has an impact on the stream system and the net difference in stream-aquifer interactions, drawing 46,000 AFY on average from streamflow to the aquifer.

A graphical representation of simulated changes to evapotranspiration, surface deliveries, and groundwater pumping are presented in Figure 5-50 though Figure 5-52 below, and 4 There are various approaches to estimating the effects of climate change on local hydrology. The 2070 Central Tendency used in this GSP according to DWR guidelines for GSP submittal may differ from local studies or certain Flood-MAR scenarios.
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complete water budgets for the climate change scenario are shown in Figure 5-53 though Figure 5-55.

Figure 5-50: Simulated Changes in Evapotranspiration due to Climate Change (Scenario minus Baseline)
Figure 5-51: Simulated Changes in Surface Water Supplies due to Climate Change (Scenario minus Baseline)
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Figure 5-52: Simulated Changes in Groundwater Production due to Climate Change (Scenario minus Baseline)
Figure 5-53: Agricultural Land and Water Use Budget – C2VSimTM Climate Change Scenario
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Figure 5-54: Urban Land and Water Use Budget – C2VSimTM Climate Change Scenario
Figure 5-55: Groundwater Budget – C2VSimTM Climate Change Scenario #25..
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Table 5-12: -13: Average Annual Water Budget Under Climate Change – Stream Systems, Modesto Subbasin (AFY)
Component Projected
Condition
Water Budget
Climate Change
Water Budget
Hydrologic Period WY 1969 - 2018 WY 1969 - 2018
Stream Inflows 2,650,000 2,739,000
Stanislaus River 536,000 626,000
Tuolumne River 812,000 818,000
San Joaquin River 1,302,000 1,295,000
Tributary Inflow1 6,000 5,000
Stream Gain from Groundwater 104,000 96,000
Modesto Subbasin 50,000 45,000
Stanislaus River – South2
12,000 13,000
Tuolumne River - North 27,000 22,000
San Joaquin River - East 11,000 11,000
Other Subbasins 54,000 50,000
Stanislaus River - North 12,000 13,000
Tuolumne River - South 31,000 27,000
San Joaquin River - West 11,000 11,000
Surface Runoff to the Stream System3 60,000 72,000
Return Flow to Stream System3 113,000 114,000
Total Inflow 2,934,000 3,025,000
San Joaquin River Outflows 2,717,000 2,774,000
Diverted Surface Water4 33,000 33,000
Stream Seepage to Groundwater 146,000 177,000
Modesto Subbasin 76,000 91,000
Stanislaus River - South 36,000 44,000
Tuolumne River - North 38,000 45,000
San Joaquin River - East 2,000 2,000
Other Subbasins 71,000 86,000
Stanislaus River - North 31,000 39,000
Tuolumne River – South 38,000 45,000
San Joaquin River - West 2,000 2,000
Native & Riparian Uptake from Streams 37,000 41,000
Total Outflow 2,934,000 3,025,000
Note: sub-categories may not sum together due to rounding error 1 Tributary inflow include surface water contributions from small watersheds 2 Represents the location of the Modesto Subbasin relative to the stream, i.e., “North” represents the gains/losses of that stream to the Modesto Subbasin to the North.
3 Includes runoff/return flow from all subbasins adjacent to the stream system, not just the Modesto Subbasin. 4 Some surface water diversions are upstream of the Tuolumne River or Stanislaus River inflows and thus not included in this stream and canal water budget. #25..
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Table 5-13: -14: Average Annual Water Budget Under Climate Change – Land Surface System, Modesto Subbasin (AFY)
Component
Projected
Condition
Water Budget
Climate Change
Water Budget
Hydrologic Period WY 1969 - 2018 WY 1969 - 2018
Agricultural Areas Precipitation 139,000 147,000
Agricultural Water Supply 497,000 525,000
Agency Surface Water 241,000 238,000
Agency Groundwater 25,000 25,000
Private Groundwater 230,000 262,000
Urban Areas Precipitation 38,000 40,000
Urban Water Supply 111,000 112,000
Groundwater 60,000 62,000
Surface Water 51,000 50,000
Native Areas Precipitation 92,000 97,000
Native & Riparian Uptake from Stream 22,000 24,000
Total Supplies 900,000 945,000
Agricultural ET 402,000 430,000
Agricultural ET of Precipitation 82,000 84,000
Agricultural ET of Surface Water 159,000 160,000
Agricultural ET of Agency Groundwater 16,000 17,000
Agricultural ET of Private Groundwater 146,000 170,000
Agricultural Percolation 201,000 202,000
Agricultural Percolation of Precipitation 45,000 46,000 Agricultural Percolation of Surface Water 75,000 70,000 Agricultural Percolation of Agency Groundwater 8,000 7,000 Agricultural Percolation of Private Groundwater 73,000 79,000 Agricultural Runoff & Return Flow 31,000 36,000
Urban Runoff & Return Flow 91,000 93,000
Urban ET 38,000 40,000
Urban Percolation 20,000 19,000
Native Runoff 12,000 15,000
Native ET 95,000 98,000
Native Percolation 7,000 8,000
Total Demands 898,000 941,000
Land Surface System Balance 2,000 4,000
Land Surface System Balance (% of supplies) 0.2% 0.4%
Note: sub-categories may not sum together due to rounding error #25..
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Table 5-14: -15: Average Annual Water Budget Under Climate Change – Groundwater System, Modesto Subbasin (AFY)
Component
Projected
Condition
Water Budget
Climate Change
Water Budget
Hydrologic Period WY 1969 - 2018 WY 1969 - 2018
Gain from Stream 76,000 91,000
Gain from Stanislaus River 36,000 44,000
Gain from Tuolumne River 38,000 45,000
Gain from San Joaquin River 2,000 2,000
Canal & Reservoir Recharge 47,000 47,000
Deep Percolation 228,000 229,000
Subsurface Inflow 77,000 80,000
Flow from the Sierra Nevada Foothills 9,000 8,000
Eastern San Joaquin Subbasin Inflows 28,000 8,000
Turlock Subbasin Inflows 33,000 33,000
Delta Mendota Subbasin Inflows 7,000 32,000
Total Inflow 428,000 446,000
Discharge to Stream 50,000 45,000
Discharge to Stanislaus River 12,000 13,000
Discharge to Tuolumne River 27,000 22,000
Discharge to San Joaquin River 11,000 11,000
Subsurface Outflow 75,000 70,000
Eastern San Joaquin Subbasin Outflows 35,000 5,000
Turlock Subbasin Outflows 34,000 31,000
Delta Mendota Subbasin Outflows 6,000 35,000
Groundwater Production 314,000 347,000
Agency Ag. Groundwater Production 25,000 25,000
Private Ag. Groundwater Production 229,000 260,000
Urban Groundwater Production 60,000 62,000
Total Outflow 438,000 463,000
Change in Groundwater in Storage (11,000) (17,000)
Note: sub-categories may not sum together due to rounding error Table 5-166 shows the minimum, maximum and averages numbers by Water Year Type for the groundwater budget components in the Climate Change scenario. The net change in groundwater storage indicates a maximum increase in storage of 157,800 AF in a wet year and a worst-case scenario decrease in storage of 183,200 AF in a critically dry year.

Compared to the Projected Conditions, there is more groundwater storage loss as a result of higher temperatures and evapotranspiration rates, and less precipitation. #25..
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Table 5-16. Average and Range of annual values for components of Groundwater Budget by Water Year Type under the Climate Change Scenario (AFY) Component Wet Above
Normal
Below
Normal Dry Critical
Net Stream Seepage
(+)
Min 27,900 20,600 400 1,100 200
Avg 63,200 37,400 38,200 27,500 40,300
Max 125,400 62,300 65,000 79,400 73,100
Canal and Reservoir
Recharge (+)
Min 43,900 46,900 45,200 43,700 43,100
Avg 47,000 48,300 47,400 46,500 45,200
Max 48,700 49,500 50,100 49,400 47,800
Deep Percolation (+)
Min 218,700 206,700 193,000 171,600 156,500
Avg 284,300 237,200 203,000 201,000 180,400
Max 339,400 264,900 214,800 235,800 206,300
Net Subsurface Flows
(+)
Min -200 -11,700 -2,000 -5,200 -10,500
Avg 15,200 1,000 11,400 6,800 9,100
Max 39,200 17,000 30,800 34,300 32,100
Groundwater
Pumping (-)
Min 272,300 297,700 301,600 296,000 350,800
Avg 315,000 329,700 339,600 342,900 399,800
Max 357,300 357,000 380,900 387,300 474,800
Change in
Groundwater Storage
Min 3,500 -67,600 -51,800 -111,500 -183,200
Avg 94,700 -5,800 -39,500 -61,100 -124,800
Max 157,800 41,700 -31,100 24,200 -57,300

5.2.3.4. Opportunities for Future Refinement
The climate change approach developed for this GSP is based on the methodology in DWR’s guidance document (DWR, 2018b) and uses “best available information” related to climate change in the Modesto Subbasin. There are limitations and uncertainties associated with the analysis. One important limitation is that CalSim II does not fully simulate local surface water operations. Thus, the analysis conducted for this GSP may not fully reflect how surface and groundwater basin operations would respond to the changes in water demand and availability caused by climate change. For this first GSP iteration, use of a regional model and the perturbation factor approach were deemed appropriate given the uncertainties in the climate change analysis.

A recommendation for future refinements of this analysis is utilization of the local surface water operations model, the Tuolumne Reservoir Simulation (TRS) model. Use of this model would allow for greater resolution in the simulation of Tuolumne River flows and surface water supply based on local management. Additionally, utilization of TRS will allow for analysis of the localized climate conditions effecting snowpack and its implications on reservoir operations and streamflow. Further monitoring and adaptive management should be considered for the next update of the GSP along with improvements in DWR’s climate change data.
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5.3. SUSTAINABLE YIELD ESTIMATE
Sustainable yield is defined for SGMA purposes as “the maximum quantity of water, calculated over a base period representative of long-term conditions in the basin and including any temporary surplus, that can be withdrawn annually from a groundwater supply without causing an undesirable result.” (CWC §10721(w)). Sustainable yield for the Modesto Subbasin was calculated through development of a C2VSimTM scenario in which the long-term (50-year) SGMA sustainability indicators are met either directly or by groundwater levels as a proxy as outlined in Chapter 6: Sustainable Management Criteria. • Reduction of Groundwater in Storage – An Undesirable result is defined as significant and unreasonable reduction of groundwater in storage that would occur if the volume of groundwater supply is at risk of depletion and is not accessible for beneficial use, or if the Subbasin remains in a condition of long-term overdraft based on projected water use and average hydrologic conditions. in a manner that cannot be readily managed or mitigated.
• Chronic Lowering of Groundwater Levels – Undesirable results are defined as significant and unreasonable groundwater level declines – either due to multi-year droughts or due to chronic declines where groundwater is the sole supply – such that water supply wells are adversely impacted in a manner that cannot be readily managed or mitigated.
• Depletion of Interconnected Surface Water – An Undesirable Result is defined as significant and unreasonable adverse impacts to the beneficial uses of surface water caused by groundwater extraction.

The sustainable yield water budget is based on the Projected Conditions Baseline and is analyzed by reducing groundwater production through changes in the agricultural demand of the net groundwater extractors in Modesto Subbasin. Net-contributing and net-extracting users in the Subbasin are divided into the two groups shown in Figure 5-56. Group 1 users predominately rely on both surface and groundwater, while users in Group 2 predominantly rely on groundwater.

Group 1: Surface and Groundwater Users
• Modesto Irrigation District
• Oakdale Irrigation District
• Non-District West (riparian surface water users)
Group 2: Groundwater Only Users
• Non-District East
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Figure 5-56: Modesto Subbasin Sustainability Groups
The Sustainable Yield Scenario varies from the Projected Conditions Baseline in its volume of agricultural water demand. These demands were reduced by decreasing agricultural land use via a global reduction in projected cropped acreage at the element level.

The sustainable yield water budget is intended to estimate future supply, demand, and aquifer response in the Modesto Subbasin under sustainable conditions achieved with a demand reduction scenario. To meet the goals set forth by the sustainability indicators listed above, Group 2 agricultural users would need to reduce demand by 58-percent from the projected baseline levels. This reduction in groundwater usage results in a sustainable yield of approximately 267,000 acre-feet per year for the Subbasin.

The methodology for reducing Subbasin-wide pumping to estimate sustainable yield is developed solely to estimate the subbasin’s sustainable yield and is not intended to prescribe or describe how pumping would be reduced in the basin during GSP implementation to achieve sustainability. The reduction of groundwater demand to sustainable levels would be implemented in close coordination among the various Subbasin zones. The groundwater demand reduction is only one and/or part of the overall management actions that would result in groundwater sustainability within the Subbasin; factors such as water rights, beneficial uses, needs, and human right to water should also be #25..
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considered. The status of plans for implementing management actions related to pumping reductions is further discussed in Chapter 8 - Projects and Management Actions.

Table 5-15 provides a detailed listing of the water flow components of the Modesto Subbasin’s groundwater system for the historical, projected conditions baseline and sustainable yield conditions. To achieve sustainability and maintain minimum groundwater level thresholds, the Subbasin needs to experience an average annual net gain of groundwater in storage of 11,000 AFY. These conditions are met through 213,000 AFY of deep percolation, 47,000 AFY of canal and reservoir recharge, and 20,000 AFY of net subsurface inflow from the Sierra Nevada foothills and the neighboring Turlock, DeltaMendota, and Eastern San Joaquin Subbasins. Outflows from the subbasin include 266,000 AFY of pumping and 14,000 AFY of net groundwater discharge to the surface water bodies.

The major flow components are represented graphically in Figure 5-57 and Figure 5-58, on an annual and average annual basis.

Figure 5-59 and Figure 5-60 show the groundwater recharge and extraction and net recharge for the Modesto Subbasin. Under sustainable conditions, the Modesto Subbasin is expected to maintain an average net extraction of 7,000 AFY, compared to a net extraction of 39,000 AFY under projected conditions. This reduction in net extraction is attributed to the reduction of groundwater pumping, which is reduced from 314,000 AFY under the Baseline to 267,000 AFY under sustainable yield, combined with an overall reduction in percolation of agricultural applied water of 14,000 AFY between the two scenarios. #25..
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Table 5-15: -17: Sustainable Yield Average Annual Water Budget Groundwater System – Modesto Subbasin
Component Projected
Conditions
Sustainable
Conditions
Hydrologic Period Hydrology from
WY 1969 - 2018
Hydrology from
WY 1969 - 2018
Gain from Stream 76,000 58,000
Gain from Stanislaus River 36,000 27,000
Gain from Tuolumne River 38,000 29,000
Gain from San Joaquin River 2,000 1,000
Canal & Reservoir Recharge 47,000 47,000
Deep Percolation 228,000 213,000
Subsurface Inflow 77,000 83,000
Flow from the Sierra Nevada Foothills 9,000 9,000
Eastern San Joaquin Subbasin Inflows 28,000 9,000
Turlock Subbasin Inflows 33,000 29,000
Delta Mendota Subbasin Inflows 7,000 37,000
Total Inflow 428,000 401,000
Discharge to Stream 50,000 71,000
Discharge to Stanislaus River 12,000 18,000
Discharge to Tuolumne River 27,000 40,000
Discharge to San Joaquin River 11,000 14,000
Subsurface Outflow 75,000 63,000
Eastern San Joaquin Subbasin Outflows 35,000 4,000
Turlock Subbasin Outflows 34,000 30,000
Delta Mendota Subbasin Outflows 6,000 30,000
Groundwater Production 314,000 267,000
Agency Ag. Groundwater Production 25,000 25,000
Private Ag. Groundwater Production 229,000 181,000
Urban Groundwater Production 60,000 60,000
Total Outflow 438,000 401,000
Change in Groundwater in Storage (11,000) -
Note: sub-categories may not sum together due to rounding error Table 5-188 shows the minimum, maximum and averages numbers by Water Year Type for the groundwater budget components in the Sustainable Yield scenario. The net change in groundwater storage indicates a maximum increase in storage of 194,100 AF in a wet year and a worst-case scenario decrease in storage of 150,400 AF in a critically dry year.

Compared with the Projected Conditions, there is a greater increase in groundwater storage as a result of the reduction in water demand.
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Table 5-18. Average and Range of annual values for components of Groundwater Budget Under the Sustainable Yield by Water Year Type (AFY) Component Wet Above
Normal
Below
Normal Dry Critical
Net Stream Seepage
(+)
Min -14,900 -39,500 -33,400 -56,200 -62,700
Avg 8,800 -20,200 -20,400 -34,700 -22,500
Max 55,600 -8,400 -8,400 9,000 6,500
Canal and Reservoir
Recharge (+)
Min 45,200 46,900 45,700 45,100 43,500
Avg 47,100 48,400 47,800 48,300 46,200
Max 48,600 49,600 50,100 50,000 48,800
Deep Percolation (+)
Min 211,100 184,800 172,900 158,700 144,400
Avg 264,900 221,200 190,000 187,700 165,300
Max 348,900 250,200 227,900 214,500 192,300
Net Subsurface Flows
(+)
Min 9,000 100 12,700 5,400 -200
Avg 26,900 13,100 21,900 15,500 18,700
Max 48,000 26,900 39,700 43,600 42,800
Groundwater
Pumping (-)
Min 208,900 226,400 223,400 218,800 255,900
Avg 242,800 254,300 257,600 249,900 314,800
Max 279,300 284,100 298,900 298,300 390,900
Change in
Groundwater Storage
Min 34,300 -33,800 -44,800 -88,200 -150,400
Avg 105,000 8,300 -18,200 -33,200 -107,200
Max 194,100 57,900 46,000 33,000 -46,400
Figure 5-57: Sustainable Yield Average Annual Water Budget Groundwater System – Modesto Subbasin
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Figure 5-58: Sustainable Yield Water Budget Groundwater System – Modesto Subbasin
Figure 5-59: Sustainable Yield Water Budget Groundwater Recharge and Extraction – Modesto Subbasin
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Figure 5-60: Sustainable Yield Water Budget Net Recharge – Modesto Subbasin

SUMMARY
The sustainable yield of the Modesto Subbasin is developed by methodically reducing groundwater demand for the net groundwater extractors (Sustainability Group 2) in the Subbasin. The goal of this groundwater demand reduction is to reduce groundwater pumping to a level that would result in no undesirable results if continued in the long-term.

The presence of undesirable results is evaluated by analyzing sustainability indicators produced by the numerical model, including groundwater in storage, groundwater levels, and interconnected stream systems. It is assumed that by using groundwater levels as proxy for other applicable sustainability indicators (i.e., groundwater quality and land subsidence), the sustainable yield would address all applicable sustainability indicators in the Modesto Subbasin.

This analysis results in a sustainable yield of 267,000 AFY for the Modesto Subbasin.

The sustainable yield is based on the current and latest data and information for the subbasinSubbasin. It is expected that the sustainable yield estimate would be updated for the next GSP update in 2027, as additional data and information become available on the operation of the Subbasin, implementation of projects and management actions, groundwater levels, storage, and quality, and as updates to the tools and technology, such as updates to the integrated numerical model are implemented. #25..
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6. SUSTAINABLE MANAGEMENT CRITERIA
GSP regulations provide a framework for locally-defined and quantitative sustainable management criteria, which allowsallow the GSAs to quantitatively measure and track ongoing sustainable management. These criteria include a sustainability goal, which has been developed as a mission statement for the GSP. Additional criteria include specific terminology from SGMA; a brief summary1 of these terms – and the application of each – are provided below:
• Undesirable Results (URs2) – significant and unreasonable adverse conditions for any of the six sustainability indicators defined in the GSP regulations. • Minimum Threshold (MT2) – numeric value used to define undesirable results for each sustainability indicator at representative monitoring sites. • Measurable Objective (MO2) – numeric goal to track the performance of sustainable management at representative monitoring sites.
• Interim Milestone (IM2) – target numeric value representing measurable groundwater conditions, in increments of five years, as set by the GSAs as part of the GSP.

Collectively, these criteria define sustainable groundwater management by:
• quantifying groundwater conditions to avoid, along with associated warning signs (URs and MTs);
• identifying favorable groundwater conditions and operational parameters (MOs); and
• providing targets for monitoring Subbasin progress toward achieving the sustainability goal (MTs, MOs, and IMs).

6.1. SUSTAINABILITY GOAL
A sustainability goal provides a mission statement for what the GSAs wish to achieve through sustainable management. GSP regulations provide requirements for a GSP Sustainability Goal, as follows:
1 Sustainable management criteria are more fully defined in SGMA (CWC 10721(a) – (ab) and GSP regulations (§351(a) – (an)).
2 Because of the frequency of use, and to facilitate review of the text, the terms “undesirable results” “minimum threshold,” “measurable objective,” and “interim milestone” are abbreviated as “UR”, “MT”, “MO”, and “IM” respectively, throughout remaining sections of the GSP. However, the terms are spelled out in un-abbreviated form where helpful for context and clarity or when contained in a direct quotation.
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Each Agency shall establish in its Plan a sustainability goal for the basin that culminates in the absence of undesirable results within 20 years of the applicable statutory deadline. The Plan shall include a description of the sustainability goal, including information from the basin setting used to establish the sustainability goal, a discussion of the measures that will be implemented to ensure that the basin will be operated within its sustainable yield, and an explanation of how the sustainability goal is likely to be achieved within 20 years of Plan implementation and is likely to be maintained through the planning and implementation horizon. (§354.24).

In the Best Management Practices (BMPs) document on sustainable management criteria, DWR recommends that one succinct, common sustainability goal be developed for the entire Subbasin.

The requirements and guidance for a GSP sustainability goal were reviewed in a public meeting of the STRGBA GSA Technical Advisory Committee (TAC) in February 2021. That meeting was followed with a technical memorandum prepared by the technical team, in part, to assist TAC members with development of a goal. The memorandum summarized GSP requirements and how the sustainability goal fits within the overall sustainable management criteria process.

Based on TAC feedback, DWR guidance, and GSP requirements, the TAC Planning Group3 developed a draft sustainability goal reviewed by the TAC at a public meeting on May 12, 2021. At that meeting, additional comments on the sustainability goal were received from stakeholders and TAC members. Those comments were incorporated into the draft sustainability goal presented below.

The Sustainability Goal of the Modesto Subbasin GSP is to provide a sustainable groundwater supply for the local community and for the economic vitality of the region.

Groundwater levels, storage volume, and quality will be actively managed by the STRGBA GSA to:
• Operate the Subbasin within its sustainable yield to support beneficial uses including municipal, domestic, agricultural, industrial, and environmental; • Maintain a reliable, accessible, and high-quality groundwater supply, especially during droughts;
• Manage groundwater levels such that beneficial uses of interconnected surface water are not adversely impacted by groundwater extractions; • Optimize conjunctive management of local surface water and groundwater resources;
• Avoid adverse impacts from future potential land subsidence associated with groundwater level declines;
3 The TAC Planning Group is a small working group composed of representatives from the TAC to guide the GSP process and provide recommendations to the full TAC. #25..
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• Cooperate and coordinate with GSAs in neighboring subbasins to avoid undesirable results along the shared Subbasin boundaries.

This goal will be achieved within the 20-year implementation period and maintained throughout the planning horizon through a robust monitoring program and a series of projects and management actions that involve groundwater recharge, in lieu surface water use, conservation, stormwater management, and other strategies to be developed and modified over time through adaptive management.

The sustainability goal is supported by information provided in GSP chapters on the plan area (Chapter 2) and basin setting (Chapters 3 and 5). Specific information used to inform the sustainability goal included the identification of land and water use in the Subbasin (Chapter 2), ongoing conjunctive management of surface water and groundwater (Chapter

2.), delineation of the base of fresh water and groundwater in storage (Section 3.1.3), the establishment of Principal Aquifers (Section 3.1.4), groundwater conditions (Sections 3.2), and historical and projected water budgets (Chapter 5). Additional considerations of basin conditions that support the sustainability goal are described in the following section.

6.2. SELECTION OF SUSTAINABLE MANAGEMENT CRITERIA
Six sustainability indicators are defined in the GSP regulations to represent groundwater conditions that, when determined to be significant and unreasonable, cause undesirable results. The avoidance of undesirable results is the foundation for sustainable groundwater management. Accordingly, these sustainability indicators are analyzed in the Modesto Subbasin to define undesirable results and other sustainability criteria, including MTs, MOs, and IMs. A representative monitoring network is established for each applicable indicator to track these conditions throughout the implementation and planning horizon.

Those six indicators and their associated icons developed by DWR are illustrated below.

6.2.1. Sustainability Considerations in the Modesto Subbasin As explained in subsequent sections, this GSP analyzes conditions related to the six sustainability indicators that support definitions for undesirable results. SGMA legislation #25..
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states that the GSAs are not required to address undesirable results that occurred before – and have not been corrected by – January 1, 2015 (§10727.2 (b)(4)). Accordingly, the focus for several indicators is to avoid future conditions that could lead to undesirable results.

Basin conditions as of 2015 and management considerations for each sustainability indicator are summarized in Table 6-1, along with the respective GSP section where each indicator is analyzed. General locations for the conditions described in the table are shown on Figure 6- 1 with certain areas highlighted by the sustainability indicator icons for reference.

Table 6-1: Sustainability Considerations for Modesto Subbasin Basin Conditions
Undesirable Results
in Modesto Subbasin
as of 2015?
Management Considerations
GSP
Sect.

Declining water levels are occurring,
primarily in the eastern Subbasin.

Other local areas experienced water
level declines during drought.

Yes Adverse impacts to public and
domestic water supply wells caused by
declining water levels. Water levels will
be managed to avoid future impacts.

6.3 Overdraft conditions, primarily in
areas where groundwater is the
primary source of supply.

Yes Over-pumping in certain areas has
caused water level declines, which
impact beneficial uses of both
groundwater and surface water. GSP
will arrest overdraft conditions.

6.4 Not applicable to this inland Subbasin. No None 6.5 Groundwater concentrations for
certain constituents of concern are
exceed drinking water standards over
widespread areas of the Subbasin.

Groundwater extractions, GSA
projects, and GSA management
actions may have the potential to
degrade water quality in the future.

No Historical water quality impacts have
not been caused by GSA management
activities, and therefore are not
undesirable results as defined in this
GSP. GSAs need to manage Subbasin
groundwater so as not to further
degrade groundwater quality.

6.6 No documented impacts from land
subsidence in Subbasin; potential for
compressible clays to cause land
subsidence in the future.

No If groundwater levels are managed at
or near historic low levels, the
potential for future undesirable results
can be avoided.

6.7 Streamflow depletions have increased
over time, especially on the Tuolumne
and Stanislaus rivers. All 3 river
boundaries remain interconnected,
and no current impacts to surface
water rights have been identified.

Modeling predicts increased
depletions in the future.

No GSAs are not responsible for correcting
conditions before 2015. However,
modeling projectspredicts future
streamflow depletions that may lead
to undesirable results. GSAs will
manage water levels to reduce future
increases in streamflow depletions.

6.8 #25..
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As indicated in Table 6-1, the Modesto Subbasin has experienced undesirable results associated with chronic lowering of water levels and reduction of groundwater in storage.

These conditions have occurred primarily within and around the Non-District East Management Area (NDE MA) as shown on Figure 6-1. Over the historical study period, agricultural production has expanded in the eastern Subbasin where groundwater is the primary source of water supply. Over-pumping in this area has led to water level declines expanding into other areas, which exacerbated conditions during the 2014-2016 drought and caused impacts to both public and domestic water supply wells. During this time, more than 150 domestic wells failed (indicated on Figure 6-1 by the small black dots). As explained in Section 6.3, most of the impacted wells appear to have been replaced with deeper wells. Nonetheless, some wells remain vulnerable to future multi-year droughts, including two areas highlighted on Figure 6-1.

As indicated in Table 6-1, the GSAs have determined that the seawater intrusion sustainability indicator, as described in GSP regulations, does not apply to the Modesto Subbasin; as such, no sustainable management criteria have been selected for this indicator (see Section 6.5).

As indicated in Table 6-1, undesirable results have not been experienced for the degraded water quality sustainability indicator even though numerous constituents of concern have been detected above drinking water standards over time. Undesirable results for this indicator refer to water quality impacts specifically caused by GSA management (see Section

6.6.1), which has not yet been initiated. The water quality icon on Figure 6-1 is located in the City of Modesto where water quality is actively managed through groundwater extractions, wellhead treatment, and other operational strategies. Future GSA management will focus on protection against further degradation that could be caused by GSA activities.

As indicated in Table 6-1, no impacts from land subsidence have been observed in the Subbasin. However, basin conditions indicate that land subsidence could occur if water levels continue to decline. Compressible clay layers within and below the Corcoran Clay have been associated with land subsidence in other portions of the Central Valley. Areas within the extent of the Corcoran Clay are highlighted on Figure 6-1 as most susceptible to land subsidence.

The Stanislaus, Tuolumne, and San Joaquin rivers are all interconnected surface water as defined by SGMA (see icons on Figure 6-1). Projected water budget analyses indicate increased streamflow depletion will occur in the future, which could lead to undesirable results unless water level declines are arrested (see Section 6.8).

The overall process for developing sustainable management criteria is discussed in the following section. Subsequent sections document the sustainable management criteria for each sustainability indicator (Section 6.3 through 6.8). #25..
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6.2.2. Public Process for Sustainable Management Criteria An interactive and public process was established by the STRGBA GSA to develop sustainable management criteria for the Modesto Subbasin. The Tuolumne GSA participated through an agreement with Stanislaus County, a member agency of the STRGBA GSA. The STRGBA GSA formed a technical advisory committee (TAC) composed of GSA member agencies, who reviewed and commented on technical presentations throughout the GSP development process. The TAC formed a small planning group to guide development of technical analyses to support the process.

TAC meetings generally followed the monthly STRGBA GSA meetings (typically held on the 2 nd Wednesday of each month at 1:30pm). The STRGBA GSA Chair led the TAC public meetings – with input from stakeholders – for development of recommended sustainable management criteria to be incorporated into the GSP. TAC meetings were held according to the Brown Act and technical presentations on sustainable management criteria were typically posted on the STRGBA GSA website prior to the meetings. In general, presentations provided information on the following topics relating to sustainable management criteria:
• requirements from the GSP regulations,
• relevant hydrogeological conditions in the Modesto Subbasin, • recommendations from the DWR BMP on Sustainable Management Criteria, and • examples from adjacent or other relevant subbasins.

Steps taken during this process were provided in a technical memorandum in February 2021 – information from which has been incorporated into this GSP chapter. The steps are summarized below:

1. Analyze the six Sustainability Indicators, applying conditions from the Basin Setting.

2. Define Undesirable Results (URs) as specific groundwater conditions to avoid.

3. Assign minimum threshold (MTs) for each indicator as a metric that can be used to define undesirable results.

4. Select measurable objectives (MOs) for each indicator as an operational target metric to avoid operating too close to the MT and to avoid undesirable results.

5. Develop interim milestones (IMs) that show progress toward each MO over the 20- year planning horizon.

6. Develop a Sustainability Goal that culminates in the absence of undesirable results (Section 6.1).

The sustainability indicators were introduced at the public GSP kickoff meeting on September 12, 2018 and were considered during development of the technical portions of the Plan Area (Chapter 2) and basin setting (Chapters 3 and 5). A TAC meeting focused solely on the sustainable management criteria was held on November 13, 2019, when the TAC considered examples of sustainable management criteria from neighboring subbasins. #25..
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Historical water budgets, zone budgets, and projected future water budgets were developed, presented, and discussed throughout 2020 (see details on the water budgets in Chapter 5).

More than 15 public TAC meetings were focused on sustainable management criteria, monitoring networks, and management areas. During these meetings, undesirable results were established, and MTs and MOs were selected. Sustainable management criteria, including undesirable results, MTs and MOs were quantified for each representative monitoring site for all three principal aquifers and the four management areas.

6.2.3. Management Areas
Regulations allow for the establishment of management areas within a Subbasin to facilitate implementation of the GSP. A management area can be operated differently from the others and can also define different sustainable management criteria. The GSP must explain the reason for creating each management area and provide rationale for the proposed operation of each; in particular, operation of one management area cannot cause undesirable results in other areas.

In the Modesto Subbasin management areas have been developed to facilitate GSP implementation of projects and are based on areas of similar water supplies and similar ongoing water management activities. Four management areas have been established in the Modesto Subbasin as shown on Figure 6-2 and listed below (approximate acres as calculated in GIS):
• Modesto ID Management Area (101,914 acres)
• Oakdale ID Management Area (49,893 acres)
• Non-District East Management Area (77,218 acres)
• Non-District West Management Area (15,777 acres)
Boundaries of the first two management areas coincide with the current service area boundaries of Oakdale ID and Modesto ID (Figure 6-2). These areas also include most of the urban areas within the Subbasin including Modesto, Oakdale, most of Waterford, and parts of Riverbank. In these two management areas, surface water is available for conjunctive use and supplements groundwater supply for beneficial uses. Specifically, Oakdale ID conjunctively manages Stanislaus River water and groundwater within the Oakdale ID Management Area. Similarly, Modesto ID manages Tuolumne River water and groundwater conjunctively throughout the Modesto ID Management Area.

Surface water supply in these management areas was originally developed for agricultural uses but has been expanded over time to also provide drinking water supplies (e.g., City of Modesto) or non-potable urban uses. As a result, close coordination and partnerships already exist between STRGBA GSA member agencies within the Modesto ID and Oakdale ID management areas. Delineation of management areas coincident with current Modesto ID #25..
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and Oakdale ID service area boundaries allow for seamless coordination of ongoing management activities with new management responsibilities under SGMA.

The Non-District East Management Area and Non-District West Management Area are located on lands outside of the two large irrigation district boundaries where management is currently coordinated through Stanislaus County4as a member agency of the STRGBA GSA. The Non-District West Management Area is the smaller of the two and contains lands between the rivers and Modesto ID and Oakdale ID management areas along the rim of the western Subbasin. Surface water is also available in this management area through riparian rights along the river boundaries. Delineation of these lands as a separate management area combines areas of similar water supply activities in the western Subbasin to facilitate GSA management.

The Non-District East Management Area is defined as lands in the eastern Subbasin outside of the Oakdale ID and Modesto ID management areas. Unlike the other management areas, surface water has not been widely available for water supply; groundwater has served as the primary water supply for the expanding agricultural production in the Non-District East Management Area.

As described above and explained in more detail in subsequent sections of Chapter 6, the Non-District East Management Area is the primary area with declining water levels in the Subbasin. Accordingly, projects and management actions are prioritized for this management area in order to achieve the Subbasin’s Sustainability Goal.

Most of the infrastructure required for GSP projects will need to be developed in the NonDistrict East Management Area by local landowners. The Non-District East Management Area will need to develop agreements and partnerships with both the Modesto ID and the Oakdale ID management areas to bring additional water supply into the area.

As indicated by the information above, the delineation of management areas shown on Figure 6-2 facilitates the future management activities anticipated by the GSP.

6.2.4. Organization of Sustainability Indicators
Each sustainability indicator is discussed separately in Sections 6.3 through 6.8 below.

Information within each of these sections is organized similarly and tracks the order of GSP requirements provided in Subarticle 3. Sustainable Management Criteria. Headings and subheadings in the subsequent sections are as follows:
• Introduction including regulatory definitions
• Definition of Undesirable Results along with quantitative criteria that are used to define when and where undesirable results would occur.
o Causes of Undesirable Results
4 As mentioned previously, Stanislaus County also represents the Tuolumne GSA by agreement. #25..
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o Potential Effects on Beneficial Uses and Users of Groundwater • Quantification of minimum thresholds (MTs) followed by the six requirements for MT analysis in the regulations
o Justification and support for MTs
o Relationship of MTs to other sustainability indicator MTs and how GSAs determined that undesirable results would be avoided
o Impacts of MTs on adjacent subbasins
o Effects of MTs on beneficial uses and users of groundwater o Consideration of State, Federal, or local standards in MT Selection o Quantitative measurement of MTs
• Quantification of interim milestones (IMs).
• Quantification of measurable objectives (MOs)
The description of the Plan Area (Chapter 2) was used to provide the context for groundwater wells and the overall water resources for the Subbasin. The hydrogeologic conceptual model and groundwater analyses (Chapter 3) were used to understand the basin conditions relevant to sustainability. The historical, current, and projected future water budgets (Chapter 5) were used to analyze overdraft conditions, streamflow depletions, and subsurface flows with adjacent subbasins. Water budgets were also used to establish a sustainable yield for the Subbasin that analyzed sustainable management criteria required to avoid undesirable results.

Collectively, these analyses informed and supported the selection of sustainable management criteria as discussed for each sustainability indicator below.

6.3. CHRONIC LOWERING OF GROUNDWATER LEVELS
SGMA defines an undesirable result for the chronic lowering of groundwater levels as a “significant and unreasonable depletion of supply if continued over the planning and implementation horizon” (§10721 (x)(1)). As described in Section 3.2.4, DWR estimated the amount of fresh groundwater supply beneath the Modesto Subbasin at about 14 million acre feet (MAF) in 1961. An analysis of the historical water budget (WY 1991 – WY 2015) estimates a depletion of about 1.1 MAF of this supply over the 25-year period (about 43,000 AFY, see Figure 5-20 and Table 5-8), about 8 percent of the estimated total supply. Most of the deficit likely occurred in recent years with increases in agricultural water demand; this indicates that about 13 MAF of groundwater remains in storage.

Although significant amounts of fresh groundwater remain in the Subbasin, the chronic lowering of groundwater levels has created adverse impacts to numerous water supply wells. Because wells are the primary method for accessing groundwater for beneficial uses, adverse impacts to water supply wells can lead to undesirable results. As such, the emphasis of this sustainability indicator is depletion of accessible supply and focuses on adverse impacts to Subbasin supply wells. This emphasis is also consistent with GSP regulations, #25..
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which note that depletion of supply should be considered “at a given location” (§354.28(c)(1)), such as at a well.

The SGMA definition of chronic lowering of groundwater levels also addresses water level declines within the context of overdraft and storage as shown below:

Overdraft during a period of drought is not sufficient to establish a chronic lowering of groundwater levels if extractions and groundwater recharge are managed as necessary to ensure that reductions in groundwater levels or storage during a period of drought are offset by increases in groundwater levels or storage during other periods. (§10721 (x)(1)).

This definition allows for water level declines during drought as long as such declines do not result in undesirable results and as long as water levels recover to acceptable levels over average hydrologic conditions. Accordingly, the analysis of the chronic lowering of groundwater levels focuses on long-term trends of water level declines that do not recover during wet periods.

Undesirable results, including causes and impacts to beneficial uses, are described below in Section 6.3.1. The undesirable result definition, along with criteria to quantify where and when undesirable results will occur, is provided in Table 6-3 at the end of Section 6.3.1.

Section 6.3.2 describes the quantification of minimum thresholds (MTs). Section 6.3.3 provides the approach and selection of measurable objectives (MOs). Interim milestones that cover all of the applicable sustainability indicators are described in Section 6.9.

6.3.1. Undesirable Results for Chronic Lowering of Groundwater Levels As summarized previously, groundwater level declines in the Modesto Subbasin are the combined results of overdraft and multi-year drought conditions. Over-pumping, primarily in the Non-District East Management Area (NDE MA) (Figure 6-1), has contributed to a historical Subbasin overdraft of about 43,000 AFY (Section 5.1.4 and Table 5-6).

Groundwater level declines associated with this overdraft have propagated outside of the NDE MA and affected water levels in adjacent areas to the west where additional water supply wells have been impacted (see estimated areas of vulnerable domestic wells on Figure 6-1).

Impacts to water supply wells are exacerbated during droughts. Chronic declines in groundwater levels are accelerated due to less availability of surface water for water supply, decreased recharge from decreases in precipitation and runoff, and/or increased irrigation demand due to higher temperatures. If groundwater declines are not arrested following a drought, future droughts will begin with even lower water levels, resulting in increased impacts to water supply wells and beneficial uses that worsen with each drought.

In addition to impacts to wells as described below, the lowering of groundwater levels may also lead to undesirable results for the other sustainability indicators such as reduction of #25..
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groundwater in storage, land subsidence, depletions of interconnected surface water and adverse impacts to groundwater dependent ecosystems (GDEs). These impacts are summarized in Section 6.3.2.2 and described separately for each indicator in remaining sections of this chapter.

6.3.1.1. Causes of Undesirable Results – Adverse Impacts to Wells The combination of over-pumping and drought caused widespread adverse impacts to Subbasin water supply wells during drought conditions WY 2014 – WY 2017, resulting in undesirable results. Even though well owners appear to have mitigated most of these impacts, GSAs intend to arrest water level declines so that future widespread impacts to water supply wells can be avoided or mitigated. Adverse impacts to water supply wells caused by chronic lowering of groundwater levels are discussed below.

In general, lower water levels increase pumping costs. If water levels fall below the pump intake, costs may be incurred for pump lowering and/or other well modifications. Further declines can result in water levels falling below the top of well screens, potentially decreasing capacity or well integrity due to geochemical changes, biological clogging, and/or air entrainment. Water level declines can also damage wellbore equipment, such as pumps or casing, from cavitation or other mechanisms. If water levels fall below the bottom of the well and do not sufficiently recover, the well is dewatered and would require replacement.

Older wells, shallow wells, and/or wells with casing integrity issues typically have a higher risk of failure.

In the Modesto Subbasin, the STRGBA GSA member agencies responsible for public drinking water supplies documented numerous adverse impacts to public supply wells caused by declining water levels during drought (WY 2014 to WY 2017). During that period, declining water levels provided an opportunity to observe impacts associated with the historic low levels throughout much of the Subbasin. Most agencies observed a decrease in capacity and well efficiency. Some drinking water wells failed due to collapsed casing or other problems.

More than 150 domestic wells were also adversely impacted (locations on Figure 6-1).

Significant adverse impacts to water supply wells in the Modesto Subbasin during this drought period are summarized in Table 6-2 as follows.
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Table 6-2: Adverse Impacts to Wells Associated with Declining Groundwater Levels Adverse Impacts to Water Supply Wells
from 2014 – 2017 Agencies Reporting Impacts
159 dry1 or failed domestic wells (most were more
than 50 years old and less than 100 feet deep)
Stanislaus County
Loss of capacity in municipal wells
(pump replaced and lowered)
City of Waterford
Replace or deepen pumps in 3 agency wells; OID
landowners also complained of well issues
Oakdale Irrigation District
1 For purposes of this table, a “dry” domestic well does not necessarily mean that water levels in the aquifer have declined below the bottom of the well; well failures are also associated with water levels falling below a shallow pump intake or below the top of well screens such that capacity is adversely affected.

As indicated in Table 6-2, not all beneficial users of groundwater wells in the Modesto Subbasin experienced adverse impacts during the 2014 to 2017 drought. During this period, the cities of Riverbank and Oakdale were able to operate their deep drinking water supply wells without interruption. Similarly, Modesto ID did not experience well problems. The City of Modesto did not experience well impacts directly related to the drought but had water quality problems that could be exacerbated if groundwater levels continue to decline in the Subbasin. In the western Subbasin, groundwater levels experienced relatively small declines (less than 10 feet) and recovered quickly after 2016.

Most well impacts in Table 6-2 occurred in the central-eastern Subbasin due to the presence of numerous water supply wells in areas of more significant water level declines (Figure 6-1; see also hydrographs on Figure 3-25). Although the 159 reported domestic well failures occurred throughout the Subbasin, most failures were concentrated in the eastern half of the Subbasin (Figure 6-1). Although most of these domestic wells appear to have been replaced, areas with vulnerable domestic wells have been identified along the Tuolumne and Stanislaus rivers (dashed areas on Figure 6-1). More details and analyses of failed and replacement domestic wells are provided in Section 2.3.3.

The City of Waterford is located within the vulnerable area along the Tuolumne River, where one of its primary water supply wells required replacing and lowering of a well pump during the 2015 drought (Table 6-2). Near the vulnerable area along the Stanislaus River, Oakdale ID reported water level declines of 20 feet to 50 feet from 2005 to 2020 in its deep water supply wells. Since 2016, water levels have continued to decline about 1.3 feet per year in the main service area and 2 to 4 feet per year in eastern OID. These declines caused adverse impacts to Oakdale ID deep agency wells. In addition, many landowners complained to Oakdale ID regarding private well issues.

Finally, the outreach team noted impacts to a few private wells as reported on the Modesto Subbasin Stakeholder Survey (see Chapter 4). Out of 12 responses from well owners, two reported either capacity or water quality issues with their well; the remaining 10 responders did not report well issues during the 2014-2017 drought. #25..
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6.3.1.2. Potential Effects on Beneficial Uses
Adverse impacts described above affect all beneficial uses of groundwater accessed through wells including municipal, domestic, industrial, and agricultural water supply. Any of these impacts can also affect property interests.

For agricultural users, impacts can increase costs, delay irrigation operations, and result in damage to crops. For industrial users, well issues can affect operational costs, delay goods and services, or adversely affect industrial processes relying on a specific groundwater quality. For public water suppliers, well impacts can increase wellfield operational costs, reduce pressure in distribution systems, cause water quality concerns, or even jeopardize the ability to provide a reliable and safe drinking water supply.

Impacted domestic well owners during the 2014-2017 drought reported the need for trucked water, use of temporary or permanent storage tanks, purchase of bottled water, lowering of well pumps, drilling of replacement wells, and other measures. A valley-wide shortage of drillers caused significant delay in the ability to lower a pump or otherwise modify/replace a well. In addition, domestic well owners in the Modesto Subbasin are often without financial resources necessary to replace their household water supply. Many domestic wells are located in underrepresented and economically disadvantaged communities where wells are the only available drinking water source.

Although this sustainability indicator is focused on adverse impacts to wells, chronic lowering of groundwater levels can also adversely impact environmental uses of groundwater, including GDEs (Section 3.2.8). Given that GDEs in the Modesto Subbasin are primarily located along the three river boundaries, GDE impacts are also affected by the interconnected surface water sustainability indicator, as discussed in Section 6.8.

Many of these adverse well impacts that occurred during the 2014-2017 drought appear to have been mitigated. Public water suppliers have secured groundwater supply from new or modified wells. Proposed GSP projects will increase surface water deliveries for municipal supply in both Waterford and Modesto (see Chapter 8).

Most of the failed domestic wells appear to have been replaced. DWR well completion records indicate that about 236 new domestic wells have been drilled since 2015 – about

1.5 times the number of previously-reported failed wells. Although data are insufficient to provide a one-to-one match, most new wells are near the estimated location of a failed well and appear to be replacement wells5.

Since 2016, only three domestic wells have been reported as being impacted from lower water levels. These domestic wells were reported to be dry as of August and September 2021 as indicated on the DWR Household Water Supply Shortage Reporting System 5 The DWR database of domestic wells has been recognized to be incomplete, with uncertainty associated with numbers of wells, exact location, and well construction (including screen intervals, pump settings, or total depth. See analysis of domestic wells in Section 2.3.2. #25..
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(Household Water Supply Shortage Reporting System (ca.gov)). Of those three wells, the two in the City of Modesto were shallow wells with total depths of 29 feet and 79 feet. The reported failed well in the City of Oakdale had a total depth of 149 feet.

SGMA does not require the protection of all groundwater wells or the correction of historical undesirable results. For this GSP, the widespread impacts to water supply wells during the 2014-2017 drought (which were caused by then-historic groundwater level declines) are considered to be undesirable results. Although impacts appear to be mostly mitigated at current groundwater levels, the GSP strives to avoid similar undesirable results in the future by arresting chronic groundwater level declines in the Subbasin.

To assess potential undesirable results in the future, an analysis was conducted in 2024 of potential impacts to existing water supply wells if additional groundwater level declines occur. This analysis addressed potential impacts to water supply wells of groundwater levels declining to the MT groundwater elevations and to the IMs established for WY 2027, where the 2027 IMs are below the MTs. The methodology and results of this analysis are described in Section 6.3.3.1.

6.3.1.3. Modesto Subbasin Definition of Undesirable Results Based on the information summarized above and additional information presented in previous sections of this GSP (especially Sections 2.3.2 and 3.2), the definition of undesirable results focuses on maintaining access to groundwater supply through Subbasin wells.

Regulations also require that the undesirable result definition include quantitative criteria defining when and where groundwater conditions can cause an undesirable result (§354.26(b)(2)). These criteria include the number of monitoring sites/events where MT exceedances may create those conditions; criteria recognize that a single MT exceedance at one monitoring site during one monitoring event may not be sufficient to cause an undesirable result. This framework allows for clear identification as to when an undesirable result is triggered.

The undesirable result definition for the Modesto Subbasin, along with the criteria that may lead to an undesirable result, is summarized in the table below. #25..
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Table 6-3: Undesirable Results for Chronic Lowering of Groundwater Levels Undesirable Results Definition
Principal
Aquifer(s)
Chronic
Lowering of
Groundwater
Levels
Undesirable results are defined as significant and
unreasonable groundwater level declines – either due to multi-year droughts or due to chronic declines where
groundwater is the sole supply – such that water supply wells are adversely impacted in a manner that cannot be readily managed or mitigated.

An undesirable result will occur when at least 33% of
representative monitoring wells exceed the MT for a
principal aquifer in 3 consecutive Fall monitoring events.

All
As indicated in the criteria above, an undesirable result is triggered when a third or more of the monitoring wells in each principal aquifer exceed the MT during three consecutive Fall monitoring events. To provide context for these criteria, additional Subbasin considerations are provided below.

At this time, the monitoring network for chronic lowering of water levels contains 61 wells distributed among the three principal aquifers. Maps of these representative monitoring well locations are provided in Chapter 7 (Figures 7-1, 7-2, and 7-3). The number of wells in each principal aquifer are summarized below along with the number of wells that could trigger an undesirable result (i.e., 33 percent):
• Western Upper Principal Aquifer: 17 wells (33% - 6 wells) • Western Lower Principal Aquifer: 5 wells (33% - 2 wells) • Eastern Principal Aquifer: 39 wells (33% - 13 wells)
The number of representative monitoring wells that could trigger an undesirable result condition is relatively small (i.e., between 2 and 13 wells for each principal Aquifer), which provides protection for water supply wells in the Subbasin. The number of wells allowed to exceed the MTs are commensurate with the area of the aquifer being monitored. For example, the western aquifers cover about 56,000 acres while the Eastern Principal Aquifer is about three times as large (190,000 acres). Therefore, the number of wells associated with exceedances in the Eastern Principal Aquifer is much larger.

In addition, the areas that could cause undesirable results represent a relatively small percentage of the Subbasin – about 8 percent for exceedances in the western aquifers and about 25 percent of the Subbasin for exceedances in the Eastern Principal Aquifer. This indicates that undesirable results will be triggered when a relatively small area of the #25..
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Subbasin exceeds the MT. In this manner, the undesirable results definition and criteria are protective against widespread exceedances of the MT.

Data gaps are recognized in the monitoring networks for both the Eastern Principal Aquifer and the Western Lower Principal Aquifer. Additional wells are planned for these networks in the initial years of GSP implementation (see Chapter 8). Accordingly, the number of wells with MT exceedances required to trigger undesirable results may need to be revised going forward.

The number of monitoring events with MT exceedances is also considered in the undesirable results definition in Table 6-3. This provides some flexibility for future drought conditions whereby wells are allowed to exceed the MT in drought as long as periods of decline are relatively short, and ongoing projects/management actions support subsequent water level recovery above the MTs. The use of three consecutive Fall semi-annual monitoring events is based on observation that three critically dry years (WY 2013 – WY 2015, see Figure 3-2) lead to previous undesirable results. Most of the adverse impacts to wells used to define undesirable results began at the end of this three-year period (i.e., Fall 2015) and extended throughout 2016. As described above, previous impacts to wells have been managed and mitigated for current (2021) groundwater elevations. The undesirable results criteria above are selected to avoid undesirable results during future multi-year droughts.

Even though monitoring will be conducted on a semi-annual basis (i.e., Spring and Fall), criteria limit the MT exceedances to Fall monitoring events. This focuses GSP management on long-term trends rather than seasonal fluctuations and is more protective against undesirable results. A partial Spring recovery above the MT may not indicate an improvement to an overall declining water level trend. When considered in the context of water year type, a comparison of Fall events allows for a better management tool for differentiating a short-term decline versus a longer term decline below the MT.

Collectively, these criteria provide a reasonable management approach for avoidance of undesirable results for chronic lowering of groundwater levels in the Modesto Subbasin.

6.3.2. Minimum Thresholds for Chronic Lowering of Groundwater Levels Regulations require that the quantitative MT metric for this indicator be “the groundwater elevation indicating a depletion of supply at a given location that may lead to undesirable results” (§354.28 (c)(1). In the Modesto Subbasin, MTs are quantified as the low groundwater elevation from WY 1991 – WY 2020 at representative monitoring sites for all three Principal Aquifers.

While water levels have continued to decline in eastern portions of the Subbasin, the MT period contains the historic low water level for much of the Subbasin. Many of the selected MTs occurred in the 2015-2016 time period associated with drought conditions (Figure 6-1). #25..
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However, some areas of the western Subbasin reached a historic low during the early to mid-1990s before surface water was available to the City of Modesto.

Table 6-5 states the selected approach for the MTs; the MT value at each representative monitoring well is presented in Chapter 7, which describes the GSP monitoring network (see Section 7.1.1). Hydrographs of all monitoring network wells with MTs and MOs are provided in Appendix F.

Table 6-4: Minimum Thresholds for Chronic Lowering of Groundwater Levels Minimum Thresholds
Principal
Aquifer(s)
Chronic Lowering
of Groundwater
Levels
Minimum thresholds are set as the historic low
groundwater elevation observed or estimated during
WY 1991 – WY 2020 at each representative
monitoring location, based on available data.

All
Information from the basin setting used to support these MTs are summarized in the following section.

6.3.2.1. Justification and Support for Minimum Thresholds GSP regulations require that MTs for this indicator be supported by:
• The rate of groundwater elevation decline based on historical trends, water year type, and projected water use in the basin.
• Potential effects on other sustainability indicators. (§354.28 (c)(1)(A)(B)).

Historical declines in groundwater levels across the Subbasin are discussed throughout Section 3.2 and specifically in Section 3.2.2; associated water year types are based on the detailed information in Section 4.2.2.1 (also see Figure 3-2). Figures 3-21 through 3-25 present hydrographs showing rates of decline in selected wells with relatively long water level records across the Subbasin. Figure 6-1 provides locations of failed domestic wells from 2014 to 2017, representing undesirable results caused by groundwater level declines (also discussed in Section 2.3.3 and shown on Figure 2-15). Figure 2-17 shows the location of new and/or replacement domestic wells drilled since the 2015 drought.

As indicated by the hydrographs on Figures 3-24 and 3-25, water level declines become progressively larger from west to east in the Subbasin, especially since recent drought conditions began in WY 2013. Although wells with water level data are sparse in the NDE MA, groundwater levels in eastern-most wells have declined about 40 feet over the last seven years (decline rate of about 5.7 feet per year; see hydrograph 20 on Figure 3-25).

Rates of groundwater level declines are summarized briefly by principal aquifer below. #25..
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• Western Upper Principal Aquifer (Figures 3-21 and 3-22): Water levels in this principal aquifer have been relatively shallow and stable throughout the study period with minimal – but observable – declines during drought. Water levels have recovered to near pre-drought levels in almost every well shown and no significant long-term water level declines have been observed. Depth to water ranges from less than 10 feet bgs to about 40 feet bgs. Most of historic low water levels occurred during 2015-2016 drought conditions. Some wells near the City of Modesto exhibit historic low water levels during the 1990s drought when groundwater was primarily the City’s sole water supply (see hydrographs 7 and 8 on Figure 3-22). The availability of surface water to supplement the City’s drinking water supply allowed water levels to recover. During more recent droughts, water levels in these wells have generally remained above the previous historic low levels. • Western Lower Principal Aquifer (Figure 3-23): Although water levels have been tracked in numerous wells in the western Subbasin, many wells are screened in both the Western Upper Principal Aquifer (unconfined) and the Western Lower Principal Aquifer (confined). Wells known to be screened only in the Western Lower Principal Aquifer are sparse; nonetheless, water levels appear to be relatively stable with small declines during drought (about 10 feet to 20 feet) followed by recovery in post-drought years. The decline and recovery for hydrograph 11 on Figure 3-23 is due to the change in surface water availability for the City of Modesto as described above. Larger seasonal fluctuations are observed on the hydrographs due to the confined nature of the aquifer and its use by active pumping wells. • Eastern Principal Aquifer (Figures 3-24 and 3-25): Overall declines are observed in the Eastern Principal Aquifer, with increasing rates of decline and total declines from west to east. For wells in the western portion of the aquifer, long-term declines are relatively small (less than about 10 feet) over the study period (see hydrographs 12 and 13 on Figure 3-24). Wells slightly farther to the east exhibit declines during the 2015 drought of about 20 feet with only partial recovery (hydrographs 14, 15, and 16 on Figure 3-24).

Wells in the eastern Subbasin have experienced the largest declines, both during drought and over the long term since at least the mid-2000s (Figure 3-25). As shown by hydrograph 20 on Figure 3-25, eastern wells have overall declines of about 40 feet during the recent drought and long-term declines since the mid-2000s. During that time, water demand in the eastern Subbasin increased due to the expansion of irrigated agriculture and changes in cropping patterns (see discussion in Section 2.2 and Figure 2-8). In the eastern Subbasin, long-term rates of decline are up to about #25..
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2.7 feet/year; rates of decline during drought are up to about 6 feet/year (Figure 3- 25).

Water level declines in the eastern Subbasin occur primarily in the NDE MA (Figure 6-1).

However, local over-pumping in that area appears to have propagated westward, causing water level declines in other management areas – especially in eastern Oakdale ID MA. The area of water level declines also appears to be expanding to the north and south, intercepting groundwater that would typically be flowing toward the river boundaries.

The GSP intends to arrest these high rates of expanding water level declines by establishing MTs at the historic low water level observed (or estimated, if data are not available) during WY 1991 – WY 2020. Using this time period, MTs were selected for the 61 wells in the representative monitoring network for chronic lowering of groundwater levels; those MTs are discussed in Section 7.1.1, posted on Figures 7-1, 7-2, and 7-3, and listed in Table 7-1.

Almost all of the selected MTs represent one of three time periods:
• Fall 2015 groundwater elevation (most western Subbasin wells) • Fall 1991 groundwater elevation (a few wells near the City of Modesto) • Fall 2020 groundwater elevations (most eastern Subbasin wells) For most western wells, the MT was typically defined by 2015-2016 water levels. Even if water levels continue to decline in the eastern Subbasin while the GSP is being implemented, projects and management actions will have to be sufficient for water levels to recover back to the selected MT. The following conditions were considered when setting the MT at the historic low groundwater elevation:
• Replacement wells and other well improvements appear to have mitigated impacts from low water levels during the 2015-2016 drought conditions. • The large number of new and deeper domestic wells drilled since 2015 can reasonably be assumed to accommodate current low water levels, with some tolerance for future droughts.
• The analysis in Section 2.3.3 indicates that MTs will avoid the widespread failures of about five percent of the total domestic wells drilled in the Subbasin that occurred during the 2015 drought conditions. Uncertainties associated with data gaps regarding domestic wells limit the ability to accurately identify the exact number of wells subject to impacts (see also Section 9.5.3).
• The Subbasin is not currently experiencing widespread adverse impacts to water supply wells that occurred in 2015-2016 and formed the basis for its undesirable result definition.
• Most of the MTs are commensurate with recent Fall 2020 water levels; no additional undesirable results were identified during that Fall period. #25..
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• As of Spring 2021, groundwater levels are within about 10 feet of the MT; several wells are below the MT.

Collectively, these considerations support the selection of the MTs for chronic lowering of groundwater levels.

6.3.2.2. Relationship between MTs of Each Sustainability Indicator Regulations require a description of the relationship between the MTs for each sustainability indicator and how the GSAs have determined that basin conditions at each MT will avoid undesirable results (§354.28(b)(2)). To facilitate a comparison between MTs, a summary table of MTs for each sustainability indicator is provided below. Justification for the approach to the MTs for each indicator is provided in subsequent GSP sections, as indicated in the table.

Table 6-5: Summary of Minimum Thresholds by Sustainability Indicator Sustainability
Indicator
Minimum Threshold (MT) GSP Section
Chronic Lowering of
Groundwater Levels
Low groundwater elevation WY 1991 – WY 2020 6.3.2
Reduction of
Groundwater in Storage
Low groundwater elevation WY 1991 – WY 2020 6.4.2
Seawater Intrusion Not applicable 6.5
Degraded Water Quality
MCL of each Constituent of Concern 6.6.2
Land Subsidence Low groundwater elevation WY 1991 – WY 2020 6.7.2 Interconnected Surface
Water
Fall 2015 groundwater elevation 6.8.2
As indicated in the table above, the historic low groundwater elevation – as observed or estimated during the period WY 1991 – WY 2020 – has been selected as the MT for three of the six sustainability indicators (chronic lowering of groundwater levels, reduction of groundwater in storage, and land subsidence).

Groundwater elevations are also used as a proxy for interconnected surface water MTs but are set differently from other water level MTs. To be more protective of basin conditions along the three river boundaries, MTs for interconnected surface water are set as the Fall 2015 groundwater elevations. This approach is consistent with the need to guard against projected increases in streamflow depletion by the water budget modeling analyses (Section 5.1.4.3). In particular, projected increases in average streamflow depletions from the Stanislaus and Tuolumne rivers could lead to undesirable results. This approach is discussed in more detail in Section 6.8.
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As discussed previously and indicated in the table above, the seawater intrusion indicator has been determined by the GSAs as not applicable to the inland Modesto Subbasin.

Accordingly, no MTs have been set for seawater intrusion.

A different approach to MTs was used for the degraded water quality sustainability indicator. MTs for that indicator are set as the California drinking water standard for water quality constituents of concern most applicable to the Modesto Subbasin. This MT approach will not conflict with the other MTs for the Subbasin. Further, the MTs set for the other sustainability indicators are supportive of the MTs for degraded water quality, as described in more detail in Section 6.6.

The interrelatedness of the MTs among the four sustainability indicators with groundwater levels as a proxy are summarized below.
• MTs for chronic lowering of groundwater levels are used as a proxy for reduction of groundwater in storage and land subsidence for all three Principal Aquifers.

Therefore, the MTs will not present conflicts between these three indicators. • As explained in Sections 6.4, the use of groundwater elevations as a proxy for reduction of groundwater in storage is supported by the sustainable yield analysis (Section 5.3), whereby the historic low water levels are correlated directly to a sustainable yield volume for the Subbasin (267,000 AFY), which avoids undesirable results and also meets the requirement to use a volume as the metric for the reduction of groundwater in storage indicator (see Section 6.4.2). • As explained in Section 6.7, the historic low water level is also an appropriate MT for land subsidence. By preventing significant groundwater level declines below the historic low level, the depressurization/dewatering of compressible subsurface clay layers can be avoided (see Section 6.7). Because this mechanism has been the primary cause of land subsidence in the Central Valley, the use of MTs for chronic lowering of groundwater levels as a proxy is supported (Section 6.7.2). • The MTs for interconnected surface water are sufficiently close to the MTs for chronic lowering of water levels. Many of the MTs for chronic lowering of water levels are either the same or within only a few feet of the MTs for interconnected surface water. Accordingly, there are no conflicts between these two MT data sets.

The use of water levels as a proxy for the interconnected surface water MTs is supported by the sustainable yield analysis in Section 5.3 and demonstrates the ability of the aquifer to meet selected MTs for both sustainability indicators under the same basin conditions (see also Section 6.8).
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Although presentation and review of technical information and selection of MTs by the TACs generally occurred one sustainability indicator at a time, basin conditions and sustainable yield analyses support the interrelatedness of the MTs. (Basin conditions that supported chronic lowering of water levels were discussed in Section 6.3.2.1 above). Sustainable yield analyses were conducted interactively for future conditions and sustainable management criteria to determine how MTs could be achieved on a Subbasin-wide basis (Section 5.3). By first setting MTs to correct overdraft conditions and arrest future groundwater elevation declines, all of the other sustainability indicators in the Modesto Subbasin could be supported. The application of consistent methodologies in each principal aquifer and in each of the four management areas (Figure 6-2) allow the collective MTs to work well together to avoid undesirable results and support sustainable groundwater management.

Notwithstanding the protective MTs above, preventing all impacts to water supply wells may be difficult where large numbers of densely spaced water supply wells are pumping at maximum capacities during drought conditions. Closely spaced pumping wells can cause interference with other wells, even if basin-wide water levels are managed at reasonable levels. Well interference between two closely spaced wells is not included in the undesirable results definition and will be managed locally, as needed. By setting MTs at historic low groundwater elevations across most of the Subbasin, regional long-term declines will be arrested and significant and unreasonable adverse impacts to water supply wells can be avoided.

6.3.2.3. Impacts of MTs on Adjacent Subbasins
Regulations require consideration of how Modesto Subbasin MTs impact the ability of an adjacent subbasin to achieve its sustainability goal. Significant technical similarities among the Modesto Subbasin and its three neighboring subbasins facilitate this process. For example, all of the subbasins have delineated principal aquifers in the same manner. In addition, all of the adjacent subbasins are linked to the Modesto Subbasin by a shared river boundary (i.e., Turlock Subbasin south of the Tuolumne River, Eastern San Joaquin Subbasin north of the Stanislaus River, and the Delta-Mendota Subbasin west of the San Joaquin River, see Figure 6-1). Due to the shared interconnected surface water along these rivers, MTs in each of the subbasins have been set in a similar manner.

There is also significant inter-basin coordination occurring among GSAs and member agencies across all of these subbasins. Multiple member agencies are actively involved in the GSP process in both the Modesto Subbasin and one of the adjacent subbasins.

For example, in the Eastern San Joaquin (ESJ) Subbasin to the north, both Oakdale ID and Stanislaus County are member agencies of ESJ GSAs and actively participated in GSP development for that subbasin. Oakdale ID has service areas and operations in both the Modesto and the ESJ subbasins, located along a large portion of the boundary between the two. Stanislaus County also provides consistent coordination in the Delta Mendota Subbasin to the west. In addition, members of the technical consulting team and outreach team in the Modesto Subbasin were also involved in GSP development in both the ESJ and Delta Mendota subbasins.
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In the Turlock Subbasin to the south, several member agencies are represented in both the Turlock and Modesto subbasins, including Stanislaus County, City of Modesto (with pumping wells in the Turlock Subbasin), and the City of Waterford (which operates the water supply system for Hickman in the Turlock Subbasin). Also, Turlock ID and Modesto ID coordinate on diversions from the Tuolumne River to provide a large supply of Tuolumne River water to both subbasins. Finally, the GSP technical consulting team is the same in both Turlock and Modesto subbasins and has developed one integrated surface water-groundwater model for coordinated GSP analyses.

Through coordination activities by these member agencies, additional coordination meetings with adjacent subbasin representatives, and review of draft and completed GSPs, the MTs selected for chronic lowering of water levels in the three adjacent subbasins have been considered together. In brief, the Modesto Subbasin MTs are not expected to either cause undesirable results or adversely impact GSP implementation in adjacent subbasins, as summarized below.

6.3.2.3.1. Eastern San Joaquin Subbasin
The MTs for chronic lowering of water levels in the ESJ Subbasin are defined as the shallower groundwater elevation of the following (ESJGWA, 2019):
• the deeper of 1992 and 2015-2016 historical groundwater levels with a buffer of 100 percent of the historical range applied, or
• the 10th percentile domestic well total depth of wells within a 3-mile radius of the monitoring well.

MTs have been set for 20 representative monitoring wells in the ESJ Subbasin, four of which are within about three miles from the shared boundary with the Modesto Subbasin (02S07E31N001, 02S08E08A001, Burnett-OID4, and 01S10E26J001M; see Figure 3-2 in ESJGWA, 2019). All of the MTs set for the ESJ monitoring wells appear to be lower than the closest Modesto Subbasin MTs.

For example, the closest ESJ Subbasin well to the Modesto Subbasin is Burnett (OID4), located across the Stanislaus River from Modesto Subbasin monitoring wells Allen (OID1) and Birnbaum (OID3). The Burnett MT is 60.7 feet msl (Table 3-1 in ESJGWA, 2019) and the Birnbaum and Allen MTs are 74 and 75 feet msl, respectively (see Figure 7-7). MTs for all three wells are based on 2015 groundwater elevations, although the ESJ monitoring well has a buffer equal to the historical water level range (see first bullet above). As indicated by these values, MTs in the ESJ Subbasin are lower, but close to the MTs in the Modesto Subbasin. Accordingly, the MTs do not appear to conflict across the Subbasin boundary and MTs in the Modesto Subbasin are not expected to adversely impact GSP implementation in the ESJ Subbasin.

ESJ Subbasin MTs for chronic lowering of water levels are also used as a proxy for the reduction of groundwater in storage, land subsidence, and interconnected surface water.

Therefore, these MTs represent the best MTs for evaluation of potential impacts across the #25..
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shared Stanislaus River boundary. Finally, as noted above, Oakdale ID operates within its service areas on both sides of this boundary and has GSP monitoring and management responsibilities in both subbasins. This close coordination allows the tracking of potential impacts in each subbasin going forward.

6.3.2.3.2. Delta-Mendota Subbasin
Sustainable management criteria in the adjacent Delta-Mendota Subbasin are provided in the Northern & Central Delta-Mendota Regions GSP (W&C and P&P, 2019). In that GSP, the MTs for water levels are defined as the hydrologic low groundwater level for the Upper Principal Aquifer and 95 percent of the hydrologic low groundwater level for the Lower Principal Aquifer. Because these low groundwater levels generally occurred in WY 2015, and MTs along the San Joaquin River in the Modesto Subbasin are also set at WY 2015 levels (for interconnected surface water – see Table 6-5), there should be no conflict in MTs along this boundary.

Because the shared San Joaquin River boundary between the Delta-Mendota Subbasin and the Modesto Subbasin is relatively short, there are no representative monitoring wells in the Delta-Mendota Subbasin along that boundary. The two closest wells are 06-004 (Upper Aquifer) and 06-003 (Lower Aquifer), both located about three miles to the southwest from the southwestern corner of the Modesto Subbasin. MTs for those two wells are 14.8 feet msl and -8.6 feet msl, respectively.

In the Modesto Subbasin, the closest representative monitoring wells in equivalent principal aquifers are Canfield 90 (Western Upper Principal Aquifer) and MRWA-3 (Western Lower Principal Aquifer). MTs for chronic lowering of water levels in those wells are 32 feet msl and 28 feet msl, respectively. Given the higher elevations and distance from representative monitoring locations, the MTs in these two subbasins do not conflict and are not expected to adversely impact GSP implementation in either Subbasin.

6.3.2.3.3. Turlock Subbasin
By selecting MTs for the chronic lowering of groundwater levels at the historic low groundwater elevations, MTs in the inland portions of the Subbasin are slightly lower in some places than in the Turlock Subbasin. However, the methodology for selecting MTs along the shared Tuolumne River boundary is identical for both subbasins. Along that boundary MTs are set at the Fall 2015 groundwater elevations in the Modesto Subbasin for interconnected surface water (Table 6-65; see also Section 6.8). Sustainable yield analyses indicate very small subsurface flows between the two subbasins (within about 1,000 AFY) along the approximate 35-mile river boundary (see Table 5-15 for the net subsurface flows between the two subbasins). These conditions suggest that there will be no adverse impacts on GSP implementation from MTs in the Modesto Subbasin on Turlock Subbasin MTs.

6.3.2.4. Effects of MTs on Beneficial Uses and Users of Groundwater By arresting groundwater level declines in the Subbasin, long-term use of groundwater will become more sustainable and provide benefits to all beneficial uses of groundwater in the #25..
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Subbasin. However, there are consequences to maintaining these MTs for some current beneficial uses of groundwater.

In brief, the current level of groundwater use will not be able to be sustained without sufficient projects or management actions to replenish the Subbasin. This will require maintenance of water levels in deep wells that could otherwise accommodate additional declines. In the NDE MA, where growers are currently reliant on groundwater for agricultural beneficial uses, significant investment in projects and supplemental water will be required to support the current level of agricultural production. If projects cannot meet the sustainable yield, demand reduction will need to be considered, which could negatively affect property interests in the Subbasin.

Conversely, the beneficial uses of public water suppliers and domestic well owners will be supported by the MTs. Although water levels will be allowed to decline somewhat during drought conditions, the Subbasin will not be subject to the continual historic lows that would occur with deeper MTs. With improved long-term maintenance of water levels, municipal water suppliers will avoid the loss of expensive public drinking water supply wells as has been documented in public meetings (e.g., by the City of Waterford). The need for widespread domestic well replacements can also be avoided (see Table 6-1).

The prevention of further water level declines will also support the potential GDEs that have been identified in the Subbasin, most of which are located along the river boundaries (see Section 3.2.8). Even more protective MTs have been set along the rivers as described in more detail in Section 6.8.2.

6.3.2.5. Consideration of State, Federal, or Local Standards in MT Selection GSP regulations require that GSAs consider how the selection of MTs might differ from other regulatory standards. For the chronic lowering of groundwater levels, the MT consists of quantified water levels in each representative monitoring well, which present no conflicts with regulatory standards.

6.3.2.6. Quantitative Measurement of Minimum Thresholds As stated above, the MTs for the chronic lowering of groundwater levels will be monitored by quantitatively measuring water levels in representative monitoring well networks for each principal aquifer as described in Chapter 7 (Monitoring Network) of this GSP (see Section 7.1.1, Table 7-1, and Figures 7-1 through 7-3. Monitoring will occur on a semiannual basis, in Spring and Fall, to represent the seasonal high and low water level and to adhere to basin-wide water level sampling protocols (Section 7.2.4).

6.3.3. Interim Milestones for Chronic Lowering of Groundwater Levels GSP regulations define an interim milestone (IM) as “a target value representing measurable groundwater conditions, in increments of five years, set by an Agency as part of a Plan.” For the Modesto Subbasin, water levels are used as a metric for the IMs, consistent with the #25..
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metric being used for MTs and MOs for all sustainability indicators except degraded water quality.

IMs provide a glide path for the Modesto Subbasin to reach its sustainability goal. The incremental approach recognizes that the path to sustainability is determined by the timing and effectiveness of GSP implementation, including projects and management actions designed to avoid undesirable results. For the Modesto Subbasin, a glide path provides needed flexibility for MAs of the Subbasin that will continue to have declines – at rates dependent on future hydrologic conditions – until projects and management actions are implemented.

The following graphic prepared by DWR illustrates the concept of how IMs relate to the MT and MO. As shown, the IMs provide a glide path to sustainable management whereby MTs and MOs are maintained to avoid undesirable results.

In this conceptual graphic, the pink area represents water levels below the MT as designated in a representative monitoring well (i.e., an MT exceedance). In this example, water levels are expected to continue to decline after the GSP is adopted while projects are brought online. This concept acknowledges that the aquifer response to projects and management actions will take time. Interim milestones are illustrated in increments of five years following Plan adoption to define the glide path from undesirable results to the MO and achieving sustainable management by 2042.

In the Modesto Subbasin, long-term declines have occurred in NDE MA (Figure 6-1) and have expanded into the Oakdale ID MA (Figure 6-2). Accordingly, 2027 target values below the MT have been developed for representative monitoring wells in these management areas.
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The amount of the anticipated declines between adoption and 2027 is dependent on future unknown hydrologic conditions. Since drought conditions began in WY 2013, dry hydrologic conditions have persisted in the Subbasin. Water year types as categorized by the DWR San Joaquin Valley indices since 2014 are summarized in the following table.

Table 6-6: Water Year Hydrologic Classification Indices Since 2014 Water Year Water Year Type
San Joaquin Valley Water Year Index
2014 Critically Dry
2015 Critically Dry
2016 Dry
2017 Wet
2018 Below Normal
2019 Wet
2020 Dry
Source : https://cdec.water.ca.gov/reportapp/javareports?name=WSIHIST As shown in the table, five out of seven water years between WY 2014 and WY 2020 have been categorized as below normal, dry, or critically dry. Water level declines associated with the last seven years may continue if hydrologic conditions do not improve, and/or if the aquifer response to GSP project implementation is delayed.

In order to plan for a worst-case scenario, a 2027 IM has been developed for declining wells based on the declines observed over the last seven years. By 2032, projects and management actions are expected to support water level recovery and the 2032 IM is set as the MT. If needed, the IM for 2037 is defined as the halfway point between the MT and MO.

This trajectory is similar to the DWR conceptual diagram illustrated above. The 2027 IMs are provided in Chapter 7 (see Table 7-1 and Table 7-3) and shown on the hydrographs in Appendix F.

IMs have been designated conservatively for monitoring wells in the Oakdale ID MA and the NDE MA but will not be used to defer implementation of GSP projects or management actions. A summary of the projects and management actions is provided in Chapter 8.

6.3.3.1. Impacts to Wells and Beneficial Uses in the Modesto Subbasin For GSP revision in 2024, an analysis was conducted to determine the potential impacts to water supply wells in the Modesto Subbasin due to groundwater levels decreasing to MTs and reaching IMs (established for 2027), where the 2027 IMs are below the MTs.

The well impacts analysis addresses all water supply wells with construction information in the Modesto Subbasin. Records of municipal, industrial, domestic and agricultural water supply wells were compiled into a database from three sources: the GSP data management system (DMS), wells added to the C2VSimTM model since the GSP was submitted in January 2022, and DWR’s Online System of Well Completion Reports (OSWCR, DWR February 2024). #25..
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The analysis includes 4,563 water supply wells with construction information in the Modesto Subbasin. Error! Reference source not found.Figure 6-3 shows the locations of the Representative Monitoring Wells (RMWs) in the chronic lowering of groundwater level monitoring network and the water supply wells included in this analysis. On this figure, the water supply wells used in the analysis are shown as gray dots, the RMWs are shown as blue circles and the RMWs with 2027 IMs below the MTs are shown as blue circles with green halos.

The well records were combined into a database for evaluation in comparison to water levels in the RMWs. The well records were mapped and then grouped according to the nearest RMW in the same principal aquifer unit. Figure 6-4 illustrates the well groups associated with each RMW.

The depth of the MT elevation and the depth of the 2027 IM elevation at the RMW was compared to the depth of each well associated with that RMW. A well was considered dry at the MT if the depth to the MT at the RMW is below the total depth of the well. Wells whose total depths were shallower than the MT elevation were considered to have been dry.

Similarly, a well was considered dry if the depth to the 2027 IM elevation at the RMW is below the total depth of the well.

A summary of the well impacts analysis results is provided in Table Table 6-7. The analysis indicates that 126 wells went dry at the MT groundwater elevation, which corresponds to

2.8 percent of the wells in the analysis. All but one of the wells dry at the MT groundwater elevation are located in the Eastern Principal Aquifer and one is located in the Western Upper Principal Aquifer. No wells dry at the MT groundwater elevation are located in the Western Lower Principal Aquifer. Wells impacted at the MT are shown on Figure 6-5. Please note that the points on the map may represent locations of more than one well because some wells from the OSWCR database are commonly located at the center of the section.

Table 6-7. Summary of Well Impacts Analysis Results
Western
Upper
Western
Lower
Eastern
Count of Wells 953 280 3,330 4,563
N Wells with MT Exceedance 1 0 125 126
N Wells with IM Exceedance 1 0 154 155
N Additional Wells with IM Exceedance 0 0 29 29
% of Wells with MT Exceedance 0.1% 0% 3.8% 2.8%
% of Wells with IM Exceedance 0.1% 0% 4.6% 3.4%
% of Additional Wells with IM Exceedance 0.0% 0% 0.9% 0.6% Statistic
Principal Aquifer
Subbasin
Total
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The analysis indicates that 29 additional wells could go dry at the 2027 IM elevation, where the IM elevation is below the MT elevation. This corresponds to 0.6 percent of the wells in the analysis and represents the impact of lowering groundwater levels below the MT to the 2027 IM. All these impacted wells are located in the Eastern Principal Aquifer. Wells impacted at the 2027 IM, where below the MT, are shown on Figure 6-6. Similar to Figure 6- 5, the points on the map may represent the location of more than one well.

In general, impacted wells are older and shallower than the average age and depth of wells in the Subbasin. These well characteristics are summarized in Table Table 6-8. Wells in the Subbasin are on average 32 years old, while wells dry at the 2027 IM, where below the MT, are on average 46 years old. The average depth of the wells in this analysis is 219 feet, while the average depth of wells impacted at the 2027 IM, where below the MT, is 162 feet.

Table 6-8: Well Age and Depth Characteristics
As discussed in Section 6.3.2, the MTs are the historic low groundwater elevation observed from WY 1991 to WY 2020. In total, 126 wells with construction are indicated as dry at the MT, representing 2.8 percent of wells in Subbasin in this analysis. As shown on Figure 6-5, the wells indicated dry at the MT are primarily in the Eastern Principal Aquifer. The MTs defined at RMWs in the Eastern Principal Aquifer represent measured groundwater elevations between Fall 2015 and Fall 2020.

Section 2.3.3 documented that 159 domestic wells were reported as dry or failed from 2014 to 2017 in Stanislaus County. The 126 wells identified as dry at the MT in this analysis are comparable, with differences resulting from limitations of the available information. For example, the dataset for this analysis includes only those wells with construction information. Second, the wells included in this analysis represent all records of wells with construction information, regardless of well age or status. Third, this analysis includes all All Analysis Wells Additional Wells Dry at IM
Number of Wells 4,563 29
Number of Wells with Age 3,626 23
Oldest 76 67
Mean Age 32 46
Median Age 34 47
Youngest 1 10
Shallowest Well Depth 20 96
Mean Depth 219 162
Median Depth 187 163
Deepest Well Depth 1,512 236
Count Age (years old) Depth (ft BGS)
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water supply wells, not just domestic wells. Finally, not all wells that went dry between 2014 and 2017 may have been reported to the counties.

Fourteen RMWs in the monitoring network for chronic lowering of groundwater levels monitoring network have 2027 IMs that are below MTs. For these RMWs, the 2027 IM below the MT represents a hypothetical future condition.

The GSAs recognize the potential for groundwater levels to decline below the MT while projects and management actions are being implemented. The GSAs are in the process of developing a Well Mitigation Program to mitigate potential impacts to water supply wells.

For the Revised GSP, the GSAs have committed to implementing the Well Mitigation Program through a signed Resolution, provided in Appendix X. The program will begin no later than January 31, 2026, will cover eligible mitigation claims accrued after January 31, 2022 (the date the original GSP was adopted) and will continue for the duration of the GSP Implementation Period or until groundwater sustainability is achieved.

As described in Chapter 8, the GSAs will implement projects and management actions to ensure that declining groundwater levels reach their inflection point in 2027, and then recover and rise to meet their MT elevation by 2032. The GSAs will respond promptly to dry well claims through its Well Mitigation Program and therefore dry wells will be addressed without delay. Because dry wells will be mitigated promptly through the Well Mitigation Program, potential impacts to land uses and property interests will also be mitigated.

6.3.3.1.1. Limitations of Well Impacts Analysis
This well impacts analysis has the following limitations:
• The analysis is limited to wells whose records include well construction information.

Well records without construction information are not included in this analysis. • Well records with construction information most commonly include total depth, but not screened interval. Therefore, water levels are compared to the total depth of the well.
• Well records do not indicate well status and therefore, it is assumed that all wells are active water supply wells. Older shallower wells have not been removed from the analysis even though some may no longer be active.
• Well records do not include information on pumping equipment, so assessment of the effects of water level changes on pumping costs is not possible. • Well locations based on well records from OSWCR are uncertain. Over 90 percent of the well records included in this analysis are from OSWCR. Approximately 60 percent of these well records are located by address and the remainder are located based on the Public Land Survey System (PLSS) section centers. #25..
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6.3.3.2. Impacts to Other Sustainability Indicators
Analyses were conducted to evaluate potential impacts of lowering groundwater levels below the MTs to the 2027 IMs, where below the MTs, on other sustainability indicators including degradation of water quality, land subsidence and depletion of interconnected surface water.

6.3.3.2.1. Degradation of Water Quality
An analysis was conducted to evaluate potential impacts on the degradation of water quality sustainability indicator of lowering groundwater levels below the MTs to the 2027 IMs, where 2027 IMs are below the MTs. Groundwater quality monitoring in the Modesto Subbasin focuses on seven regionally important constituents of concern (COCs) that have the highest potential to cause undesirable results: nitrate, tetrachloroethene (PCE), 1,2,3- trichloropropane (TCP), arsenic, uranium, total dissolved solids (TDS), and dibromochloropropane (DBCP).

Water quality data for the COCs monitored in the Modesto Subbasin were downloaded and evaluated from the State Water Resources Control Board (SWRCB) Groundwater Ambient Monitoring and Assessment Program (GAMA) portal website. The water quality data were filtered to include 207 GAMA wells with known construction and sufficient data of at least one COC. The locations of these GAMA wells and the RMWs are presented on Figure 6-7.

Groundwater levels at each RMW were compared to concentrations of the COCs at the nearest five GAMA wells to assess correlation between changes in groundwater elevations in RMWs and COC concentrations in monitored wells. Specifically:
• For each RMW, a hydrograph of groundwater levels over time was compared to time-concentration plots of the COCs at the five closest GAMA wells. • The screened interval depths for each RMW were compared to the screened interval depths of the five closest GAMA wells.
• Other factors were also considered, including the distance between the RMW and the GAMA well and the length of hydrograph record.
• Groundwater level trends were compared to water quality concentrations at GAMA wells screened in the same aquifer and in close proximity to the RMW. • The comparison focused on visual identification of potential trends and/or relationships between water levels in the RMW and COC concentrations in the closest five GAMA wells.

The ability to compare water quality to groundwater levels was limited by the number and location of GAMA wells. As shown on Figure 6-7, most of the GAMA wells are concentrated within the municipalities. Within the Corcoran Clay extent, most of the GAMA wells are in the Western Lower Principal Aquifer, with few in the Western Upper Principal Aquifer. The GAMA wells in the Eastern Principal Aquifer are primarily in the western region of the #25..
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aquifer within the City of Modesto and along the river boundaries. Many of the RMWs with 2027 IMs below the MTs are not near GAMA wells.

The ability to compare groundwater quality to groundwater levels was also limited by the availability of groundwater level data. For example, several GAMA wells with increasing nitrate, uranium, and TDS trends are located in the Western Lower Principal Aquifer.

However, there are only five RMWs in the Western Lower Principal Aquifer, with sparse groundwater elevation data before 2020. The lack of groundwater elevation data makes it difficult to compare groundwater levels to water quality.

A clear relationship between concentrations of any of the COCs and groundwater levels at RMWs was not apparent. At most GAMA wells, nitrate was the only COC with a sufficient number of detections above the reporting limit to compare to groundwater levels. Several GAMA wells near RMWs had detections of arsenic, uranium, TDS, and DBCP over the past 20 years. Few GAMA wells had a sufficient number of detections of TCP or PCE to evaluate.

There was no clear relationship between nitrate concentrations and groundwater levels.

Several GAMA wells showed increasing nitrate concentrations over the past 20 years.

However, nitrate concentrations in other nearby wells within a similar distance from the same RMW had declining trends or did not have a clear trend. For example, three GAMA wells within one mile of RMW Riverbank OID-13 had increasing nitrate trends, while two GAMA wells within one mile of the same RMW had decreasing nitrate trends.

Several factors may cause increasing nitrate concentrations. Nitrate concentrations are often higher in shallower portions of an aquifer due to the surface discharge of nitrate from septic tanks or the infiltration of nitrate from animal operations or fertilizer. Groundwater level declines can cause increased nitrate concentrations at depth by intensifying the downward migration of nitrate-rich shallow groundwater. However, even without declining groundwater levels, historical nitrate discharge (i.e., legacy loading) can be transported to deeper parts of the aquifer. Additionally, wells that are screened both above and below the Corcoran Clay can serve as conduits for nitrate-rich water.

Several GAMA wells had detections of arsenic over the past 20 years. However, in most of these wells, arsenic concentrations fluctuated over time and did not show a clear trend.

While most wells with uranium or TDS detections did not show a clear trend, several wells in the Western Lower Principal Aquifer had increasing uranium, TDS and nitrate trends.

However, these GAMA wells were miles from an RMW with historical water level measurements, making a comparison to groundwater levels impossible.

The increasing uranium, TDS, and nitrate concentrations are likely due to the drawdown of shallow, bicarbonate and nitrate-rich water to deeper portions of the aquifer. This process is described in a USGS study of Modesto public supply wells by Jurgens et al., 2008. Saline, nutrient-rich water from irrigation return flow can be transported to deeper parts of the #25..
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aquifer through the intermittent operation of public supply wells screened both above and below the Corcoran Clay.

Several GAMA wells in the Eastern Principal Aquifer show declining DBCP concentrations.

These declining concentrations do not appear to correlate with water level trends.

The review of historical water quality information and groundwater elevations in the Modesto Subbasin using best available data showed no clear relationship between COC concentrations and groundwater levels. The absence of a relationship, especially between declining groundwater levels and COC concentrations, suggests that lowering groundwater levels from the MTs to the 2027 IMs, where the 2027 IMs are below the MTs, should not affect the degradation of water quality sustainability indicator.

6.3.3.2.2. Land Subsidence
An analysis was conducted to evaluate whether there is a significant effect on the land subsidence sustainability indicator from lowering groundwater levels below the MT to the 2027 IM, where the 2027 IM is below the MT.

As described in Section 3.2.6, no impacts from inelastic land subsidence are known to occur in the Modesto Subbasin. Significant rates of land subsidence are not occurring. As presented on Figure 3-60 and discussed in Section 3.2.6, InSAR data from June 2015 to October 2020 indicate no land subsidence over most of the Subbasin. One small area of land subsidence is indicated within the Corcoran Clay extent in the northwest corner of the Subbasin (up to 0.24 inches/year). Small amounts of vertical displacement are also indicated within the central and eastern Subbasin (up to 0.36 inches/year).

The western Subbasin is considered most susceptible to future land subsidence because it is underlain by the Corcoran Clay. The Corcoran Clay is known as a key subsidence factor throughout the San Joaquin Valley. The Eastern Principal Aquifer is less susceptible to subsidence because it is more consolidated with no known regional clay zones like the Corcoran Clay. The GSP presents a strategy for minimizing subsidence in the western principal aquifers by maintaining groundwater levels at or above historical low levels (the MTs).

The RMWs with 2027 IMs below the MTs are located within the Oakdale ID and NDE Management Areas. These wells are shown on Figure 7-3. These RMWs are within the Eastern Principal Aquifer and far from the edge of the Corcoran Clay. Lowering groundwater levels from the MT to the 2027 IM at these RMWs will not affect groundwater levels within the extent of the Corcoran Clay or near the Corcoran Clay boundary. There are no RMWs with 2027 IMs below the MTs within the Corcoran Clay extent in either the Western Upper Principal Aquifer or the Western Lower Principal Aquifer.

Because the RMWs with 2027 IMs below the MTs are in the Eastern Principal Aquifer and not close to the edge of the Cocoran Clay, lowering groundwater elevations to the 2027 IMs will not result in groundwater elevations declining to below the top of the Corcoran Clay. #25..
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Therefore, it is unlikely that groundwater at the 2027 IMs, where below the MTs, will have an impact on land subsidence.

6.3.3.2.3. Depletion of Interconnected Surface Water
An analysis was conducted to evaluate whether there would be a significant effect on the depletion of interconnected surface water sustainability indicator from lowering groundwater levels below the MT to the 2027 IM, where the 2027 IM is below the MT.

As stated in Section 6.8, the Stanislaus River, Tuolumne River and San Joaquin River represent interconnected surface water. Groundwater occurs above the base elevation of the channel on an average basis, allowing groundwater to interact with surface water.

C2VSimTM results indicate that the groundwater system and river system remain connected throughout the 50-year implementation and planning horizon. If depletion increased significantly more than indicated from modeling, groundwater could become disconnected from surface water. Future projected increases in streamflow depletion results in a net loss of streamflow from the river systems compared to a net gain in streamflow over historical conditions. Beneficial uses could be adversely impacted at these predicted levels of streamflow depletion even if the groundwater and surface water remain connected.

Accordingly, the projections for future streamflow depletions are considered undesirable results.

As described in Section 9.5.1.3, data gaps exist for monitoring and management of interconnected surface water along the river boundaries. A management action to improve the monitoring network provides for additional shallow monitoring wells to be installed along the rivers over time. Since GSP submittal in January 2022, the Modesto Subbasin GSAs have developed a plan for installing additional monitoring wells throughout the Subbasin to fill data gaps in the GSP monitoring network. This plan includes additional monitoring wells along the river boundaries for the interconnected surface water monitoring network.

The methodology for the interconnected surface water analysis focused on the RMWs with 2027 IMs below their MTs that are within the interconnected surface water monitoring network. There are five RMWs in the interconnected surface water monitoring network with 2027 IMs below their MTs: three along the Stanislaus River (Allen OID-01, Birnbaum OID-03, and Marquis OID-10) and two along the Tuolumne River (Quesenberry 223 and MW-9).

There are no RMWs along the San Joaquin River with 2027 IMs below the MTs. The locations of these RMWs are shown on Figure 7-5.

The analysis included a comparison of the MT and 2027 IM elevations at these RMWs to the elevation of the nearest stream node invert elevation. The stream node invert elevations are from the C2VSimTM model, where stream nodes are spaced approximately one-half mile apart from one another along the Stanislaus and Tuolumne rivers. The invert elevations represent the base of the stream channel, or thalweg. The analysis also includes an evaluation of the groundwater elevation change from the MT to the 2027 IM and the distance between the RMW and the river.
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A summary of this analysis is provided in Table 6-9. The results show that the MT and 2027 IM elevations are either both above or both below the nearest stream node invert elevation. There are no RMWs where the MT elevation is above the nearest stream node invert elevation and the IM elevation is below the nearest stream node. This means that it is unlikely that lowering groundwater levels from the MT to the 2027 IM will result in groundwater levels declining from above the base of the river channel to below the base of the river channel.

Table 6-9: Summary of Interconnected Surface Water Analysis Along the Stanislaus River, the MT and 2027 IM elevations are both above the stream node invert elevation at Marquis OID-10. At Allen OID-01 and Birnbaum OID-03, both the MT and 2027 IM elevations are below the nearest stream node invert elevations. Allen OID-01 is approximately 7,200 feet from the Stanislaus River and has a 14-foot elevation change from the MT to the 2027 IM. Birnbaum OID-03 is approximately 5,700 feet from the Stanislaus River and has a 13-foot elevation change from the MT to the 2027 IM.

Along the Tuolumne River, the MT and 2027 IM elevations are both above the stream node invert elevations at both RMWs.

It is uncertain whether lowering groundwater levels 13 or 14 feet at two RMWs more than a mile from the Stanislaus River will significantly increase streamflow depletion. This will depend on local hydrogeology and river stage. The GSP recognizes groundwater conditions along the river boundaries as a data gap and this data gap will need to be filled to help answer this question. Furthermore, DWR plans to issue future guidance documents about interconnected surface water that may help the GSAs fill this data gap.

6.3.3.6.3.4. Measurable Objectives for Chronic Lowering of Groundwater Levels GSP regulations define measurable objectives (MOs) as “specific, quantifiable goals for the maintenance or improvement of specified groundwater conditions that have been included in an adopted Plan to achieve the sustainability goal for the basin” (§351(s)). The MO is used Representative
Monitoring Well
Minimum
Threshold
(MT)
Interim
Milestone
(IM)
Nearest Stream
Node Invert
Elevation
(feet MSL)
Distance from
Well to Nearest
Stream Node
(ft)
MT Above or
Below Nearest
Stream Node
IM Above or
Below Nearest
Stream Node
Stanislaus River
Allen OID-01 75 61 86 7,162 below below
Birnbaum OID-03 74 61 85 5,728 below below
Marquis OID-10 86 78 78 5,783 above above
Tuolumne River
Quesenberry 223 89 72 67 4,205 above above
MW-9 150 138 119 5,637 above above
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to target desired groundwater conditions and provide a margin of operational flexibility above the MTs.

For chronic lowering of water levels, the MT represents a “floor” for maintenance of low water levels, with allowance for short-term exceedances by less than a third of representative monitoring wells during droughts (see Table 6-5). Accordingly, water levels will be managed generally between the MT and anticipated high water levels that occur during wet periods.

This operational range is represented by the midpoint between the MT and high water levels observed over average hydrologic conditions. Using the average hydrologic condition for the historical water budget study period of WY 1991 – WY 2015, the MO is defined as the midpoint between the selected MT and the high water level during that period (usually observed in 1998) for each representative monitoring location as summarized in the following table.

Table 6-6:-10: Measurable Objectives for Chronic Lowering of Groundwater Levels Measurable Objectives Principal
Aquifer(s)
Chronic
Lowering of
Groundwater
Levels
Measurable objectives are established as the midpoint
between the historical high groundwater elevation and
the MT at each representative monitoring location.

All
Each representative monitoring well is assigned a quantitative MO; these data are provided in Chapter 7 (see Table 7-1).

Setting the MO at the midpoint between the MT and the high-water level results in a very small margin of operational flexibility for some western Subbasin wells screened in the Western Upper Principal Aquifer. In the far western areas of the Subbasin, water levels are shallow, and historical water levels have not fluctuated significantly. As a result, the MO is close to the MT; in some portions of the western Subbasin, there are only a few feet between the MO and the MT in representative monitoring wells. Setting the MO higher would not be consistent with the need to manage shallow groundwater such that existing agricultural land use can be preserved. MOs and MTs may require future adjustment to allow for more operational flexibility in the future.

It is also recognized that this methodology may be setting MOs higher than may be easily attained if ongoing drought conditions persist. At the time of preparation of this GSP, most years since the end of the historical study period (WY 2015) have been dry; these conditions may have reset the range of future expected high water levels in the Subbasin.

Nonetheless, this approach to MO selection provides a reasonable method to quantify desired groundwater conditions using best available data. Compliance with selected #25..
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sustainable management criteria will be reported in GSP Annual Reports and revisited in the five-year GSP evaluation for possible adjustment as needed.

6.4. REDUCTION OF GROUNDWATER IN STORAGE
SGMA defines an undesirable result for the groundwater in storage sustainability indicator as “significant and unreasonable reduction of groundwater storage.” (§10721 (x)(2)). GSP regulations require that the MT for the reduction of groundwater in storage be set as “a total volume of groundwater that can be withdrawn from the basin without causing conditions that may lead to undesirable results” (§354.28(c)(2)). This requirement contains almost identical language as the SGMA definition of sustainable yield.6In addition, regulations require the MT for this indicator to be supported specifically by the sustainable yield. The sustainable yield analysis for the Modesto Subbasin is presented in Section 5.3 and discussed in the context of this indicator throughout the remaining subsections of Section 6.4, as well as throughout the remaining sections of Chapter 6.

Although the Modesto Subbasin is not at risk of depleting a large percentage of its total volume of groundwater supply, the ongoing depletion due to pumping larger volumes from the groundwater basin than can be reasonably replenished (overdraft conditions) requires mitigation to meet the Subbasin sustainability goal. As discussed in Section 6.3, the chronic lowering of groundwater levels in the Modesto Subbasin is caused primarily by overdraft conditions, illustrating the close relationship between these two indicators.

As explained in subsequent subsections, sustainable management criteria for chronic lowering of groundwater levels are used as a proxy for the reduction of groundwater in storage criteria. GSP regulations allow for use of groundwater elevations as a proxy metric when there is a significant correlation between groundwater levels and the metric for the other indicator (DWR, 2017). In this case, that metric is the volume of groundwater that can be extracted without causing undesirable results.

The definition of undesirable results for reduction of groundwater in storage, including causes and impacts to beneficial uses, is described in Section 6.4.1 below, along with additional criteria to quantify where and when undesirable results occur. Section 6.4.2 describes the selection and quantification of MTs, along with the justification and rationale.

Section 6.4.3 provides the approach and selection of MOs. Interim milestones that cover all of the applicable sustainability indicators are described in Section 6.9. 6 SGMA defines sustainable yield as “the maximum quantity of water, calculated over a base period representative of long-term conditions in the basin and including any temporary surplus, that can be withdrawn annually from a groundwater supply without causing an undesirable result.” (§10721(w)). #25..
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6.4.1. Undesirable Results for Reduction of Groundwater in Storage As described in Chapter 5, the historical reduction of groundwater in storage is estimated at about 43,000 AFY (see Table 5-8). This reduction is primarily related to overdraft7, which is determined to be unsustainable and thereby an undesirable result in this GSP.

Modeling analyses of projected future conditions indicate that historical overdraft conditions could potentially improve to about 11,000 AFY but would do so at the expense of significant streamflow depletion of the rivers along the Subbasin boundaries (compare net gains/discharges to streams from historical to projected conditions in Table 5-8). These increases in projected streamflow depletions have also been determined to be an undesirable result.

The causes of groundwater conditions that lead to undesirable results for the reduction of groundwater in storage are described below. Impacts to beneficial uses are also discussed.

6.4.1.1. Cause of Undesirable Results
In the Modesto Subbasin, the reduction of groundwater in storage is caused by overpumping primarily in the NDE MA in the eastern Subbasin (Figure 6-1). In this area, surface water is generally not available, and groundwater has provided the primary supply for the expansion of irrigated agriculture and conversion to crops with higher water demand. Overpumping has caused lowering of water levels in this area.

Because overdraft conditions cause chronic lowering of groundwater levels, overdraft contributes to all of the undesirable results associated with that indicator (Section 6.3.1.1 and 6.3.1.3). Overdraft also contributes directly to undesirable results for each of the remaining applicable sustainability indicators.

Ongoing overdraft conditions are expected to expand the area of low groundwater levels to the north and south beneath the Stanislaus and Tuolumne rivers, resulting in significant and unreasonable streamflow depletions and impacts to surface water uses (see Section 6.8.1.1 and 6.8.1.3). Overdraft conditions can lower water levels in areas where poorer groundwater quality occurs at depth and contribute to undesirable results for the degradation of water quality (see Section 6.6.1.1 and 6.6.1.3). Finally, overdraft conditions can also contribute to undesirable results for land subsidence if the lowering of water levels depressurize or dewater subsurface compressible clays. Where this occurs, significant amounts of land subsidence could be triggered and ultimately cause significant and unreasonable impacts to land uses and/or critical infrastructure – defined in this GSP as undesirable results (see Section 6.7.1.1 and 6.7.1.3)
7 Other causes of reduction of groundwater in storage include net subsurface outflows or contributions to baseflow in rivers or streams.
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6.4.1.2. Potential Effects on Beneficial Uses
The reduction of groundwater in storage causes lowering of water levels, which in turn, affects beneficial uses of groundwater and wells. As such potential effects on beneficial uses for reduction of groundwater in storage also includes the potential effects for chronic lowering of water levels as documented in Sections 6.3.1.2 and 6.3.1.3.

Recognizing that the volume of usable groundwater in the Modesto Subbasin is relatively large, and the base of freshwater is deep, a large groundwater supply would be accessible with sufficiently deep wells. However, the increased costs associated with installation and pumping lifts could ultimately place limits on beneficial uses of groundwater. With the large number of wells in the Subbasin, increased costs could be substantial and could also negatively impact land use and property interests.

Operating the Subbasin at significantly deeper levels also has the potential to pump groundwater with increased constituents of concern at depth. Deeper groundwater is often confined and subject to a geochemical environment that can impact the quality of drinking water supplies, increase public agency operational costs, and increase the potential for water quality impacts on water aesthetics such as odor or taste. Certain constituents, such as iron and manganese, can also cause impacts to groundwater conveyance pipes and fixtures. In addition, depth-related constituents can be associated with health effects if drinking water standards are exceeded (see also Section 6.6.1.2).

If overdraft contributes to land subsidence, beneficial users could experience adverse impacts to the physical ground surface, affecting surface operations, land uses, and potentially affecting property interests. Costs to repair or maintain infrastructure could increase; damage to roads or bridges may be associated with public safety concerns (see Section 6.7.1.2).

If overdraft results in inducing additional surface water from rivers, streamflow depletions could increase, potentially affecting all surface water beneficial uses including habitat, surface water rights holders, riparian vegetation, among others (see Section 6.8.1.2).

6.4.1.3. Modesto Subbasin Definition of Undesirable Results Based on the information summarized above and supported in other chapters of this GSP, a definition of undesirable results has been developed for Reduction of Groundwater in Storage in the Modesto Subbasin.

Regulations require that the undesirable result definition include quantitative criteria used to define when and where groundwater conditions can cause an undesirable result (§354.26(b)(2)). These criteria address the number of monitoring sites and events that an MT can be exceeded before causing an undesirable result. These criteria recognize that a single MT exceedance at one monitoring site may not indicate an undesirable result. This framework also allows clear identification for when an undesirable result is triggered under the GSP. The undesirable result and associated criteria are provided in the following table. #25..
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Table 6-7:-11: Undesirable Results for Reduction of Groundwater in Storage Undesirable Results Definition Principal
Aquifer(s)
Reduction of
Groundwater in
Storage
An undesirable result is defined as a significant and
unreasonable reduction of groundwater in storage that
would occur if the volume of groundwater supply is at risk of depletion and is not accessible for beneficial use, or if the Subbasin remains in a condition of long-term
overdraft based on projected water use and average
hydrologic conditions.

An undesirable result will occur when at least 33% of
representative monitoring wells exceed the MT for a
principal aquifer in 3 consecutive Fall monitoring events.

All
The use of 33 percent of the representative monitoring wells is based on the chronic lowering of groundwater levels criteria as discussed in Section 6.3.1.3. The use of three Fall events for triggering undesirable results recognizes that short-term declines during drought are anticipated as long as reductions of groundwater in storage are eliminated over average hydrologic conditions. SGMA allows for reduction of groundwater in storage during droughts if water levels subsequently recover (see introductory paragraphs in Section 6.3 above; see also Section 6.3.1.3).

The change in groundwater in storage is a required element for the GSP annual reports and will be documented annually in those reports over time. Over average hydrologic conditions, this element can be used to substantiate the correlation of overdraft conditions to the combination of MT exceedances for each principal aquifer as provided in the definition above.

The MTs selected for this indicator use MTs from the chronic lowering of water levels as a proxy, as presented in the following section.

6.4.2. Minimum Thresholds for Reduction of Groundwater in Storage As indicated in the previous sections, reductions of groundwater in storage resulting from overdraft can be partially offset by inducing recharge from rivers (baseflow) or increasing subsurface inflows from other subbasins. Each of these can cause undesirable results relating to either streamflow depletions or adverse impacts to adjacent beneficial uses of groundwater. However, overdraft conditions can be corrected through projects and management actions such that undesirable results are avoided as demonstrated by an analysis of sustainable yield using the integrated surface water-groundwater model developed for the GSP (C2VSimFG-TM).
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Under such an analysis – presented in Section 5.3 – groundwater demand is reduced iteratively in areas of over-pumping until sustainable management criteria is met. The resulting sustainable yield for the Subbasin is used to inform and confirm the sustainable management criteria selected for the sustainability indicators. The sustainable yield is also used to guide locations and volumes required for projects and management actions.

For the Modesto Subbasin, the analysis estimated a sustainable yield of about 267,000 AFY (see the total volume of groundwater production in Table 5-15). Given that future projected groundwater production in the Subbasin has been estimated at 314,000, an increase in supply or reduction in demand that adds approximately 47,000 AFY is required to bring the Subbasin into sustainability.

The sustainable yield modeling analysis incorporated the sustainable management criteria for chronic lowering of water levels and was also shown to eliminate overdraft in the Subbasin over the 50-year implementation and planning horizon (Section 5.3; see Figure 5- 58). Accordingly, both the chronic lowering of water levels criteria and elimination of overdraft are correlated to the sustainable yield of 267,000 AFY. This volume can be applied as a metric for reduction of groundwater in storage and linked directly to management criteria for the chronic lowering of groundwater levels indicator.

In this manner, the selection of a volume as the required metric for the reduction of groundwater in storage indicator is met (i.e., 267,000), and justification is provided by the sustainable yield modeling that the chronic lowering of water levels criteria can be applied as a proxy for the reduction of groundwater in storage sustainability indicator.

Table 6-8:-12: Minimum Thresholds for Reduction of Groundwater in Storage Minimum Thresholds Principal
Aquifer(s)
Reduction of
Groundwater in
Storage
Minimum thresholds are defined as the historic low
groundwater elevation observed or estimated during
WY 1991 – WY 2020 at each representative
monitoring location, based on available data.
(Chronic Lowering of Groundwater Levels MT as a
proxy.)
All
It is recognized that sustainable yield is not a fixed number and will vary over time with changes in land use, hydrologic conditions, and GSP implementation of projects and management actions. Nonetheless, this sustainable yield represents the current best available estimate to use as a required metric for the MT of this indicator.

6.4.2.1. Justification and Support for Minimum Thresholds In the BMP on sustainable management criteria, DWR lists several technical topics to consider when selecting an MT for reduction of groundwater in storage. Those #25..
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considerations, along with a summary of relevant information from the basin setting (and other related portions of the GSP), are provided below:
• Historical trends, water year types, and projected water use: In the Modesto Subbasin the historical conditions of overdraft were analyzed annually over a 25- year period and summarized for conditions in each of the management areas. As indicated on Figure 5-3, 17 of the 25 years experienced a net reduction of groundwater in storage, primarily due to overdraft. As indicated in Table 5-9, this imbalance even occurred in water year types of above normal precipitation. As indicated on Figure 5-16, much of this imbalance occurs in the NDE MA where annual water budgets indicated a new extraction from groundwater in storage in this area. Specifically, only 3 of the 25 years indicate more recharge than extraction in the NDE MA. Net extractions occurred in the NDE MA during every year since 1991. Water level declines described in Section 6.3.2.1 support the water budget analysis in the NDE MA (see also Figure 3-25).

Projected water budgets are shown annually for the 25-year period on Figure 5-40 and confirm the continuation of overdraft conditions into the future. As indicated in the discussion on sustainable yield above, the avoidance of undesirable results estimated over-pumping of about 47,000 AFY, primarily in the NDE MA, as compared to the projected future water use in the Subbasin (see Table 5-15). • Groundwater reserves needed to withstand future droughts: During recent drought conditions from WY 2013 through WY 2020, groundwater declines in the Subbasin were observed to range from less than 10 feet in the western Modesto ID MA to more than 40 feet in some areas of the NDE MA (see Figures 3-21 through 3-25).

With about 13 MAF of fresh groundwater in storage to depths of more than 1,000 feet in some areas, groundwater reserves will be available to meet future demands under sustainable yield conditions.
• Whether production wells have ever gone dry: As described in Section 2.3.2, more than 150 domestic wells failed during the 2014 – 2016 drought of record. Additional adverse impacts to public supply wells related to water level declines were also documented (see Section 6.3.1.1 and Table 6-2 above). Since that time, well impacts appear to have been mitigated with the installation of more than 200 new and typically deeper domestic wells. Accordingly, the MTs are set at historical low groundwater levels and projects and management actions have been developed to avoid widespread well failures in the future (see Chapter 8). • Effective storage of the basin: As mentioned previously, the Subbasin contains more than about 13 MAF of fresh groundwater in storage and overall depletion of #25..
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groundwater supply is unlikely (Section 3.2.4. Figure 3-18 illustrates the thickness of fresh groundwater in storage (between current groundwater level and the base of freshwater) across the Subbasin.
• Understanding of well construction and potential impacts to pumping costs: Well construction was considered in adverse impacts to public water supply wells summarized in Section 6.3.1.3 above. Most of those wells were sufficiently deep for water supply during the 2015 drought; however, adverse impacts associated with declining water levels were documented (Section 6.3.1. and Table 6-2). By setting MTs close to current levels, existing Subbasin wells are supported. • Adjacent Subbasin MTs: MTs for chronic lowering of groundwater levels are compared to and analyzed for each adjacent subbasin in Sections 6.3.2.3.1 through

6.3.2.3.3 above. The Modesto Subbasin and all adjacent subbasins are using these MTs as a proxy for the reduction of groundwater in storage indicator; accordingly, those analyses apply to both indicators.

6.4.2.2. Relationship between MTs of Each Sustainability Indicator Regulations require a description of the relationship between the MTs for each sustainability indicator and how the GSAs have determined that basin conditions for each MT will avoid undesirable results (§354.28(b)(2)). As previously discussed, the MTs for each sustainability indicator are summarized in Table 6-5 and discussed in Section 6.3.2.2.

Section 6.3.2.2 also describes the relationship between the MT for chronic lowering of water levels and the MTs for each of the remaining sustainability indicators. Because the MTs for reduction of groundwater in storage are the same as the MTs for chronic lowering of water levels, that discussion would be identical for the reduction of groundwater in storage. As such, please refer to Section 6.3.2.2 for this required component of the GSP.

6.4.2.3. Impacts of MTs on Adjacent Subbasins
Regulations require consideration of how Modesto Subbasin MTs impact the ability of an adjacent subbasin to achieve its sustainability goal. For the reduction of groundwater in storage sustainability indicator, all three adjacent subbasins – the ESJ Subbasin, the DeltaMendota Subbasin and the Turlock Subbasin – are also using the MTs for the chronic lowering of groundwater levels as a proxy. Therefore, the considerations of how Modesto Subbasin MTs impact adjacent subbasin MTs are already analyzed for this sustainability indicator through the proxy. As such, please refer to Section 6.3.2.3 for this required component of the GSP (see Sections 6.3.2.3.1 through 6.3.2.3.3 on each of the three adjacent subbasins).

6.4.2.4. Effects of MTs on Beneficial Uses and Users of Groundwater Benefits of these MTs on the beneficial uses and users of groundwater provide a balanced groundwater basin and eliminate overdraft conditions. As such, groundwater level declines #25..
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are generally arrested. Long term benefits include a more sustainable groundwater supply for all beneficial uses, including municipal, industrial, domestic, agricultural, and environmental uses.

The effects of these conditions on beneficial uses and users of groundwater are similar to those stated for the chronic lowering of groundwater levels; as such, please refer to Section

6.3.2.4 for this required component of the GSP.

6.4.2.5. Consideration of State, Federal, or Local Standards in MT Selection GSP regulations require that GSAs consider how the selection of MTs might differ from other regulatory standards. For the reduction of groundwater in storage indicator, the MT consists of quantified water levels in each representative monitoring well. Accordingly, there are no conflicts with regard to other regulatory standards.

6.4.2.6. Quantitative Measurement of Minimum Thresholds As stated above, the MTs for the chronic lowering of groundwater levels are used as a proxy for monitoring reduction of groundwater in storage. Accordingly, the representative monitoring network, along with individual MTs and MOs, for chronic lowering of water levels are also applied to the reduction of groundwater in storage indicator.

MTs will be monitored by quantitatively measuring water levels in representative monitoring wells for each principal aquifer as described in Chapter 7 (Monitoring Network – see Section 7.1.2). Monitoring will occur on a semi-annual basis, in Spring and Fall, to represent the seasonal high and low water level and adhere to water level sampling protocols (Section 7.2.4). Table 7-1 provides the quantitative MTs for each representative monitoring well used to monitor both chronic lowering of groundwater levels and reduction of groundwater in storage. Representative monitoring wells for both indicators are shown on Figures 7-1 through 7-3.

6.4.3. Interim Milestones for Reduction of Groundwater in Storage As described previously, the chronic lowering of water levels criteria are applied as a proxy for the reduction of groundwater in storage sustainability indicator. By extension, the interim milestones for chronic lowering of water levels are used as a proxy for the reduction in groundwater in storage sustainability indicator.

6.4.3.6.4.4. Measurable Objectives for Reduction of Groundwater in Storage In the same manner that the MTs for chronic lowering of groundwater levels are used as a proxy for the reduction in groundwater in storage, the same MOs are also applied to this indicator, as provided in the following table.
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Table 6-9:-13: Measurable Objectives for Reduction of Groundwater in Storage Measurable Objectives Principal
Aquifer(s)
Reduction of
Groundwater in
Storage
Measurable objectives are established at the midpoint
between the historical high groundwater elevation and the MT at each representative monitoring location. (Using
Chronic Lowering of Groundwater Levels as a proxy).

All
Even though GSP regulations note that reduction in groundwater in storage is controlled by a single value for the Subbasin (in this case, 267,000 AFY), the management of that single value is manifested by applying chronic lowering of water levels criteria as a proxy for reduction of groundwater in storage including both the MTs and MOs at the same representative monitoring wells. MOs are listed for representative monitoring wells on Table 7-1 for chronic lowering of groundwater levels, which are used as a proxy for reduction of groundwater in storage.

6.5. SEAWATER INTRUSION
GSP regulations define Seawater Intrusion as “the advancement of seawater into a groundwater supply that results in degradation of water quality in the basin and includes seawater from any source.” The minimum threshold for the indicator “shall be defined by a chloride concentration isocontour…where seawater intrusion may lead to undesirable results.” Further, the seawater intrusion minimum threshold must consider the effects of “current and projected sea levels” (§354.28 (c)(3) emphasis added).

Typically, these conditions would occur in a coastal groundwater basin where aquifers are in hydraulic communication with the open ocean, either directly or indirectly by interconnected waterways such as bays, deltas, or inlets. As an inland basin, the Modesto Subbasin is not directly or indirectly connected to the open ocean. The Subbasin aquifers are separated from the Pacific Ocean by the bedrock units of the Coast Ranges; further Subbasin aquifers are more than 10 miles upgradient from the edge of the Sacramento-San Joaquin Delta and not influenced by deltaic seawater intrusion. GSAs in the Eastern San Joaquin Subbasin to the north have determined that seawater is not occurring nor is likely to occur in that subbasin, even though elevated salinity has been encountered in groundwater and the subbasin is closer the Sacramento-San Joaquin Delta. Elevated salinity conditions do not exist in the Modesto Subbasin such that a chloride concentration isocontour could be developed and used for the MT as required by the regulations.

GSP regulations state that if GSAs are “able to demonstrate that undesirable results related to one or more sustainability indicators are not present and are not likely to occur…” then sustainable management criteria are not required to be established (§354.26 (d)). To assess the applicability of the seawater intrusion indicator to the Modesto Subbasin, the technical team provided both a public presentation to the TAC (January 2021) as well as a technical #25..
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memorandum on the issues (March 23, 2021). At a public meeting of the STRGBA GSA on April 14, 2021, the GSAs made the determination “that seawater intrusion does not exist and is not likely to occur in the future, and therefore a seawater intrusion sustainability indicator is not applicable in the Modesto Subbasin (Resolution 2021-2).

6.6. DEGRADATION OF WATER QUALITY
Degraded water quality is unique among the sustainability indicators in that other regulatory agencies have the primary responsibility for groundwater quality. SGMA does not authorize or mandate GSAs to duplicate these efforts. The GSAs are not responsible for enforcing drinking water requirements or for remediating groundwater quality problems caused by others (Moran and Belin, 2019). Similar to the other sustainability indicators, GSAs are not required to correct degraded water quality that occurred before January 1, 2015. Further, the existing regulatory framework does not require the GSAs to take affirmative actions to manage existing groundwater quality.

However, SGMA does give the GSAs the authority to regulate groundwater extractions and groundwater levels. In addition, GSAs are responsible for development and implementation of projects and management actions to bring the Subbasin into sustainable groundwater conditions. Given these authorities, GSA activities have the potential to impact groundwater quality; this GSP focuses on avoidance of these potential impacts. • To protect against GSA impacts to water quality in the future, the GSAs intend to:
• track water quality annually through existing monitoring programs, • assess the potential for GSA impacts to water quality, and • confer and coordinate with other regulatory water quality agencies and regulated water quality coalitions in the Subbasin to ensure ongoing protection groundwater quality in the Subbasin.

Because most of the public drinking water suppliers in the Modesto Subbasin are also member agencies of the GSAs, there is already close coordination between water quality regulators and GSA members including the cities of Modesto, Riverbank, Oakdale, and Waterford.

The undesirable results associated with degraded water quality, including causes and impacts to beneficial uses, are described in Section 6.6.1 below. Section 6.6.2 describes the quantification of minimum thresholds (MTS), along with justification on how MTs avoid undesirable results. Section 6.6.3 provides the approach and selection of MOs. Interim milestones (IMs) are described in Section 6.9 but are not set for this sustainability indicator.

6.6.1. Undesirable Results for Degraded Groundwater Quality SGMA defines an undesirable result for the water quality sustainability indicator as “significant and unreasonable degraded water quality, including the migration of #25..
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contaminant plumes that impair water supplies.” (§10721 (x)(4)). GSP guidance clarifies that GSAs are only responsible for degraded water quality caused by GSA management activities including regulation of pumping and water levels, along with projects and management actions (Moran and Belin, 2019). Such GSA activities that could lead to undesirable results are described in more detail below.

6.6.1.1. Causes of Undesirable Results
GSA management could potentially affect groundwater quality in several ways. GSAs could allow groundwater level declines in areas where poorer quality groundwater occurs at depth. In those areas, groundwater quality in water supply wells could be adversely impacted. In addition, GSA-allowed groundwater extractions could alter hydraulic gradients and local groundwater flow directions such that degraded water quality could spread laterally into un-impacted areas. Groundwater pumping can also induce the vertical migration of constituents of concern into un-impacted deeper aquifers.

High salinity groundwater is inferred to exist in the Modesto Subbasin below the base of fresh water. Although the base of fresh water is designated as the bottom of the groundwater basin, deep pumping could induce groundwater with elevated total dissolved solids (TDS) to migrate vertically into a well and/or into the freshwater zone of the aquifer.

These actions could locally impair water supply and potentially reduce the amount of freshwater in the Subbasin. Deep wells that pump elevated concentrations of constituents of concern may also need to be abandoned to prevent conduits for migration of low quality groundwater.

GSP-related projects and management actions also have the potential to impact groundwater quality. For example, recharge projects could introduce water with constituents of concern or affect the migration of existing constituents. GSP regulations specifically require consideration of whether projects or management actions could inadvertently exacerbate the migration of contaminant plumes.

In the Modesto Subbasin, public water suppliers have noted some deterioration in water quality during recent drought conditions, especially constituents of concern arsenic and TDS; these observations suggest that concentrations of these constituents may be elevated at depth. However, nitrate, which is sourced from the surface has also increased in many areas, perhaps in wells with deeper screens that now pull from shallower, nitrate-impacted groundwater. The City of Modesto has conducted numerous investigations of water quality issues in their wellfields and notes that correlations between constituent concentration and depth are complex.

Degraded water quality can impair groundwater supplies, causing restrictions and/or costs for operation of drinking water supply wells. Increasing costs to provide a reliable and safe drinking water supply could lead to undesirable results. Costs and impacts for domestic wells are also a concern because those wells often represent the sole water supply for the household. Impacts to other beneficial uses other than drinking water supply could also lead to undesirable results. Certain constituents can harm crops, limit water supply for certain #25..
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industrial processes, harm pipes, cause accelerated corrosion or clogging of fixtures, cause staining on bathtubs and sinks, produce bad taste or odor, and cause acute or chronic health effects.

In the Modesto Subbasin, seven constituents of concern have been identified as having the most likely potential for causing undesirable results based on widespread exceedances of MCLs and adverse impacts on public water suppliers in the Subbasin. Those constituents have been of most concern to STRGBA GSA member agencies as documented in a July 2019 public workshop on Subbasin water quality.

The constituents of concern are associated with a variety of sources including both naturally occurring (geogenic) conditions and human related (anthropogenic) activities. The naturally occurring constituents of concern may be elevated at certain depths or in certain aquifer layers and may be of most use in tracking impacts from GSA management of groundwater levels.

The anthropogenic constituents of concern, including nitrate, TCP and PCE (and some sources of TDS), are likely sourced at or near the ground surface where human-related activities occur. This suggests that shallow aquifers are more often impacted from these constituents. However, pumping can cause downward migration of these constituents into deeper aquifers either through more permeable portions of an aquitard or in conduits such as wells.

GSA management activities that cause degraded water quality and lead to significant operations costs and impaired groundwater supply are incorporated into the GSP definition of undesirable results. Specific impacts on beneficial users of groundwater from these conditions are summarized below.

6.6.1.2. Potential Effects on Beneficial Uses
As summarized above, degraded water quality can impair water supply and create considerable operational costs or constraints on public water suppliers. Public water suppliers may need to inactivate or abandon impacted wells, re-distribute wellfield pumping, blend contaminants with clean wells or surface water, drill additional wells, install wellhead or regional treatment facilities, and/or make other operational changes.

Immediate notifications to customers may also be required.

If constituents of concern impact domestic wells, residents may lose their water supply; if water quality is not well known in domestic wells, impacts to public health and safety could occur. Agricultural and industrial uses of groundwater could also be adversely impacted as summarized in the previous section. Finally, environmental beneficial uses of groundwater could be impacted; for example, if pumping caused the migration of high salinity groundwater into freshwater areas, GDEs could be affected.

For the Modesto Subbasin, six of the seven constituents of concern have primary maximum contaminant levels (MCLs) that are associated with health concerns such as toxicity (i.e., #25..
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nitrate, uranium) or carcinogens (i.e., arsenic, TCP, DBCP, and PCE). Accordingly, elevated concentrations of these constituents in drinking water can cause deleterious health effects.

Wellhead treatment has been installed on numerous drinking water supply wells to manage these constituents. In particular, the City of Modesto has removed numerous water supply wells from service over time to manage local water quality issues (as indicated by the water quality icon on Figure 6-1). Constituents with concentrations above the health-based MCLs significantly affect operations and costs for public water suppliers to ensure a safe drinking water supply.

The regulatory drinking water standard for TDS is not health based and is referred to as a secondary MCL, which is related to aesthetics of the water such as taste or odor. However, public water suppliers incur costs for managing TDS concentrations to provide low salinity groundwater for customer satisfaction. In addition, elevated TDS concentrations in groundwater can also impact agricultural beneficial users by limiting crop yields and causing other operational problems. TDS can also limit industrial beneficial uses for industrial processes requiring low salinity water.

6.6.1.3. Modesto Subbasin Definition of Undesirable Results Based on the information summarized above and presented in the basin setting, a definition for undesirable results has been developed for degraded water quality in the Modesto Subbasin. Regulations also require that the undesirable result definition include quantitative criteria used to define when and where groundwater conditions can cause an undesirable result (§354.26(b)(2)). This framework allows clear identification for when an undesirable result is triggered under the GSP; definition and criteria are provided below.

Table 6-10:-14: Undesirable Results for Degraded Water Quality Undesirable Results Definition Principal
Aquifer(s)
Degraded
Water Quality
An Undesirable Result is defined as significant and unreasonable adverse impacts to groundwater quality as indicated by a new (first-time) exceedance of, or further exceedance from, an MCL for a constituent of concern that is caused by GSA projects, management actions, or management of groundwater levels or extractions such that beneficial uses are affected and well owners experience an increase in operational costs.

An undesirable result will occur when a Subbasin potable water supply well in the defined monitoring network reports a new (first-time) exceedance of an MT or an increase in concentration above the MT for a Modesto Subbasin constituent of concern that results in increased operational costs and is caused by GSA management activities as listed above.

All
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The undesirable result is highly protective in that it requires analysis of every first-time exceedance of an MT or an increase above the MCL of an MT for any of the seven constituents of concern in each potable supply well monitored for that constituent. These criteria ensure that all key data are analyzed with respect to GSA activities. The GSAs will conduct this analysis on an annual basis.

To accomplish this annual analysis, historical data for each potable water supply well in the network must be reviewed on an annual basis to determine if the constituent has been exceeded in that well in the past. Each new (i.e., first-time) exceedance or increase in concentration above the MT – occurring after GSP adoption – must be tracked and analyzed separately to determine if such a concentration could have been caused by GSA regulated groundwater levels, extractions, or projects/management actions, and if additional operational costs are incurred by the well owner. If so, the concentration represents an undesirable result by definition.

This analysis will consider the recent groundwater elevations and extractions near each impacted well. Data will be analyzed in the context of the historical record to establish correlations between groundwater levels, monitoring well locations and construction, and water quality analyses. Changes in water levels and water quality in nearby wells will be incorporated into the analysis. Each constituent of concern will be analyzed using information on sources, historical records of nearby and regional wells, and occurrence/concentrations with respect to the principal aquifer and well screens.

Increases in concentration will also be tracked to comply with the MO described in Section

6.6.3 below. Hydrographs and chemographs will be used to support the analyses, as needed. Analyses will be coordinated with local public agencies providing drinking water supply including member agencies of the GSAs. Data and analyses will be summarized in annual reports and coordinated with the regulatory agencies responsible for water quality.

Any undesirable results will be identified, and GSAs will coordinate with regulatory agencies on options and mitigation measures for water quality impacts.

The MTs are quantified in the following section. The MOs are quantified in subsequent Section 6.6.3.

6.6.2. Minimum Thresholds for Degraded Water Quality
GSP regulations require that the MT metric for degraded water quality be set at the water quality measurement that indicates degradation at the monitoring site (DWR, 2017).

Regulations also require the consideration of state and federal standards and Basin Plan water quality objectives when setting the MT.

The seven constituents of concern have already exceeded MCLs over a relatively widespread area in Subbasin principal aquifers. Accordingly, MCLs (including primary and secondary MCLs) are set as the MTs and are expressed as follows.
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Table 6-11:-15: Minimum Thresholds for Degraded Water Quality Minimum Thresholds Principal
Aquifer(s)
Degraded
Water Quality
Minimum thresholds are set as the primary or secondary
California maximum contaminant level (MCL) for each of
seven (7) constituents of concern:
• Nitrate (as N) - 10 mg/L
• Arsenic - 10 ug/L
• Uranium - 20 pCi/L
• Total dissolved solids (TDS) - 500 mg/L
• Dibromochloropropane (DBCP) - 0.2 ug/L
• 1,2,3-Trichloropropane (TCP) - 0.005 ug/L
• Tetrachloroethene (PCE) - 5 ug/L.

All

6.6.2.1. Justification and Support for Minimum Thresholds Analysis of existing groundwater quality conditions in the Modesto Subbasin is provided in Section 3.2.5 as part of the basin setting. As explained in the text, the analysis included potential constituents of concern based on a review of the water quality database, local knowledge of constituents of concern from previous studies, and identified by GSA member agencies and stakeholders at a public TAC meeting in July 2019. Public water suppliers, including the City of Modesto, shared information on constituents of concern that have been identified in their drinking water wells over the historical study period. Other GSA members identified other potential constituents of concern that had been the target of several ongoing water quality programs including the Irrigated Lands Regulatory Program (ILRP) and Central Valley Salinity Alternatives for Long-Term Sustainability (CV-SALTS).

As presented in Section 3.2.5, data for these potential constituents of concern were analyzed over a 25-year study period based on available data. Analyses included development and posting of average and recent water quality data on Subbasin maps, along with various statistical analyses for concentration distribution, temporal trends and occurrence by principal aquifers (when known) (see Tables 3-4, 3-5, and 3-6).

Based on these analyses seven constituents of concern were selected for assignment of an MT and further characterization on an annual basis based on elevated concentrations over a relatively widespread area of the Subbasin. These constituents have been the most difficult to manage according to public water suppliers. The constituents also include a variety of sources and occurrences across the Subbasin to provide a more comprehensive tracking of groundwater quality. Specifically, the constituents include:
• naturally occurring constituents (arsenic, uranium, TDS) • special constituents with widespread areas of multiple non-point sources (nitrate,

TCP, DBCP)
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• constituents associated with industrial point sources and environmental investigations (PCE).

Data were evaluated for all three principal aquifers in the Subbasin because all are used for drinking water supply. The City of Modesto is the largest drinking water supplier and has wells in all three principal aquifers. The cities of Riverbank, Oakdale, and Waterford have municipal supply wells in the Eastern Principal Aquifer (see Figure 2-13). In addition to these providers, more than 75 smaller water systems scattered throughout the Subbasin also have wells in each of the principal aquifers. Numerous domestic wells also occur in both western and eastern principal aquifers. However, very few wells or drinking water systems are located in the eastern third of the Subbasin, (i.e., generally east of Waterford and Oakdale.

See Figures 2-10, 2-13, 2-14, and 6-1).

Summary information is provided below on the seven constituents of concern assigned an MT; more detailed information is provided in Section 3.2.5.3 including statistical analyses and temporal trends over a 25-year study period (1995 through 2019) and numerous water quality distribution maps on Figures 3-35 through 3-52.

6.6.2.1.1. Nitrate
Nitrate is the most widespread constituent of concern in both the California Central Valley and the Modesto Subbasin (see Section 3.2.5). Because of its serious health effects, the MCL of 10 mg/L of nitrate as N is selected as the MT. Sources, median and maximum concentrations, and occurrence of nitrate in Modesto Subbasin groundwater are described in Section 3.2.5.3 and shown on Figures 3-35 and 3-36. Elevated nitrate concentrations are detected in all principal aquifers, including the confined Western Lower Principal Aquifer below the Corcoran Clay. Nitrate concentrations have exhibited a slightly increasing trend over the 25-year study period.

The widespread occurrence of nitrogen in California’s Central Valley is being regulated by the Central Valley RWQCB under several programs (in addition to individual site regulatory orders). Those programs include the General Dairy Order (Dairy Order), the ILRP, and CVSALTS. Nitrate concentrations in domestic wells are being mitigated through the Nitrate Control Program, which involves management areas with mandates to provide safe drinking water to impacted well owners (Section 2.4.4).

6.6.2.1.2. Arsenic
Arsenic is a naturally occurring trace element in the rocks, soils, and groundwater of the Modesto Subbasin. Given its toxicity, the MT has been set at the arsenic MCL of 10 micrograms per liter (µg/L). Other water quality investigations have indicated that arsenic concentrations are higher in older and deeper groundwater samples (see Section 3.2.5.3).

Although elevated arsenic has been detected in all principal aquifers, average concentrations are much higher in the Western Upper Principal Aquifer and Wester Lower Principal Aquifer than in the Eastern Principal Aquifer. Arsenic concentrations appear to be decreasing in Subbasin wells over the 25-year study period. Additional information on the #25..
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occurrence and concentrations of arsenic in Modesto Subbasin groundwater is included in Section 3.2.5.3 and shown on Figures 3-39 and 3-40.

6.6.2.1.3. Uranium
Uranium is another naturally occurring trace element largely derived from granitic rocks in the Sierra Nevada. It is toxic and associated with health effects; the MT is set at the MCL of 20 picocuries per liter (pCi/L). Uranium has been detected at or above the MCL in shallow and intermediate depth wells in the City of Modesto wellfield; about nine wells have been taken offline due to elevated uranium concentrations. In general, concentrations of uranium are higher in the Western Upper Principal Aquifer compared to the other two aquifers. This occurrence is consistent with the geochemical conditions that lead to mobilization of uranium in the aquifers (Section 3.2.5.3). Over the 25-year study period, uranium concentrations have exhibited an increasing trend in Modesto Subbasin groundwater.

Additional information on the occurrence and concentrations of uranium is included in Section 3.2.5.3 and shown on Figures 3-41 and 3-42.

6.6.2.1.4. Total Dissolved Solids
TDS represents the total concentration of anions and cations in groundwater and is a useful indicator of mineralization, salt content, and overall groundwater quality. TDS generally meets drinking water standards in the Subbasin with only 14 percent of the TDS samples exceeding the upper limit California Secondary MCL of 1,000 mg/L. Most samples also meet the MT recommended secondary MCL for drinking water of 500 mg/L. The lower secondary MCL is used as the MT to address recommended concentrations for both drinking water and irrigation of some Modesto Subbasin crops (see Section 3.2.5.3) and to provide for a more protective water quality analysis.

Average and recent concentrations of TDS in groundwater samples are provided on Figures 3-37 and 3-38, respectively. As indicated on the maps, TDS concentrations are generally lowest in the central Subbasin, especially in the urban areas around Modesto, Oakdale, Riverbank, and Waterford. Elevated concentrations occur in the western Subbasin (in the San Joaquin National Wildlife Refuge) and in southwest Modesto.

Even though elevated TDS is inferred to occur in deeper portions of the Subbasin (below the base of freshwater), the statistical analysis in Section 3.2.5.3 indicates that the highest TDS concentrations have been observed in the Western Upper Principal Aquifer (i.e., in the western Subbasin as indicated above). However, these high concentrations were not necessarily widespread and may indicate local point sources of TDS, especially near the San Joaquin River.

Additional information on the occurrence and concentrations of TDS in Modesto Subbasin groundwater is included in Section 3.2.5.3 and shown on Figures 3-37 and 3-38.

6.6.2.1.5. 1,2,3-Trichloropropane (TCP)
TCP is a manufactured chlorinated hydrocarbon used for degreasing and previously associated with soil fumigants, which were widely used in agriculture through most of the #25..
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1980s. The chemical was banned in the 1990s. The MT is set at the MCL of 0.005 µg/L, which was only recently established (effective 2018). As a result, historical data for TCP in groundwater are sparse.

Elevated TCP concentrations have been detected in mostly urban areas, including Modesto, Riverbank, and Waterford, likely due to the increased sampling in drinking water supply wells. Even though TCP has been associated with relatively widespread application throughout the Central Valley, elevated concentrations are relatively sparse and localized in the Modesto Subbasin. This may indicate a lack of historical use in the Subbasin with just a few local point sources indicated. Elevated concentrations have not been detected in the Western Lower Principal Aquifer, indicating a surficial source and local protection against vertical migration by the Corcoran Clay.

Additional information on the occurrence and concentrations of TCP in Modesto Subbasin groundwater is included in Section 3.2.5.3 and shown on Figures 3-49 and 3-50.

6.6.2.1.6. Dibromochloropropane (DBCP)
DBCP was a widely used pesticide (nematocide and soil fumigant) in the Central Valley prior to being banned in the late 1970s.Due to its mobility and toxicity, the MT is set at the MCL of 0.2 ug/L.

Concentrations are relatively low in the Modesto Subbasin with about 14 percent of the samples from the historical database exceeding the MCL. Similar to TCP, DBCP has not been detected in the Western Lower Principal Aquifer. In addition, data indicate a declining trend of concentrations over time, likely due to its long-term ban. Additional information on the occurrence and concentrations of DBCP in Modesto Subbasin groundwater is included in Section 3.2.5.3 and shown on Figures 3-47 and 3-48.

6.6.2.1.7. Tetrachloroethene (PCE)
PCE is a volatile organic compound (VOC) developed as an industrial solvent. PCE has been widely useused in a variety of industrial applications including as a dry cleaning fluid.

Discharges from a number of dry cleaners in the City of Modesto have resulted in local contaminant plumes of PCE, all of which are being managed by other local regulatory agencies responsible for water quality. PCE has also been detected at Modesto Subbasin landfills and other sites under regulatory investigations and remediation. At least seven City of Modesto wells have installed wellhead treatment systems for managing PCE impacts. The MT is set at the California and Federal MCL of 5 ug/L.

Elevated concentrations of PCE are generally associated with point sources of the contaminant including industrial and commercial sites. Similar to TCP and DBCP, PCE has not been detected in the Western Lower Principal Aquifer, indicating surficial sources and protection by the Corcoran Clay.

Additional information on the occurrence and concentrations of PCE in Modesto Subbasin groundwater is included in Section 3.2.5.3 and shown on Figures 3-51 and 3-52. #25..
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6.6.2.2. Relationship between MTs of Each Sustainability Indicator Regulations require a description of the relationship between the MTs for each sustainability indicator and how the GSAs have determined that basin conditions at each MT will avoid undesirable results (§354.28(b)(2)). To facilitate a comparison between MTs, a summary of MTs for each sustainability indicator was provided in Table 6-5 and discussed previously in Section 6.3.2.2.

As provided in Section 6.3.2.2, the MCLs for each constituent of concern – selected as the MTs – would not interfere with the MTs for the other sustainability indicators. All other MTs consist of groundwater elevations that are at or above the historic low water in the Subbasin. As such, the groundwater level MTs are protective against increases in constituents of concern that occur primarily at depth. Further, because these groundwater level MTs are similar to recent water levels across the Subbasin, hydraulic gradients would not be altered substantially that might cause migration of constituents into previously unimpacted areas.

In this manner, the MTs for the other sustainability indicators are supportive of the MTs for degraded water quality and cause no conflicts for groundwater management. The constituents will be tracked on an annual basis and analyzed with respect to changes in groundwater levels and extractions to determine if GSA management activities might be impacting groundwater quality.

GSA member agencies have already been coordinating with regulatory agencies responsible for drinking water quality in the Subbasin. In addition, these agencies are actively engaged with regulated water quality coalitions that have ongoing monitoring programs for certain Modesto Subbasin constituents of concern including the Nitrate Control Program and CVSalts. Representatives from the Valley Water Collaborative – a coalition responsible for implementing the Nitrate Control Program (NCP) – provided a presentation at a public TAC meeting in December 2020. Many Subbasin landowners are directly participating in the NCP, providing additional opportunities for coordination.

Finally, as previously stated, multiple GSA member agencies are responsible for drinking water quality and routinely coordinate with water quality regulatory agencies. Because the drinking water standard (MCLs) are the target for both the water quality coalitions mentioned above and the water quality regulatory agencies, the selection of the MCLs as the MTs is consistent with other water quality programs. In this manner, the GSAs have determined that the MTs will avoid undesirable results.

6.6.2.3. Impacts of MTs on Adjacent Subbasins
Regulations require consideration of how Modesto Subbasin MTs impact the ability of an adjacent subbasin to achieve its sustainability goal. As summarized in more detail in Section

6.3.2.3, similar principal aquifers, shared interconnected surface water boundaries, and multiple GSA member agencies that overlap both the Modesto Subbasin and adjacent subbasins have facilitated setting MTs in the Modesto Subbasin that will not adversely impact adjacent subbasins GSP implementation.
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Additional water quality considerations for MTs in each adjacent subbasin are summarized below.

6.6.2.3.1. Eastern San Joaquin Subbasin
The MT for degraded water quality in the ESJ Subbasin is defined as a TDS concentration of 1,000 mg/L TDS in representative monitoring wells, none of which occur along the shared subbasin boundary with the Modesto Subbasin. Rather, water quality monitoring is focused along the western rim of the ESJ Subbasin where TDS concentrations are of most concern in the ESJ Subbasin. The closest water quality monitoring well more than six miles north of the Modesto Subbasin. In addition, MTs for interconnected surface water, set at 2015 groundwater elevations along the Stanislaus River, are set similarly in both subbasins.

Finally, water budget analyses for sustainable yield conditions indicate that subsurface flow is relatively small and occurs from the ESJ Subbasin into the Modesto Subbasin. Therefore, MTs in the Modesto Subbasin are not expected to conflict or affect the MTs in the ESJ Subbasin.

6.6.2.3.2. Delta-Mendota Subbasin
The Delta-Mendota Northern & Central GSP focused on constituents that are linked to groundwater elevations or other groundwater-management activities. Undesirable results are to be triggered if TDS, nitrate, or boron exceed the MCL or water quality objectives (WQOs) in three consecutive sampling events in non-drought years or additional degradation where current groundwater quality already exceeds the MCLs or WQOs. An undesirable result would also occur if a recharge project exceeded 20 percent of the aquifer’s assimilative capacity without justification of a greater public benefit.

MTs were set at each monitoring site based on these criteria. As indicated in the GSP, there are no representative monitoring sites adjacent to the shared river boundary with the Modesto Subbasin (see the Delta-Mendota representative monitoring wells for degraded water quality on Figures 6-4 and 6-5 in W&C and P&P, 2019). The closest monitoring wells are 06-004 in the Upper Aquifer and 0-003 in the Lower Aquifer, located about three miles to the southwest of the southwestern corner of the Modesto Subbasin.

At those wells, the MTs for TDS are 4,000 mg/L and 2,000 mg/L for the Upper Aquifer and Lower Aquifer, respectively. The MTs for nitrate (as N) are 80 mg/L and 50 mg/L for the Upper Aquifer and Lower Aquifer, respectively. These MTs are much higher than the MCLs established for the MTs in the Modesto Subbasin. In addition, the closest monitoring wells are upgradient and would not be impacted by any degraded groundwater quality in the Modesto Subbasin.

In addition, water budget analyses indicate a net subsurface inflow from the Delta Mendota Subbasin into the Modesto Subbasin for projected future and sustainable yield conditions (Table 5-15). Collectively, the 3-mile distance from the nearest monitoring well, the upgradient location of the Delta-Mendota wells, the higher MTs for TDS and nitrate in the Delta-Mendota Subbasin, and the indicated subsurface flow direction into the Modesto #25..
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Subbasin indicate that MTs in the Modesto Subbasin will not impact MTs for degraded water quality or impact GSP implementation in the Delta-Mendota Subbasin.

6.6.2.3.3. Turlock Subbasin
The Turlock Subbasin has defined undesirable results for degraded water quality in a similar manner to the Modesto Subbasin, using MCLs for six of the seven Modesto Subbasin constituents of concern as the MTs. Both subbasins have similar water quality issues and will coordinate the tracking and analysis across the Tuolumne River boundary.

In addition to the coordination of sustainable management criteria, two member agencies in the Modesto Subbasin - the City of Modesto and the City of Waterford8 – monitor for groundwater quality in both subbasins, allowing for close coordination of any water quality issues along the Tuolumne River boundary. Water quality data for both subbasins will be analyzed annually using similar data sources and methods, which will allow for close coordination of any degraded water quality across the two subbasins. Analyses in both subbasins will be conducted to determine if GSA management of groundwater extractions, levels, or GSP projects/management actions are impacting groundwater quality. These analyses will be presented in Annual Reports for each subbasin.

6.6.2.4. Effects of MTs on Beneficial Uses and Users of Groundwater The setting of MCLs as the MTs is protective with respect to the avoidance of undesirable results. By protecting drinking water quality, the long-term quality and quantity of useable groundwater for all beneficial uses will be preserved.

The City of Modesto has been historically impacted by water quality problems in their wellfields. About 18 water supply wells had to be removed from service for impacts related to arsenic, nitrate, or uranium (see Section 3.2.5.3). Another 9 water supply wells have been taken offline due to TCP or PCE contamination. To address these issues, the City has conducted numerous water quality studies and is currently completing a wellfield investigation and feasibility study to identify remedial options for wellfield management.

Those independent studies and Subbasin-wide annual tracking of groundwater quality will each inform the other, providing a better understanding of degraded water quality in the Subbasin.

The commitment to analyze a large groundwater quality dataset across the Subbasin on an annual basis will improve GSA understanding of water quality in each Principal Aquifer and lead to better management practices. Expanded and ongoing data collection and analysis will also support ongoing regulatory monitoring, allowing others to evaluate their local water quality monitoring data in the context of Subbasin-wide water quality. For example, an improved understanding of water quality with depth allows future wells to be sited and 8 The City of Waterford operates drinking water supply wells for the community of Hickman in the Turlock Subbasin.
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designed such that water quality is optimized. Overall, these improvements will support all beneficial uses of groundwater in the Subbasin.

6.6.2.5. Consideration of State, Federal, or Local Standards in MT Selection In setting MTs for degraded water quality, GSP regulations require that GSAs consider local, state, and federal water quality standards applicable to the Subbasin (354.28(c)(4)). As provided above, the degraded water quality sustainability indicator relies on California MCLs for the MT; in this manner, the MT adheres to drinking water quality standards set by California, which are either as protective or more protective than federal standards. The MCLs are also consistent with the local standards and water quality objectives (WQO) in the Central Valley RWQCB Basin Plan for the San Joaquin River Basin (2018). Accordingly, there are no conflicts with regard to regulatory standards.

6.6.2.6. Quantitative Measurement of Minimum Thresholds As stated above, the MTs for the degradation of water quality will be quantitatively monitored through existing monitoring programs that are being managed by the SWRCB and uploaded to the public GeoTracker website. These water quality data are monitored by public agencies, regulated coalitions, and others in representative monitoring wells for each Principal Aquifer using regulatory-approved sampling protocols. Data will be downloaded from the State GeoTracker water quality website and supplemented with data from the salt and nutrient regulatory programs in the Subbasin (see Section 2.4.4). Water quality data will be analyzed for constituents of concern in each Principal Aquifer as described in Chapter 7 (Monitoring Network) of this GSP (see Section 7.1.4). Analyses will be included in the Subbasin GSP annual reports.

These data are considered comprehensive for characterization of water quality in the Subbasin. More than 300 wells with water quality data for Modesto Subbasin constituents of concern were available from GeoTracker from January 2020 to May 2021; these water quality monitoring sites are shown on Figure 7-4 as part of the GSP monitoring network and tabulated in Appendix G. As shown on Figure 7-4, wells are distributed throughout the Subbasin but focused in areas of drinking water supply wells (see Figure 2-10). This is appropriate given the emphasis on drinking water supply impacts (i.e., MCL exceedances) in the definition of undesirable results.

Although monitored wells will change from year to year based on regulatory monitoring requirements, public water suppliers generally monitor and report water quality data for all active drinking water wells (see Figure 2-13). GeoTracker also includes water quality monitoring data from sites with contaminant plumes as a part of the RWQCB regulatory programs (see summary data on Figure 4-57). As indicated in Appendix G, monitoring sites consist of municipal supply wells, monitoring wells, and domestic wells. Although most domestic wells are currently sampled for nitrate only (Appendix G), the SWRCB is planning to expand water quality monitoring in those wells, adding additional constituents of concern including most of those in the Modesto Subbasin.
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Additional wells from supplemental regulatory programs are also either included on GeoTracker or available for public download to allow for a broad analysis of water quality on an annual basis. Monitoring programs for TDS and nitrate are conducted by the Eastern San Joaquin Water Quality Coalition (ESJWQC) in coordination with the CV-SALTS program and the Nitrate Control Program, which requires growers in management zones to ensure safe drinking water supplies for well owners impacted by nitrate concentrations (see Section

2.4.4). As a result of this large dataset, the GSAs are not planning to develop a separate GSP water quality monitoring network, and no water quality sampling will be conducted by the GSAs.

However, GSAs will ensure that projects and management actions comply with regulatory water quality requirements. GSAs will consider appropriate constituents, MCLs, and water quality objectives (WQOs) as projects are initiated to avoid undesirable results. Potential water quality considerations for currently proposed projects will be evaluated through the CEQA process as projects are implemented.

6.6.3. Interim Milestones for Degraded Water Quality
Interim milestones are not defined for the degraded water quality sustainability indicator.

6.6.3.6.6.4. Measurable Objectives for Degraded Water Quality To avoid exacerbation of the nature and extent of current groundwater quality by management activities, the GSAs are using the MOs to establish a target water quality condition whereby GSA management does not cause an increase in historical concentrations of constituents of concern (i.e., further degradation of water quality). This target is managed by the definition of measurable objectives for degraded water quality as follows.

Table 6-12:-16: Measurable Objectives for Degraded Water Quality Measurable Objectives Principal
Aquifer(s)
Degraded Water
Quality
Measurable objectives are defined as the historical
maximum concentration of each constituent of
concern at each representative monitoring location.

All
The same monitoring data summarized in Section 6.6.2.6 above will be used to analyze MOs for the constituents of concern (see also Figure 7-4).

6.7. LAND SUBSIDENCE
SGMA defines an undesirable result for land subsidence as “significant and unreasonable land subsidence that substantially interferes with surface land uses” (§10721 (x)(5)). In general, land subsidence can interfere with land use by causing damage to either the natural #25..
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land surface (e.g., surface fissures) or to structures on the land surface (e.g., roads or pipelines). Potential impacts from land subsidence are documented in Section 3.2.6 and summarized in Section 6.7.1.1 below.

As described in Section 3.2.6, there have been no known impacts from inelastic land subsidence in the Modesto Subbasin. Land subsidence associated with groundwater extraction has been documented across large segments of the San Joaquin Valley since the 1950s, but these areas are located significant distances to the south of the Modesto Subbasin (see Figure 3-58).

However, as explained in the remainder of Section 6.7, the potential for future land subsidence in the Subbasin cannot be dismissed, given the presence of the Corcoran Clay, the decline of groundwater levels in certain management areas, and the results of recent GPS station monitoring and remote sensing data. As a protective measure, sustainable management criteria for the land subsidence sustainability indicator have been selected for all principal aquifers in the Modesto Subbasin.

Because there have been no known impacts from land subsidence, it is difficult to determine what rates of subsidence would lead to undesirable results. For the Modesto Subbasin, the sustainable management criteria for chronic lowering of water levels were developed to arrest groundwater level declines caused by groundwater extraction (Section

6.3). As such, those criteria would protect against future land subsidence (see Section

6.7.1.1). Accordingly, the sustainable management criteria, including MTs set as the historical low groundwater levels for WY 1991 through WY 2020, are used as a proxy for land subsidence sustainable management criteria.

Potential undesirable results, including causes and impacts to beneficial uses, are described in Section 6.7.1 below, with a definition of undesirable results provided at the end of the section. Section 6.7.2 describes the quantification of minimum thresholds (MTs) and provides additional information on rationale and coordination of MTs in adjacent subbasins.

Section 6.7.3 provides the approach and selection of measurable objectives (MOs). Interim milestones that cover all of the applicable sustainability indicators are described in Section

6.9. 6.7.1. Undesirable Results for Land Subsidence
Vertical displacement of the land surface can be caused by a variety of mechanisms, including extraction of oil and gas, the wetting of collapsible soils, piping of sediment from underground pipeline or tank leaks, collapse from underground mining facilities, tectonic activity along geological faults, and other conditions. This GSP only focuses on land subsidence related to groundwater extraction. The following sections summarize the physical processes that could potentially cause future land subsidence in the Modesto Subbasin as well as the related causes and effects of potential undesirable results. #25..
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6.7.1.1. Causes of Undesirable Results for Land Subsidence Areas of the San Joaquin Valley have had impacts from land subsidence related to groundwater pumping, which has lowered water levels within and below the thick and compressible Corcoran Clay. For example, land subsidence in the Merced Subbasin to the south occurred in this manner (W&C, 2019) (see Figure 3-58).

As pumping removes groundwater from storage, the pore pressure and support of the aquifer framework are reduced, and sediments can be realigned and compacted at depth.

This compaction is typically associated with thick and compressible clay layers. Subsurface compaction reduces the volume of subsurface sediments, causing the ground surface to depress. The processes and mechanisms that result in land subsidence are more complex than summarized herein, but the concept of subsurface compaction is typically used to provide a general understanding of the process. Additional information is summarized in Section 3.2.6 and illustrated on Figure 3-57.

The western Modesto Subbasin within the extent of the Corcoran Clay is thought to be the area most susceptible to future land subsidence (see red striped area on Figure 6-1). Recent processing of satellite data to analyze vertical displacement – referred to as InSAR9 – suggests that no land subsidence has recently occurred in the western Subbasin (see Figure 3-59). However, data show some small amounts of vertical displacement in the eastern Modesto Subbasin (see Figure 3-59). It is not known whether this vertical displacement is related to groundwater extraction or other mechanisms described in Section 6.7.1 above.

Nonetheless, the hydrogeological conditions in the western Subbasin and the InSAR data in the eastern Subbasin highlight the need for monitoring and management. Because groundwater drains slowly from compacted clay layers, there is a time lag between the triggering mechanisms that cause land subsidence and the actual depression on the land surface. A slow and small rate of decline in the land surface can go unnoticed until disruption of infrastructure or other physical manifestation of the problem occurs.

The processes above describe the causes of potential land subsidence, but the causes of undesirable results are related to the adverse impacts that land subsidence could have on land uses. For example, the documented land subsidence in the California Central Valley has caused numerous adverse impacts that could lead to undesirable results if they occurred in the Modesto Subbasin. Land subsidence could interfere with land use through a physical alteration of the ground surface, such as fissures, cracks, or depressions or by damaging physical structures on the ground surface such as buildings or infrastructure.

Adverse impacts are likely to occur in urban areas where numerous buildings, utilities, and pipelines are present. In addition, areas of groundwater wells could experience casing or other wellbore damage. Impacts have also been documented along surface water canals and transportation corridors, with damage to canals, roads, freeways or bridges. These impacts could cause an interruption to vital services or increase risks to public health and 9 InSAR refers to Interferometric Synthetic Aperture Radar data. #25..
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safety. In addition to physical damage, land subsidence can also affect gravity drainage in sewers, pipelines, or water conveyance canals and can also increase the risk of flooding (LSCE, 2014; W&C, 2019; W&C and P&P, 2019).

In consideration of these adverse impacts, the Modesto Subbasin GSAs incorporated impacts to infrastructure into its undesirable result definition. Definitions from GSPs in adjacent subbasins, including the Delta-Mendota and the Eastern San Joaquin subbasins, were also reviewed (W&C and P&P, 2019; ESJGWA, 2019). The definition of undesirable results for the Modesto Subbasin is provided in Section 6.7.1.3 below.

6.7.1.2. Effects on Beneficial Uses of Groundwater
Two commonly cited effects on almost all beneficial users of groundwater in the Central Valley include damage to casings in water supply wells and interference with water canal capacity and conveyance (LSCE, 2014). Widespread collapse of well casings resulting from land subsidence have been well-documented in numerous areas. Near El Nido, California, well casings have been observed protruding above the land surface, in some cases with the connected concrete well pad suspended in the air (LSCE, 2014). Casing damage typically requires well replacement, resulting in significant costs to beneficial users of groundwater.

Given the close linkage between groundwater and surface water use in the Central Valley, land subsidence impacts on water conveyance facilities can have a negative impact on the beneficial uses and users of groundwater. Land subsidence has reduced freeboard and flow capacity in large water conveyance canals such as the Delta-Mendota Canal, the California Aqueduct, and the Friant-Kern Canal. Repairs to restore conveyance capacity along critical segments of the Friant-Kern Canal alone is estimated to cost as much as $200 million or more (FWA, 2018). In the Merced Subbasin GSP, undesirable results for land subsidence were related primarily to the viability of the Eastside Bypass Canal, where subsidence has caused a reduction in freeboard and capacity over the last 50 years. These impacts to surface water canals can result in an increase in groundwater pumping, often from groundwater basins already experiencing overdraft conditions, which can lead to a depletion in water supply.

Subsurface compaction of clay layers also causes permanent removal of groundwater from storage. Although the usable storage capacity of an aquifer is not substantially impacted by the dewatering and compaction of clay layers, there is some amount of groundwater that is permanently lost. Pumping an identical amount of groundwater after this loss can result in a lower water level than before the clay layer was drained. Lower groundwater levels can result in higher pumping lift costs and other negative effects on beneficial uses of groundwater (see Section 6.3.1.2) (LSCE, 2014).

Land subsidence could also disrupt activities on the land surface including agricultural production. Changes to the land surface, such as with fissures or depressions, could affect how both surface water and groundwater is conveyed onto and within productive agricultural parcels. These effects could create inefficiencies in beneficial groundwater use or interferences with agricultural land uses.
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Finally, any of the above activities that lead to increased groundwater pumping would also have the potential to affect environmental users of groundwater including potential GDEs (see Section 3.2.8 and Figure 3-60).

6.7.1.3. Modesto Subbasin Definition of Undesirable Results In consideration of the land use and infrastructure impacts summarized above, an undesirable result has been developed for the Modesto Subbasin. Regulations require that the undesirable result definition include quantitative criteria used to define when and where groundwater conditions can cause an undesirable result (§354.26(b)(2)). These criteria address the number of monitoring sites and events that an MT can be exceeded before causing an undesirable result while recognizing that a single MT exceedance at one monitoring site may not indicate an undesirable result. Criteria also allow for a clear identification when an undesirable result is triggered.

The definition of undesirable results is provided as follows.

Table 6-13:-17: Undesirable Results for Land Subsidence Undesirable Results Definition Principal
Aquifer(s)
Land
Subsidence
An Undesirable Result is defined as significant and
unreasonable inelastic land subsidence, caused by
groundwater extraction and associated water level
declines, that adversely affects land use or reduces the viability of the use of critical infrastructure.

An undesirable result will occur when 33 percent of
representative monitoring wells exceed the MT in three
consecutive Fall monitoring events.

All
The criteria for triggering an undesirable result were developed for the chronic lowering of water levels indicator as discussed in Section 6.3.1.3 and are applied as a proxy for the land subsidence sustainability indicator.

Accordingly, the monitoring networks for both land subsidence and chronic lowering of water levels are identical. As stated in Section 6.3.1.3, 33 percent is equivalent to 6 of 17 wells in the Western Upper Principal Aquifer, 2 of 5 wells in the Western Lower Principal Aquifer, and 13 of 39 wells in the Eastern Principal Aquifer.

MT exceedances are limited to 3 consecutive Fall monitoring events to avoid the potential seasonal component of elastic land subsidence. Elastic subsidence may occur in the fall, during low water level conditions, only to rebound during the spring, during high water level conditions. Data from a GPS station in the Subbasin illustrates this seasonal rebound (see Section 3.2.6, information on existing GPS stations). If groundwater elevations are managed #25..
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at or above the MTs on a regional and multi-year basis, potential undesirable results for land subsidence should be avoided.

Water level monitoring will be supplemented by annual screening of InSAR data. These data will be re-evaluated with the water level monitoring network in the five-year GSP evaluation. If InSAR data indicate increasing rates of subsidence, the monitoring network will be bolstered by additional monitoring, such as the installation of GPS stations, in targeted areas of the Subbasin. In addition, the criteria could also be adjusted to be more protective.

6.7.2. Minimum Thresholds for Land Subsidence
As provided in the GSP regulations, the MT for land subsidence “shall be the rate and extent of subsidence that substantially interferes with surface land uses and may lead to undesirable results” (§354.28(c)(5)). Given the lack of undesirable results associated with land subsidence in the Modesto Subbasin, it is not possible to correlate a rate of subsidence to undesirable results. As explained in more detail below, available data sets indicate no land subsidence over most of the Subbasin. InSAR data indicate very low rates of vertical displacement in the central and eastern Subbasin, but this may also be due to irrigation on clay-rich soils or other land surface modifications associated with agricultural operations (see Figure 3-6). Additional supporting technical information on land subsidence in the Modesto Subbasin is provided in Section 3.2.6 and summarized below in Section 6.7.2.1.

Because the greatest risk for land subsidence in the Modesto Subbasin is the dewatering/depressurization of clays, setting MTs at historic low groundwater levels (WY 2015 – WY 2020) was viewed as a reasonable strategy for minimizing future subsidence. In this manner, groundwater levels would be protective against worsening conditions that could lead to future undesirable results for land subsidence. Because the chronic lowering of water level MTs were developed to arrest water level declines in the Subbasin, they serve as reasonable MTs for avoidance of undesirable results for land subsidence. As such, chronic lowering of water levels MTs are used as a proxy for directly monitoring for land subsidence as follows.

Table 6-14:-18: Minimum Thresholds for Land Subsidence
Minimum Thresholds Principal
Aquifer(s)
Land
Subsidence
Minimum thresholds are defined as the historic low groundwater elevation observed or estimated during WY 1991 – WY 2020 at each representative monitoring location, based on available data. (Using Chronic Lowering of Groundwater Levels as a proxy.) All
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Additional support and justifications for the MTs, along with the quantitative criteria for the combination of MT exceedances provided in the undesirable results definition, are discussed in the following section.

6.7.2.1. Justification and Support for Minimum Thresholds GSP regulations require that the MTs for land subsidence be supported by:
• Identification of land uses and property interests that have been affected or are likely to be affected by land subsidence, including an explanation of how these uses and interests were determined.
• Rationale for establishing MTs in consideration of the above effects • Maps and graphs showing the extent and a rate of land subsidence in the basin that defines the MT and MO.

With regards to the identification of land uses and property interests that are likely to be affected by land subsidence, potential effects of land subsidence on property interests are mentioned above in Sections 6.7.1.1 and 6.7.1.2. These effects on beneficial uses are general and hypothetical because no effects on beneficial uses caused by land subsidence have been identified in the Subbasin.

As mentioned previously, InSAR data published by DWR provides the best available vertical displacement data for the Subbasin. Figure 3-60 illustrates cumulative vertical displacement over more than five years, from June 2015 through October 2020. As indicated by the dark gray areas, there is no negative vertical displacement (land subsidence) over most of the Subbasin. Only one small area of land subsidence is indicated within the extent of the Corcoran Clay. This area, located in the northwest corner of the Subbasin in the San Joaquin Wildlife Refuge, indicates a rate of land subsidence of up to 0.24 inches per year.

InSAR data indicate larger rates of vertical displacement in the central-southeastern Subbasin (orange and brown on Figure 3-60). Data in this area indicate a vertical displacement rate of about 0.12 inches per year with rates up to about 0.36 inches per year in two small, isolated areas (Figure 3-60). This area is outside of the Corcoran Clay and is characterized by relatively shallow, consolidated aquifers (i.e., Mehrten Formation) that would be less likely to experience significant land subsidence than areas with compressible clays.

In addition, there are clay-rich soils and multiple restrictive layers (e.g., duripan) in the eastern Subbasin that could be the cause of these small rates of vertical displacement (rather than groundwater extractions) (see Figure 3-6). For example, clay soils can be subject to swelling when wetted. In addition, the disruption of restrictive layers on agricultural lands could also result in small local differences in surface elevation, as can other agricultural operations. However, this area is also associated with increasing groundwater extractions over the historical study period, and the potential for land subsidence associated with these extractions cannot be ruled out at this time. #25..
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The map on Figure 3-59 also shows the locations of three existing global positioning system (GPS) stations10 along Highway 99, within the extent of the Corcoran Clay. The two northern stations are in Salida, and the southern station is in Modesto. These existing stations, monitored by other programs, provide highly accurate ground surface elevation data. Data available from the northern (August 2006 to December 2007) and southern (November 2006 to July 2001) GPS stations indicate that there has been no inelastic land subsidence at those locations. The central station indicates a rate of land subsidence of about 0.048 inches per year (less than 5 inches over 100 years), for the period of August 2008 to June 2014 (see Section 3.2.6 for more information).

Increased rates of subsidence are often triggered during drought conditions (LSCE, 2014); the available recent land subsidence data in the Modesto Subbasin were collected during the long-term (and ongoing) drought conditions that resulted in historic low water levels throughout the Subbasin. It is not possible to know whether the current rates will continue beyond the drought.

Collectively, these data suggest that significant rates of land subsidence are not occurring in the Modesto Subbasin. Accordingly, MTs are selected to be protective against triggering significant rates of subsidence in the future. All of the information and data reviewed to date indicate that undesirable results from land subsidence could be avoided by arresting the ongoing water level declines in the Subbasin. By setting MTs at the historical low, water level declines are controlled, and any current land subsidence is not exacerbated. As indicated above, the MTs for chronic lowering of groundwater levels are being used as a proxy for land subsidence MTs because these MTs manage groundwater levels near or above historic low groundwater levels (WY 1991 – WY 2020).

As an additional protective measure, the GSAs intend to download and review DWR’s InSAR data on an annual basis, for screening purposes. As illustrated on Figure 3-59, the InSAR data cover the entire extent of the Subbasin. Data will be used for ongoing evaluation of the rate and extent of land subsidence. The data will be re-evaluated for the five-year evaluation in 2027. If significant rates of subsidence have occurred between 2022 and 2027, additional monitoring, such as additional wells or GPS stations, will be installed in areas of concern.

In this manner, the GSAs will ensure that the potential for impacts to land uses from land subsidence is not missed. This approach is reasonable, based on the best available data in the Modesto Subbasin.

6.7.2.2. Relationship between MTs of Each Sustainability Indicator Regulations require a description of the relationship between the MTs for each sustainability indicator and how the GSAs have determined that basin conditions at each MT 10 Installed and operated by the U.S. Bureau of Reclamation in connection with the San Joaquin River Restoration Program.
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will avoid undesirable results (§354.28(b)(2)). To facilitate this comparison, MTs for each sustainability indicator were summarized in Table 6-5, as discussed above in Section 6.3.2.2.

Because the MTs for chronic lowering of groundwater levels are being used as a proxy for land subsidence, the interaction between the MTs for land subsidence and the other MTs is the same as for chronic lowering of water levels. As such, please refer to Section 6.3.2.2 above for meeting this regulatory requirement for the land subsidence sustainability indicator. These sustainability indicators are also analyzed separately in other subsections of Chapter 6, as referenced in Table 6-45.

6.7.2.3. Impacts of MTs on Adjacent Subbasins
Regulations require consideration of how Modesto Subbasin MTs impact the ability of an adjacent subbasin to achieve its sustainability goal. As summarized in more detail in Section

6.3.2.3, similar principal aquifers, shared interconnected surface water boundaries, and multiple GSA member agencies that overlap both the Modesto Subbasin and adjacent subbasins have facilitated setting MTs in the Modesto Subbasin that will not adversely impact adjacent subbasins GSP implementation. Additional details relevant to each adjacent subbasin are summarized below.

6.7.2.3.1. Eastern San Joaquin Subbasin
ESJ Subbasin MTs for chronic lowering of water levels are also used as a proxy for the reduction of groundwater in storage, land subsidence, and interconnected surface water.

Therefore, the analysis presented for the chronic lowering of water levels in Section

6.3.2.3.1 provides the technical rationale for concluding that MTs in the Modesto Subbasin for land subsidence will not adversely affect GSP implementation in the ESJ Subbasin.

6.7.2.3.2. Delta Mendota Subbasin
Land subsidence is a prevalent issue in the Delta-Mendota Subbasin, with impacts to infrastructure of statewide importance (such as the California Aqueduct and the DeltaMendota Canal). However, no significant land subsidence has been documented near the Modesto Subbasin. Most of the subsidence maps in the Northern & Central Delta-Mendota GSP either do not contain data or do not indicate significant amounts of land subsidence along its shared San Joaquin River boundary with the Modesto Subbasin (see Figures 5-113, 5-114, and 5-116 in W&C and P&P, 2019). The closest UNAVCO GPS station (P255) along the Delta-Mendota Canal is located approximately nine miles to the west of the Modesto Subbasin, and data from 2007 to 2018 at that station did not indicate inelastic land subsidence.

For the Northern & Central Delta-Mendota GSP, land subsidence MTs in the management area adjacent to the Modesto Subbasin are based on an acceptable loss in distribution capacity in subbasin canals, to be determined in a future study (W&C and P&P, 2019). The closest subsidence monitoring station to the Modesto Subbasin is more than two miles to the southwest of the Modesto Subbasin boundary (04-002), and the MT had not yet been quantified. However, given that MTs are set at the historical low groundwater levels, no impacts on land subsidence in the Delta-Mendota Subbasin would be anticipated. In #25..
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addition, MTs for interconnected surface water are the Fall 2015 groundwater elevations along the San Joaquin River, providing even more protection for the adjacent subbasin (see Section 6.8.2.3.2). Given these conditions, no impacts are expected on GSP implementation in the Delta-Mendota Subbasin.

6.7.2.3.3. Turlock Subbasin
Both the Turlock Subbasin and Modesto Subbasin have approved MTs for interconnected surface water that are based on Fall 2015 water levels along both sides of the Tuolumne River (see Section 6.8.2.3.3). In that manner, the two GSPs are coordinating on MTs and avoiding undesirable results for streamflow depletion. Accordingly, MTs in the Modesto Subbasin for land subsidence will not have an adverse impact on GSP implementation in the Turlock Subbasin.

6.7.2.4. Effects of MTs on Beneficial Uses and Users of Groundwater The setting of MTs is protective with respect to the avoidance of undesirable results.

However, the MTs place operational constraints on agricultural wells and other water supply wells, especially during long-term multi-year droughts. Because the MTs for chronic lowering of water levels are used as a proxy for land subsidence, all of the same effects on beneficial uses and users of groundwater discussed previously also apply to this indicator (see Section 6.3.2.4).

Shallow groundwater levels in the Western Upper Principal Aquifer create operational issues for agriculture and groundwater pumping is required in some areas to drain fields and allow access for farming. Given the small fluctuations in these wells, maintaining water levels at MTs may impose restrictions on these extractions and limit beneficial uses of groundwater.

However, the definition of undesirable results allows for short-term declines and criteria for undesirable results focus on the lowest seasonal levels (Fall). These criteria will assist with the necessary operational pumping of shallow groundwater in the western Subbasin.

Notwithstanding the constraints placed on various well owners, groundwater users would benefit from the control and mitigation of potential impacts from land subsidence in the future. Those impacts could negatively affect agricultural or urban land uses or other beneficial uses of groundwater as explained in Section 6.7.1 above.

6.7.2.5. Consideration of State, Federal, or Local Standards in MT Selection GSP regulations require that GSAs consider how the selection of MTs might differ from other regulatory standards. For land subsidence, the MT consists of managing water levels in each representative monitoring well, which would not conflict with other regulatory standards.

6.7.2.6. Quantitative Measurement of Minimum Thresholds As stated above, the MTs for land subsidence will be monitored by quantitatively measuring water levels as a proxy in representative monitoring well networks for each applicable Principal Aquifer as described in Section 7.1.5 of this GSP. Monitoring will occur on a semi- #25..
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annual basis, in Spring and Fall, to represent the seasonal high and low water level and adhere to water level sampling protocols (Section 7.2).

For land subsidence, supplemental monitoring is also planned. To provide a backstop for the uncertainties associated with future rates and extents of land subsidence, the GSAs also intend to use the annual DWR-published InSAR data as a screening tool. Those data cover the entire extent of the Subbasin and will provide a valuable tool for evaluating future vertical displacement. When combined with the annual data on groundwater extractions and groundwater elevations, the InSAR data can be used to identify areas where vertical displacement rates are changing and provide areas of the Subbasin where additional monitoring may be warranted. Data from existing GPS stations will be incorporated in the annual analysis, as available. Collectively, InSAR and GPS stations will serve as future land subsidence screening tools and, if necessary, will help identify optimal locations for either additional wells or future GPS stations.

6.7.3. Interim Milestones for Land Subsidence
As described previously, the chronic lowering of water levels criteria are applied as a proxy for the land subsidence sustainability indicator. By extension, the interim milestones for chronic lowering of water levels are used as a proxy for the land subsidence sustainability indicator.

6.7.3.6.7.4. Measurable Objectives for Land Subsidence
The MO for land subsidence is the midpoint between the MT and the historical high water level (WY 1991 – WY 2020). This is the same approach as for chronic lowering of water levels and is developed at the same representative monitoring sites.

Table 6-15:-19: Measurable Objectives for Land Subsidence Measurable Objectives Principal
Aquifer(s)
Land
Subsidence
Midpoint between the historical high groundwater elevation and the MT at each representative monitoring location.
(Using Chronic Lowering of Groundwater Levels as a proxy) All

6.8. DEPLETION OF INTERCONNECTED SURFACE WATER
SGMA defines an undesirable result for the interconnected water sustainability indicator as “depletions of interconnected surface water that have significant and unreasonable adverse impacts on beneficial uses of the surface water.” (§10721 (x)(6)). In the Modesto Subbasin, the Tuolumne, Stanislaus, and San Joaquin rivers are all interconnected surface water. Along these boundary rivers, groundwater occurs above the channel invert elevation on an #25..
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average basis, allowing groundwater to interact with surface water. All three rivers are interconnected during historical, current, and projected future conditions (Figure 6-1).

STRGBA GSA member agencies Modesto ID and Oakdale ID manage surface water supplies from the Tuolumne River and Stanislaus River, respectively. The districts provide local management of diversions and conveyance of surface water for municipal drinking water (City of Modesto), non-potable municipal uses, and agricultural supply. Agency experience was used to guide the analysis of streamflow depletions and undesirable results. Both agencies provided information and data to incorporate into the integrated surface watergroundwater model (C2VSim-TM) for streamflow depletion analyses under historical, current, and projected future water budgets (see Chapter 5). Agencies also provided expertise on potential undesirable results for surface water rights. Modesto ID and the consultant team also coordinated with TID on information along the Tuolumne River; TID operates New Don Pedro Dam for releases to the Tuolumne River for water supply.

The undesirable results, including causes and impacts to beneficial uses, are described in Section 6.8.1 below, with a definition of undesirable results at the end of the section that includes criteria to quantify where and when undesirable results would occur. Section 6.8.2 describes the quantification of MTs. Section 6.8.3 provides the approach and selection of MOs. IMs that cover all of the applicable sustainability indicators (except degraded water quality) are described in Section 6.9.

6.8.1. Undesirable Results for Interconnected Surface Water Analyses of groundwater conditions and water budget modeling in the Modesto Subbasin highlight the linkages between groundwater extractions, reduction of groundwater in storage, and interconnected surface water. In its Water Budget BMP, DWR notes that increases in groundwater extraction will initially result in a decline in groundwater in storage. However, over time, this decline in storage will be ultimately balanced by decreases in groundwater flow to streams (DWR, 2016a). This condition will induce groundwater recharge, removing water from the rivers (streamflow depletion). Although beneficial to water levels and storage, this streamflow depletion may impact beneficial uses of surface water including municipal, agricultural, and environmental uses.

Modeling shows that increased streamflow depletion (i.e., net groundwater recharge) along the Modesto Subbasin boundaries is associated with groundwater level declines. This observation indicates that water levels along the rivers can be used as a proxy for streamflow depletions if the water level declines can be shown to be protective against undesirable results.

Groundwater level monitoring for this purpose is best accomplished with a series of shallow monitoring wells adjacent to and transitioning away from the river. Although not ideal, current GSP monitoring wells are relatively close to the rivers and are screened in the unconfined aquifers that are connected to the rivers. When coupled with stream gage data and ongoing modeling, current wells are likely to be sufficient for monitoring surface water- #25..
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groundwater conditions in the short term (see Section 7.1.6, Table 7-2, and Figure 7-5).

Over time, additional monitoring wells will be added to the interconnected surface water monitoring network. A management action to improve the monitoring network provides for additional shallow monitoring wells to be installed along the rivers over time (Chapter 8).

6.8.1.1. Causes of Undesirable Results
In the Modesto Subbasin, groundwater extractions – primarily in the NDE MA – have lowered groundwater levels locally and in adjacent areas to the west. These extractions intercept groundwater that would have naturally flowed toward the river boundaries, depleting some amount of baseflow to the rivers. This streamflow depletion increases over time during the historical study period (note the declining amounts of stream/aquifer interaction as groundwater outflow, as shown in blue on Figure 5-20).

Modeling of projected future conditions suggests that the area of groundwater level declines will expand to the north and south toward the Stanislaus and Tuolumne rivers and cause increases in streamflow depletion (compare the net river gains/losses between historical and projected conditions in Table 5-8). Groundwater extractions in other parts of the Subbasin also contribute to this depletion, especially along the rivers. In the projected conditions scenario, both the Tuolumne and Stanislaus rivers transition from net gaining streams to net losing streams, a continuation of a trend that began in recent years.

If depletion increased significantly more than indicated from the modeling, the groundwater system could become disconnected from the surface water system. At that point, groundwater would no longer contribute baseflow to the river. Lower groundwater levels would induce more recharge from the river, significantly depleting flows; these conditions would produce an undesirable result.

In the Modesto Subbasin, integrated surface water-groundwater modeling indicates that the groundwater system and river system remain connected through the 50-year implementation and planning horizon under future projected conditions. This indicates that even if future water levels declined to the extent estimated, connection between the two systems could be maintained. The projected streamflow depletions average about 26,000 AFY, only about one percent of the total river outflows from the Subbasin.

Nonetheless, these future projected increases in streamflow depletion result in a net loss of streamflow from the river systems compared to a net gain in streamflow over historical conditions. In addition, beneficial uses could be adversely impacted at these predicted levels of streamflow depletion even if the groundwater and surface water systems remain connected (see Section 6.8.1.2 below). Accordingly, the projections for future streamflow depletions are considered undesirable results in this GSP.

GSAs are not required to correct undesirable results that occurred prior to January 1, 2015.

Rather, the GSAs intend to protect against future projected increases in depletions and set a “floor” at 2015 conditions. In this manner, future projected declines in groundwater #25..
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elevations will be managed, and future projections for streamflow depletion will be reduced.

6.8.1.2. Potential Effects on Beneficial Uses
Beneficial uses of the three Modesto Subbasin rivers are provided in the Basin Plan for the Sacramento River Basin and the San Joaquin River Basin (CVRWQCB, 2018). All three rivers are associated with almost all categories of beneficial uses including municipal (including potential uses), agricultural, and/or industrial supply; recreation; freshwater habitat, migration, and spawning; and wildlife habitat. The three rivers also support large riparian corridors. A preliminary evaluation of vegetative and wetland areas mapped by TNC as natural communities commonly associated with groundwater (NCCAG) indicates potential GDEs along most of the river reaches in the Modesto Subbasin (DWR, 2018d) (see Section

3.2.8).

Although predicted future streamflow over the 50-year baseline conditions are not precise, the predicted depletions result in lower streamflow during low flow conditions. These changes could exacerbate drought conditions on the rivers and adversely affect all beneficial uses that rely on surface water.

Both Modesto ID and Oakdale ID noted that more water would have to be released over time to meet current downstream flow requirements. This would make operation of the river more difficult, especially during low-flow conditions, and provide less water supply for municipal and agricultural beneficial uses during times when water demands are high.

In addition to adverse impacts to surface water rights holders, these conditions could also adversely impact flows needed to support fish and other wildlife. The large riparian corridors along the river could be adversely impacted. Lower groundwater levels adjacent to the rivers could impact GDEs and other environmental uses of groundwater that occur along the Subbasin river boundaries.

6.8.1.3. Modesto Subbasin Definition of Undesirable Results The definition of undesirable results for interconnected surface water in the Modesto Subbasin is based on the causes and effects discussed above, along with additional information from the basin setting and water budgets (Chapters 3 and 5). Regulations also require that the undesirable result definition include quantitative criteria used to define when and where groundwater conditions can cause an undesirable result (§354.26(b)(2)).

These criteria set the number of monitoring sites and events to determine where and when an MT can be exceeded before causing undesirable results. This framework recognizes that a single MT exceedance at one monitoring site may not indicate an undesirable result. The criteria also allow clear identification for when an undesirable result is triggered under the GSP.

The definition of undesirable results along with the quantitative combination of MT exceedances that cause undesirable results are provided below. #25..
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Table 6-16:-20: Undesirable Results for Interconnected Surface Water Undesirable Results Definition Principal
Aquifer(s)
Interconnected
Surface Water
An Undesirable Result is defined as significant and
unreasonable adverse impacts to the beneficial uses of
surface water caused by groundwater extraction.

An undesirable result will occur on either the Tuolumne or Stanislaus rivers when 33% of representative
monitoring wells for that river exceed the MT in three
consecutive Fall monitoring events.

An undesirable result will occur on the San Joaquin River when 50% of representative monitoring wells for that
river exceed the MT in three consecutive Fall monitoring events.

All
The 50% criterion for the San Joaquin River is because there are only two representative monitoring wells along the San Joaquin River, and MT exceedances in both wells (100%) is difficult to justify. This criterion may change when additional wells are added to the monitoring network along the San Joaquin River. An exceedance in only one well may not lead to undesirable results as being set in this GSP, so incorporating additional wells is a priority for improvements to the monitoring network. This and other improvements are included as an implementation action in Chapter 9.

The total number of current wells and the number of MT exceedances allowed by the undesirable result definition are summarized below. The monitoring network is described in Chapter 7 and shown on Figure 7-5.
• Tuolumne River: 10 wells (33% - 3 wells)
• Stanislaus River: 8 wells (33% - 3 wells)
• San Joaquin River: 2 wells (50% - 1 well)
The MT exceedance is limited to three consecutive Fall events (semi-annual monitoring).

Spring events will be monitored but not used in the criterion because the increase in Spring water levels would not be representative of potential negative impacts during low flows on the rivers.

These criteria were incorporated into the sustainable yield modeling (Section 5.3), which demonstrated that these criteria could be met using simulated hydrographs at wells along the river. Sustainable yield conditions indicate significant decreases in streamflow depletion at each of the three rivers as discussed below.
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6.8.2. Minimum Thresholds for Interconnected Surface Water GSP regulations require the metric for interconnected surface water MTs to be “the rate or volume of surface water depletions caused by groundwater use that has adverse impacts on beneficial uses of the surface water and may lead to undesirable results” (§354.28(c)(6)) (emphasis added). As explained in Section 6.8.1.1, the modeling projections of future volumes of streamflow depletion have been determined by the GSAs to be undesirable results and is caused by lower groundwater levels. Therefore, specific groundwater levels can be directly correlated to these volumes of streamflow depletion and used as a proxy for interconnected surface water MTs.

The link between streamflow depletion volume and groundwater levels is confirmed by a sustainable yield modeling analysis described in Section 5.3. For this analysis, groundwater extractions were reduced to test aquifer response to groundwater level MTs, resulting in a reduction in projected surface water depletions and elimination of net depletions over the Subbasin. That is, there was a net contribution to streamflow from the groundwater system at the Subbasin outflow (i.e., the downstream point past the confluence of the Stanislaus and San Joaquin rivers). By managing water levels at or near the Fall 2015 groundwater elevations, modeling showed that the projected net depletions could be eliminated.

Accordingly, MTs for this sustainability indicator are defined at the 2015 groundwater elevations as follows.

Table 6-17:-21: Minimum Thresholds for Interconnected Surface Water Minimum Thresholds Principal Aquifer(s)
Interconnected
Surface Water
Minimum Thresholds are defined as the low
groundwater elevation observed in Fall 2015 at
each representative monitoring location.

Western Upper and
Eastern Principal
Aquifers

6.8.2.1. Justification and Support for Minimum Thresholds GSP regulations require that the MTs be supported by:
• Location, quantity, and timing of depletions of interconnected surface water • A description of the groundwater and surface water model used to quantify surface water depletion (§354.28(c)(6)(A)(B)).

Background information for the interconnected surface water analysis is provided in Section

3.2.7, followed by a preliminary analysis of potential GDEs, which occur along the river boundaries (Section 3.2.8 and Figure 3-60). The historical, projected, and sustainable yield water budgets provide a detailed assessment of groundwater-surface water interaction and are presented in Chapter 5. As described above in Section 6.8.2, the sustainable yield analysis in Section 5.3 was used to support the selection of MTs for this indicator. These collective analyses are summarized below.
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In brief, the Tuolumne, Stanislaus, and San Joaquin rivers are interconnected surface water as defined by SGMA. The surface water-groundwater interaction is dynamic, with recharge and baseflow varying along segments of the river both seasonally and over time. This dynamic system of mixed gaining and losing segments along the Tuolumne and Stanislaus rivers is the result of both natural interactions and managed operations. As mentioned previously, both rivers are actively managed to provide critical water supplies for the Modesto, Turlock, and Eastern San Joaquin subbasins. The San Joaquin River has less variability and has the largest flows of the three Subbasin rivers. The segment of the San Joaquin River along the western Modesto Subbasin can be characterized as a net gaining reach during both historical and projected future conditions.

The location, quantity, and timing of deletions of these interconnected rivers were analyzed using the integrated surface water-groundwater model C2VSimTM. This local model is based on the DWR regional C2VSimFG-BETA2 model, which has been revised to include local water budget data for both the Turlock and Modesto subbasins in order to simulate the river boundary more accurately. Local surface water and groundwater data from the Eastern San Joaquin Subbasin to the north was also incorporated into the modeling analyses. These revisions provided increased ability and accuracy for modeling interconnected surface water across the northern and southern river boundaries. Documentation of the revised C2VSimTM model is provided in Appendix C of this GSP.

Interconnected surface water was analyzed with C2VSimTM for historical, current, and future projected water budget conditions including separate average annual water budgets for the Modesto Subbasin surface water systems (see Table 5-2). Total surface water inflows into the Subbasin historically have averaged about 2,547,000 AFY11 for all three river systems, with about one-half consisting of the San Joaquin River flows. Surface water outflows are estimated at 2,770,000 AFY under historical conditions as measured at the confluence of the Stanislaus River and the San Joaquin River at the northwest corner of the Modesto Subbasin (Table 5-2).

During historical conditions, all three rivers were net gaining on an average annual basis with baseflow contributions of about 61,000 AFY (see the net of the Modesto Subbasin total gains from groundwater (baseflow) and losses to groundwater (seepage/recharge) under historical conditions in Table 5-2). Under future conditions, streamflow seepage is projected to increase in all three rivers, resulting in net depletions on both the Tuolumne and Stanislaus rivers over the 50-year period of analysis. Smaller streamflow depletions are projected to occur along the San Joaquin River, but the river remains a net gaining stream overall.

Historical conditions represent an average over a 25-year period. During that time, streamflow depletions increased along each of the Subbasin rivers as groundwater extractions increased, especially after about 2005. Figure 5-20 illustrates this increase by 11 As footnoted in Table 5-2, some diversions occur upstream of the inflow measurement point into the Subbasin and are not included in these totals.
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showing overall smaller groundwater outflows to the surface water system from WY 2005 to WY 2015 (see annual estimates represented by the stream/aquifer interaction shaded blue on Figure 5-20). Figure 5-25 shows the relatively small amount of total streamflow that is affected by the groundwater system.

To reduce the potential for projected future depletions to cause undesirable results, groundwater level declines associated with groundwater extractions need to be arrested. By managing groundwater at or above 2015 groundwater levels, sustainable yield modeling predicts significant improvements in the future projections. A summary of these improvements is shown in the following table.

Table 6-18:-22: Improvements to Interconnected Surface Water under Sustainable Yield Conditions
Modesto
Subbasin
Surface Water
Projected Future
Baseline Conditions
(AFY)
Sustainable Yield
Conditions
(AFY)
Increase in Baseflow*
under Sustainable Yield
Conditions
(AFY) (%)
Total GW-SW
Interaction
26,000 -15,000 41,000 158%
San Joaquin
River
-9,000 -13,000 4,000 44%
Tuolumne
River
11,000 -11,000 22,000 200%
Stanislaus
River
24,000 9,000 15,000 63%
Positive numbers represent net recharge from surface water to groundwater (i.e., streamflow depletion, also referred to as a net losing river) over average hydrologic conditions.

Negative numbers represent a net contribution to surface water (SW) from groundwater (GW) (i.e., net baseflow, also referred to as a net gaining river) over average hydrologic conditions. *”Increase in baseflow” refers to the larger contributions to surface water from groundwater (i.e., lower amounts of streamflow depletion) under Sustainable Yield Conditions.

As shown in the table above, net streamflow depletion in the Modesto Subbasin rivers is estimated at 26,000 AFY under the projected future baseline conditions. Under sustainable yield conditions, which incorporated the 2015 groundwater elevation MTs, the projected future streamflow depletion is eliminated, and the overall surface water system returns to a net gaining condition. Sustainable yield conditions indicate an increase of 41,000 AFY of baseflow over projected future conditions. Additional details of these data are provided in Section 5.1.4.4 for projected conditions (see also Table 5-2 and Figure 5-24); additional details on the sustainable yield analysis are provided in Section 5.3 (see Table 5-15 and Figure 5-24).
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6.8.2.2. Relationship between MTs of Each Sustainability Indicator Regulations require a description of the relationship between the MTs for each sustainability indicator and how the GSAs have determined that basin conditions at each MT will avoid undesirable results (§354.28(b)(2)). Table 6-5 summarizes the MTs for the sustainability indicators.

The use of 2015 groundwater levels as a proxy for interconnected surface water coordinates well to the other sustainability indicators, most of which are also tied to similar or identical water levels. The relationship between the MTs for interconnected surface water and the other MTs are summarized below:

MTs for interconnected surface water are either identical or a few feet higher than the MTs selected for chronic lowering of water levels to allow more protection against streamflow depletions along the rivers. For the 20 wells along the rivers that are included in the monitoring networks for both the chronic lowering of groundwater levels and interconnected surface water indicators, MTs vary by four feet or less (compare Figures 7-1 and 7-3 with Figure 7-5). These differences are not sufficient to create a conflict for GSP implementation and management.

MTs for reduction of groundwater in storage and land subsidence are the same as those for the chronic lowering of water levels. As such, interaction of those MTs with interconnected surface water MTs occurs in the same manner as discussed above (see also Section 6.4.2 and 6.5.2).

MTs have not been selected for the Seawater Intrusion indicator because it is not applicable to the inland Turlock Subbasin (see Section 6.5).

MTs for interconnected surface water will not affect water quality and, as such, will not conflict with degraded water quality MTs. In addition, by setting MTs at the Fall 2015 groundwater levels along the rivers, groundwater will continue to contribute fresh water to the rivers. (see also Section 6.6).

Although these MTs were considered and approved separately for each of the sustainability indicators separately, the TAC reviewed technical presentations on how the MTs for each indicator coordinates with the others. Technical information and modeling analyses were reviewed both by mangers and representatives in the TAC planning group as well as in public TAC meetings held in tandem with monthly STRGBA GSA meetings.

6.8.2.3. Impacts of MTs on Adjacent Subbasins
Regulations require consideration of how Modesto Subbasin MTs impact the ability of an adjacent subbasin to achieve its sustainability goal. As summarized in more detail in Section

6.3.2.3, similar principal aquifers, shared interconnected surface water boundaries, and multiple GSA member agencies that overlap both the Modesto Subbasin and adjacent subbasins have facilitated setting MTs in the Modesto Subbasin that will not adversely #25..
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impact adjacent subbasins GSP implementation. Additional details relevant to each adjacent subbasin are summarized below.

6.8.2.3.1. Eastern San Joaquin Subbasin
ESJ Subbasin MTs for chronic lowering of water levels are also used as a proxy for the reduction of groundwater in storage, land subsidence, and interconnected surface water.

Given that the MTs for interconnected surface water are either the same or only a few feet higher than the MTs for the chronic lowering of water levels, the previous analysis in Section

6.3.2.3.1 is applicable to this indicator. Information in that section provides the technical rationale for concluding that MTs in the Modesto Subbasin for interconnected surface water will not adversely affect GSP implementation in the ESJ Subbasin.

6.8.2.3.2. Delta-Mendota Subbasin
The Delta-Mendota Northern & Central GSP defines undesirable results for interconnected surface water as a percentage increase in streamflow depletions that is to be determined within the first five years of GSP implementation. A quantitative MT is not set due to insufficient data. The data to be incorporated into the evaluation will be collected from two wells along the San Joaquin River south of the Modesto Subbasin (see wells 03-001 and 03- 003 on GSP Figure 6-7 in W&C and P&P, 2019). In the interim, the GSP selects a narrative MO, which states “no increased depletions of surface water occur as a result of groundwater pumping.” (W&C and P&P, 2019).

In the absence of a quantitative MT for interconnected surface water, the MT for the Modesto Subbasin seems sufficiently high to not interfere with the Delta-Mendota Subbasin achieving its sustainability goal. As mentioned previously, MTs for chronic lowering of water levels have been set similarly in both subbasins adjacent to the San Joaquin River.

Sustainable yield modeling shows that MTs for the San Joaquin River in the Modesto Subbasin are correlated to conditions that contribute a net baseflow of 13,000 AFY (Table 6- 1822), an amount that differs from the average historical net baseflow of only 1,000 AFY (i.e., 14,000 AFY; subtract outflows from inflow for the San Joaquin River on Table 5-8). With this contribution to baseflow and MTs from 2015 conditions on both sides of the river, the MT for interconnected surface water in the Modesto Subbasin would not be expected to negatively impact implementation of the Delta-Mendota Northern & Central GSP.

6.8.2.3.3. Turlock Subbasin
MTs selected in both subbasins are Fall 2015 groundwater levels for the interconnected surface water sustainability indicator along the shared Tuolumne River boundary.

Representatives from both subbasins have determined that future projected depletions of streamflow on the Tuolumne River may lead to undesirable results and have selected groundwater levels as a proxy for monitoring interconnected surface water and avoiding those future conditions (see Table 6-1822 above).

Further, GSAs in both subbasins have tested the MTs through similar sustainable yield modeling analyses (Section 5.3) to ensure that interconnected surface water conditions are protected. Results of the sustainable yield modeling indicate similar net contributions to #25..
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baseflow on both sides of the river (16,200 AFY from Turlock Subbasin compared to 11,000 AFY from Modesto Subbasin).

6.8.2.4. Effects of MTs on Beneficial Uses and Users of Groundwater The setting of MTs is protective with respect to the avoidance of undesirable results related to streamflow depletion. By arresting groundwater level declines along the river boundaries, the net future projected streamflow depletions can be substantially reduced or eliminated at each of the Modesto Subbasin rivers, and long-term use of groundwater can become more sustainable. Environmental uses of surface water and groundwater would also be supported.

However, there will be consequences on current uses of groundwater. The MTs will not be able to be achieved without sufficient projects or management actions to raise and maintain water levels along the Subbasin river boundaries. This will require significant investment in projects to replenish the Subbasin. Although projects identified in Chapter 8 of this GSP appear to provide sufficient supplemental water supply to achieve the MTs, a management action of demand reduction is included in the GSP as a backstop in the event that projects and associated aquifer response are not as expected. In that case, both agricultural beneficial uses and property interests could be negatively impacted if demand reduction is required to meet the Subbasin sustainability goal.

6.8.2.5. Consideration of State, Federal, or Local Standards in MT Selection GSP regulations require that GSAs consider how the selection of MTs might differ from other regulatory standards. For interconnected surface water, the MT consists of water levels quantified at each representative monitoring well. Surface water rights holders on the Stanislaus and Tuolumne rivers estimate that the MTs will not adversely impact surface water rights and will allow for compliance with state and federal requirements. Accordingly, there are no conflicts with regard to other regulatory standards.

6.8.2.6. Quantitative Measurement of Minimum Thresholds As stated above, the MTs for interconnected surface water will be monitored by quantitatively measuring water levels in representative monitoring wells along the river boundaries as described in Chapter 7 (see Section 7.1.6, Table 7-2, and Figure 7-5).

Monitoring will occur on a semi-annual basis, in Spring and Fall, to represent the seasonal high and low water level and will adhere to water level sampling protocols (Section 7.2).

6.8.3. Interim Milestones for Interconnected Surface Water The chronic lowering of water levels criteria are applied as a proxy for the interconnected surface water sustainability indicator. By extension, the interim milestones for chronic lowering of water levels are used as a proxy for the interconnected surface water sustainability indicator.

As described in Section 6.3.3, 2027 IMs below the MT were developed for RMWs in the OID and NDE Management Areas. There are five RMWs in the interconnected surface water #25..
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monitoring network along the Stanislaus River and the Tuolumne River that have 2027 IMs below the MTs. No RMWs along the San Joaquin River have 2027 IMs below the MTs.

6.8.3.6.8.4. Measurable Objectives for Interconnected Surface Water Similar to the other sustainability indicators, the MO for interconnected surface water is set as the midpoint between the high groundwater elevation and the MT in each of the representative monitoring wells. As explained in Section 6.3.3, the MTs represents a “floor” for maintenance of low water levels, with allowance for short-term exceedances during droughts. Accordingly, water levels will be managed over an operational range generally occurring between the MT (with temporary exceedances) and anticipated high water levels that occur during wet periods.

Table 6-19:-23: Measurable Objectives for Interconnected Surface Water Measurable Objectives Principal
Aquifer(s)
Interconnected
Surface Water
Measurable objectives are established at the midpoint
between the MT and the historical high groundwater
elevation at each representative monitoring site.

Western Upper
and Eastern
Principal
Aquifers

6.9. INTERIM MILESTONES
GSP regulations define an interim milestone (IM) as “a target value representing measurable groundwater conditions, in increments of five years, set by an Agency as part of a Plan.” For the Modesto Subbasin, water levels are used as a metric for the IMs, consistent with the metric being used for MTs and MOs for all sustainability indicators except degraded water quality.

IMs provide a glide path for the Modesto Subbasin to reach its sustainability goal. The incremental approach recognizes that the path to sustainability is determined by the timing and effectiveness of GSP implementation, including projects and management actions designed to avoid undesirable results. For the Modesto Subbasin, a glide path provides needed flexibility for MAs of the Subbasin that will continue to decline – at rates dependent on future hydrologic conditions – until projects and management actions are implemented.

The following graphic prepared by DWR illustrates the concept of how IMs relate to the MT and MO. As shown, the IMs provide a glide path to sustainable management whereby MTs and MOs are maintained to avoid undesirable results.
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In this conceptual graphic, the pink area represents water levels below the MT as designated in a representative monitoring well (i.e., an MT exceedance). In this example, water levels are expected to continue to decline after the GSP is adopted while projects are brought online. This concept acknowledges that the aquifer response to projects and management actions will take time. Interim milestones are illustrated in increments of five years following Plan adoption to define the glide path from undesirable results to the MO and achieving sustainable management by 2042.

In the Modesto Subbasin, long-term declines have occurred in NDE MA (Figure 6-1) and have expanded into the Oakdale ID MA (Figure 6-2). Accordingly, 2027 target values below the MT have been developed for representative monitoring wells in the management areas.

The amount of the anticipated declines between adoption and 2027 is dependent on future unknown hydrologic conditions. Since drought conditions began in WY 2013, dry hydrologic conditions have persisted in the Subbasin. Water year types as categorized by the DWR San Joaquin Valley indices since 2014 are summarized in the following table. #25..
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Table 6-20: Water Year Hydrologic Classification Indices Since 2014 Water Year Water Year Type
San Joaquin Valley Water Year Index
2014 Critically Dry
2015 Critically Dry
2016 Dry
2017 Wet
2018 Below Normal
2019 Wet
2020 Dry
Source : : https://cdec.water.ca.gov/reportapp/javareports?name=WSIHIST As shown in the table, five out of seven water years between WY 2014 and WY 2020 have been categorized as below normal, dry, or critically dry. Water level declines associated with the last seven years may continue if hydrologic conditions do not improve, and/or if the aquifer response to GSP project implementation is delayed.

In order to plan for a worst-case scenario, a 2027 IM has been developed for declining wells based on the declines observed over the last seven years. By 2032, project implementation is expected to support water level recovery and the 2032 IM is set as the MT. If needed, the IM for 2037 is defined as the halfway point between the MT and MO. This trajectory is similar to the DWR conceptual diagram illustrated above. The 2027 IMs are provided in Chapter 7 (see Table 7-1 and Table 7-3) and shown on the hydrographs in Appendix F.

IMs have been designated conservatively for monitoring wells in the Oakdale ID MA and the NDE MA but will not be used to defer implementation of GSP projects or management actions. Other projects and/or management actions may also be needed during the first five years of GSP implementation to avoid undesirable results near wells if water levels reach the IMs.

To provide protection against IMs causing undesirable results, the following projects and management actions are being included in the GSP:
• A Group 2 project provides treated surface water to the City of Waterford to reduce pumping near interconnected surface water and in areas where domestic wells have previously failed (see Figure 6-1).
• Group 2 projects providing surface water as in lieu supply or for direct recharge are scheduled to begin immediately upon GSP adoption through coordination with, and actions by, landowners in the NDE MA to secure agreements and to plan for infrastructure with Oakdale ID and Modesto ID.
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6.10.6.9. SUMMARY OF SUSTAINABLE MANAGEMENT CRITERIA AND ADAPTIVE

MANAGEMENT
Collectively, the sustainable management criteria discussed in this GSP chapter provide a robust set of criteria to avoid undesirable results and achieve the Modesto Subbasin sustainability goal. Sustainable management criteria provided in multiple tables above are summarized in Table 6-2124, including the definition of undesirable results, minimum thresholds (MTs), and measurable objectives (MOs) for all sustainability indicators applicable to the Modesto Subbasin GSP.

Modesto Subbasin GSAs note that this initial sustainable management criteria employs new SGMA terminology and represents reasonable estimates for sustainable management of groundwater through the planning horizon. Nonetheless, it is recognized that sustainable management criteria – including the definition of undesirable results – may require adjustment in the future.

Improvements to the GSP monitoring network including new installations of monitoring wells are incorporated into this GSP. As the GSAs implement the GSP and monitoring network, additional information will be routinely compiled and analyzed to evaluate aquifer response to the initial sustainable management criteria.

GSAs recognize that monitoring results may indicate that the initial undesirable results definition and MTs require adjustment in the future. Actual MTs that lead to undesirable results may be higher or lower than those selected in Table 6-2124 as projects and management actions are implemented. Consistent with the concept of adaptive management, the GSAs report compliance and GSP implementation in Annual Reports. The GSAs will also re-evaluate the criteria in the five-year GSP evaluation and make appropriate adjustments to ensure that the Subbasin meets its sustainability goal within the GSP implementation period as required.
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Table 6-21:-24: Sustainable Management Criteria Summary #25..
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80 40
60 100 120
60 8
0 120
40 1
0 0
40 40
60 40
40 20
20 ("N
0 3
Miles
Tuolumne River
Stanislaus River
San Joaquin River
#
# #
# #
#
#
Note:Contours based on Fall 2015 water levels in Western Upperand Eastern principal aquifers – see Figure 3-27 #
#
Figure 6-1
Sustainability
Considerations for the
Modesto Subbasin
December 2021
Chronic Lowering of Groundwater LevelsReduction of Groundwater in StorageDegraded Water QualityInelastic Land SubsidenceInterconnected Surface WaterSeawater Intrusion Not applicable
Legend
Failed Domestic WellGroundwater Elevation Contour (feet msl)Groundwater Elevation Contour (feet msl), EstimatedCorcoran Clay ExtentModesto SubbasinAreas Most Susceptible to Land SubsidenceNon-District East Management Area Estimated area of most
vulnerable domestic wells
Estimated area
of most vulnerable
domestic wells
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Modesto ID
Management Area
Oakdale ID
Management Area
Non-District East
Management Area
Non-District West
Management Area
Path: T:\Projects\Modesto Revised GSP 74313\GIS\Maps\Figures\Revised GSP Figures\Figure 6-2 Management Areas Modesto Subbasin.mxd Figure 6-2
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Modesto Subbasin
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03 Miles
Legend
Corcoran Clay Extent
Management Area
Modesto ID
Oakdale ID
Non-District West
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Path: T:\Projects\Modesto Revised GSP 74313\GIS\Maps\Figures\Revised GSP Figures\Figure 6-4 RMWs and Wells w-Construction.mxd Figure 6-4
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Path: T:\Projects\Modesto Revised GSP 74313\GIS\Maps\Figures\Revised GSP Figures\Figure 6-6 Wells Impacted at Interim Milestone.mxd Figure 6-6
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Corcoran Clay Extent (Burow)
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Path: T:\Projects\Modesto Revised GSP 74313\GIS\Maps\Figures\Revised GSP Figures\Figure 6-7 GAMA Wells with Construction.mxd Figure 6-7
GAMA Wells with
Construction
May 2024
(
"N
0 3
Miles
Legend
#* GAMA Wells with Construction
! Representative Monitoring Well (RMW)
!!
Representative Monitoring Well (RMW) with
2027 Interim Milestone (IM) below Minimum
Threshold (MT)
Corcoran Clay Extent (Burow)
Modesto Subbasin
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8. PROJECTS AND MANAGEMENT ACTIONS
To achieve the sustainability goals for the Modesto Subbasin by 2042, andThe GSA acknowledges that during the 20-year GSP implementation period it will be necessary to implement Projects and Management Actions (PMA)s to achieve and maintain sustainable groundwater conditions in the Subbasins by or before 2042. Therefore, multiple PMAs have been identified and considered by the GSAs that are designed to avoid undesirable results over the remainder of a 50-year planning horizon, as required by SGMA regulations, multiple Projects and Management Actions (PMAs) have been identified and considered by the Modesto Subbasin Groundwater Sustainability Agencies (GSAs) in this Groundwater Sustainability Plan (GSP)..

A descriptionDescriptions of PMAs that will contribute to the achievement of sustainability goals in the Modesto Subbasin isare provided herein. PMAs are described in accordance with §354.42 and §354.44 of the SGMA regulations. An evaluationEvaluations of the benefits and/or impacts of various planned projects on groundwater levels and storage volumes isare also provided for their respective projects. “Projects” generally refer to physically constructed (structural) features whereas “Management Actions” generally refer to non‐structural programs or policies designed to incentivize reductions in groundwater pumping or optimize management of the Subbasin.

The PMAs discussed in this chapter are intended to help the GSAs progress toward meeting the sustainability goals and Measurable Objectives (MOs), as well as avoid Minimum Thresholds (MTs) and undesirable results identified for the Subbasin in Chapter 6:

Sustainable Management Criteria (SMCs). The subsequent Chapter Error! Reference source not found.9: Plan Implementation of the GSP describes the plan for implementing the PMAs detailed in this chapter.

Recognizing the data gaps identified in the GSP and uncertainties in the basin setting (per §354.44(d)), PMA development and implementation in the Modesto Subbasin applies an adaptive management approach informed by continued monitoring of groundwater conditions. The adaptive approach includes two categories:
1 PMAs developed for implementation at this time that would help to achieve and maintain groundwater sustainability while supporting other local goals. These PMAs include:
o PMAs that are in place and will continue to be implemented by specific participating agencies, that will support groundwater management and GSP implementation.
o PMAs that are currently planned and will be implemented by specific participating agencies, that will contribute to attainment of the Subbasin sustainability goal and will support GSP implementation #25..
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Other PMAs to be implemented as needed to gather and evaluate monitoring and investigation data as well as achieve and maintain long-term sustainable groundwater management across the Modesto Subbasin. These potential PMAs will be managed adaptively via further evaluation and initiation during GSP implementation if the GSAs finds that established Interim Milestones (IMs) or MOs cannot be achieved and/or if MTs are being approached.The STRGBA GSA adopted a resolution on July 10, 2024, to develop and implement management actions in order to arrest groundwater level declines by 2027 and raise groundwater levels after 2027, and to manage the Subbasin in a sustainable manner.

The GSAs committed to developing management actions by January 31, 2026, and implementing these management actions by January 31, 2027. The resolution approves the revised Modesto Subbasin GSP, commits to developing and implementing a well mitigation plan, and commits to developing and implementing management actions. The full text of the resolution can be found in Appendix YYY.

The management actions to be considered include, but are not limited to:
• A groundwater allocation and pumping management program • A groundwater extraction and surface water reporting program • Groundwater extraction fees
• A groundwater pumping credit market and trading program • Voluntary conservation/land fallowing, and
• Conservation practices
• A dry well mitigation program
Management actions will be developed to include triggers, based on sustainable management criteria established in the GSP, so the GSAs have the ability to readily respond to changing hydrologic conditions within the Subbasin. Development of management actions and their components are discussed in Section 8.4.

A range of PMAs isare presented to allow the GSAs flexibility in their response to changing hydrologic and groundwater conditions. However, itIt is anticipated that not all PMAs will need to be implemented, or that some PMAs will be implemented by one GSA but not the other. Adaptive implementationa subset of PMAs will be informed by ongoing monitoringprojects will provide the Subbasin with a suitable amount of groundwater conditions using needed for the monitoring network and methods described inSubbasin to achieve its sustainability goal. As a result, certain PMAs may not need to be implemented for the GSP. Any adverse groundwater conditions or challenges in maintaining groundwater sustainability will be addressed by scaling and implementing PMAs in a targeted and proportional manner, consistent with conditions observed in the SubbasinSubbasin, however, the GSAs will consider these PMAs for future initiatives or as means to achieve local goals and support the sustainability goal. Given their commitment to tangible results by 2027, the GSAs will place highest priority on implementation of PMAs with most rapid results, to be demonstrated with empirical data.
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PMAs will be evaluated periodically assessed during the GSP implementation period. As planning is at very PMAs, specifically management actions, are in early stages of development, complete. Complete information on construction requirements, operations, costs, permitting requirements, and other details are not uniformly available for all the PMAs. Potential timingImplementation schedules, costs, and funding of PMAsmechanisms are described under provided for each PMA where known. Other implementation and funding efforts will be determined and reported if/whenbased on the PMA is evaluated and selected for implementation. Thislatest information will be reportedavailable. Information related to PMAs still in annual reports and five-year updates to development will be reported in Annual Reports and Periodic Evaluations of the GSP when known. For more detailed information, refer to Chapter Error! Reference source not found.9: Plan Implementation.

8.1. MANAGEMENT ACTIONS
This section identifies and describes proposed Management Actions (MA) that will be undertaken by the GSAs as an element of GSP implementation. Management Actions refer to non-structural programs or policies designed to incentivize or enforce reductions in groundwater pumping, optimize management of the Subbasin, or implement GSA management authorities. Table 8-Table 8-1 shows a list of the six MAs organized into two categories: pumping management framework (Section 8.1.18.1.1) and demand reduction strategies (Section 8.28.1.2). The pumping management framework provides a suite of administrative procedures, programs, and policies that describe how the GSAs will manage and monitor groundwater extractions. Implementation activities such as monitoring, annual reporting, and GSP updates are discussed in further detail in Chapter 9. Demand reduction strategies are a broad and strategic set of actions intended to reduce water demand, some of which may be incentivized by State programs or policies, or by a pumping management framework.

As described in Chapter 5, the Subbasin has experienced overdraft conditions. Per § 354.44(b)(2), the GSP must describe Projects or MAs, including a quantification of demand reduction or other methods, for the mitigation of overdraft. Several Projects identified in earlier sections of this chapter would increase the available water in the Subbasin through increased recharge or use of alternate supplies and are expected to reduce the groundwater deficit sufficiently to achieve the Subbasin’s sustainability goal. Additionally, Group 3 projects may contribute to further improve groundwater conditions. MAs presented in the GSP are designed for the GSAs to promptly implement, while Projects are being designed and implemented. The extent and effectiveness of the MAs described in Sections 8.1.18.1.1 and 8.28.1.2 are not yet known, however, these programs will be developed for the GSAs to readily arrest groundwater level decline and storage deficits.

While the tools described in this section will be available for implementation Subbasin-wide, implementation may be prioritized in areas based on groundwater conditions. As such, it is anticipated that responsibility for implementing MAs will correspond with the relative #25..
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contribution of each Management Area to overdraft and impacts associated with other sustainability criteria.

Multiple MAs are presented to allow the GSAs flexibility in their response to changing groundwater conditions and as data gaps and uncertainties are addressed during GSP implementation. However, it is anticipated that not all MAs will need to be applied during the GSP implementation period. In addition, implementation and/or escalation of MAs will be based on ongoing monitoring of groundwater conditions using the monitoring network.

Monitoring data will be used to assess the need for MAs in the Subbasin as a whole and in specific areas. In general, the potential for undesirable results to be approached, exceedances of minimum thresholds, and poor Project performance will serve as triggers for scaling and implementing MAs in both a targeted and proportional manner, consistent with conditions observed in the Subbasin. The full scope of MAs including program descriptions, triggering criteria, GSA authorities, costs and funding, management of water sources, monitoring processes, and applicable areas will be developed by January 31, 2026, in accordance with the resolution.

Table 8-Table 8-1 lists the MAs described in the subsections that follow. Each MA description is organized to address the applicable regulatory requirements:
• Management Action Description: 23 CCR §354.44(b)
• Public Notice: 23 CCR §354.44(b)(1)(B)
• Permitting and Regulatory Process: 23 CCR §354.44(b)(3) • Expected Benefits: 23 CCR §354.44(b)(4), §354.44(b)(5) • Implementation Criteria, Status, and Plan: 23 CCR §354.44(b)(1)(A); §354.44(b)(4); §354.44(b)(6)
• Water Source and Reliability: 23 CCR §354.44(b)(6)
• Legal Authority: 23 CCR §354.44(b)(7)
• Estimated Costs and Funding Plan: 23 CCR §354.44(b)(8) • Management of Groundwater Extractions and Recharge: 23 CCR §354.44(b)(9) Summary of Criteria for Project Implementation (23 CCR §354.44(b)(1)(A)) MAs described in this section will be fully developed into MA-specific policies, resolutions, and/or implementation plans during the first years of GSP implementation as discussed in the subsequent sections. These MAs will be implemented by the GSAs, indicated by forthcoming triggering criteria, to achieve and maintain long-term sustainable groundwater management across the Subbasin. The GSAs will prioritize development of the Pumping Management Framework MAs. These MAs are based on authorities granted to the GSAs through SGMA as a means to establish groundwater extraction limitations and allocations, regulate the pumping of groundwater, and implement special taxes, assessments, and user fees. The Pumping Management Framework provides the GSAs with readily implementable methods to restrict groundwater extraction throughout the entire or portions of the Subbasin. This approach will be informed by continued monitoring of groundwater #25..
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conditions, using the monitoring network and methods that will be established in forthcoming MA-specific policies, resolutions, and/or implementation plans. MA’s and MAspecific policies will be developed with public participation and input from stakeholders within the Subbasin.

Table 8-1: List of Management Actions
Category Number Proponent2 Management
Action
Primary
Mechanism(s)1 Partner(s)
Pumping
Management
Framework
1 Modesto
Subbasin
GSAs
Groundwater
Allocation and
Pumping
Management
Program
Pumping
Reduction N/A
2 Modesto
Subbasin
GSAs
Groundwater
Extraction and
Surface Water
Reporting Program
Pumping
Reduction N/A
3 Modesto
Subbasin
GSAs
Groundwater
Extraction Fee
Pumping
Reduction N/A
4 Modesto
Subbasin
GSAs
Groundwater
Pumping Credit
Market and Trading
Program
Pumping
Reduction N/A
Demand
Reduction
Strategies
5 Modesto
Subbasin
GSAs
Voluntary
Conservation
and/or Land
Fallowing
Conservation/
Land Fallowing N/A
6 Modesto
Subbasin
GSAs
Conservation
Practices Conservation N/A
Dry Well
Mitigation 7
Modesto
Subbasin
GSAs
Dry Well Mitigation
Program (multiple) N/A
1The primary mechanism of the MA as conceptualized. MAs may support groundwater sustainability through multiple mechanisms during implementation.
2 It is anticipated that MAs will be implemented by the GSAs or by each GSA member agency as needed to mitigate overdraft within their jurisdictional areas and assure that the SMC adopted in Chapter Error! Reference source not found.6 are met.

8.1.1. Pumping Management Framework
The Pumping Management Framework consists of four tiered MAs that would be implemented in a prioritized order as determined by the GSAs. Not all MAs may be needed – Subbasin conditions will be evaluated against the sustainability management criteria when considering whether an additional tiered MA is needed. The tiered order of implementing Pumping Management Framework MAs is:
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1. Groundwater Allocation Program (MA 1) – see Section 8.1.1.1

2. Groundwater Extraction and Surface Water Accounting Reporting or Monitoring Program (MA 2) – see Section 8.1.1.18.1.1.2

3. Groundwater Extraction Fee (MA 3) – see Section 8.1.1.18.1.1.3

4. Groundwater Pumping Credit Market and Trading Program (MA 4) – see Section

8.1.1.38.1.1.4 8.1.1.1. Groundwater Allocation Program (Management Action 1)

8.1.1.1.1. Management Action Description
As previously discussed, the Subbasin has overdraft conditions. While the Projects identified in Sections 8.2 may provide the Subbasin with water necessary to achieve the sustainability goal, management actions will be necessary. As a result, GSAs will develop a Groundwater Allocation Program (Management Action) to allocate the sustainable yield of native groundwater in the Subbasin as a policy-driven approach to arrest groundwater level declines. The GSAs are currently in the process of evaluating and developing methods for the Management Action. In accordance with the resolution, management actions will be developed by January 31, 2026, and implemented by January 31, 2027.

Outlined here is a framework for how the Modesto Subbasin GSAs might develop and implement pumping allocations in the Subbasin based on the magnitude of projected overdraft estimated by Subbasin modeling.

There are four key steps to developing pumping allocations:

1. Identify the sources of water contributing to the native yield and estimate the quantity of native yield for the Subbasin annually (see Chapter 5 of this GSP)

2. Estimate the amount of native yield that can be used annually consistent with the Sustainable Yield

3. Allocate native yield to groundwater right holders based on:
a. Priority of right
b. Prescription
c. Other legal principles, such as reasonable use

4. Determine how to account for new/additional supplies.

5. Develop a timeline for reducing pumping to achieve allocations over time.

The Groundwater Allocation Program is currently conceptual and actively being evaluated and developed. There are numerous ways to structure and implement an allocation program which will need to be further evaluated, developed, and refined by the GSAs prior to implementation.
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8.1.1.1.2. Public Noticing
Development of a Groundwater Allocation Program requires substantial public input to understand the potential impacts of groundwater allocations and baseline needs that should be accounted for. The Modesto Subbasin GSAs anticipates that public outreach would include multiple public workshops and meetings, potential website and/or email announcements, along with other public notices for the workshops. The Groundwater Allocation Program would be circulated for public comment before finalized, though final approval of the plan would be made by the Modesto Subbasin GSAs in partnership with their respective member agencies. Implementation of the program may be confined to specific Management Areas.

8.1.1.1.3. Permitting and Regulatory Process
Development of a Groundwater Allocation Program would not require any permitting but would require consideration of existing water rights and applicable permits and regulations associated with groundwater pumping in the Subbasin.

8.1.1.1.4. Expected Benefits
Benefits to Sustainability Indicators
Sustainability indicators benefitting from the Groundwater Allocation Program include:
• Chronic lowering of groundwater levels – By reducing groundwater demand, this MA would reduce pumping and pumping-related contributions to chronic lowering of groundwater levels.
• Reduction of groundwater storage – Reduced pumping throughout the Subbasin contributes to a smaller rate of reduction in groundwater storage. • Degraded water quality – This MA does not address this sustainability indicator. • Land subsidence – Reduced groundwater pumping may reduce the risk of subsidence associated with lowering of groundwater levels. • Depletion of interconnected surface water – Reduced pumping would reduce the potential for negative impacts to surface water flows associated with lowering groundwater levels.

Benefits to Disadvantaged Communities
Benefits to disadvantaged communities overlap with the benefits described above for sustainability indicators.

Volumetric Benefits to Subbasin Groundwater System
The volumetric benefit to the groundwater system will depend on the structure of the allocation framework and will be further studied when the program is fully developed by the GSAs.

8.1.1.1.5. Implementation Criteria, Status, and Plan
The allocation program and its criteria for implementation are still under development. It is anticipated that the program will be implemented after groundwater conditions in the Subbasin do not improve as expected in conjunction with implementation of Group 1 and #25..
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Group 2 Projects. These conditions may include unstable groundwater levels, groundwater levels observed consistently nearing interim milestones, continued overdraft conditions, or increased amounts of pumping beyond the sustainable yield.

The program will be developed by January 31, 2026, and implemented by January 31, 2027, in accordance with the resolution. The intent is that groundwater users will have a year to adapt and adjust their pumping operations as necessary to meet the requirements of the program. This resolution was adopted by the STRGBA GSA and can be found in Appendix X.

The progress of this program will be presented in Annual Reports and is expected to be completed by the forthcoming periodic evaluation.

8.1.1.1.6. Water Source and Reliability
This program does not rely on the supplies from outside the Subbasin because it is a planning effort that will result in conservation. It will support overall supply reliability by reducing overdraft in the Subbasin and moving the Subbasin towards sustainability.

8.1.1.1.7. Legal Authority
Under SGMA, GSAs have authority to establish groundwater extraction allocations.

Specifically, SGMA authorizes GSAs to control groundwater by regulating, limiting, or suspending extractions from individual wells or extractions in the aggregate.1SGMA and GSPs adopted under SGMA cannot alter water rights.

8.1.1.1.8. Estimated Costs and Funding Plan
Development and initiation of an allocation program is expected to include upfront costs to conduct the analysis, set up the tracking system, and conduct outreach. Costs to implement the plan would depend on the level of enforcement required to achieve allocation targets and the level of outreach required annually to remind users of their allocation for a given year. The Groundwater Allocation Program would also include an annual cost that covers ongoing enforcement and implementation. Because the Groundwater Allocation Program is in the preliminary stages of development, no costs have been estimated. Sources of funding will be determined during the development of the program.

8.1.1.1.9. Management of Groundwater Extractions and Recharge The Groundwater Allocation Program would include provisions for the recovery of groundwater levels and groundwater storage during non-drought periods.

8.1.1.2 Groundwater Extraction and Surface Water Accounting Reporting or Monitoring Program (Management Action 2)

8.1.1.2.1 Management Action Description
As required in SGMA regulations, groundwater extractions have been calculated by the GSAs for this GSP using the CV2SIM-TM model (Appendix C). Presently, the GSAs intend to continue with their current data collection and groundwater extraction monitoring 1 California Water Code § 10726.4(a)(2)
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techniques. This MA is provided as an alternative to allow the GSAs flexibility and additional options in the event more or alternative forms of data are needed in the future.

There are several ways that this MA could be implemented by the GSAs. For this plan, two potential components have been developed which include a voluntary program and a comprehensive program. However, these two potential components are provided only as options, and likely would be implemented in Management Areas if the triggering criteria is met. If initiated, the GSAs will further develop options before implementation. • Voluntary program - This program is intended to provide an annual reporting of groundwater use by agricultural and other well owners and surface water transfers for in-lieu use. The Data Management System will be set up with appropriate input data forms for voluntary reporting of groundwater use as well as other relevant information, such as irrigated acreage, crop type, and sources of water. • Comprehensive program - This program is a more robust and elaborate strategy for reporting groundwater extraction that is intended to cover all groundwater users and surface water transfers for in-lieu use. Implementation of this program may incorporate satellite imagery to estimate the evapotranspiration of crops by parcel.

Additionally, this strategy can take the form of requiring the installation of meters at all agricultural and other non-exempt wells.

The Groundwater Extraction Reporting Program would exclude de minimis extractors (domestic use of 2 AF or less per year) but may also include surface water accounting in the Subbasin due to the amount of surface water transferred from MID and OID to the NDE area used for in-lieu and direct recharge.

8.1.1.2.2 Public Noticing
Successful implementation of either component of this program would require the support and coordination of member agencies, well owners throughout the Subbasin, and other stakeholders.

The voluntary program would be noticed via public outreach and education about the logistics of participating in the program as well as the purpose and importance of doing so.

Outreach may include public notices, meetings, potential website presence and email announcements.

The comprehensive program would involve more of a robust planning process. The Modesto Subbasin GSAs anticipate that public outreach and education on the potential structure of this program would be necessary, including public notices, meetings, potential website presence and email announcements.
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8.1.1.2.3 Permitting and Regulatory Process
The Groundwater Extraction Reporting Program is not expected to require any permitting or regulatory involvement.

8.1.1.2.4 Expected Benefits
Benefits to Sustainability Indicators
Direct measurement of groundwater extractions may not have direct impacts on sustainability indicators but would improve future water budget and sustainable yield refinement. The accurate and widespread collection of extraction data would provide the Modesto Subbasin GSAs with critical information to assist in management of the Subbasin, development of additional MAs, and monitoring the success of the GSP against the sustainable management criteria.

Benefits to Disadvantaged Communities
The Groundwater Extraction Reporting Program would exclude de minimis extractors, including those in disadvantaged communities.

Volumetric Benefits to Subbasin Groundwater System
Additional measurements and reporting of groundwater extractions would provide a higher resolution of groundwater use in the Subbasin. The addition of these data would provide the GSAs with the ability to further improve current and projected water budgets and basin storage calculations.

8.1.1.2.5 Implementation Criteria, Status, and Plan
The Modesto Subbasin GSAs will develop Annual Reports to evaluate progress toward meeting the sustainability goal. If monitoring efforts demonstrate that the Projects and MAs being implemented are not effective in achieving stated targets, the GSAs will convene a working group to evaluate the implementation of additional supply-side and demand-side actions, such as the implementation of tiered approaches of the Water Accounting Framework.

8.1.1.2.6 Water Source and Reliability
This management action is an accounting and monitoring program and as such does not rely on water availability. The Groundwater Extraction and Surface Water Accounting Reporting or Monitoring Program is a planning effort that will support overall supply reliability by providing additional information for better management of the Subbasin and moving the Subbasin towards sustainability.

8.1.1.2.7 Legal Authority
SGMA provides GSAs with the authority to regulate the pumping of groundwater in order to stabilize the region’s water supply and recharge aquifers. As such, the GSAs have the authority to: “control groundwater extractions by regulating, limiting, or suspending extractions from individual groundwater wells or extractions from groundwater wells in the aggregate, . . . or otherwise establishing groundwater extraction allocations” (CWC, §10726.4(a)).
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8.1.1.2.8 Estimated Costs and Funding Plan
The estimated costs for the Groundwater Extraction Reporting Program would vary depending on the components that are implemented:
• The costs for the voluntary component are minimal and include:
o One-time costs for initial public outreach and setup of tools and procedures to receive and compile voluntary submitted data
o Ongoing annual administrative costs to review and compile the voluntarily submitted data as well as continued outreach
• The costs for implementing the more comprehensive program would be larger as they may include:
o One-time costs to develop a remote sensing system or a more comprehensive program to track and monitor well meters, in addition to public outreach
o Ongoing annual costs to administer the program, whether via purchase and analysis of the latest remote sensing data or to track and collect data from well meters
The Groundwater Extraction Reporting Program is in the preliminary stages of discussion and possible consideration. Therefore, no costs have been estimated for its development and implementation. Such costs would be developed should the Modesto Subbasin GSAs decide to pursue a program in the future.

8.1.1.2.9 Management of Groundwater Extractions and Recharge This program would directly develop and expand the reporting of groundwater extractions, including during both dry and wet periods, to support better management of the Subbasin.

8.1.1.3 Groundwater Extraction Fee (Management Action 3)

8.1.1.3.1 Management Action Description
This strategy entails setting up a Groundwater Extraction Fee structure for each groundwater user. The fee structure could work in conjunction with the groundwater allocation and reporting programs, such that groundwater use above a certain allocation can be subject to a fee. This strategy could be implemented within areas of the Subbasin where triggering criteria has been met, as needed to achieve the sustainability goals.

Revenue from these fees could then be used to pay for a variety of activities, such as the construction of water infrastructure, protection of groundwater, proper construction and destruction of wells to prevent contamination, groundwater recharge and recovery projects, purchase of imported water or other supplies to replenish the groundwater basin, and/or purchasing and permanent fallowing of marginally productive agricultural lands dependent on groundwater. Fees could also be used to pay for administration, enforcement, and implementation of the MA.
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8.1.1.3.2 Public Noticing
Development of a Groundwater Extraction Fee would require substantial public input to understand the potential impacts and needs that should be considered. The Modesto Subbasin GSAs anticipate that public outreach would include multiple public workshops and meetings, potential website and/or email announcements, along with other public notices for the workshops. The Groundwater Extraction Fee framework would be circulated for public comment before being finalized, though final approval of the plan would be made by the Modesto Subbasin GSAs in partnership with their member agencies.

Additional noticing for the public would be conducted consistent with permitting requirements in the case of the enactment of fees. GSA outreach may include public notices, meetings, website or social media presence, and email announcements. Prior to implementing any fee or assessment program, the GSAs would complete a rate assessment study or other analysis if required by the regulatory requirements.

Per Water Code §10730, prior to imposing or increasing a fee, a groundwater sustainability agency shall hold at least one public meeting, at which oral or written presentations may be made as part of the meeting. Notice of the time and place of the meeting shall include a general explanation of the matter to be considered and a statement that the data required by this section is available. The notice shall be provided by publication pursuant to §6066 of the Government Code, by posting notice on the Internet Web site of the groundwater sustainability agency, and by mail to any interested party who files a written request with the agency for mailed notice of the meeting on new or increased fees. A written request for mailed notices shall be valid for one year from the date that the request is made and may be renewed by making a written request on or before April 1 of each year. At least 20 days prior to the meeting, the groundwater sustainability agency shall make available to the public data upon which the proposed fee is based. Any action by a groundwater sustainability agency to impose or increase a fee shall be taken only by ordinance or resolution.

8.1.1.3.3 Permitting and Regulatory Process
Fees imposed pursuant to Water Code §10730 shall be adopted in accordance with all applicable laws.

A fee or charge shall not be extended, imposed, or increased by any agency unless it meets all of the following requirements:
• Revenues derived from the fee or charge shall not exceed the funds required to provide the property related service.
• Revenues derived from the fee or charge shall not be used for any purpose other than that for which the fee or charge was imposed.
• The amount of a fee or charge imposed upon any parcel or person as an incident of property ownership shall not exceed the proportional cost of the service attributable to the parcel.
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• No fee or charge may be imposed for a service unless that service is actually used by, or immediately available to, the owner of the property in question. Fees or charges based on potential or future use of a service are not permitted. Standby charges, whether characterized as charges or assessments, shall be classified as assessments and shall not be imposed without compliance with Section 4 (Water Code §10730).
• No fee or charge may be imposed for general governmental services including, but not limited to, police, fire, ambulance or library services, where the service is available to the public at large in substantially the same manner as it is to property owners.

8.1.1.3.4 Expected Benefits
Benefits to Sustainability Indicators
Collection of groundwater extraction fees incentivizes the use of supplemental or alternative water supplies where fees can also fund activities/projects that increase groundwater supplies, such as groundwater recharge, thus reducing declines in groundwater elevations and groundwater storage. Other sustainability indicators benefitting from the Groundwater Extraction Fee program include:
• Degraded water quality – Funded activities and projects can also reduce degradation of groundwater quality (such as proper construction/destruction of wells to prevent contamination).
• Land subsidence – Reduced groundwater pumping would reduce the risk of subsidence associated with lowering of groundwater levels. • Depletion of interconnected surface water – Reduced pumping would reduce the potential for negative impacts to surface water flows associated with lowering groundwater levels.

Benefits to Disadvantaged Communities
Any fees would comply with CWC, §10730(a) and shall exclude de minimis extractors from fees, where appropriate.

Volumetric Benefits to Subbasin Groundwater System
The volumetric benefit to the groundwater system would depend on the framework of the fee implemented and would be further studied as the Groundwater Extraction Fee framework was developed by the GSAs.

8.1.1.3.5 Implementation Criteria, Status, and Plan
The Modesto Subbasin GSAs will develop Annual Reports to evaluate progress toward meeting the sustainability goal. If monitoring efforts demonstrate that the Projects and MAs being implemented are not effective in achieving stated targets, the GSAs will convene a working group to evaluate the implementation of additional supply-side and demand-side #25..
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actions, such as the implementation of tiered approaches in the Water Accounting Framework.

8.1.1.3.6 Water Source and Reliability
The Groundwater Extraction Fee program will apply in both drought and non-drought periods.

8.1.1.3.7 Legal Authority
The GSAs possess the legal authority to implement special taxes, assessments, and user fees within the Project proponent service area or area of Project benefit. Fees imposed include fixed fees and fees charged on a volumetric basis, including, but not limited to, fees that increase based on the quantity of groundwater produced annually, the year in which the production of groundwater commenced from a groundwater extraction facility, and impacts to the basin.

8.1.1.3.8 Estimated Costs and Funding Plan
While there are certain administrative costs anticipated with the development and implementation of a Groundwater Extraction Fee, the Groundwater Extraction Fee itself is a potential mechanism to fund the costs of groundwater management. This includes, but is not limited to, the following:
• Administration, operation, and maintenance, including a prudent reserve • Acquisition of lands or other property, facilities, and services • Supply, production, treatment, or distribution of water • Other activities necessary or convenient to implement the plan

8.1.1.3.9 Management of Groundwater Extractions and Recharge This program, in conjunction with the Groundwater Extraction Reporting Program, would directly develop and expand the reporting of groundwater extractions, including during both drought and non-drought periods, to support better management of the Subbasin.

8.1.1.4 Groundwater Pumping Credit Market and Trading Program (Management Action 4)

8.1.1.4.1 Management Action Description
Groundwater credit markets and trading programs can be used to exchange and trade the allocation of groundwater use by each landowner within the Subbasin. This strategy is contingent upon implementation of the groundwater reporting and allocation programs (MAs 1 and 2), so that the credit and trading market can monitor the exchange of groundwater allocations among the landowners and/or the GSAs. Should the Modesto Subbasin GSAs decide to pursue a program in the future, they would seek guidance from agencies with experience in water markets to identify options for communications and outreach with stakeholders, program design, and mechanisms to ensure that nonparticipating stakeholders are not adversely impacted by the program. #25..
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8.1.1.4.2 Public Noticing
Development and implementation of a Groundwater Pumping Credit Market and Trading Program would require substantial public input to understand the potential impacts and nuances of implementing such a program. The Modesto Subbasin GSAs anticipate that public outreach would include multiple public workshops and meetings, potential website and/or email announcements, along with other public notices for the workshops. The program plan would be circulated for public comment before being finalized, though final approval of the plan would be made by the Modesto Subbasin GSAs in partnership with their member agencies.

8.1.1.4.3 Permitting and Regulatory Process
Permitting and other regulatory compliance issues will be identified and addressed when the program is being further explored and developed, consistent with SGMA §10726.4 (a) (3 & 4).

8.1.1.4.4 Expected Benefits
Benefits to Sustainability Indicators
Sustainability indicators benefitting from the Groundwater Pumping Credit Market and Trading Program include:
• Chronic lowering of groundwater levels – By reducing groundwater demand, this MA would reduce pumping and pumping-related contributions to chronic lowering of groundwater levels.
• Reduction of groundwater storage – Reduced pumping throughout the Subbasin contributes to a smaller rate of reduction in groundwater storage. • Degraded water quality – This MA does not address this sustainability indicator. • Land subsidence – Reduced groundwater pumping would reduce the risk of subsidence associated with lowering of groundwater levels. • Depletion of interconnected surface water – Reduced pumping would reduce the potential for negative impacts to surface water flows associated with lowering groundwater levels.

Benefits to Disadvantaged Communities
Benefits to disadvantaged communities overlap with the benefits described above for sustainability indicators.

Volumetric Benefits to Subbasin Groundwater System
The volumetric benefit to the groundwater system will depend on the framework of the credit market and trading program implemented and would be further studied when the program was developed by the GSAs.
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8.1.1.4.5 Implementation Criteria, Status, and Plan
The Modesto Subbasin GSAs will develop Annual Reports to evaluate progress toward meeting the sustainability goal. If monitoring efforts demonstrate that the Projects and MAs being implemented are not effective in achieving stated targets, the GSAs will convene a working group to evaluate the implementation of additional supply-side and demand-side actions, such as the implementation of tiered approaches in the Pumping Management Framework.

8.1.1.4.6 Water Source and Reliability
The Subbasin area will be the source of groundwater and will be limited by the hydrology of the region.

8.1.1.4.7 Legal Authority
SGMA §10726.4 (a) (3 & 4) provide legal authority for groundwater transfer and accounting programs.

8.1.1.4.8 Estimated Costs and Funding Plan
The Groundwater Pumping Credit Market and Trading Program is in preliminary stages of discussion and possible consideration. Therefore, no costs have been estimated for its development and implementation. Such costs would be developed should the Modesto Subbasin GSAs decide to pursue a program in the future. Costs would likely include additional staffing required to administer the program and would be borne by the participants.

8.1.1.4.9 Management of Groundwater Extractions and Recharge The implementation of a Groundwater Pumping Credit Market and Trading Program will include provisions for the recovery of groundwater levels and groundwater storage during non-drought periods.

8.1.2. Demand Reduction Strategies
Demand reduction strategies will be developed to manage the agricultural and urban water demands in the Subbasin. These strategies could be implemented in the form of voluntary conservation and/or land fallowing (see Section 8.1.2.18.1.1.1) or other urban and agricultural conservation practices (see Section 8.1.2.28.1.1.2). While conservation practices are expected to be implemented throughout GSP implementation, specific strategies are in preliminary stages of discussion and possible consideration. Should the Modesto Subbasin GSAs decide to pursue a program in the future, the program would be implemented as necessary in a targeted and proportional manner consistent with conditions observed in the Subbasin. Similarly, the Conservation Practices MA is expected to be implemented adaptively.
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8.1.2.1. Voluntary Conservation and/or Land Fallowing (Management Action 5)

8.1.2.1.1. Management Action Description
Voluntary Conservation and/or Land Fallowing covers several strategies that can be designed to achieve both temporary and permanent water demand reduction. Should the Modesto Subbasin GSAs decide to pursue such strategies, this MA would assess options and develop a program to incentivize voluntary conservation and/or fallowing strategies in close coordination and collaboration with the landowners. Examples of this strategy could include repurposing of lands growing lower value crops. These lands could be dry farmed, fallowed in rotation, or used for recreation, habitat restoration, groundwater recharge, or solar power generation. This MA would also try to prioritize those lands that are more favorable for groundwater recharge projects.

Temporary or permanent land fallowing could also be combined with recharge projects through the application of surplus surface water supplies to the fallowed lands.

8.1.2.1.2. Public Noticing
A successful Voluntary Conservation and/or Land Fallowing program will require a comprehensive and strategic outreach effort, including multiple public workshops and meetings, potential website and/or email announcements, along with other public notices for the workshops. The outreach will be targeted to both potential participants of the program (landowners) as well as other stakeholders who may be impacted by changes to land and water use.

8.1.2.1.3. Permitting and Regulatory Process
Preparation of a CEQA evaluation for a fallowing program will identify potential environmental impacts and identify feasible alternatives or feasible mitigation measures.

Establishment of a voluntary land fallowing program is expressly authorized under SGMA (CWC, §10726.2(c)). The fallowing program, including program standards, will be developed and undergo CEQA review as necessary.

8.1.2.1.4. Expected Benefits
Benefits to Sustainability Indicators
Sustainability indicators that could benefit from Voluntary Conservation and/or Land Fallowing include:
• Chronic lowering of groundwater levels – By reducing groundwater demand, this MA would reduce pumping and pumping-related contributions to chronic lowering of groundwater levels.
• Reduction of groundwater storage – Reduced pumping throughout the Subbasin contributes to a smaller rate of reduction in groundwater storage. • Land subsidence – Depending on the location of land fallowing or conservation, reduced pumping stress on local aquifer(s) may reduce the potential for subsidence. #25..
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• Depletion of interconnected surface water – Reduced pumping would reduce the potential for negative impacts to surface water flows associated with lowering groundwater levels.

Benefits to Disadvantaged Communities
Benefits to disadvantaged communities overlap with the benefits described above for sustainability indicators. Land repurposing can also provide other ancillary benefits to local communities, such as recreation.

Volumetric Benefits to Subbasin Groundwater System
The volumetric benefit to the groundwater system would depend on the extent to which a Voluntary Conservation and/or Land Fallowing program is adopted and would be further studied when the program is implemented by the GSAs.

8.1.2.1.5. Implementation Criteria, Status, and Plan
Temporary fallowing is a quick way to reduce demand with no capital costs or infrastructure needed. Because it is inexpensive, it can be implemented earlier and quicker while other long-term solutions like land repurposing are investigated. The Modesto Subbasin GSAs may explore options for encouraging voluntary and temporary fallowing during GSP implementation while developing a more structured program and exploring funding opportunities.

The Voluntary Conservation and/or Land Fallowing program is in preliminary stages of discussion and consideration. Should the Modesto Subbasin GSAs decide to pursue a program in the future, the program would be implemented as necessary in a targeted and proportional manner consistent with conditions observed in the Subbasin. To maximize recharge potential, the preservation lands that are more favorable for recharge projects could be prioritized while developing this MA. The implementation timeline has yet to be determined but will be provided in GSP Annual Reports and five-year updates when known.

Any future changes in implementation would be communicated with the public and other agencies and would be documented in GSP Annual Reports and five-year updates.

8.1.2.1.6. Water Source and Reliability
This program does not rely on the supplies from outside the Subbasin because it is a planning effort that will result in conservation. It will support overall supply reliability by reducing overdraft in the Subbasin and moving the Subbasin towards sustainability.

8.1.2.1.7. Legal Authority
The GSAs have authority to “provide for a program of voluntary fallowing of agricultural lands or validate an existing program” (CWC, §10726.2(c)).

This MA carries forward the policy of the state and satisfies SGMA requirements by establishing a voluntary program that encourages water within the Subbasin to be dedicated to beneficial uses of water in a manner designed to achieve the sustainability goals and to protect against undesirable results.
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8.1.2.1.8. Estimated Costs and Funding Plan
The Voluntary Conservation and/or Land Fallowing program is in preliminary stages of discussion and possible consideration. Therefore, no costs have been estimated for its development and implementation. Such costs would be developed, should the Modesto Subbasin GSAs decide to pursue a program in the future. Separately, multiple funding programs exist as a potential source of revenue for individual landowners looking at options for land repurposing, including (EDF, 2021):
• Mitigation or Conservation Banks
• Conservation Easements
• Solar Rentals
• Grazing Leases
• Converting to Low Water Intensity Crops
• Federal and State Grant Funding Programs

8.1.2.1.9. Management of Groundwater Extractions and Recharge This MA encourages the conservation of water; this will be applicable during both drought and non-drought conditions.

8.1.2.2. Conservation Practices (Management Action 6)

8.1.2.2.1. Management Action Description
This MA would create a program to support the use of conservation practices in both urban and agricultural sectors.

Urban water suppliers are already obligated to consider demand reduction and conservation efforts during dry periods. These demand MAs are described in their respective Urban Water Management Plans (UWMPs). These include:
• City of Modesto Urban Water Management Plan (West Yost Associates, 2016b) o https://www.modestogov.com/860/Urban-Water-Management-Plan • Modesto Irrigation District Urban Water Management Plan (West Yost Associates, 2021)
o https://wuedata.water.ca.gov/public/uwmp_attachments/2173444449/R% 20-%20418%20-%20City%20of%20Modesto_MID%20-
%20Final%202020%20UWMP%20%20-%2006-23-21.pdf
• City of Riverbank Urban Water Management Plan (KSN Inc, 2016) o https://www.riverbank.org/610/Urban-Water-Management-Plan-WSCP • City of Oakdale Urban Water Management Plan (MCR Engineering, 2015) o https://cadwr.app.box.com/s/hg3k8bc9vuka689jkh1x4f9i1n58ey9a/file/521 558561581
• City of Waterford (covered under City of Modesto 2015 UWMP) #25..
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In addition, SB 606 and AB 1668, both signed into law in May 2018, are laws that introduce conservation mandates that will cap indoor residential use and set a target for efficient outdoor landscape irrigation based on local climate and size of landscaped areas. Urban water suppliers will be required to report on progress to meeting urban water use objectives beginning in 2023 and comply with them beginning in 2028.

In addition to meeting urban water use objectives, this MA could include changing standards for storm drainage so that storm flows do not discharge straight to a river, creek, or canal, as contemplated by the City of Modesto as a potential Group 3 Project. This would help increase the sustainability footprint of the City of Modesto as it grows. Currently approximately 36% of the City of Modesto area drains to a river or canal, while approximately 64% is captured for local recharge. If the City of Modesto adopts new Storm Drain Standards, 100% of runoff from newly developed areas would reach a retention system and contribute to recharge.

Agricultural water suppliers serving more than 25,000 irrigated acres must adopt an Agricultural Water Management Plan (AWMP) that include reports on the implementation status of specific Efficient Water Management Practices required by the Water Conservation Act of 2009 (SB X7-7). Agencies that have developed AWMPs include:
• Modesto Irrigation District Agricultural Water Management Plan o https://www.mid.org/water/awmp/default.html
• Oakdale Irrigation District Agricultural Water Management Plan o https://wuedata.water.ca.gov/public/awmp_attachments/3350354850/OID %202020%20AWMP%20FINAL%20210323.pdf
The Modesto Subbasin GSAs may choose to evaluate the existing UWMPs and AWMPs in the Subbasin and either expand upon minimum requirements to increase the impact of such programs or implement similar conservation practice programs in other areas of the Subbasin that may not be covered under an UWMP or AWMP.

Notably, conservation practices must be considered in the greater context of the Subbasin water budget, especially at the nexus between on-farm water use and groundwater sustainability. In areas where groundwater is the primary or sole water supply, conservation practices that reduce water demand may also reduce groundwater consumption, but conservation practices may also have unintended consequences that impede water conservation and sustainable groundwater management. Some of these consequences directly result from irrigation efficiency improvements: applying less water to an area and reducing the gap between irrigation and consumptive use also reduces deep percolation and seepage to the groundwater system. Other consequences may stem from behavioral responses and changes in irrigation resulting from these technologies and policies. If less water can be used to produce the same amount of a crop product, growers may be inclined to use the same amount of water and produce more (Lankford, et al., 2020). Additional considerations on the promises, pitfalls, and paradoxes of irrigation efficiency in water management planning are described by Lankford et al. (2020). #25..
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Further details on any expansion of the Conservation Practices program are preliminary as of the time of publishing and would need to be developed and refined further during GSP implementation.

8.1.2.2.2. Public Noticing
The Modesto Subbasin GSAs anticipates that public outreach and education on the potential structure of the Conservation Practices program, as well as feasible monitoring and enforcement mechanisms, would be necessary to enable a successful program. Outreach may include public notices, meetings, potential website presence and email announcements. Initial program implementation would likely focus on voluntary compliance while the GSAs or GSAs member agencies consider the necessary elements to begin enforcing the program potentially by 2027 (five years after adopting and submitting the GSP). This date is contingent upon monitoring results and achievement of Interim Milestones.

8.1.2.2.3. Permitting and Regulatory Process
Development of a Conservation Practices program is not a Project as defined by the California Environmental Quality Act (CEQA) and National Environmental Policy Act (NEPA) and would therefore not trigger either.

8.1.2.2.4. Expected Benefits
Benefits to Sustainability Indicators
Sustainability indicators benefitting from Conservation Practices include:
• Chronic lowering of groundwater levels – By reducing groundwater demand, this MA would reduce pumping and pumping-related contributions to chronic lowering of groundwater levels.
• Reduction of groundwater storage – Reduced pumping throughout the Subbasin contributes to a smaller rate of reduction in groundwater storage. • Degraded water quality – This MA does not address this sustainability indicator. • Land subsidence – Depending on the location of Conservation Practices, reduced pumping stress on local aquifer(s) may reduce the potential for subsidence. • Depletion of interconnected surface water – Reduced pumping would reduce the potential for negative impacts to surface water flows associated with lowering groundwater levels.

Benefits to Disadvantaged Communities
Benefits to disadvantaged communities overlap with the benefits described above for sustainability indicators. Depending on how they are structured, urban conservation programs may also provide a financial benefit to individual users who reduce their water consumption, either via a lower water bill or reduced demand on a domestic well. #25..
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Volumetric Benefits to Subbasin Groundwater System
The volumetric benefit to the groundwater system will depend on the extent to which a Conservation Practices program is implemented and will be further studied if a program is developed by the GSAs.

8.1.2.2.5. Implementation Criteria, Status, and Plan
The implementation timeline has yet to be determined but will be provided in GSP Annual Reports and five-year updates when known. Any future changes in implementation would be communicated with the public and other agencies and would be documented in GSP Annual Reports and five-year updates.

8.1.2.2.6. Water Source and Reliability
This MA does not rely on the supplies from outside the Subbasin because it is a planning effort that will result in conservation benefits. It will support overall supply reliability by reducing groundwater demand in the Subbasin and moving the Subbasin towards sustainability.

8.1.2.2.7. Legal Authority
The Modesto Subbasin GSAs have the authority to develop a Conservation Practices program and may perform implementation and enforcement of practices via implementation of fees for noncompliance or through metering or other methods to quantify groundwater use. Mechanisms for enforcement would be outlined in the Conservation Practices program once developed and are expected to be enforced by the Modesto Subbasin GSAs and/or member agencies.

8.1.2.2.8. Estimated Costs and Funding Plan
Costs for UWMP and AWMP report preparation and submittals are ongoing for urban and agricultural water suppliers, respectively. Any future costs related to additional programming or program enforcement have yet to be developed.

8.1.2.2.9. Management of Groundwater Extractions and Recharge This MA encourages the conservation of water; this will be applicable during both wet and dry conditions.

8.1.3. Dry Well Mitigation (Management Action 7)
This MA will develop and implement a well mitigation program to address and mitigate impacts from groundwater level declines that may occur when water levels drop below the MTs.

Management Action Description
This Dry Well Mitigation Program provides mitigation measures for water supply wells that have experienced adverse impacts due to declining groundwater levels, as described in Section 6.3.3.1. It will cover eligible mitigation claims accrued after January 31, 2022, the date the original GSP was adopted. This program will specify mitigation measures, organization of the program, estimated costs and means of funding. As stated in the #25..
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Resolution, this program will be developed and implemented no later than January 31, 2026, and will continue into perpetuity unless otherwise directed by the STRGBA GSA.

Dry Well Mitigation Program Measures
This Dry Well Mitigation Program will describe potential short-term and long-term measures to mitigate impacts to domestic wells. Mitigation measures may include, but are not limited to:
• Short-term emergency solutions, such as delivery of bottled water and/or water tanks. (Considered only for temporary mitigation while other actions are in progress.)
• Setting well pump at deeper depths, replacement of well pump, well rehabilitation or replacement of wells (including abandonment of existing wells). • Connection to a public water system.

Long-term management actions and projects may include, but are not limited to:
• Reduction of groundwater demand around communities reliant on groundwater for drinking water, e.g., create buffer zones for drinking water users. • Support for managed aquifer recharge near affected communities.

Development of the Dry Well Mitigation Program
The Dry Well Mitigation Program will be developed with potential elements including:
• One or more committees to develop and implement the program on behalf of the

STRGBA GSA,
• A fund to support dry well mitigation and implementation of the program, • Public outreach to publicize this program,
• Definition of eligibility criteria to guide well owners in considering a claims application for mitigation, such as well failure or diminished well yield due to groundwater levels declining below MTs,
• Definition of an application process, including application submittal, review and investigation of an application, decision-making, reporting, and agreements for approved applications.

8.1.3.1. Public Notice
Public outreach and notice will be included in the Dry Well Mitigation Program. In addition, it is anticipated that the program plan will be circulated for public comment prior to being finalized, although final approval of the plan will be made by STRGBA GSA.

8.1.1.1.8.1.3.2. Permitting and Regulatory Process
Permitting and other regulatory compliance issues will be identified as the program is developed, consistent with CWC §10726.4 (a) (3 & 4).
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8.1.3.3. Expected Benefits
Benefits to Sustainability Indicators
This Management Action provides a program for direct mitigation of impacts to domestic wells during early years of GSP implementation.

Benefits to Disadvantaged Communities
The Dry Well Mitigation Program provides significant direct benefits to disadvantaged communities who rely on groundwater and supply wells and additional potential benefits for other sustainability indicators (see analyses in Section 6.3.3.2).

Volumetric Benefits to Subbasin Groundwater System
The Dry Well Mitigation Program provides benefits to users of the groundwater basin storage who rely on reliability of groundwater and supply wells.

8.1.3.4. Implementation Criteria, Status, and Plan
These components are described in Section Error! Reference source not found.8.1.3.1. The Dry Well Mitigation Program will be developed and implemented no later than January 31, 2026, and will continue into perpetuity unless otherwise directed by the STRGBA GSA.

8.1.1.2.8.1.3.5. Water Source and Reliability
This program provides mitigation measures for domestic water supply wells that have experienced adverse impacts due to declining groundwater levels occurring after January 31, 2022, the date of adoption of the Joint GSP. It supports reliable access to groundwater in response to eligible claims at the discretion of the STRGBA GSA.

8.1.3.6. Legal Authority
Legal authority for implementation of this action is provided by STRGBA GSA Resolution No. 2024-XX.

8.1.3.7. Estimated Costs and Funding Plan
As stated in the 2024 Resolution, the GSA is establishing baseline funding amounting to $300,000 no later than January 31, 2026.

8.1.3.8. Management of Groundwater Extractions and Recharge This program involves mitigation for well failures or diminished well yields of existing domestic water supply wells. It is not intended to provide a net increase beyond original well yield. Accordingly, no long-term net increase in groundwater extractions is planned as part of this program. Long-term management actions and projects associated with this program may include support for managed aquifer recharge or in lieu recharge near impacted wells or areas vulnerable to wells going dry.

8.2. PROJECTS OVERVIEW
This section describes the Projects that are in place, planned, or may be considered for implementation in the Modesto Subbasin. In accordance with 23 CCR §354.44, Projects #25..
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were developed to help achieve and maintain the Subbasin sustainability goal by 2042 and avoid undesirable results over the GSP planning and implementation horizon. Broadly, Projects provide tools that can be used to achieve and maintain groundwater sustainability.

Projects were developed, where possible, to be aligned with State grant program preferences and the Governor’s Water Action Plan, by providing multiple. Projects, where possible, were designed to provide benefits, embracing innovation and new technologies, and benefitting to surface water users, groundwater users, and disadvantaged communities (DACs) and environmental water users. This Planembrace innovation and new technologies.

The GSP prioritizes Projects that contain multi-benefit approaches thatto address multiple needs and stressexpand the utilization of natural infrastructure, including the Subbasin itself for storage and the natural waterways and floodplains as recharge areas. An emphasis is also placed on Projects that are located in targeted areas to achieve maximum recharge results and address water level decline. are a point of emphasis for the Subbasin to achieve its sustainability goal. Additionally, the Plan stressesPMAs prioritize coordination among users, STRBGA GSA member agencies, and neighboring basins to improve the region's groundwater condition and achieveconditions while achieving sustainability.

Projects were identified in the Modesto Subbasin through a several-month process involving the STRGBA GSA Technical Advisory Committee. Project information was provided by the STRGBA GSA and compiled into a draft list. The initial set of projects was reviewed further, and a final list of 13 possible projects was identified for inclusion in the GSP, representing a variety of. The project types includingpresented in the GSP a include direct and in-lieu recharge, water recycling, and advancementsimprovements to metering infrastructure.

Projects are classified into three groupscategories based on project status: Group 1, Group 2, and Group 3, as defined below.
• Group 1 – Projects that are in place and will continue to be implemented and expanded upon by specific participating agencies within the Modesto Subbasin to support groundwater management and GSP implementation.
• Group 2 – Projects that are, generally, readily implementable but may still be in the planning stages of development and may be pursued by specific participating agencies within phase but are anticipated to be implemented shortly after adoption of the Modesto Subbasin which will GSP. Group 2 Projects are expected to greatly contribute to attainment of Sustainable Management Criteria (SMC) and will support GSP implementationachieving the Subbasin’s sustainability goal and continue supporting GSP implementation efforts. Project statuses and implementation schedules are presented in Sections 8.2.2 and 8.2.3 of the GSP. • Group 3Supplemental Projects – Projects which have been identified for consideration in the Modesto Subbasin in thefor future subject to GSA activities.

Supplemental Projects are not currently planned for implementation; however, the GSAs will continue assessing their feasibility. These projects to support local goals.

Should these Projects be implemented, they would providesupport Group 1 and #25..
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Group 2 Projects’ benefits in contributing to the attainment of the sustainability goal and Sustainable Management Criteria (SMCs) and would otherwise support GSP implementationthe sustainability goal.

Group 1 and Group 2 Projects are summarized in Section 8.38.23: Projects Developed for Implementation. These Projects were analyzed as part of scenarios using the C2VSimTM model to estimate their benefit to the groundwater system over the projected planning period. The results of the model scenarios are discussed in Section 8.5: Plan for Achieving Sustainability.

Group 3Supplemental Projects are summarized in Section 8.3: Conceptual8.4:

Supplemental Projects to be Implemented as Needed. Group 3. These Projects are currently not evaluated in detail, and are described at a more general level, reflecting their conceptual nature and planning status at this time.. Additional feasibility studies and details for these Projects willmay be developed in the future, as needed. and their progress will be reported in Annual Reports and Periodic Evaluations should they be implemented.

The proposed Projects identified in this chaptersection will be either directly funded and implemented by the Project Proponent or will be subject to grant funding requests through state and federal funding opportunities. Project proponents are listed in Table 8-Table 8-12.

Each individual Project proponent will manage the permitting and other specificoversee implementation oversight for itstheir own Projects. Inclusion of Projects in this GSP does not forego any obligations regarding individual Project implementation under local, state, or federal regulatory programs. While the GSAs do not have an obligation to oversee progress towards groundwater sustainability, they are not the primary regulator of land use, water quality, or environmental Project compliance. It is the responsibility of the Projectimplementing agencies of planned Projects to ensure that they are collaborating with outside trusteetrustees and responsible regulatory agencies to ensure theirthe Projects are in compliance with all applicable laws and permitting requirements.

The GSAs will collaborate with Project proponents and partners to track progress and support Project implementation. The implementation of PMAs will be enhanced by the development of clear policypolicies and guidance by the GSAs that lays outconsider applicable sustainable management criteria (as described in Chapter 6: Sustainable Management Criteria) as well asSMCs and establish PMA-specific monitoring and reporting frameworks to facilitate adaptive management toward Subbasin protection and sustainability. The. GSP implementation will include guidelines and protocols to coordinate implementation of Projects in such a way that the Subbasin sustainability is achieved in athrough coordinated environment inefforts between the GSAs, with the Project proponents and sponsors, and other stakeholders.

Table 8-Table 8-1 shows the Projects withwithin their respective groups. This represents an initial list of Projects that will be further refined as; additional Projects are identifiedmay be added during GSP implementation, with updates included in Annual Reports and the GSP #25..
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updates, as appropriate. A descriptionPeriodic Evaluations. Detailed descriptions of each Project in more detail is are provided in Sections 8.28.3 [Projects Developed for Near-Term Implementation (Groups 1 and 2)Projects Developed for Near-Term Implementation (Groups 1 and 2)] and Section 8.3 [Other8.4 [Supplemental Projects to be Implemented as Needed].
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Table 8-1:-2: List of Projects
Number Proponent(s) Project Name Primary Mechanism(s)1 Partner(s) Group Included in
Modeling
Scenario
Urban Projects
1 City of
Modesto
Growth Realization of Surface Water
Treatment Plant Phase II
In-lieuLieu Groundwater
Recharge N/A 1 Baseline
2 City of
Modesto
Advanced Metering Infrastructure Project
(AMI) Conservation N/A 1 ×
3 City of
Modesto
Storm Drain Cross Connection Removal
Project Stormwater Capture N/A 2 ×
4 City of
Waterford
Project 3: Waterford/Hickman Surface
Water Pump Station and Storage Tank
In-lieuLieu Groundwater
Recharge
City of
Modesto, MID 2 ×
In-Lieu & Direct Recharge Projects
5 Non-District
East Areas
Modesto Irrigation District In-lieu and
Direct Recharge Project
Direct orand In-lieuLieu
Groundwater Recharge Modesto ID 2 ×
6 NDE Areas Oakdale Irrigation District In-lieu and
Direct Recharge Project
Direct orand In-lieuLieu
Groundwater Recharge OID 2 ×
Flood Mitigation Projects
7 NDE Areas Tuolumne River Flood Mitigation and
Direct Recharge Project
Direct Groundwater
Recharge Modesto ID 2 ×
8 NDE Areas Dry Creek Flood Mitigation and Direct
Recharge Project
Direct Groundwater
Recharge
Stanislaus
County 2 ×
Potential FutureSupplemental Projects
9 NDE Areas Stanislaus River Flood Mitigation and
Direct Recharge Project
Direct Groundwater
Recharge
Stanislaus
County
3 10 City of
Modesto
Detention Basin Standards Specifications
Update Groundwater Recharge N/A 3
11 NDE Areas Recharge Ponds Groundwater Recharge N/A 3
12 City of Oakdale OID Irrigation and Recharge to Benefit City of Oakdale
Direct or In-lieuLieu
Groundwater Recharge N/A 3
13 MID MID FloodMARFlood-MAR Projects
Direct Groundwater
Recharge N/A 3
These Projects are considered as potential projects to support the GSP implementation. They are currently considered as alternative options and are not directly analyzed in this Chapter.
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8.3. PROJECTS DEVELOPED FOR NEAR-TERM IMPLEMENTATION (GROUPS 1 AND 2) This section describes the Projects that were developed for near-term implementation in the Modesto Subbasin, organized and are categorized by proponent. This includes all Group 1 and 2 Projects identified in Table 8-Table 8-1. These Projects are either:
• Currently in place and will continue to be implemented by specific participating agencies, or are with future expansions planned, or
• Currently planned and will be implemented or started by specific participating agencies in the next five years.

The Projects developed for near-term implementation were modeled in the C2VSimTM to estimate their potential benefit to the groundwater system over the projected future water budget period. Applicable assumptions used to model each Project are described in each Project description. The results of these model scenarios are discussed in Section 8.5: 8.5:

Plan for Achieving SustainabilityPlan for Achieving Sustainability.. C2VSimTM modeling results of Group 1 and Group 2 Projects indicate that Projects developed for near-term implementation are expected to be sufficient in the Subbasin for reaching its sustainability goal. However, the GSAs understand that assumptions used in modeling may differ from actual conditions. As a result, the GSAs have begun developing Management Actions that will be implemented to arrest groundwater level declines by 2027 and raise groundwater levels after 2027. These Management Actions currently under development are presented in Section 8.1: Management Actions.

As described above, the Group 1 and Group 2 Projects presented in this section are either currently in place or are planned to be initiated within five years. Projects that are currently in place will continue to be implemented over the 2042 Plan horizon.

Table 8-As described above, the Group 1 and Group 2 PMAs described in this section are either currently in place or are planned to be initiated within 5 years. Those PMAs that are currently in place will continue to be implemented over the 2042 Plan horizon.

Table 8-23 lists all Group 1 and Group 2 PMAsProjects described in the subsections that follow. Each Project description is organized to address the applicable regulatory requirements:
• Project Description: 23 CCR §354.44(b)
• Public Noticing: 23 CCR §354.44(b)(1)(B)
• Permitting and Regulatory Process: 23 CCR §354.44(b)(3) • Expected Benefits: 23 CCR §354.44(b)(4), §354.44(b)(5) • Implementation Criteria, Status, and Plan: 23 CCR §354.44(b)(1)(A); §354.44(b)(4); §354.44(b)(6)
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• Water Source and Reliability: 23 CCR §354.44(b)(6)
• Legal Authority: 23 CCR §354.44(b)(7)
• Estimated Costs and Funding Plan: 23 CCR §354.44(b)(8) • Management of Groundwater Extractions and Recharge: 23 CCR §354.44(b)(9) Summary of Criteria for Project Implementation (23 CCR §354.44(b)(1)(A)) As described above, the Group 1 and Group 2 PMAs describedProjects presented in this section are either currently in place or are planned to be initiated within 5five years. Those PMAsProjects that are currently in place will continue to be implemented over the 2042 Plan horizon.

Table 8-2: List of-3: Projects Developed for Near-Term Implementation in the Modesto Subbasin
Location (Proponent) Project Name Primary Mechanism(s)1 City of Modesto
Project 1: Growth Realization of
Surface Water Treatment Plant
Phase II
In-Lieu Recharge
Project 2: Advanced Metering
Infrastructure Project (AMI) Water Conservation
Project 3: Storm Drain Cross
Connection Removal Project Stormwater Capture
City of Waterford Project 4: Waterford/Hickman Surface
Water Pump Station and Storage Tank Water Conservation
NDE Areas
Project 5: Modesto Irrigation District
In-lieu and Direct Recharge Project
In-lieu and Direct Recharge
Project
Project 6: Oakdale Irrigation District Inlieu and Direct Recharge ProjectIn-lieu and Direct Recharge Project
Project 7: Tuolumne River Flood
Mitigation and Direct Recharge Project
Flood control and Direct
Recharge Project
Project 8: Dry Creek Flood Mitigation
and Direct Recharge Project
Flood control and Direct
Recharge Project
1 The primary mechanism of the Project as conceptualized, although during implementation Projects may be used for multiple functions to support groundwater sustainability and multiple other benefits.
1The primary mechanism of the Project as conceptualized, although during implementation Projects may be used for multiple functions to support groundwater sustainability and beneficial users. #25..
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8.3.1. Urban and Municipal Projects
PMAsProjects developed for implementation by urban and municipal proponents in the Modesto Subbasin are summarized in the sections below.

8.3.1.1. Growth Realization of Surface Water Treatment Plant Phase II (Project 1)

8.3.1.1.1. Project Description
This projectThe Growth Realization of Surface Water Treatment Plant Phase II project (Project) continues the water purchase agreement between Modesto Irrigation District (MID) and the City of Modesto to meet urban demands. It utilizes the expansion from Phase II of the Modesto Regional Water Treatment Plant (MRWTP).

The Modesto Irrigation District operates the MRWTP to treat surface water for use within the City of Modesto and has been expanding its capacity to meet growing and future water demands from its customers. The Initial Phase (first phase) of the MRWTP Project included the construction of a 30 million-gallon per day (mgd) surface water treatment plant, two 5- million-gallon (MG) terminal storage tanks and associated pumping facility. The pump station delivered water into the MID transmission system for distribution into either the Del Este or City of Modesto water distribution systems through several MID turnouts. The City of Modesto now owns the Del Este water system. Figure 1-1 shows the existing transmission mains and turnouts constructed as part of the Phase One MRWTP Project The Expansion Phase of the MRWTP Project (second phasePhase II) included the construction of a new parallel treatment process consisting of low-pressure membranes, ozone disinfection system, a dissolved air flotation thickener and a new Supervisory Control and Data Acquisition (SCADA) system. The total capacity available at the MRWTP with the completion of the MRWTP Phase TwoII Expansion Project is 60 MGDmgd with a maximum annual supply of up to 67,200 AFY.

The City of Modesto currently operates its treatment and conveyance systems at capacity and has not been able to utilize any additional surface water supply. However, recently the City of Modesto has taken several proactive steps to increase its infrastructural optimization, particularly its water utilization and storage. Some of these steps include: (1) the submittal of a conceptual grant application to modify up to four recharge basins to dilute aquifer contaminants, increase aquifer storage, and improve water quality, and (2) hiring an outside consultant to study system optimization and (3) investigate the feasibility of integrating additional surface water supply for recharge in wet years.

8.3.1.1.2. Public Noticing
The public and other agencies will be notified of the planned or ongoing implementation of PMAProject activities through the outreach and communication channels identified in the GSP and during updates presented at regularly scheduled STRGBA GSA meetings. Noticing will occur as potential activities are being considered for implementation, and as ongoing #25..
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and planned activities are implemented. Noticing will inform the public and other agencies what that the proponent is considering or will be implementingrelated to the PMAProject and will provide a description of the actions that will be taken.

Public and/or inter-agency noticing may be facilitated through the STRGBA GSA’s boardGSA meetings and/or City and Agencylocal agency meetings, associated website(s), inter-basin coordination meetings, other public meetings hosted by the GSAs, GSP annual reportsAnnual Reports and five-year updatesPeriodic Evaluations, public scoping meetings, and/or environmental/regulatoryapplicable permitting notification processes.

8.3.1.1.3. Permitting and Regulatory Process
This Project includes the continued transfer of water purchased between MID and the City of Modesto, and therefore, permitting, and regulatory requirements have already been completed. Future permitting and regulatory processes, if needed to continue or expand Project activities, will be managed through MID and the City of Modesto.

8.3.1.1.4. Expected Benefits
Benefits to Sustainability Indicators
Utilization of purchased water for urban water demands is expected to offset groundwater pumping demands, with through in-lieu groundwater recharge benefits to the Subbasin. The sustainability indicators expected to benefit from this Project are groundwater levels, groundwater in storage, land subsidence, and interconnected surface water, and possibly land subsidence. All. Project benefits to sustainability indicators in the Modesto Subbasin will be evaluated through monitoring groundwater monitoring at nearbylevels within the GSP's representative monitoring sites, identified in the GSPnetwork.

Benefits to Disadvantaged Communities
Water supplied through this Project directly benefits areas within the City of Modesto’s contiguous water service areas within the Modesto Subbasin, most of which isare classified as a DACDACs. By supplementing and diversifying their drinking water supply, this Project will provide an alternate drinking water source and operational flexibility to remove or blend production wells with treated surface water to comply with safe drinking water regulations and meeting Maximum Contaminant Levels (MCLs).MCLs. The additional surface water supply will also reduce groundwater pumping and increase groundwater levels near the communities which can reduce pumping costs and potentially mitigate some groundwater quality concerns. Additionally, benefits to groundwater conditions in the Modesto Subbasin are also expected to broadly benefit all local DACs, SDACs (Severely Disadvantaged Communities), and EDAs (Economically Distressed Areas) in the Modesto Subbasin.).

Volumetric Benefits to the Subbasin Groundwater System
The expected yield of the benefits from the Growth Realization of Surface Water Treatment Plan Phase II Project was estimated by simulating this Projectsimulations performed in the #25..
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C2VSimTM model. General information and assumptions used to simulate this Project are summarized in the Implementation section below. Additional information is provided in Section 8.5: Plan for Achieving SustainabilityPlan for Achieving Sustainability.

This Project has provided an estimated additional 10 mgd (11,200 AFY) starting in 2016 and continuing at 10 mgd through 2020, and then is anticipated to gradually increase to an additional 30 mgd (33,600 AFY) by 2050.

Evaluation of benefits will be based on analysis of without-Project and with-Project measurements supported by modeling. Measured parameters will include surface water deliveries, groundwater levels, and other parameters to be determined. Modeling will be done with the C2VSimTM model used forin developing the GSP development.

8.3.1.1.5. Implementation Criteria, Status, and Strategy Implementation Strategy and Timeline
This Project is being implemented by the City of Modesto and MID and is expected to provideprovides 10 mgd initially and will eventually increase to 30 mgd. This Project includes the expansion of current water transfers between MID and the City of Modesto. Updates to the status and continuation of this agreement and Project will be provided in GSP Annual Reports and Five-Year GSP updatesPeriodic Evaluations.

Implementation Assumptions for Modeling
Impacts to the Subbasin from the Growth Realization of Surface Water Treatment Plan Phase II Project were already captured in the Projected Conditions Baseline and thus no additional changes were needed to simulate this Project in the PMAmodeling scenarios.

Baseline conditions include both the expansion of the City of Modesto’s footprint and the resulting increase of surface water available for urban use.

8.3.1.1.6. Water Source and Reliability
This Project would use water from MID to supplement water for the City of Modesto for urban demands. ThisThe Project has provided an estimated additional 10 mgd (11,200 AFY) starting in 2016 and continuing at 10 mgd through 2020, and then is anticipated to gradually increase to an additional 30 mgd (33,600 AFY) by 2050. These assumptions are included in the model development. The exact volumeVolumetric groundwater benefits will be reported in Annual Reports and GSP Five-Year Update Reports when knownPeriodic Evaluations.

8.3.1.1.7. Legal Authority
The GSAs, Districts, and individual Project proponents have the authority to plan and implement Projects through consultation with applicable governing agencies. MID has the authority to construct and continue to operate its water treatment plant and to continue to transfer water to the City of Modesto.
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8.3.1.1.8. Estimated Costs and Funding Plan
The Growth Realization of Surface Water Treatment Plant Phase IIThe Project is a continuation of water transfers from MID to the City of Modesto. Because an agreement and water transfers have already commenced, the estimated costs of this Project are low and include agreement/coordination costs and yearly costs. Infrastructure for this Project has already been constructed and thereforeadditional infrastructure is not needed.

Continuedrequired. The ongoing capital cost for this Project is $4.1M annually, which will increase to $8.3M in FY 2024 when payment towards the principal balance begins. The City of Modesto has been utilizing the Water Fund as a funding sourcessource to cover Project costs as part of Project development and continuation. Other funding sources may be identified in the future including, such as grants (e.g., Prop 1, Prop 68m, NRCS), fees, local cost share, and loans, and other assessmentsmay be pursued in the future if needed.

8.3.1.1.9. Management of Groundwater Extractions and Recharge Per 23 CCR § 354.44(b)(9), all PMAs developed for implementation are targeted to maintain the balance of groundwater extractions and recharge to help ensure that lowering of groundwater levels or depletion of supply during periods of drought is offset by increases in groundwater levels and storage in other years.

In-lieu recharge benefits of this Project are expected to increase the use and recharge of available surface water supplies during wetter years, helping to offset potential increases in groundwater pumping during drought when surface water supplies are limited.

8.3.1.2. Advanced Metering Infrastructure Project (AMI) (Project 2) The Advanced Metering Infrastructure (AMI) Project (Project) involves the installation of AMI throughout the City of Modesto. The City of Modesto is in the initial stages of on installing AMI smart meters to support water reduction goals. Smart meters will assist the City of Modesto in notifying residents of leaking pipes and helping to reduce overall domestic water consumption through improved and direct consumer data.

8.3.1.2.1. Project Description
The City of Modesto is planning onin progress of upgrading 75,000 meters to AMI smart meters to support water reduction goals. Smart meters will assist the City of Modesto in providing analytical tools to manage water usage better such as identifying potential leaks sooner and providing customers with more usable and user -friendly data to manage their water usage.

8.3.1.2.2. Public Noticing
Public and/or inter-agency noticing willmay be facilitated through GSAs, City Council or District BoardSTRGBA GSA meetings, GSAs and/or districtlocal agency meetings, associated website(s), GSAs and/or district newsletters, inter-basin coordination meetings, GSP other public meetings hosted by the GSAs, Annual Reports and Five-Year Assessment #25..
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ReportsPeriodic Evaluations, public scoping meetings, and environmental/regulatory/or applicable permitting notification processes.

8.3.1.2.3. Permitting and Regulatory Process
Required permitting and regulatory review will be Project-specific and initiated through consultation with applicable governing agencies. Governing agencies for which consultation will be initiated may include, but isare not limited to: DWR, SWRCB, CDFW, Flood Board, Regional Water Boards, USFWS, NMFS, LAFCO, County of Stanislaus, and CARB.

8.3.1.2.4. Expected Benefits
Benefits to Sustainability Indicators
The sustainability indicators expected to benefit from the Project are groundwater levels, groundwater in storage, land subsidence, and depletion of interconnected surface water.

Benefits to Disadvantaged Communities
This Project would apply to and benefit all water customers served by the City of Modesto, most of which are considered a DAC or SDAC.

Volumetric Benefits to the Subbasin Groundwater System
ThisThe Project is currently in development and AMI is actively being installed throughout the early conceptual stage. ThusProject area. As a result, the expected yield of this Project has yet to benot been determined and . Volumetric benefits will be reported in GSP Annual Reports and Five-Year Assessment Reports when knownPeriodic Evaluations. However, the Project is expected to reduce water use in the City of Modesto to meet future water use mandates and conservation goals.

Evaluation of benefits will be based on analysis of without-Project and with-Project effects on the SGMA sustainability indicators. Each Project is evaluated as part of a scenario and the C2VSimTM ismodeling scenarios used to assess the benefits and impacts on the subbasin sustainabilitySubbasin.

8.3.1.2.5. Implementation Criteria, Status, and Strategy Implementation Strategy and Timeline
This Project would install AMI smart meters to support water reduction goals, by helping the City toof Modesto obtain the analytical tools to manage water usage better. The planning phase is scheduled forwas completed between 2022 throughand 2023 with implementation occurring from. Installation activities began in 2024 throughand Project completion is anticipated in 2026.
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Implementation Assumptions for Modeling
The Advanced Metering Infrastructure Project has beenwas modeled in the C2VSimTM model. Additional information abouton Project-related modeling is described in Section 8.5:

Plan for Achieving Sustainability.

The following general information and assumptions were used to simulate implementation of the Project:
• Modeled as part of scenario of ongoing conservation efforts within the City of Modesto. Simulated change includes the reduction of urban water demand from 228 gallons per person per day (GPCD) (2015 City of Modesto UWMP) to 175 GPCD (2020 City of Modesto UWMP) (West Yost Associates, 2016 & 2021).

8.3.1.2.6. Water Source and Reliability
This Project would not directly use a water source but would help to manage and enhance use of existing water City of Modesto supplies.

8.3.1.2.7. Legal Authority
The GSAs, Districts, and individual Project proponents have the authority to plan and implement Projects.

8.3.1.2.8. Estimated Costs and Funding Plan
The anticipated costscost of this Project areis estimated to be $20 million. Any updates or changes to the estimated costs will be reported in GSP Annual Reports and Five-Year Assessment Reports when knownPeriodic Evaluations. The Project proponent will identify funding sources to cover Project costs as part of Project development. These may include grants, fees, loans, and other assessments.

8.3.1.2.9. Management of Groundwater Extractions and Recharge This Project would AMI does not directly userely on a water source (e.g., no groundwater extraction or recharge is involved)), but the Project would help to manage and enhance use of existing water City of Modesto water supplies.

8.3.1.3. Storm Drain Cross Connection Removal Project (Project 3)

8.3.1.3.1. Project Description
This multi-benefit and multi-componentThe Storm Drain Cross Connection Removal Project (Project) captures, treats, and infiltrates stormwater within the City of Modesto. ProjectsThe Project components use low impact development (LID) techniques including bio-retention planters, infiltration trenches, and underground retention basins underwithin city parks to recharge the for groundwater aquifer. Otherrecharge. Additional benefits include reduced the reduction of stormwater flows to the City of Modesto’s wastewater treatment plant, reduced number of and sanitary sewer overflows, reduction of localized flooding in heavily #25..
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traveled and localized streets, and improved water quality forwithin Dry Creek and the Lower Tuolumne River (both of which are 303d water bodies).. Each Projectproject component is located within the City of Modesto jurisdiction in areas with no positive storm drainage systems within the City of Modesto's jurisdiction. The Project components are a cost effective and LID alternative to constructing detention basins in undeveloped portions of the city and constructing miles of storm drains. This Project also includes the removal of failed dry wells and storm to sanitary sewer cross connections. The Project components, status, and expected recharge benefits are included in Table 8-1Table 8-3.

Table 8-13: Storm Drain Cross Connection Removal Project Components, Status, and Expected Recharge Benefit
Component Status
Expected Recharge
Benefit
Garrison Park Completed 12 AFY
Roosevelt Park Completed 29 AFY
JM Pike Park Design in Progress 53 AFY
Catherine Everett Park Planning/Construction
(2026 completion)
29 AFY
Other Planning 125 AFY
Removal of failed dry wells
and storm to sanitary sewer
cross connection
In Progress N/A

8.3.1.3.2. Public Noticing
The public and other agencies will be notified of the planned or ongoing implementation of PMAProject activities through the outreach and communication channels identified in the GSP, during the preparation process of the PEIR (if applicable), and during updates presented at regularly scheduled STRGBA GSA meetings. Noticing will occur as potential activities are being considered for implementation, and as ongoing and planned activities are implemented. Noticing will inform the public and other agencies that the proponent is considering or will be implementingmodifications to the PMAProject and will provide a description of the actions that will be taken.

Public and/or inter-agency noticing may be facilitated through the GSA’s boardSTRGBA GSA meetings and/or City and Agencylocal agency meetings, associated website(s), inter-basin coordination meetings, other public meetings hosted by the GSAs, GSP annual reports and five-year updatesAnnual Reports and Periodic Evaluations, public scoping meetings, and/or environmental/regulatoryapplicable permitting notification processes. #25..
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8.3.1.3.3. Permitting and Regulatory Process
Required permitting and regulatory review is being initiated as necessary through consultation with applicable governing agencies. Governing agencies that may be consulted for this Project include, but are not limited to: DWR, SWRCB, the California Department of Fish and Wildlife (CDFW), the Central Valley Flood Protection Board (Flood Board), Regional Water Boards, the United States Bureau of Reclamation (Reclamation or USBR), the United States Army Corps of Engineers (USACE), the United States Fish and Wildlife Service (USFWS), the National Marine Fisheries Service (NMFS), Local Agency Formation Commissions (LAFCo), the County of Stanislaus and/or Tuolumne, and the California Air Resources Board (CARB).

8.3.1.3.4. Expected Benefits
Benefits to Sustainability Indicators
Stormwater flows going to sanitary sewer systems will be disconnected and rerouted to provide direct groundwater recharge to the Subbasin.. Sustainability indicators expected to benefit from this Project are groundwater levels, groundwater in storage, land subsidence, and interconnected surface water. All benefits to sustainability indicators in the Modesto Subbasin will be evaluated through groundwater monitoring at nearby monitoring sites, as identified in Section 7.1 of the GSP.

Benefits to Disadvantaged Communities
The City of Modesto storm drain cross connection removalThe Storm Drain Cross Connection Removal Project is expected to provide direct recharge in and around the City of Modesto. Most communities in the Modesto Subbasin are classified as DACs, SDACs, or EDAs (according to 2018 census data, evaluated by place, tract, and block group). Depending on which specific parcels receive surface water deliveries, this Project may directly benefit specific DACs in this area. In addition, maintenance or improvement of groundwater levels will help to protect beneficial groundwater use by rural domestic wells from potential adverse impacts related to chronic groundwater level decline. Benefits to groundwater conditions in the Modesto Subbasin are also expected to broadly benefit all DACs, SDACs, and EDAs.

Volumetric Benefits to the Subbasin Groundwater System
The expected yield of the City of Modesto storm drain cross connection removal Project was estimated by simulating this Projectsimulations run in the C2VSimTM model. General information and assumptions used to simulate this Project are summarized in the Implementationfollowing section below. Additional information is provided in Section 8.5:

Plan for Achieving SustainabilityPlan for Achieving Sustainability. #25..
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On average across all years, the City of Modesto storm drain cross connection removal Project is expected to provide approximately 248 AFY of recharge benefit to the Modesto Subbasin, once completed.

Evaluation of benefits will be based on analysis of without-Project and with-Project measurements supported by modeling. Measured parameters will include surface water deliveries, groundwater levels, and other parameters to be determined. Modeling may be done with the C2VSimTM model used for GSP development to evaluate volumetric benefits.

8.3.1.3.5. Implementation Criteria, Status, and Strategy Implementation Strategy and Timeline
This Project consists of several different components of a larger program which has relied on the success of previous grant funds. For the components included in this Project, work is already in progress. The JM Pike Park component is expected to bewas completed in 2023.

Overall, the final storm to sewer cross connection removals for the program are estimated to be completed in 2061.

Implementation Assumptions for Modeling
The Storm Drian Cross Connection Removal Project has been modeled in the C2VSimTM model. Additional information about Project-related modeling is described in Section 8.5:

Plan for Achieving Sustainability.

The following general information and assumptions were used to simulate implementation of the Project:
• Volume of water: 41 AFY were provided during the first 10 years of simulation, 70 AFY during the following 5 years of simulation, and 248 AFY for the rest of the simulation, distributed evenly between the months of October and April. The annual average during the 50-year simulation period would be of 189 AFY. • The total volume would be provided as direct recharge over the aquifer..

8.3.1.3.6. Water Source and Reliability
This Project would useutilize flows that becamebecome available from disconnecting storm drain flows going to sanitary sewer systems and redirecting them to rechargefor groundwater recharge. Stormwater flows are more dependent on precipitation events. It is anticipated that annual contributions from this Projectproject will collect approximately 12 AF from Garrison Park, 29 AF from Roosevelt Park, 53 AF from JM Pike Park, 29 AF from Catherine Everett, and an additional 125 AF from other areas. The precise reliability of availableChanges to water source availability will be identified as the Project is evaluated during implementation.further. This information will be reported in GSP annual reportsAnnual Reports and five-year updates when knownPeriodic Evaluations. #25..
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8.3.1.3.7. Legal Authority
The GSAs, Districts, and individual Project proponents have the authority to plan and implement Projects. Required permitting and regulatory review will be Project-specific and initiated through consultation with applicable governing agencies. Governing agencies for which consultation will be initiated may include, but is not limited to: DWR, SWRCB, CDFW, Flood Board, Regional Water Boards, USFWS, NMFS, LAFCO, Stanislaus County, and CARB.

8.3.1.3.8. Estimated Costs and Funding Plan
Potential costs of this Project include construction or improvements to new or existing recharge basin and alteration of current stormwater and sewer system connections. The currentestimated cost estimate for this Project is $40 million for all Project components. It is anticipated that the City of Modesto would identify funding sources to cover Project costs as part of Project development. These may include grants (e.g., Prop 1, Prop 68, NRCS), fees, local cost share, loans and other assessments.

8.3.1.3.9. Management of Groundwater Extractions and Recharge Per 23 CCR § 354.44(b)(9), all PMAs developed for implementation are expected to maintain the balance of groundwater extractions and recharge to ensure that lowering of groundwater levels or depletion of supply during periods of drought is offset by increases in groundwater levels and storage in other years.

8.3.1.4. Surface Water Pump Station and Storage Tank (Project 4)

8.3.1.4.1. Project Description
The Surface Water Pump Station and Storage Tank project (Project) entails connecting the City of Waterford (Waterford)Waterford’s water supply system to Modesto Irrigation District’s (MID)MID’s water treatment plant and potable surface water supply system. The Project includes several components, described in order of the flow of the surface water.

Surface water will be diverted from MID’s distribution network at a pipeline turn-out located at the corner of Tim Bell and Vineyard Road, northeast of the City of Waterford. The surface water will be piped into a one-million-gallon storage tank that will be constructed at this intersection. A pump station at this location and transmission line will also be constructed that transports the water to Yosemite Boulevard in the City of Waterford. This project involves water supply agreements between Modesto Irrigation District, the City of Modesto, and the City of Waterford, the details of which are currently being negotiated.

As partAnother component of a separatethis Project, by entails combining the endCity of 2023 Waterford is planning to combine itsWaterford’s distribution network and provideproviding water to the disadvantaged community of Hickman, located in the Turlock Subbasin by 2023. While Hickman is in the Turlock Subbasin, supplying surface water to the community would support the Modesto Subbasin’s sustainability goals of mitigating stream depletions along the Tuolumne River and protecting domestic wells by reinforcing groundwater levels along the Subbasin boundary.
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8.3.1.4.2. Public Noticing
The public and other agencies will be notified of the planned or ongoing implementation of PMAProject activities through the outreach and communication channels identified in the GSP, during the preparation process of the PEIR (if applicable), and during updates presented at regularly scheduled STRGBA GSA meetings. Noticing will occur as potential activities are being considered for implementation, and as ongoing and planned activities are implemented. Noticing will inform the public and other agencies that the proponent is considering or will be implementingmodifications to the PMAProject and will provide a description of the actions that will be taken.

Public and/or inter-agency noticing may be facilitated through the STRGBA GSA board meetings and/or MID boardlocal agency meetings, the Modesto Subbasin and/or MIDassociated website(s), the MID newsletter, inter-basin coordination meetings, other public meetings hosted by the STRGBA and/or MID, GSP annual reports and five-year updatesGSAs, Annual Reports and Periodic Evaluations, public scoping meetings, and/or environmental/regulatoryapplicable permitting notification processes.

8.3.1.4.3. Permitting and Regulatory Process
Required permitting and regulatory review would be initiated through consultation with applicable governing agencies. Governing agencies for which consultation will be initiated may include, but are not limited to: DWR, SWRCB, CDFW, Flood Board, Regional Water Boards, USFWS, NMFS, LAFCO, Counties of Stanislaus and/or Tuolumne, and CARB. Specific permitting and regulatory processes that may potentially affect the construction of Projectrelated infrastructure include, but are not limited to:
• USACE Section 404 Permits (potential exemption under Section 404(f)(1)(C) of Clean Water Act)
• Regional Water Quality Control Board Section 401 Water Quality Certification (not required if exempt from USACE Section 404)
• SWRCB Construction General Permit and Storm Water Pollution Prevention Plan (SWPPP)
• State Historic Preservation Office (SHPO) and National Historic Preservation Act (NHPA) Section 106 Coordination
• CEQA Environmental Review Process
• California Endangered Species Act (CESA) Consultation • Endangered Species Act (ESA) Compliance
• National Environmental Policy Act (NEPA) Compliance (expected to require either an Environmental Impact Report and Negative Declaration or Mitigated Negative Declaration)
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8.3.1.4.4. Expected Benefits
Benefits to Sustainability Indicators
Utilization of surface water for urban water demands in Waterford and Hickman is expected to offset groundwater pumping demands, with in-lieu groundwater recharge benefits to the Modesto Subbasin. Because a single water Waterford and Hickman use a combined system, Hickman (which lies in the Turlock Subbasin) will also benefit. Benefits in this area are seen in Tuolumne River stream depletionsconditions and will further protect domestic wells in both the Modesto and Turlock Subbasins. The sustainability indicators expected to benefit from this Projectproject are groundwater levels, groundwater in storage, land subsidence, and interconnected surface water, and possibly land subsidence.. All benefits to sustainability indicators in the Modesto Subbasin will be evaluated through groundwater monitoring at nearby monitoring sites, identified in the GSP.

Benefits to Disadvantaged Communities
The Waterford/Hickman Surface Water Pump Station and Storage Tank Project directly benefits Waterford and Hickman, both classified as a DACs, by supplementing and diversifying their drinking water supply. This Project will provide an alternate drinking water source in case of infrastructure or contamination concerns with the communities’ groundwater production wells. The additional surface water supply will also reduce groundwater pumping and increase groundwater levels near the communities which can reduce pumping costs, decrease the likelihood of dewatering domestic wells, and potentially mitigate some groundwater quality concerns. Additionally, benefits to groundwater conditions in the Modesto Subbasin are also expected to broadly benefit all DACs, SDACs, and EDAs in the Modesto Subbasin.

Volumetric Benefits to the Subbasin Groundwater System
The expected yield of the Waterford/Hickman Surface Water Pump Station and Storage Tank was estimated by simulating this Projectthrough simulations in the C2VSimTM model.

General information and assumptions used to simulate this Project are summarized in the Implementation section below. Additional information is provided in Section 8.5: Plan for Achieving SustainabilityPlan for Achieving Sustainability.

It is assumed that MID will provide 900 AF/yearAFY to Waterford and Hickman, except for critical years which will provide a partial allotment (approximately 750 AF/yearAFY in critical years).

Evaluation of benefits will be based on analysis of without-Project and with-Project measurements supported by modeling. Measured parameters will include surface water deliveries, groundwater levels, and other parameters to be determined. Modeling will be done with the C2VSimTM model used for GSP development.
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8.3.1.4.5. Implementation Criteria, Status, and Strategy Implementation Strategy and Timeline
The Waterford/Hickman Surface Water Pump Station and Storage Tank will be implemented by the City of Waterford. Waterford will oversee the Project financing and funding, permitting, and construction. The Project will require an agreement(s) between MID and the City of Modesto to purchase treated surface water. Negotiations are underway but have not been concluded. Once negotiations are finalized and financing is secured, then design and subsequent construction will begin. This PMAProject is currently in the early conceptual stage. Thus, the start and completion dates for this PMAProject have yet to be determined and will be provided in GSP annual reportsAnnual Reports and five-year updatesPeriodic Evaluations when known. Once the Project construction is complete, it is expected that MID would provide 900 AF/yearAFY to Waterford and Hickman in all water years except critical years which will provide a partial allocation.

Implementation Assumptions for Modeling
The Waterford/Hickman Surface Water Pump Station and Storage Tank has been modeled in the C2VSimTM model. Additional information about Project-related modeling is described in Section 8.5: Plan for Achieving SustainabilityPlan for Achieving Sustainability.

The following general information and assumptions were used to simulate implementation of the Project:
• Estimated volume of surface water deliveries: Proportional to the MID irrigation water allotment based on water year type, not to exceed 900 AFY. The surface water deliveries are distributed throughout the months proportional to monthly urban demands.
• Area receiving surface water deliveries: Surface water is delivered to the jurisdictional extent of the Hickman and Waterford communities, consistent with the extent in the historical C2VSimTM model. Surface water is distributed between Waterford and Hickman proportionalproportionally to simulated demands of each community.
• Water source: It is assumed that all surface water is diverted from MID’s distribution system, with no adjustment to modeled MID diversions, spillage, and seepage. • Groundwater pumping: It is assumed that groundwater production is reduced by the volume of surface water deliveries which is distributed evenly among all wells in Waterford and Hickman.

8.3.1.4.6. Water Source and Reliability
The Waterford/Hickman Surface Water Pump Station and Storage Tank will use water diverted from MID’s surface water distribution network. MID has existing water rights on the Tuolumne River and existing storage and conveyance facilities that afford secure surface #25..
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water supplies. Surface water is expected to be available for this Project in all hydrologic years, proportional to MID irrigation allotment, while still meeting the demand of existing MID customers.

8.3.1.4.7. Legal Authority
The GSAs, Districts, and individual Project proponents have the authority to plan and implement Projects through consultation with applicable governing agencies. MID and the City of Modesto have the authority to sell surface water to the City of Waterford.

8.3.1.4.8. Estimated Costs and Funding Plan
Costs of this Project include right of way purchase, environmental permitting, design, construction, and Project management costs. The estimateestimated cost is approximately $8.5 million. However, this Project is currently in the early conceptual stage and a more refined cost can be reported in GSP annual reportsAnnual Reports and five-year updatesPeriodic Evaluations when known. It is anticipated that the City of Waterford would identify grant funding sources to cover Project costs as part of Project development.

8.3.1.4.9. Management of Groundwater Extractions and Recharge Per 23 CCR § 354.44(b)(9), all PMAs developed for implementation are expected to maintain the balance of groundwater extractions and recharge to ensure that lowering of groundwater levels or depletion of supply during periods of drought is offset by increases in groundwater levels and storage in other years.

In particular, in-lieu recharge benefits offrom this Project are expected to increase the use and recharge of available surface water supplies, helping to offset any potential increases in groundwater pumping during drought when surface water supplies are limited.

8.3.2. In-Lieu & Direct Recharge Projects

8.3.2.1. Modesto Irrigation District In-Lieu and Direct Recharge Project (Project 5)

8.3.2.1.1. Project Description
The Modesto Irrigation District In-lieuLieu and Direct Recharge Project (Project)), also known as the Long-Term Groundwater Replenishment Program (GRP), is intended to be a cooperative long-term Project between Modesto Irrigation District (MID) and the nondistrict east (NDE) landowners. The purpose of this Project is to allow MID to facilitate recharge for NDE landowners during times and conditions that will not impact MID’s existing agricultural and urban customers. ThisThe Project would be operated separately but coordinated with the Oakdale Irrigation District In-lieu and Direct Recharge Project, which shares a similar goal of facilitating groundwater sustainability in the NDE areas.

Currently developedDeveloped agriculture in the NDE areas of the Modesto subbasinSubbasin is estimated to be approximately 36,000 acres, of which approximately 30,000 acres isare permanent crops such as deciduous fruits and nuts (permanent crops).. #25..
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With limited exception, the entire NDE area is solely reliant on groundwater from the Modesto subbasin. TheSubbasin. This Project involves the delivery of approximately 60,000 AF of surface water from the Tuolumne River in Wet and Above Normal water years (WYs)).

Deliveries would be supplied through a limited number of new points of diversions offon MID’s existing irrigation conveyance infrastructure and subsequent conveyance through newly constructed private irrigation conveyance infrastructure for. Water supply would be provided to NDE during the growing season in the form of in-lieu and direct recharge during the growing season.. Historically (1972-2020), Wet and Above Normal WYs have occurred approximately 47% of the time onwithin the Tuolumne RiverSubbasin. Under the current Final Environmental Impact Statement for the relicensing of Don Pedro Reservoir, there is estimated to be approximately 1,500,000 AF of surface water in Wet WYs and 620,000 AF of surface water in Above Normal WYs in the Tuolumne River above and beyond that. These water volumes exceed what is necessary to meetingmeet existing customer demands (all Tuolumne River Partners) and the recommended instream flow obligations. As a result, 60,000 AF of Tuolumne River surface water to applicable NDE areas amounts to approximately 4% and 10% of available surface water supply respectively, for Wet and Above Normal WYs., respectively. Project operation is intended to make surface water delivery available to applicable NDE areas in mostAbove Normal and Wet WYs.

8.3.2.1.2. Public Noticing
The public and other agencies will be notified of the planned or ongoing implementation of PMAProject activities through the outreach and communication channels identified in the GSP, during the preparation process of the PEIR (if applicable), and during updates presented at regularly scheduled STRGBA GSA meetings. Noticing will occur as potential activities are being considered for implementation, and as ongoing and planned activities are implemented. Noticing will inform the public and other agencies that the proponent is considering or will be implementing the PMAProject and will provide a description of the actions that will be taken.

Public During the development of the Revised GSP, MID has held multiple meetings and workshops to promote the program:
• Landowner Meeting held March 4, 2024, at MID Downtown Office • Landowner Meeting held March 5, 2024, at Waterford Council Chambers • Long-term Groundwater Replenishment Program Workshop held April 23, 2024, at MID Downtown Office
• Long-term Groundwater Replenishment Program Workshop held April 24, 2024, at Waterford Council Chambers
Additionally, MID has promoted the program at STRGBA meetings and Stanislaus County meetings, as well as issued social media notifications and postcard fliers. #25..
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Additional public and/or inter-agency noticing may be facilitated through the GSA’s boardSTRGBA GSA meetings and/or Districtlocal agency meetings, associated website(s), inter-basin coordination meetings, other public meetings hosted by the GSAs, GSP annual reports and five-year updatesAnnual Reports and Periodic Evaluations, public scoping meetings, and/or environmental/regulatoryapplicable permitting notification processes.

8.3.2.1.3. Permitting and Regulatory Process
Required permitting and regulatory review would be initiated as necessary through consultation with applicable governing agencies. Surface water would be diverted for this Project by MID through existing pre- and post-1914 water rights. Governing agencies that may be consulted for this Project include but are not limited to the State Water Resources Control Board (SWRCB), the County(ies)Stanislaus and/or Tuolumne Counties of Stanislaus and/or Tuolumne, and DWR.

If necessary for field flooding, the Project proponent will obtain land grading permits from the County(ies). Recharge Projects may also require an environmentalStanislaus and/or Tuolumne Counties. Environmental review process under CEQA. may also be required for the Projects.

8.3.2.1.4. Expected Benefits
Benefits to Sustainability Indicators
Surface water deliveries during the non-irrigation season are expected to provide direct groundwater recharge to the Subbasin. For fields that are irrigated using groundwater, surface water deliveries during the irrigation season are expected to offset groundwater demand and provide in-lieu groundwater recharge benefits. In both cases, the sustainability indicators expected to benefit from this Project are groundwater levels, groundwater in storage, land subsidence, and interconnected surface water, and land subsidence (depending on where recharge occurs).. All benefits to sustainability indicators in the Modesto Subbasin will be evaluated through groundwater monitoring at nearby monitoring sites, identified in the GSP.

Benefits to Disadvantaged Communities
The MID in-lieuIn-Lieu and direct rechargeDirect Recharge Project is expected to provide direct or in-lieu recharge for use in the NDE area. Most communities in the Modesto Subbasin, particularly in the NDE area, are classified as DACs, SDACs, or EDAs (according to 2018 census data, evaluated by place, tract, and block group). Depending on which specific parcels receive surface water deliveries, this Project may directly benefit specificcertain DACs in this area. In addition, maintenance or improvement of groundwater levels will help to protect beneficial groundwater use by rural domestic wells from potential adverse impacts related to chronic groundwater level decline. Benefits to groundwater conditions in the Modesto Subbasin are also expected to broadly benefit all DACs, SDACs, and EDAs. #25..
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Volumetric Benefits to the Subbasin Groundwater System
The expected yield of the MID in-lieu and direct recharge Project was estimated by simulating this Projectthrough simulations in the C2VSimTM model. General information and assumptions used to simulate this Projectproject are summarized in the Implementationfollowing section below. Additional information is provided in Section 8.5:

Plan for Achieving SustainabilityPlan for Achieving Sustainability.

On average across all years, the MID in-lieuIn-Lieu and direct rechargeDirect Recharge Project is expected to provide an average annual benefit of 28,800 AFY of recharge benefit to the Modesto Subbasin. These benefits would accrue in years with Wet or Above Normal hydrologic conditions when sufficient water is expected to be available for on-farm recharge (approximately 5047 percent of years historically). In those yearsAbove Normal and Wet WYs, approximately 60,000 AFY of groundwater recharge is expected to occurbe supplied.

Evaluation of benefits will be based on analysis of without-Project and with-Project measurements supported by modeling. Measured parameters will include surface water deliveries, groundwater levels, and other parameters to be determined. Modeling mayEvaluation of benefits will be based on analysis of without-Project and with-Project measurements supported by modeling. Measured parameters will include surface water deliveries, groundwater levels, and other parameters to be determined. Modeling will be done with the C2VSimTM model used for GSP development.

8.3.2.1.5. Implementation Criteria, Status, and Strategy Implementation Strategy and Timeline
The Project involves the delivery of surface water from the Tuolumne River in Wet and Above Normal water years (WYs) through a limited number of new points of diversions offon MID’s existing irrigation conveyance infrastructure and subsequent conveyance through existing and newly constructed private irrigation conveyance infrastructure for inlieu and direct recharge during the growing season.

It is anticipatedModeling results indicate that that most of the surface water made available will be used to meet agricultural demand during the irrigation season throughout the NDE area. This in-lieu use is intended to reduce the pumping needed in this area of the subbasinSubbasin in wetWet and above normal yearsAbove Normal WYs.

This Project is currently inOn January 23, 2024, the early conceptual stage. Thus,MID Board of Directors approved the startimplementation of the GRP and completion dates for this Project have yetadopted an Addendum to the Modesto Irrigation District Comprehensive Water Resources Management Plan Final Programmatic Environmental Impact Report which incorporated the Long-term Groundwater Replenishment Program. Project activities, such as surface water deliveries, are anticipated to begin January 2024. #25..
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The GRP is planned to be determined andexpanded to parcels outside of the MID Service Boundary but within the Modesto Subbasin following adoption of the Revised GSP. Project updates will be provided in GSP Annual Reports and Five-Year Assessment Reports when known. Periodic Evaluations.

Implementation Assumptions for Modeling
The MID In-lieu and Direct Recharge Project has been modeled in the C2VSimTM model.

Additional information about Project-related modeling is described in Section 8.5: Plan for Achieving Sustainability.

The following general information and assumptions were used to simulate implementation of the Project:
• Volume of water: 60,000 AFY were provided during Wet and Above Normal yearsWYs, distributed in the months following the demand distribution. During the 50-year simulation period, the average annual water supply from this Project would be 28,800 AFY.
• One third of the total volume would be provided as direct recharge over the aquifer.

The other two thirdsand remainder would be delivered as in-lieu recharge. • The location of the in-lieu and direct recharge would be within the NDE area, located near existing MID conveyance facilities and those parcels with low/medium infrastructure requirements.

8.3.2.1.6. Water Source and Reliability
The Project involves the delivery of approximately 60,000 AF of surface water from the Tuolumne River in Wet and Above Normal water years (WYs) through a limited number of new points of diversions off MID’s existing irrigation conveyance infrastructure and subsequent conveyance through newly constructed private irrigation conveyance infrastructure for in-lieu and direct recharge during the growing season. Historically (1969- 2018), Wet and Above Normal WYs have occurred approximately 48% of the time on the Tuolumne River.WYs. Under the current Final Environmental Impact Statement for the relicensing of Don Pedro Reservoir, there is estimated to be approximately 1,500,000 AF of surface water in Wet WYs and 620,000 AF of surface water in Above Normal WYs in the Tuolumne River above and beyond that necessary to meeting existing customer demands (all Tuolumne River Partners) and the recommended instream flow obligations. As a result, 60,000 AF of Tuolumne River surface water to applicable NDE areas amounts to approximately 4% and 10% of available surface water supply respectively, for Wet and Above Normal WYs.

8.3.2.1.7. Legal Authority
The GSAs, Districts, and individual Project proponents have the authority to plan and implement Projects through consultation with applicable governing agencies. MID has the #25..
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authority to contract with and provide deliveries to non-districted east landownersparcels in the NDE area, and individual irrigators have the authority to apply surface water to their fields for on-farm recharge.

8.3.2.1.8. Estimated Costs and Funding Plan
Potential costs of this Project may include Project coordination and administration, financial, or other incentives to encourage on-farm recharge, field preparation to enhance flooding, and other potential on-field monitoring equipment. Costs per site may vary depending on changes in Project implementation and incentives. Slightly higher costs per site would likely be incurred in the first year an irrigator participates, as more coordination and site preparation may be required. The total costs of the Project will vary over time, depending on the number of sites receiving water, the extent to which irrigators require coordination and support, and any applicable Project incentives.

This Project is currently in the early conceptual stage. Thus, the anticipated costs contained herein are planning level costs and subject to change. However, high-level initial estimates are on the order of $53,340,000 – $75,000,000 of new conveyance infrastructure. Most costs are anticipated to be borne by the NDE participants; however, member agencies of the STRGBA GSA may identify funding sources to cover Project costs as part of Project development. These may include grants (e.g., Prop 1, Prop 68, NRCS, others), fees, and loans. Participating NDE landowners would ultimately be responsible for payment and installation of their private conveyance systems and the volumetric rate of MID surface water deliveries.

8.3.2.1.9. Management of Groundwater Extractions and Recharge Per 23 CCR § 354.44(b)(9), all PMAs developed for implementation are targeted to maintain the balance of groundwater extractions and recharge to help ensure that lowering of groundwater levels or depletion of supply during periods of drought is offset by increases in groundwater levels and storage in other years.

In-lieu recharge benefits of this Project are expected to increase the use and recharge of available surface water supplies during wetter years, helping to offset potential increases in groundwater pumping during drought when surface water supplies are limited.

8.3.2.2. Oakdale Irrigation District In-lieu and Direct Recharge Project (Project 6)

8.3.2.2.1. Project Description
The Oakdale Irrigation District In-lieu and Direct Recharge Project (Project) is intended to be a cooperative long-term Project between OID and the NDE east landowners. The purpose of this Project is to allow OID to facilitate recharge for NDE landowners during times and conditions that will not impact OID’s existing agricultural customers. The Project is separate from but coordinated withsimilar to the Modesto Irrigation District In-lieuLieu and Direct Recharge Project, which and shares a similar goal of facilitating groundwater sustainability in the NDE areas. Coordination between the two Districts is ongoing and these #25..
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projectsProjects may be operated in tandem, utilizing the MID-OID interconnected distribution systems to potentially work together and convey water to the NDE or others from OID.

Currently developedDeveloped agriculture in the NDE areas of the Modesto subbasinSubbasin is estimated to be approximately 36,000 acres, of which approximately 30,000 acres isare permanent crops such as deciduous fruits and nuts (permanent crops)..

With limited exception, the NDE area is solely reliant on groundwater from the Modesto subbasin.Subbasin. The Project envisions the development of up tois anticipated to provide approximately 20,000 AF of surface water from the Stanislaus River in all water years (WYs) except Critically Dry WYs. Deliveries would be supplied through a limited number of several existing and new points of diversions offon OID’s existing irrigation conveyance infrastructure and subsequent conveyance through newly constructed private irrigation conveyance infrastructure for in-lieu use. Water supply benefits would be provided to NDE between March 1st- October and September 31st in the form of in-lieu and direct recharge.

Some direct recharge is expected to occur as canal or reservoir seepage in the expanded conveyance network. OID surface water The Project will not be delivereddeliver water supply to the NDE between NovemberOctober 1
st
- March 1st. The OID Board of Directors
would consider and define the volume of water (if any) available to this Project on an annual basis in non-Critically Dry WYs. The Project is in the initial planning phase and as such, the Project terms have yet to be considered or approved by the OID Board of Directors. outside of the Project’s scope (Critically Dry WYs). Significant progress has been made with this Project since the adoption of the 2022 Modesto Subbasin GSP. The 10-Year out-of-District Water Sales Program (10-Year Program) began in 2023 and includes 4,882 irrigated acres in the Modesto Subbasin within the NDE. Under the 10-Year Program, participating landowners are required to purchase a minimum of 1.5 acre-feet per irrigated acre during each year that surplus surface water is available from OID. The landowners also have the opportunity to purchase and use additional surplus surface water throughout the irrigation season if available. Under the 10-Year Program, a minimum of 7,300 acre-feet will be purchased each year that out-of-District water is available.

The Paulsell Lateral Expansion (Paulsell Expansion) has been approved by OID and will be funded at least in part with $14.4M of SGMA Implementation Grant funding that was awarded to OID in October 2023. Working in sync with the 10-Year Program, the Paulsell Expansion will rehabilitate, automate, and expand the Paulsell Lateral, largely within the existing right of way, to accommodate an additional 150 cfs, allowing OID to deliver up to 20,000 AFY of available surface water for in-lieu and direct recharge. Infrastructure improvements will also provide further in-lieu recharge benefits by improving irrigation service to in-District lands served by OID, but that have resorted in part to pumping groundwater to supplement irrigation due to service issues on the Paulsell Lateral. In total, the Paulsell Expansion is expected to provide in-lieu and direct recharge benefits across 11,000 irrigated acres in the Subbasin.
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Historically (2010-2019), OID diverts between approximately 165,000 AF to 246,000 AF, with an overall average of approximately 208,000 AF. Given OID’s existing surface water rights of 300,000 AF from the Stanislaus (300,000 AF)River and its overall average system inflows, the surface water contemplated fordeliveries estimated from this Project amounts to approximately 22% of the total available surface water supply above and beyond that necessary to meet their existing customer demands (on an average basis).. As a result, if this Project were approved, it would provideprovides the opportunity for OID to meet a portion of the NDE area needs while retaining some volume of water for “high-value” out-of-basin water transfers..

Voluntary transfers of water have provided a basis for funding improvements to the OID distribution system under the District’s Water Resources Plan. As water is conserved and transferred, OID receives revenue and implements additional improvements, resulting in additional water conservation. More information on OID’s WRP implementation to date can be found in Section 8 of OID’s AWMP. Both the OID WRP and AWMP are available for reference on OID’s website (www.oakdaleirrigation.com). OID has participated in numerous water transfers in the past and continues to seek opportunities for mutually beneficial temporary transfer agreements with water users (agricultural, urban, and others) outside of the District.

8.3.2.2.2. Public Noticing
The public and other agencies will continue to be notified of the planned or ongoing implementation of PMAproject activities through the outreach and communication channels identified in the GSP, during the preparation process of the PEIR (if applicable), and during updates presented at regularly scheduled STRGBA GSA meetings. Noticing will occur as potential activities are being considered for implementation, and as ongoing and planned activities are implemented. Noticing will inform the public and other agencies that the proponent is considering or will be implementing the PMAproject and will provide a description of the actions that will be taken.

Public and/or inter-agency noticing may be facilitated through the GSA’s boardSTRGBA GSA meetings and/or Districtlocal agency meetings, associated website(s), inter-basin coordination meetings, other public meetings hosted by the GSAs, GSP annual reportsAnnual Reports and five-year updatesPeriodic Evaluations, public scoping meetings, and/or environmental/regulatoryapplicable permitting notification processes.

Review and approval of both the 10-Year Program and the Paulsell Expansion has occurred at public OID Board of Directors meetings. Numerous presentations of both the 10-Year Program and the Paulsell Expansion have occurred at other Board meetings, workshops, and venues. Landowners impacted by the Paulsell Expansion and those participating in the 10- Year Program continue to be updated by OID staff about project progress on a regular basis. #25..
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8.3.2.2.3. Permitting and Regulatory Process
Required permitting and regulatory review would be initiated as necessary through consultation with applicable governing agencies. Surface water would be diverted forthrough this Project by OID throughvia existing water rights. Governing agencies that may be consulted for this Project include but are not limited to the State Water Resources Control Board (SWRCB), the County(ies) of Stanislaus and/or Tuolumne Counties, USBR, and DWR.

If necessary, Project proponent will obtain any applicable permits from the County(ies).Tuolumne and/or Stanislaus Counties. Recharge projects and construction or expansion of conveyance facilities may also require an environmental review process under CEQA. CEQA review has been completed for both the 10-Year Program and the Paulsell Expansion.

8.3.2.2.4. Expected Benefits
Benefits to Sustainability Indicators
For fields that are irrigated using groundwater, surface water deliveries during the irrigation season are expected to offset groundwater demand and provide in-lieu groundwater recharge benefits. Some additional recharge is anticipated to occur from canal and reservoir seepage in the expanded conveyance network. The sustainability indicators expected to benefit from this Project are groundwater levels, groundwater in storage, land subsidence, and interconnected surface water, and land subsidence (depending on where recharge occurs).. All benefits to sustainability indicators in the Modesto Subbasin will be evaluated through groundwater monitoring at nearby monitoring sites, identified in the GSP.

Benefits to Disadvantaged Communities
The Oakdale Irrigation District in-lieuIn-Lieu and direct rechargeDirect Recharge Project is expected to provide direct orand in-lieu recharge for parcels in the NDE landowners area.

The majority ofSeveral communities in the Modesto Subbasin, including the NDE area, are classified as DACs, SDACs, or EDAs (according to 2018 census data, evaluated by place, tract, and block group). Depending on which specific parcels receive surface water deliveries, this Project may directly benefit specific DACs in this area. In addition, maintenance or improvement of groundwater levels will help to protect beneficial groundwater use by rural domestic wells from potential adverse impacts related to chronic groundwater level decline.

Benefits to groundwater conditions in the Modesto Subbasin are also expected to broadly benefit all DACs, SDACs, and EDAs.

Volumetric Benefits to the Subbasin Groundwater System
The expected yield of the Oakdale Irrigation District in-lieu and direct recharge Project was estimated by simulating this Project inthrough simulations using the C2VSimTM model.

General information and assumptions used to simulate this Project are summarized in the #25..
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Implementationfollowing section below. Additional information is provided in Section 8.5:

Plan for Achieving SustainabilityPlan for Achieving Sustainability.

On average across all years, the Oakdale Irrigation District in-lieu and direct recharge Project is expected to provide approximately 14,400 AFY of recharge benefit to the Modesto Subbasin. These benefits would accrue in all hydrologic conditions except for critically dry yearsCritically Dry WYs (approximately 72 percent of years historically). In those yearsnonCritically Dry WYs, approximately 20,000 AFY of groundwater recharge is expected to occur.

Evaluation of benefits will be based on analysis of without-Project and with-Project measurements supported by modeling. Measured parameters will include surface water deliveries, groundwater levels, and other parameters to be determined. Modeling has been done with the C2VSimTM model used for GSP development and will continue to be analyzed during plan implementation.

8.3.2.2.5. Implementation Criteria, Status, and Strategy Implementation Strategy and Timeline
The Project involves the delivery of surface water from the Stanislaus River in Wet, Above Normal, Below Normal and Dry water years (WYs) through a limited number of existing and new points of diversions off OID’s existing irrigation conveyance infrastructure and subsequent newly constructed private irrigation conveyance infrastructure for in-lieu and direct recharge during the growing season. It is The 10-Year Program has entered its second year and the 10-Year term ends in 2032, at which time the OID Board of Directors may decide to extend the program at the request of the participants for another 10-Year term.

New turnouts for participants without existing service connections have been installed on the OID canals and it is expected that most of the Project waterthose landowners will be used for in-lieu recharge on parcels that have previously purchased surface water from OID, but it is anticipated that other NDE growers will participate as additionalhave their private conveyance infrastructure is constructed.

This Project is currently in the early conceptual stage. Thus,systems connected no later than the start and completion dates for this Project have yet to be determined and will be provided in GSP Annual Reports and Five-Year Assessment Reports when knownof the 2025 irrigation season.

Construction of the Paulsell Expansion is proposed to start in Fall 2024 with completion by Spring 2026.

Implementation Assumptions for Modeling
The OID In-lieuLieu and Direct Recharge Project has been modeled in the C2VSimTM model.

Additional information about Project-related modeling is described in Section 8.5: Plan for Achieving Sustainability.
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The following general information and assumptions were used to simulate implementation of the Project:
• Volume of water: Up to 20,000 AFY of water was made available in all years except critically dry hydrologic year types. Surface water deliveries were made within the irrigation season, distributed based on agricultural demand. The annual average water supply during the 50-year simulation period would be ofapproximately 14,400 AFY.
• The location of the in-lieu and direct recharge would be within the NDE area, located near existing OID conveyance facilities and those parcels with low/med infrastructure requirements.

8.3.2.2.6. Water Source and Reliability
The Project contemplatesanticipates the delivery of approximately 20,000 AF of surface water from the Stanislaus River in all water years (WYs) except Critically Dry WYs, through a limited number of existing and new points of diversions off OID’s existing irrigation conveyance infrastructure and subsequent newly constructed private irrigation conveyance infrastructure for in-lieu and direct recharge during the growing season.. Historically (2010- 2019), OID diverts between approximately 165,000 AF to 246,000 AF, with an overall average of approximately 208,000 AF. Given OID’s existing surface water rights of up to 300,00 AF from the Stanislaus (300,000 AF) and its overall average system inflows, the surface water contemplated fordeliveries expected from this Project amounts to approximately 22% of the total available surface water supply above and beyond that necessary to meet their existing customer demands (on an average basis).. As a result, if this Project were approved, it would provideprovides the opportunity for OID to meet a portion of the NDE area demands while retaining some volume of water for “high-value” out-ofbasin water transfersneeds.

8.3.2.2.7. Legal Authority
The GSAs, Districts, and individual Project proponents have the authority to plan and implement Projects through consultation with applicable governing agencies. OID has the authority to contract with and provide deliveries to non-districted eastNDE landowners area, and individual irrigators have the authority to apply surface water to their fields for inlieu recharge.

8.3.2.2.8. Estimated Costs and Funding Plan
Potential costs of this Project may include Project coordination and administration, permitting, CEQA analysis, construction or expansion of conveyance facilities, and financial or other incentives to encourage in-lieu use. Costs per site may vary depending on proximity to OID conveyance facilities and changes in Project implementation or incentives. The total costs of the Project will vary over time depending on how many NDE landowners #25..
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participate, the amount of construction necessary, the volumetric rate of OID surface water deliveries, and the extent to which irrigators require coordination and support.

This Project is currently in Costs related to the early conceptual stage. Thus, the anticipated costs have yet to be determinednew turnout construction, CEQA process, and will be reported in GSP Annual Reports and Five-Year Assessment Reports when known. However, high-level initial estimates are on the order of $17,780,000 - $25,000,000 of new conveyanceprivate irrigation infrastructure. The majority of costs are anticipated to be for the 10-Year Program have been borne by the NDEprogram participants, however, STRGBA GSA member agencies may identify funding sources to cover Project costs as part of Project development. These may include grants (e.g., Prop 1, Prop 68, NRCS, others), fees, and loans. The .

The participating NDE landowners will ultimatelyalso be responsible for the cost of new private conveyance infrastructure andmaintenance costs of the turnout, flowmeter, and appurtenances as well as the volumetric rate of OID surface water deliveries. The volumetric price of out-of-District surface water began at $200 per acre-foot during the first year of the 10-Year Program and is subject to a rate increase of 3% each year thereafter.

The estimated cost of design and construction of all three phases of the Paulsell Expansion was $18.6M. OID received $14.4M in funding under a DWR SGMA Implementation Grant for the design of all three phases and the construction of the first phase of the Paulsell Expansion. The first phase includes most of the major construction components (tunnels, siphons, control structures), and will provide a significant improvement to the level of irrigation service. OID or NDE stakeholder groups may pursue future grant funding opportunities to complete construction of the final two phases of the Paulsell Expansion.

8.3.2.2.9. Management of Groundwater Extractions and Recharge Per 23 CCR § 354.44(b)(9), all PMAs developed for implementation are targeted to maintain the balance of groundwater extractions and recharge to help ensure that lowering of groundwater levels or depletion of supply during periods of drought is offset by increases in groundwater levels and storage in other years.

In-lieu recharge benefits of this Project are expected to increase the use and recharge of available surface water supplies during wetter years, helping to offset potential increases in groundwater pumping during drought when surface water supplies are limited.

8.3.3. Flood Mitigation Projects

8.3.3.1. Tuolumne River Flood Mitigation and Direct Recharge Project (Project 7)

8.3.3.1.1. Project Description
The Tuolumne River Flood Mitigation and Direct Recharge Project (Project) is intended to be a cooperative long-term Project between Modesto Irrigation District (MID)MID and the NDE #25..
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landowners and is designed to be implemented with no impacts to MID’s existing agricultural and urban customers. Currently developed agriculture in the NDE areas of the Modesto subbasinSubbasin is estimated to be approximately 36,000 acres, of which approximately 30,000 acres is deciduous fruits and nuts (permanent crops). With limited exception, the entire NDE area is solely reliant on groundwater from the Modesto subbasin.

The Project is different thandiffers from the Modesto Irrigation District In-lieu and Direct Recharge Project, namely from a timing perspective, and involves the delivery of approximately 20,000 AF of surface water from the Tuolumne River in Wet and Above Normal water years (WYs) through a limited number of new points of diversions off MID’s existing irrigation conveyance infrastructure and subsequent conveyance through newly constructed private irrigation conveyance infrastructure for storage and direct rechargeWYs during the non-growing season. Historically (1972-2020), Wet and Above Normal WYs have occurred approximately 47% of the time on the Tuolumne River. In addition to measurable benefits to groundwater resources within the Modesto subbasinSubbasin, this Project is intended to mitigate flood releases from Don Pedro Reservoir during the winter months whereby reducing impacts on the lower Tuolumne River (City of Modesto and growers near the confluence of the lower Tuolumne River and the San Joaquin River), the San Joaquin River and the Delta. Under the current Final Environmental Impact Statement for the relicensing of Don Pedro Reservoir, there is estimated to be approximately 1,500,000 AF of surface water in Wet WYs and 620,000 AF of surface water in Above Normal WYs in the Tuolumne River above and beyond that. These water volumes exceed what is necessary to meetingmeet existing customer demands (all Tuolumne River Partners) and the recommended instream flow obligations. As a result, 20,000 AF of Tuolumne River surface water to applicable NDE areas during the non-growing season amounts to approximately 1% and 3% of available surface water supply respectively, for Wet and Above Normal WYs. New licenses for diversions/water rights may be required for this Project.

8.3.3.1.2. Public Noticing
The public and other agencies will be notified of the planned or ongoing implementation of PMAProject activities through the outreach and communication channels identified in the GSP, during the preparation process of the PEIR (if applicable), and during updates presented at regularly scheduled STRGBA GSA meetings. Noticing will occur as potential activities are being considered for implementation, and as ongoing and planned activities are implemented. Noticing will inform the public and other agencies that the proponent is considering or will be implementing the PMAProject and will provide a description of the actions that will be taken.

Public and/or inter-agency noticing may be facilitated through the GSA’s boardSTRGBA GSA meetings and/or Districtlocal agency meetings, associated website(s), inter-basin coordination meetings, other public meetings hosted by the GSAs, GSP annual reportsAnnual Reports and five-year updatesPeriodic Evaluations, public scoping meetings, and/or environmental/regulatory applicable permitting notification processes. #25..
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8.3.3.1.3. Permitting and Regulatory Process
Required permitting and regulatory review would be initiated as necessary through consultation with applicable governing agencies. Surface water would be diverted forthrough this Projectproject by MID throughvia existing pre- and post-1914 water rights.

Governing agencies that may be consulted for this Project include but are not limited to the State Water Resources Control Board (SWRCB), the County(ies) of Stanislaus and/or Tuolumne Counties, USBR, and DWR.

If necessary for field flooding, the, Project proponentproponents will obtain land gradingany applicable permits from the County(ies).Tuolumne and Stanislaus Counties. Recharge Projectsprojects and construction or expansion of conveyance facilities may also require an environmental review process under CEQA.

8.3.3.1.4. Expected Benefits
Benefits to Sustainability Indicators
Surface water deliveries during the non-irrigation season are expected to provide direct groundwater recharge to the Subbasin. For fields that are irrigated using groundwater, surface water deliveries during the irrigation season are expected to offset groundwater demand and provide groundwater recharge benefits. In both cases, the sustainability indicators expected to benefit from this Project are groundwater levels, groundwater in storage, land subsidence, and interconnected surface water, and land subsidence (depending on where recharge occurs).. All benefits to sustainability indicators in the Modesto Subbasin will be evaluated through groundwater monitoring at nearby monitoring sites, identified in the GSP.

Benefits to Disadvantaged Communities
The Tuolumne River flood mitigation and direct recharge Project is expected to provide direct recharge for NDE landowners area. Most communities in the Modesto Subbasin, particularly the NDE area, are classified as DACs, SDACs, or EDAs (according to 2018 census data, evaluated by place, tract, and block group). Depending on which specific parcels receive surface water deliveries, this Project may directly benefit specific DACs in this area.

In addition, maintenance or improvement of groundwater levels will help to protect beneficial groundwater use by rural domestic wells from potential adverse impacts related to chronic groundwater level decline. Benefits to groundwater conditions in the Modesto Subbasin are also expected to broadly benefit all DACs, SDACs, and EDAs.

Volumetric Benefits to the Subbasin Groundwater System
The expected yield of the Tuolumne River flood mitigationFlood Mitigation and direct rechargeDirect Recharge Project was estimated by simulating this Project inthrough simulations using the C2VSimTM model. General information and assumptions used to simulate this Project are summarized in the Implementationfollowing section below.

Additional information is provided in Section 8.5: Plan for Achieving SustainabilityPlan for Achieving Sustainability.
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On average across all years, the Tuolumne River flood mitigation and direct recharge Projectproject is expected to provide approximately 9,600 AFY of recharge benefit to the Modesto Subbasin. These benefits would accrue in years with wetWet or above normalAbove Normal hydrologic conditions when sufficient water is expected to be available for on-farm recharge (approximately 50 percent of years historically). In those years, approximately 20,000 AFY of groundwater recharge is expected to occur.

Evaluation of benefits will be based on analysis of without-Projectproject and withProjectproject measurements supported by modeling. Measured parameters will include surface water deliveries, groundwater levels, and other parameters to be determined.

Modeling mayfor the Project will be done with the C2VsimTMC2VSimTM model used for GSP development.

8.3.3.1.5. Implementation Criteria, Status, and Strategy Implementation Strategy and Timeline
Project involves the delivery of surface water from the Tuolumne River in Wet and Above Normal water yearsWYs through a limited number of new points of diversions off MID’s existing irrigation conveyance infrastructure and subsequent conveyance through newly constructed private irrigation conveyance infrastructure for direct recharge. during the growing season. It is expected that fields with non-permanent crops, permeable soils, and existing flood irrigation infrastructure will be most suitable for Project participation.

This Project is currently in the early conceptual stage.The Project is planned for discussion with the MID Water Advisory Committee to determine if an implementation plan will be prepared and recommended to Board of Directors for approval. Thus, the start and completion dates for this Project have yet to be determined and will be provided in GSP Annual Reports and Five-Year Assessment Reports when knownPeriodic Evaluations.

However, once Project implementation begins, it is expected that MID would deliver surface water during wet and above normal hydrologic years (approximately 50 percent of years historically) when sufficient water is available for field flooding and on-farm recharge. MID would deliver surface water to participating fields, and irrigators would use that water to flood their fields for recharge. Subsequent analysis of projected water availability, actual annual application rates, and extent of participating lands will be necessary as Project development continues and implementation begins.

Implementation Assumptions for Modeling
The Tuolumne River Flood Mitigation Direct Recharge Project has been modeled in the C2VsimTM modelC2VSimTM. Additional information about project-related modeling is described in Section 8.5: Plan for Achieving Sustainability.

The following general information and assumptions were used to simulate implementation of the project:
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model. Additional information about project-related modeling is described in Section 8.5:

Plan for Achieving Sustainability.

The following general information and assumptions were used to simulate implementation of the Project:
• Volume of water: 20,000 AFY were provided during Wet and Above Normal years, distributed between the months of January and February for direct recharge. The annual average during the 50-year simulation period would be of 9,600 AFY.

8.3.3.1.6. Water Source and Reliability
The Project involves the delivery of approximately 20,000 AF of surface water from the Tuolumne River in Wet and Above Normal water yearsWYs through a limited number of new points of diversions off MID’s existing irrigation conveyance infrastructure and subsequent conveyance through newly constructed private irrigation conveyance infrastructure for and direct recharge during the non-growing season. Historically (1972- 2020), Wet and Above Normal WYs have occurred approximately 47% of the time on the Tuolumne River. In addition to measurable benefits to groundwater resources within the Modesto subbasin, this Project is intended to mitigate flood releases from Don Pedro Reservoir during the winter months whereby reducing impacts on the lower Tuolumne River (City of Modesto and growers near the confluence of the lower Tuolumne River and the San Joaquin River), the San Joaquin River and the Delta. Under the current Final Environmental Impact Statement for the relicensing of Don Pedro Reservoir, there is estimated to be approximately 1,500,000 AF of surface water in Wet WYs and 620,000 AF of surface water in Above Normal WYs in the Tuolumne River above and beyond that necessary to meeting existing customer demands (all Tuolumne River Partners) and the recommended instream flow obligations. As a result, 20,000 AF of Tuolumne River surface water to applicable NDE areas during the non-growing season amounts to approximately 1% and 3% of available surface water supply respectively, for Wet and Above Normal WYs. New licenses for diversions/water rights may be required for this project.

8.3.3.1.6.8.3.3.1.7. Legal Authority
The GSAs, Districts, and individual Project proponents have the authority to plan and implement projects through consultation with applicable governing agencies. MID has the authority to contract with and provide deliveries to non-districted eastNDE landowners area, and individual irrigators have the authority to apply surface water to their fields for on-farm recharge.

8.3.3.1.7.8.3.3.1.8. Estimated Costs and Funding Plan
Potential costs offor this Project may include Project coordination and administration, financial, or other incentives to encourage on-farm recharge, field preparation to enhance flooding, and other potential on-field monitoring equipment. Costs per site may vary depending on changes in Project implementation and incentives. Slightly higher costs per #25..
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site would likely be incurred in the first year an irrigator participates, as more coordination and site preparation may be required. The total costs of the Project will vary over time, depending on the number of sites receiving water, the extent to which irrigators require coordination and support, and any applicable Project incentives.

This Project is currently in the early conceptual stage. Thus, the anticipated costs have yet to be determined and will be reported in GSP Annual Reports and Five-Year Assessment Reports when known. This project shares the same infrastructural development as the Modesto Irrigation District In-Lieu and Direct Recharge Project (Project 5). However, if implemented without Project 5, high-level initial estimates are on the order of $53,340,000 – $75,000,000 of new conveyance infrastructure. It is anticipated that STRGBA GSA member agencies and/or NDE landowners would identify funding sources to cover Project costs as part of Project development. These may include grants (e.g., Prop 1, Prop 68, NRCS, others), fees, and loans.

8.3.3.1.8.8.3.3.1.9. Management of Groundwater Extractions and Recharge Per 23 CCR § 354.44(b)(9), all PMAs developed for implementation are targeted to maintain the balance of groundwater extractions and recharge to help ensure that lowering of groundwater levels or depletion of supply during periods of drought is offset by increases in groundwater levels and storage in other years.

Recharge benefits of this Project are expected to increase the use and recharge of available surface water supplies during wetter years, helping to offset potential increases in groundwater pumping during drought when surface water supplies are limited.

8.3.3.2. Dry Creek Flood Mitigation and Direct Recharge Project (Project 8)

8.3.3.2.1. Project Description
The Dry Creek Flood Mitigation and Direct Recharge Project (Project) is intended to be a cooperative long-term Project implemented by the NDE landowners and is designed to be constructed and managed in a way to prevent negative impacts to downstream users.

Currently developed agriculture in the NDE areas of the Modesto subbasin is estimated to be approximately 36,000 acres, of which approximately 30,000 acres isare deciduous fruits and nuts (permanent crops).. With limited exception, the entire NDE area is solely reliant on groundwater from the Modesto Subbasin. The Project involves the delivery of approximately 5,400 AF of surface water from Dry Creek through a limited number of new and/or existing points of diversions off Dry Creek and subsequent conveyance through new and/or existing private irrigation conveyance infrastructure for direct recharge during the non-growing season. The volume of water associated with this Project was derived from previous work done on behalf of Stanislaus County and is representative of only a fraction of modeled results for a 2-year storm event in the lower reaches of Dry Creek. As a result, both the frequency and volume of water available are conservative estimates. In addition to measurable benefits to groundwater resources within the Modesto subbasin, this Project is #25..
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intended to mitigate flood flows in Dry Creek whereby reducing impacts on the lower Tuolumne River (City of Modesto and growers near the confluence of the lower Tuolumne River and the San Joaquin River), the San Joaquin River and the Delta. New licenses for diversions/water rights may be required for this project.

8.3.3.2.2. Public Noticing
The public and other agencies will be notified of the planned or ongoing implementation of PMAProject activities through the outreach and communication channels identified in the GSP, during the preparation process of the PEIR (if applicable), and during updates presented at regularly scheduled STRGBA GSA meetings. Noticing will occur as potential activities are being considered for implementation, and as ongoing and planned activities are implemented. Noticing will inform the public and other agencies that the proponent is considering or will be implementing the PMAProject and will provide a description of the actions that will be taken.

Public and/or inter-agency noticing may be facilitated through the GSA’s boardSTRGBA GSA meetings and/or City and Agencylocal agency meetings, associated website(s), inter-basin coordination meetings, other public meetings hosted by the GSAs, GSP annualAnnual reports and five-year updatesPeriodic Evaluations, public scoping meetings, and/or environmental/regulatoryapplicable permitting notification processes.

8.3.3.2.3. Permitting and Regulatory Process
Required permitting and regulatory review would be initiated as necessary through consultation with applicable governing agencies. Surface water would be diverted through this project by MID via existing water rights. Governing agencies that may be consulted for this Projectproject include but are not limited to the State Water Resources Control Board (SWRCB), the County(ies) of Stanislaus and/or Tuolumne Counties, USBR, and DWR.

If necessary for field flooding, the, Project proponentproponents will obtain land gradingany applicable permits from the County(ies).Tuolumne and Stanislaus Counties. Recharge projects and construction or expansion of conveyance facilities may also require an environmental review process under CEQA.

8.3.3.2.4. Expected Benefits
Benefits to Sustainability Indicators
Surface water deliveries from storm events during the non-irrigation season are expected to provide direct groundwater recharge to the Subbasin. The sustainability indicators expected to benefit from this Projectproject are groundwater levels, groundwater in storage, land subsidence, and interconnected surface water, and land subsidence (depending on where recharge occurs).. All benefits to sustainability indicators in the Modesto Subbasin will be evaluated through groundwater monitoring at nearby monitoring sites, identified in the GSP.
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Benefits to Disadvantaged Communities
The Dry Creek flood mitigationFlood Mitigation and direct rechargeDirect Recharge Project is expected to provide direct recharge for NDE landowners area. Most communities in the Modesto Subbasin, including the NDE area, are classified as DACs, SDACs, or EDAs (according to 2018 census data, evaluated by place, tract, and block group). Depending on which specific parcels receive surface water deliveries, this Project may directly benefit specific DACs in this area. In addition, maintenance or improvement of groundwater levels will help to protect beneficial groundwater use by rural domestic wells from potential adverse impacts related to chronic groundwater level decline. Benefits to groundwater conditions in the Modesto Subbasin are also expected to broadly benefit all DACs, SDACs, and EDAs.

Volumetric Benefits to the Subbasin Groundwater System
The expected yield of the Dry Creek flood mitigationFlood Mitigation and direct rechargeDirect Recharge Project was estimated by simulating this Projectthrough simulations in the C2VSimTM model. General information and assumptions used to simulate this Project are summarized in the Implementationfollowing section below. Additional information is provided in Section 8.5: Plan for Achieving SustainabilityPlan for Achieving Sustainability.

On average across all years, Dry Creek flood mitigation and direct rechargethe Project is expected to provide approximately 5,400 AFY of recharge benefit to the Modesto Subbasin.

Evaluation of benefits will be based on analysis of without-Project and with-Project measurements supported by modeling. Measured parameters will include surface water deliveries, groundwater levels, and other parameters to be determined. Evaluation of benefits will be based on analysis of without-project and with-project measurements supported by modeling. Measured parameters will include surface water deliveries, groundwater levels, and other parameters to be determined. Modeling mayModeling will be done with the C2VSimTM model used for GSP development.

8.3.3.2.5. Implementation Criteria, Status, and Strategy Implementation Strategy and Timeline
The Project involvesanticipates the delivery of approximately 5,400 AF of surface water from Dry Creek through a limited number of new and/or existing points of diversions off Dry Creek and subsequent conveyance through new and/or existing private irrigation conveyance infrastructure for direct recharge during the non-growing season. At the initiation of this Project and on an ongoing basis, the GSAs and/or NDE landowners plan to identify fields that are most suitable for groundwater recharge. It is expected that fields with non-permanent crops, permeable soils, and existing flood irrigation infrastructure will be most suitable for Project participation.
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This Project is currently in the early conceptual stage. Thus, the start and completion dates for this Project have yet to be determined and will be provided in GSP Annual Reports and Five-Year Assessment Reports when knownPeriodic Evaluations.

However, once Project implementation begins, it is expected that storm water would be available for diversion during wet and above normal hydrologic years (approximately 50 percent of years historically) when sufficient water is available for field flooding and on-farm recharge. Subsequent analysis of projected water availability, actual annual application rates, and extent of participating lands will be necessary as Project development continues and implementation begins.

Implementation Assumptions for Modeling
The Dry Creek Flood Mitigation Direct Recharge Project has been modeled in the C2VSimTM. Additional information about project-related modeling is described in Section

8.5: Plan for Achieving Sustainability.

The following general information and assumptions were used to simulate implementation of the model. Additional information about project-related modeling is described in Section

8.5: Plan for Achieving Sustainability.

The following general information and assumptions were used to simulate implementation of the project:
• Volume of water: 5,400 AFY were diverted during all years, distributed evenly in the months of January and February. The annual average during the 50-year simulation period would be of 5,400 AFY.
• The total volume would be applied as direct recharge over the aquifer.

8.3.3.2.6. Water Source and Reliability
The Project involves the diversion and application of approximately 5,400 AF of surface water from Dry Creek through a limited number of new and/or existing points of diversions off Dry Creek and subsequent conveyance through new and/or existing private irrigation conveyance infrastructure for direct recharge during the non-growing season. The volume of water associated with this Project was derived from previous work done on behalf of Stanislaus County and is representative of only a fraction of modelled results for a 2-year storm event in the lower reaches of Dry Creek. As a result, both the frequency and volume of water available are conservative estimates. In addition to measurable benefits to groundwater resources within the Modesto subbasinSubbasin, this Project is intended to mitigate flood flows in Dry Creek whereby reducing impacts on the lower Tuolumne River (City of Modesto and growers near the confluence of the lower Tuolumne River and the San Joaquin River), the San Joaquin River and the Delta. New licenses for diversions/water rights may be required for this project.
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8.3.3.2.7. Legal Authority
The GSAs and individual Project proponents have the authority to plan and implement projects through consultation with applicable governing agencies. Individual irrigators have the authority to apply surface water to their fields for on-farm recharge. However, new licenses for diversions/water rights may be required for this Project.

8.3.3.2.8. Estimated Costs and Funding Plan
Potential costs of this Project may include Project coordination and administration, financial, or other incentives to encourage on-farm recharge, field preparation to enhance flooding, and other potential on-field monitoring equipment. Costs per site may vary depending on changes in Projectproject implementation and incentives. Slightly higher costs per site would likely be incurred in the first year an irrigator participates, as more coordination and site preparation may be required. The total costs of the Project will vary over time, depending on the number of sites receiving water, the extent to which irrigators require coordination and support, and any applicable Project incentives.

This Project is currently in the early conceptual stage. Thus, the anticipated costs have yet to be determined and will be reported in GSP Annual Reports and Five-Year Assessment Reports when knownPeriodic Evaluations. However, high-level initial estimates are on the order of $4,800,600 - $6,750,000 of new conveyance infrastructure. It is anticipated that STRGBA GSA member agencies would identify funding sources to cover Project costs as part of Project development. These may include grants (e.g., Prop 1, Prop 68, NRCS, others), fees, and loans.

8.3.3.2.9. Management of Groundwater Extractions and Recharge Per 23 CCR § 354.44(b)(9), all PMAs developed for implementation are targeted to maintain the balance of groundwater extractions and recharge to help ensure that lowering of groundwater levels or depletion of supply during periods of drought is offset by increases in groundwater levels and storage in other years.

In particular, recharge benefits of this Projectproject are expected to increase the use and recharge of available surface water supplies during wetter years, helping to offset potential increases in groundwater pumping during drought when surface water supplies are limited.

8.4. OTHERSUPPLEMENTAL PROJECTS TO BE IMPLEMENTED AS NEEDED (GROUP 3) This section describes potential Project(s)Projects that wouldmay be implemented if determinedin the Subbasin to support local goals and future GSA activities (Supplemental Projects, Table 8-Table 8-1). Group 3 projects are not currently planned for implementation; however, the GSAs will continue assessing their feasibility to be necessary, pending future conditions in the Modesto Subbasin (Group 3 Projects, Table 8-1). Whilesupport sustainable groundwater management. Regardless, should these Projects could contributeprojects be implemented, the projects would provide benefits in contributing to the attainment of SMCs #25..
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and the sustainability goal and support GSP implementation, they. Group 3 projects are in the early conceptual or planning stages at this time, with no specific implementation timeline established.

To the extent that future monitoring indicates the occurrence of undesirable results in the Subbasin, additional Projects will be implemented to address these changing conditions. As additional development occurs for the Projects described below or for other projects identified in the future, updates will be documented and reported in subsequent GSP Annual Reports and Five-Year Assessment Reports.

Summary of Criteria for Project Implementation (23 CCR §354.44(b)(1)(A)) Summary of Criteria for Project Implementation (23 CCR §354.44(b)(1)(A)) As described above, the Projects described in this section are still in the early conceptual or planning stages. These potential Projects could be implemented as needed to achieve and maintain long-term sustainable groundwater management across the Modesto Subbasin.

The Projects would be evaluated for implementation if, based on data gathered during GSP implementation, the GSAs find that established IMs and MOs cannot be maintained and/or if MTs are being approached. This adaptive approach will be informed by continued monitoring of groundwater conditions, using the monitoring network and methods described in the GSP. This initial list will likely be supplemented with additional projects The GSAs will continue evaluating the feasibility for implementing these projects in the future.

Additional projects may be added to this list as they are identified and reported through Annual Reports and Five-Year Assessment ReportsPeriodic Evaluations of the GSP.

In addition, there are projects that have been considered in the past as part of the Integrated Regional Water Management Planning (IRWMP) and are included in the East Stanislaus IRWMP project database2. These projects are considered as potential projects to support the GSP implementationgroundwater sustainability in the Subbasin but are currently considered as alternative options and are not directly analyzed in this Chapter.

8.4.1. Stanislaus River Flood Mitigation and Direct Recharge Project (Project 9) The Stanislaus River Flood Mitigation and Direct Recharge Project (Project) is conceptually proposed by the NDE landowners to be a cooperative long-term Project with Oakdale Irrigation District (OID) and is designed to be implemented with no impacts to OID’s existing agricultural customers. Currently developed agriculture in the NDE areas of the Modesto subbasin is estimated to be approximately 36,000 acres, of which approximately 30,000 acres is deciduous fruits and nuts (permanent crops). With limited exception, the NDE area is solely reliant on groundwaterThe project differs from the Modesto subbasin. The Project is different than the Oakdale Irrigation District In-lieu and Direct Recharge Project, namely 2 http://www.eaststanirwm.org/projects/
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from a timing perspective, and involves the delivery of approximately 5,000 AF of surface water from the Stanislaus River in Wet water years (WYs) through a limited number of new points of diversion off OID’s existing irrigation conveyance infrastructure and subsequent newly constructed private irrigation conveyance infrastructure for direct recharge during the non-growing season. Storage in New Melones is approximately 2.5 times what the watershed yields on an average annual basis and as a result, the magnitude and frequency (5,000 AF and wetWet WYs) of this Project has been limited. Nonetheless, this Project is intended to mitigate flood releases from New Melones Reservoir during the winter months whereby reducing impacts on the lower Stanislaus River (growers along the lower Stanislaus River), the San Joaquin River and the Delta. This Project may require the acquisition of a right to divert flood flows and supplemental groundwater in storage application, as well as agreements with multiple agencies potentially including but not limited to, UBSR, OID, and the SWRCB for the revised operation of existing storage facilities, water diversion and rights on the Stanislaus River.

If this project is pursued, furtherFurther analysis, consultation, and review would be neededis anticipated prior to any additional refinementdetermination of water availability and utilization given itfor the project. Additional considerations may be contingent uponinclude the terms and negotiations of a new water rights permit/license if required. Of note, historical. Historical operations of New Melones Reservoir and future water supply availability also hashave the potential to change significantly if the Lower San Joaquin River flow objectives proposed in the Bay-Delta Plan amendments and Final SED are implemented.

A summary of the Project is provided in Table 8-Table 8-45.

Table 8-4:-5: Stanislaus River Flood Mitigation and Direct Recharge Project:

Summary (23 CCR §354.44(b))
Item in GSP
Regulations Description
Implementation
Strategy and Criteria
(§354.44(b)(1)(A);
§354.44(b)(6))
Although similar toUtilizing the conveyance infrastructure provided by the OID In-lieu and Direct Recharge Project, this Project is different because of the timing perspective and the delivery ofwould provide approximately 5,000 AF of surface water from the Stanislaus River in Wet water years (WYs). This. The Project is intended to mitigate flood releases from New Melones Reservoir during the winter months whereby reducing impacts on the lower Stanislaus River, the San Joaquin River, and the Delta. ThisThe Project is currently in the conceptual stage and is a Project will continue to be evaluated by the GSAs and NDE landowners may wish to pursue in the future if additional Projects are needed to reach sustainability in lieu of Management Actions..

Timeline and
Implementation Status
This Project is currently in the early conceptual stage and thus, the start and completion dates for this Project have yet to benot been #25..
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Item in GSP
Regulations Description
(§354.44(b)(4)) determined. If it should ultimately be implemented, an updated timelineUpdates to Project activities will be provided in GSP Annual Reports and Five-Year Assessment ReportsPeriodic Evaluations.

Benefits are expected to accrue in wet hydrologic year types when flood water is available for use, potentially beginning the first year of Project implementation.

Notice to public and
other agencies
Public Noticing
(§354.44(b)(1)(B))
Public and/or inter-agency noticing willmay be facilitated through GSAs and/or district boardSTRGBA GSA meetings, GSAs and/or districtlocal agency meetings, associated website(s), GSAs and/or district newsletters, inter-basin coordination meetings, GSPother public meetings hosted by the GSAs, Annual Reports and Five-Year Assessment ReportsPeriodic Evaluations, public scoping meetings, and environmental/regulatory/or applicable permitting notification processes.

Water source &
reliabilitySource &
Reliability
(§354.44(b)(6))
ThisThe Project would use available flood water from the Stanislaus River.

This Project is currently in the early conceptual stage. The precise reliability of available water would be identified if/when the Project ishas been evaluated and selecteddeveloped for implementation. This information will be reported in GSP Annual Reports and Five-Year Assessment Reports when knownPeriodic Evaluations.

Legal authority,
permitting
processes,Authority,
Permitting and
regulatory
controlRegulatory
Processes,
(§354.44(b)(3);
§354.44(b)(7))
Required permitting and regulatory review will be projectProjectspecific and initiated through consultation with applicable governing agencies. Governing agencies for which consultation will be initiated may include, but is not limited to: OID, USBR, DWR, SWRCB, CDFW, Flood Board, RWQCBs, USFWS, NMFS, LAFCo, County(ies) of Stanislaus and/or Tuolumne, and CARB.will be identified during Project evaluation.

Benefits and benefit
evaluation
methodology
(§354.44(b)(5))
The sustainability indicators expected to benefit are would be determined during Project evaluation. Conceptually, groundwater levels, groundwater in storage, land subsidence, and depletion of interconnected surface water would benefit from this project.

This Project is currently in the early conceptual stage. Thus, theThe expected yield of this groundwater benefits from the Project hasare not yet to be determined known and will be reported in GSP Annual Reports and Five-Year Assessment Reports when known. determined during project evaluation. Evaluation of benefits will be based on analysis of without-Project project and with-Projectproject #25..
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Item in GSP
Regulations Description
effects on the SGMA sustainability indicators. Each Project may be evaluated as part of a scenario and the C2VSimTM would be used to assess the benefits and impacts on the subbasin sustainability.

Costs
(§354.44(b)(8))
This Project is currently in the early conceptual stage. Thus, the The anticipated costs of this Project have yet towill be determined and will be reported in GSP Annual Reports and Five-Year Assessment Reports when known. The NDE landowners, as theduring its evaluation. The Project proponent, would identify funding sources to cover Project costs as part of Project development. These may include grants, fees, loans, and other assessments.

8.4.2. Retention Basin Standards Specifications Update (Project 10) This ProjectThe Retention Basin Standards Specifications Update Project (Project) would aim to change standards for future storm drains so that the drains would not discharge straight to rivers, creeks, or canals but rather to retention basins. This would increase the sustainability footprint of the City of Modesto through future growth. Currently, approximately 16.37 Square miles out of 45 Square miles (Approximately 36 percent) of the surface area in the City of Modesto area draindrains to surface water, with approximately 64 percent draining and contributing to local recharge. If the City of Modesto adopts new storm drain standards, 100 percent of runoff from newly developed areas would reach a retention system with an approximate runoff coefficient of 0.7, and an average rainfall of

12.14 inches per year.

A summary of the Project is provided in Table 8-Table 8-56. #25..
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Table 8-5:-6: Retention Basin Standards Specifications Update: Summary (23 CCR §354.44(b))
Item in GSP
Regulations Description
Implementation
Strategy and Criteria
(§354.44(b)(1)(A);
§354.44(b)(6))
ThisThe Project would aim to change standards for future storm drains so that the drains would not discharge straight to rivers, creeks, or canals but rather tothan retention basins. ThisThe Project is currently in the conceptual stage and is a Project being evaluated by the GSAs may decide to pursue in the future if additional strategies are needed to reach sustainability.

Timeline and
Implementation Status
(§354.44(b)(4))
This Project is currently in the early conceptual stage and will be implemented at the discretion of the GSAs. Thus, the start and completion dates for this Project have yet to benot been determined and if the GSAs determine it should be implemented, an update.

Updates on Project activities will be provided in GSP Annual Reports and Five-Year Assessment Reports.Periodic Evaluations. Benefits are expected to accrue in all years and potentially beginning the first year of Project implementation.

Notice to public and
other agencies
Public Noticing
(§354.44(b)(1)(B))
Public and/or inter-agency noticing willmay be facilitated by the City of Modesto as well as through GSAs and/or City councilSTRGBA GSA meetings, GSAs and/or citylocal agency meetings, associated website(s), GSAs and/or district newsletters, inter-basin coordination meetings, GSPother public meetings hosted by the GSAs, Annual Reports and Five-Year Assessment ReportsPeriodic Evaluations, public scoping meetings, and environmental/regulatory/or applicable permitting notification processes.
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Item in GSP
Regulations Description
Water source &
reliabilitySource &
Reliability
(§354.44(b)(6))
This Project would use urban storm runoff flows from the City of Modesto. This Project is currently in the early conceptual stage. The precise reliability of available water would be identified if/when the Project ishas been evaluated and selected and developed for implementation. This information wouldwill be reported in GSP Annual Reports and Five-Year Assessment Reports when knownPeriodic Evaluations.

Legal authority,
permitting
processes,Authority,
Permitting and
regulatory
controlRegulatory
Processes
(§354.44(b)(3);
§354.44(b)(7))
The GSAs and individual Project proponents have the authority to plan and implement projects. Required permitting and regulatory review will be project-specific and initiated through consultation with applicable governing agencies. Governing agencies for which consultation will be initiated may include, but is not limited to: DWR, SWRCB, CDFW, Flood Board, RWQCBs, USFWS, NMFS, LAFCo,
County of Stanislaus, and CARB.will be identified during Project evaluation.

Benefits and benefit
evaluation
methodologyBenefit
Evaluation
Methodology
(§354.44(b)(5))
The sustainability indicators expected to benefit are would be determined during Project evaluation. Conceptually, groundwater levels, groundwater in storage, land subsidence, and depletion of interconnected surface water.

This Project is currently in the early conceptual stage. Thus, the expected yield of would benefit from this Project has.

The expected groundwater benefits from the project are not yet to be determined and would be reported in GSP Annual Reports and Five-Year Assessment Reports when known.
and will be determined during project evaluation. Evaluation of benefits wouldwill be based on analysis of without-Project project and withProjectproject effects on the SGMA sustainability indicators. Each Project may be evaluated as part of a scenario and the C2VSimTM would be used to assess the benefits and impacts on the subbasin sustainability.

Costs
(§354.44(b)(8))
This Project is currently in the early conceptual stage. Thus, the The anticipated costs of this Project have yet towill be determined and would be reported in GSP Annual Reports and Five-Year
Assessment Reports when known. The Projectduring its evaluation.

The project proponent would identify funding sources to cover Project costs as part of Project development. These sources may include grants, fees, loans, and other assessments.
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8.4.3. Recharge Ponds Constructed by Non-District East Landowners (Project 11) ThisThe Recharge Ponds Constructed by Non-District East Landowners Project (Project) would aim to capture some wintertime runoff from the Dry Creek Watershed by constructing detention basins. These It is anticipated the basins would be constructed by NDE Landowners. NDE participants have identified five reservoirs for direct diversion and off-stream storage through an existing water right on Dry Creek. Diversions would originate from a facility on Dry Creek, which was constructed and fully operational by February 2021, to the reservoirs for storage. Stored water would then be used during the growing season in-lieu of groundwater while also providing direct recharge benefits. Conveyance infrastructure from the diversion facility to the proposed reservoirs and receiving irrigated acreage was completed in April 2024. The Project is currently in the conceptual phase.

Project scope, implementation schedule, groundwater benefits, and costs will be evaluated further and presented in Annual Reports and Periodic Evaluations.

A summary of the Project is provided in Table 8-Table 8-67.

Table 8-6:-7: Recharge Ponds Constructed by Non-District East Landowners:

Summary (23 CCR §354.44(b))
Item in GSP Regulations Description
Implementation
Strategy and Criteria
(§354.44(b)(1)(A);
§354.44(b)(6))
This Project would aim to capture some wintertime runoff from the Dry Creek Watershed by constructing detention basins. TheseIt is anticipated the basins would be constructed by NDE Landowners.

The project is currently in the conceptual stage and is being evaluated by the GSAs.

Timeline and
Implementation Status
(§354.44(b)(4))
This Project is currently in the early conceptual stage and will be implemented at the discretion of the NDE Landowners. Thus, the start and completion dates for this Project have yet to benot been determined and if the NDE Landowners determines it should be implemented, an updated timeline. Updates on Project activities will be provided in GSP Annual Reports and Five-Year Assessment Reports.Periodic Evaluations. Benefits are expected to accrue during the winter periods when water is available for use,
potentiallymonths beginning the first year of Project implementation.

Notice to public and
other agencies
Public Noticing
(§354.44(b)(1)(B))
Public and/or inter-agency noticing may be facilitated through GSAs STRGBA GSA meetings and/or otherlocal agency meetings,
GSAsassociated website(s), GSAs newsletters, inter-basin coordination meetings, GSPother public meetings hosted by the GSAs, Annual Reports and Five-Year Assessment ReportsPeriodic Evaluations, public scoping meetings, and
environmental/regulatory/or applicable permitting notification processes.
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Item in GSP Regulations Description
Water source &
reliabilitySource &
Reliability
(§354.44(b)(6))
This Projectproject would use water from the Dry Creek Watershed.

This Project is currently in the early conceptual stage. The precise reliability of available water would be identified if/when the Project isproject has been evaluated and selected and developed for implementation. This information wouldwill be reported in GSP Annual Reports and Five-Year Assessment Reports when
knownPeriodic Evaluations.

Legal authority,
permitting
processes,Authority,
Permitting and
regulatory
controlRegulatory
Processes
(§354.44(b)(3);
§354.44(b)(7))
Required permitting and regulatory review will be project-specific and initiated through consultation with applicable governing agencies.

Governing agencies for which consultation will be initiated may include, but is not limited to: DWR, SWRCB, CDFW, Flood Board, RWQCBs, USFWS, NMFS, LAFCo, County(ies) of Stanislaus and/or Tuolumne, and CARB.will be identified during project evaluation.

Benefits and benefit
evaluation
methodologyBenefit
Evaluation Methodology
(§354.44(b)(5))
The sustainability indicators expected to benefit are would be determined during project evaluation. Conceptually, groundwater levels, groundwater in storage, land subsidence, and depletion of interconnected surface water would benefit from this project.

This Project is currently in the early conceptual stage. Thus, theThe expected yield of this Project hasgroundwater benefits from the project are not yet to be determined and would be reported in GSP Annual Reports and Five-Year Assessment Reports when known.
and will be determined during project evaluation. Evaluation of benefits wouldwill be based on analysis of without-Project project and with-Projectproject effects on the SGMA sustainability indicators.

Each Project may be evaluated as part of a scenario and the C2VSimTM would be used to assess the benefits and impacts on the subbasin sustainability.

Costs
(§354.44(b)(8))
This Project is currently in the early conceptual stage. Thus, the The anticipated costs of this Project have yet toproject will be determined and would be reported in GSP Annual Reports and Five-Year Assessment Reports when known. The NDE
landowners, as the Projectduring its evaluation. The project proponent would identify funding sources to cover Project costs as part of Projectproject development. These sources may include grants, fees, loans, and other assessments.
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8.4.4. OID Irrigation and Recharge to Benefit City of Oakdale (Project 12) This ProjectThe OID Irrigation and Recharge to Benefit City of Oakdale Project (Project) proposes to utilize surface water from OID to irrigate the City of Oakdale’s parks. The first phase of this Project is beingwas constructed at two City of Oakdale parks to assess the costs and benefits. for implementation of additional components. The two parks involved in thisthe initial phase are located within close proximity tonear an existing OID conveyance system. Surface water for irrigation would beis being provided for City of Oakdale use during the irrigation, starting as early as March 1st and ending no later than October 31st each year.

Anticipated yield of thisfrom the Project is approximately 50 AF per year. Pending results from the initial phase of the Project, expanded implementation of this Project in cooperation with OID may be subsequently considered by the City of Oakdale.

A summary of the Project is provided in Table 8-Table 8-78.

Table 8-7:-8: OID Irrigation and Recharge to Benefit City of Oakdale Summary (23 CCR §354.44(b))
Item in GSP Regulations Description
Implementation Strategy
and Criteria
(§354.44(b)(1)(A);
§354.44(b)(6))
This Project would aimaims to reduce City of Oakdale groundwater pumping by providing OID surface water for irrigation of City parks.

Construction of the first phase of implementation is currently in progress. has been completed. The City of Oakdale may decide to pursue expansion in the future if the first phase is successful and additional strategies are needed to reach sustainability.

Timeline and
Implementation Status
(§354.44(b)(4))
Construction of the first phase of the Project will likely bewas completed by the summer of 2022.An updated timeline and2023.

Updated Project results will be provided in GSP Annual Reports and Five-Year Assessment Reports. Benefits are expected to accrue in all hydrologic year types provided OID’s surface water allocation is sufficient, potentially beginning the first year of Project implementation.

Notice to public and
other agencies
Public Noticing
(§354.44(b)(1)(B))
Public and/or inter-agency noticing willmay be facilitated through GSAs and/or City/District boardSTRGBA GSA meetings, GSAs and/or districtlocal agency meetings, associated website(s), GSAs and/or district newsletters, inter-basin coordination meetings, GSPother public meetings hosted by the GSAs, Annual Reports and Five-Year Assessment ReportsPeriodic Evaluations, public scoping meetings, and environmental/regulatory/or applicable permitting notification processes.
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Item in GSP Regulations Description
Water source &
reliabilitySource &
Reliability
(§354.44(b)(6))
The City of Oakdale remains within the OID boundary and thus is entitled to receive OID surface water when it is available.

Legal authority,
permitting
processes,Authority,
Permitting and
regulatory
controlRegulatory
Processes
(§354.44(b)(3);
§354.44(b)(7))
The Districts/, Cities, and individual Project proponents have the authority to plan and implement projects. Required permitting and regulatory review will be project-specific and initiated through consultation with applicable governing agencies. Governing agencies for which consultation will be initiated will be identified during project evaluation.

Benefits and benefit
evaluation
methodologyBenefit
Evaluation Methodology
(§354.44(b)(5))
The sustainability indicators expected to benefit are groundwater levels, groundwater in storage, land subsidence, and depletion of interconnected surface water.

This first phase of the Project is currently beinghas been constructed. The anticipated yield of this Project is approximately 50 AF per year and actual; results will be reported in GSP Annual Reports and Five-Year Assessment Reports when known.Periodic Evaluations once available.

Evaluation of benefits will be based on analysis of withoutProjectproject and with-Project effectsproject impacts on the SGMA sustainability indicators. Each ProjectThe project may be evaluated as part of a scenario and the C2VSimTM would be used to assess the benefits and impacts on the subbasin
sustainability.Subbasin.

Costs
(§354.44(b)(8))
This first phase of the Project is estimated toproject cost approximately $300250,000. Costs of any future expansion have yet to benot been determined and would be reported in GSP Annual Reports and Five-Year Assessment Reports if pursued and when known.. The City of Oakdale, as the Projectproject proponent, would identify funding sources to cover Projectproject costs as part of Projectproject development. These may include grants, fees, loans, and other assessments.

8.4.5. MID FloodMARFlood-MAR Projects (Project 13)
This The MID Flood-MAR Projects (Project) would support the development of flood managed aquifer recharge (FloodMARFlood-MAR) activities in locations in the Modesto Irrigation DistrictMID boundaries where storm flows are available, or where existing surface #25..
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water facilities can be utilized to direct and control surface water for various beneficial uses.

Components of this Project would be developed privately or as coordinated efforts.

Necessary infrastructure would be installed to connect existing delivery systems to FloodMARFlood-MAR activities. ThisThe Project is astill conceptual Project and has not benefited fromundergoing evaluation, however, the next steps would likely include a feasibility analysis or any subsequentand design.

A summary of the Project is provided in Table 8-Table 8-89.

Table 8-8:-9: MID FloodMARFlood-MAR Projects Summary (23 CCR §354.44(b)) Item in GSP Regulations Description
Implementation Strategy
and Criteria
(§354.44(b)(1)(A);
§354.44(b)(6))
This Project would support the development of flood managed aquifer recharge (FloodMAR)Flood-MAR activities in locations in the Modesto Irrigation DistrictMID where storm flows are available, or where existing surface water facilities can be utilized to direct and control stormwater for various beneficial uses.

The Project may be implemented and would be monitored and quantified with respect to groundwater conditions, as needed, if sustainable levels are not reached following implementation of other PMAs.

This is currently in the conceptual stage and is a Project that may be considered in the future if additional strategies are needed to reach sustainability.

Timeline and
Implementation Status
(§354.44(b)(4))
This Project is currently in the early conceptual stage thus, the start and completion dates for this Project have yet to benot been determined. If the Project proponents determine it should be implemented, an updated timelineUpdates on project activities will be provided in GSP Annual Reports and Five-Year Assessment Reports. Periodic Evaluations.

Benefits would be expected to accrue in wetWet and above normal hydrologic yearsAbove Normal WYs when flood water is available for use, potentially beginning the first year of Project
implementation..

Notice to public and
other agencies
Public Noticing
(§354.44(b)(1)(B))
Public and/or inter-agency noticing wouldmay be facilitated through GSAs and/or district boardSTRGBA GSA meetings, GSAs and/or districtlocal agency meetings, associated website(s), GSAs and/or district newsletters, inter-basin coordination meetings, GSPother public meetings hosted by the GSAs, Annual Reports and Five-Year Assessment ReportsPeriodic Evaluations, public scoping meetings, and environmental/regulatory/or applicable permitting notification processes.
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Item in GSP Regulations Description
Water source &
reliabilitySource &
Reliability
(§354.44(b)(6))
This Project would use water from storm flows or other excess flow.

This Project is currently in the early conceptual stage. The precise reliability of storm flows or other excess flowsavailable water would be identified if/when the Project isproject has been evaluated and selected and developed for implementation. This information wouldwill be reported in GSP Annual Reports and Five-Year Assessment Reports when knownPeriodic Evaluations.

Legal authority,
permitting
processes,Authority,
Permitting and
regulatory
controlRegulatory
Processes
(§354.44(b)(3);
§354.44(b)(7))
Required permitting and regulatory review will be project-specific and initiated through consultation with applicable governing agencies.

Governing agencies for which consultation will be initiated may include, but is not limited to: MID, DWR, SWRCB, CDFW, Flood Board, RWQCBs, USFWS, NMFS, LAFCo, County(ies) of
Stanislaus and/or Tuolumne, and CARB.will be identified during project evaluation.

Benefits and benefit
evaluation
methodologyBenefit
Evaluation Methodology
(§354.44(b)(5))
The sustainability indicators expected to benefit are would be determined during project evaluation. Conceptually, groundwater levels, groundwater in storage, land subsidence, and depletion of interconnected surface water would benefit from this project.

This Project is currently in the early conceptual stage. Thus, theThe expected yield of this Project hasgroundwater benefits from the project are not yet to be determined and would be reported in GSP Annual Reports and Five-Year Assessment Reports when known.
and will be determined during project evaluation. Evaluation of benefits wouldwill be based on analysis of without-Project project and with-Projectproject effects on the SGMA sustainability indicators. Each Project may be evaluated as part of a scenario and the C2VSimTM would be used to assess the benefits and impacts on the subbasin sustainability.

Costs
(§354.44(b)(8))
This Project is currently in the early conceptual stage. Thus, the The anticipated costs of this Project have yet toproject will be determined and would be reported in GSP Annual Reports and Five-Year Assessment Reports when known. The Projectduring its evaluation. The project proponent would identify funding sources to cover Project costs as part of Projectproject development. These sources may include grants, fees, loans, and other assessments. #25..
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8.5. MANAGEMENT ACTIONS
This Section identifies and describes proposed Management Actions (MA) that may be undertaken by the Modesto Subbasin GSAs as an element of GSP implementation.

Management Actions generally refer to non-structural programs or policies designed to incentivize reductions in groundwater pumping, optimize management of the Subbasin, or implement GSA management authorities. Table 8-9 shows a list of the six MAs organized into two categories: demand reduction strategies (Section 8.4.1) and pumping management framework (Section 8.4.2). Demand reduction strategies are a broad and strategic set of actions intended to reduce water demand, some of which may be incentivized by State programs or policies, or by a pumping management framework. The pumping management framework provides a suite of administrative procedures, programs, and policies that describe how the GSAs will manage and monitor groundwater extractions. Implementation activities such as monitoring, annual reporting, and GSP updates are discussed in further detail in Chapter 9.

As described in Chapter 5, the Subbasin has experienced overdraft conditions. Per § 354.44(b)(2), the GSP must describe Projects or MAs, including a quantification of demand reduction or other methods, for the mitigation of overdraft. Several Projects identified in earlier sections of this chapter would increase the available water in the Subbasin through increased recharge or use of alternate supplies and are expected to reduce the groundwater deficit sufficiently to achieve the Subbasin’s sustainability goal. Additional Group 3 projects could be implemented to further decrease this deficit if necessary. Projects will need to be implemented as soon as feasible to prevent the need for MAs to be imposed. MAs are strategies the GSAs could additionally implement or implement in parallel to assist in achieving the sustainability goal if needed. A modeling analysis to assess the effectiveness of the current Group 1 and Group 2 projects is provided in Section 8.5. Although the C2VSimTM model used in this analysis is currently the best available tool for this analysis, its ability to accurately predict future groundwater levels is limited and the estimate is therefore approximate and subject to future refinement. In addition, the extent and effectiveness of the Group 3 projects that will be implemented in the future, and of the water conservation MAs described in Sections 8.4.1 and 8.4.2 is not yet known.

This section describes potential MAs that could be implemented in the Subbasin. While the tools described in this section will be available for implementation basin wide, implementation will be determined based upon need within each Management Area separately. PMAs implemented in one Management Area represent that Management Area’s contributions to subbasin sustainability. As such, it is anticipated that responsibility for implementing MAs will correspond with the relative Management Area contribution to overdraft and impacts associated with other sustainability criteria within that Management Area.
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A range of MAs is presented to allow the GSAs flexibility in their response to changing groundwater conditions and as data gaps and uncertainties are addressed during GSP implementation. However, it is anticipated that not all MAs will need to be implemented, or that individual MAs may be implemented by the GSAs in one Management Area but not by the other. In addition, implementation of MAs will be based on adaptive management strategies informed by ongoing monitoring of groundwater conditions using the monitoring network and methods described in the GSP. Monitoring data will be used to assess the need for PMAs in the Subbasin as a whole, in the Management Area, and at specific locations.

This will occur incrementally as monitoring data become available, the effectiveness of prior PMAs is established, and knowledge of the Subbasin improves over time. The advent or threat of undesirable results and the performance or failure of the Subbasin to meet Interim Milestones or Measurable Objectives will serve as triggers for scaling and implementing both Projects and MAs in a targeted and proportional manner, consistent with conditions observed in the Subbasin.

Table 8-9 lists the MAs described in the subsections that follow. Each MA description is organized to address the applicable regulatory requirements:
• Management Action Description: 23 CCR §354.44(b)
• Public Notice: 23 CCR §354.44(b)(1)(B)
• Permitting and Regulatory Process: 23 CCR §354.44(b)(3) • Expected Benefits: 23 CCR §354.44(b)(4), §354.44(b)(5) • Implementation Criteria, Status, and Plan: 23 CCR §354.44(b)(1)(A); §354.44(b)(4); §354.44(b)(6)
• Water Source and Reliability: 23 CCR §354.44(b)(6)
• Legal Authority: 23 CCR §354.44(b)(7)
• Estimated Costs and Funding Plan: 23 CCR §354.44(b)(8) • Management of Groundwater Extractions and Recharge: 23 CCR §354.44(b)(9) Summary of Criteria for Project Implementation (23 CCR §354.44(b)(1)(A)) Most of the MAs described in this section are presented as frameworks and will be fully developed into implementation plans during the first years of GSP implementation as indicated in the subsequent sections. These potential MAs will be implemented by the GSAs as needed to achieve and maintain long-term sustainable groundwater management across the Modesto Subbasin. They would be evaluated and selected for implementation if, based on data gathered during GSP implementation, the GSAs find that established IMs and MOs cannot be maintained and/or if MTs are being approached. This adaptive approach will be informed by continued monitoring of groundwater conditions, using the monitoring network and methods described in the GSP.
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Table 8-9: List of Management Actions
Category Number Proponent2 Management
Action
Primary
Mechanism(s)1 Partner(s)
Demand
Reduction
Strategies
1 Modesto
Subbasin
GSAs
Voluntary
Conservation and/or
Land Fallowing
Conservation/
Land Fallowing N/A
2 Modesto
Subbasin
GSAs
Conservation
Practices Conservation N/A
Water
Accounting
framework
3 Modesto
Subbasin
GSAs
Groundwater
Extraction and
Surface Water
Reporting Program
Pumping
Reduction N/A
4 Modesto
Subbasin
GSAs
Groundwater
Allocation and
Pumping
Management
Program
Pumping
Reduction N/A
5 Modesto
Subbasin
GSAs
Groundwater
Extraction Fee
Pumping
Reduction N/A
6 Modesto
Subbasin
GSAs
Groundwater
Pumping Credit
Market and Trading
Program
Pumping
Reduction N/A
1 The primary mechanism of the MA as conceptualized. MAs may support groundwater sustainability through multiple mechanisms during implementation. 2 It is anticipated that MAs will be implemented by the GSAs or by each GSA member agency as needed to mitigate overdraft within their jurisdictional areas and assure that the SMC adopted in Chapter 6 are met.

8.5.1. Demand Management Strategies
In case Projects are insufficient to manage the Subbasin in a sustainable condition, strategies may need to be developed to manage the agricultural and urban water demands in the Subbasin. These strategies could be implemented in the form of voluntary conservation and/or land fallowing (see Section 8.4.1.1) or other urban and agricultural conservation practices (see Section 8.4.1.2). While conservation practices are expected to be implemented throughout GSP implementation, specific strategies are in preliminary stages of discussion and possible consideration. Should the Modesto Subbasin GSAs decide to pursue a program in the future, the program would be implemented as necessary in a targeted and proportional manner consistent with conditions observed in the Subbasin.

Similarly, the Conservation Practices MA is expected to be implemented adaptively. #25..
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8.5.1.1. Voluntary Conservation and/or Land Fallowing (Management Action 1)

8.5.1.1.1.8.1.1.1.1. Management Action Description
Voluntary Conservation and/or Land Fallowing covers several strategies that can be designed to achieve both temporary and permanent water demand reduction. Should the Modesto Subbasin GSAs decide to pursue such strategies, this MA would assess options and develop a program to incentivize voluntary conservation and/or fallowing strategies in close coordination and collaboration with the landowners. Examples of this strategy could include repurposing of lands growing lower value crops. These lands could be dry farmed, fallowed in rotation, or used for recreation, habitat restoration, groundwater recharge, or solar power generation. This MA would also try to prioritize those lands that are more favorable for groundwater recharge projects.

Temporary or permanent land fallowing could also be combined with recharge projects through the application of surplus surface water supplies to the fallowed lands.

8.5.1.1.2.8.1.1.1.1. Public Noticing
A successful Voluntary Conservation and/or Land Fallowing program will require a comprehensive and strategic outreach effort, including multiple public workshops and meetings, potential website and/or email announcements, along with other public notices for the workshops. The outreach will be targeted to both potential participants of the program (landowners) as well as other stakeholders who may be impacted by changes to land and water use.

8.5.1.1.3.8.1.1.1.1. Permitting and Regulatory Process
Preparation of a CEQA evaluation for a fallowing program will identify potential environmental impacts and identify feasible alternatives or feasible mitigation measures.

Establishment of a voluntary land fallowing program is expressly authorized under SGMA (CWC, §10726.2(c)). The fallowing program, including program standards, will be developed and undergo CEQA review as necessary.

8.5.1.1.4.8.1.1.1.1. Expected Benefits
Benefits to Sustainability Indicators
Sustainability indicators that could benefit from Voluntary Conservation and/or Land Fallowing include:
• Chronic lowering of groundwater levels – By reducing groundwater demand, this MA would reduce pumping and pumping-related contributions to chronic lowering of groundwater levels.
• Reduction of groundwater in storage – Reduced pumping throughout the Subbasin contributes to a smaller rate of reduction of groundwater in storage. #25..
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• Land subsidence – Depending on the location of land fallowing or conservation, reduced pumping stress on local aquifer(s) may reduce the potential for subsidence. • Depletion of interconnected surface water – Reduced pumping would reduce the potential for negative impacts to surface water flows associated with lowering groundwater levels.

Benefits to Disadvantaged Communities
Benefits to disadvantaged communities overlap with the benefits described above for sustainability indicators. Land repurposing can also provide other ancillary benefits to local communities, such as recreation.

Volumetric Benefits to Subbasin Groundwater System
The volumetric benefit to the groundwater system would depend on the extent to which a Voluntary Conservation and/or Land Fallowing program is adopted and would be further studied when the program is implemented by the GSAs.

8.5.1.1.5.8.1.1.1.1. Implementation Criteria, Status, and Plan Temporary fallowing is a quick way to reduce demand with no capital costs or infrastructure needed. Because it is inexpensive, it can be implemented earlier and quicker while other long-term solutions like land repurposing are investigated. The Modesto Subbasin GSAs may explore options for encouraging voluntary and temporary fallowing during GSP implementation while developing a more structured program and exploring funding opportunities.

The Voluntary Conservation and/or Land Fallowing program is in preliminary stages of discussion and consideration. Should the Modesto Subbasin GSAs decide to pursue a program in the future, the program would be implemented as necessary in a targeted and proportional manner consistent with conditions observed in the Subbasin. To maximize recharge potential, the preservation lands that are more favorable for recharge projects could be prioritized while developing this MA. The implementation timeline has yet to be determined but would be provided in GSP annual reports and five-year updates when known. Any future changes in implementation would be communicated with the public and other agencies and would be documented in GSP annual reports and five-year updates.

8.5.1.1.6.8.1.1.1.1. Water Source and Reliability
This program does not rely on the supplies from outside the Subbasin because it is a planning effort that will result in conservation. It will support overall supply reliability by reducing overdraft in the Subbasin and moving the Subbasin towards sustainability.

8.5.1.1.7.8.1.1.1.1. Legal Authority
The GSAs have authority to “provide for a program of voluntary fallowing of agricultural lands or validate an existing program” (CWC, §10726.2(c)). #25..
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This MA carries forward the policy of the state and satisfies SGMA requirements by establishing a voluntary program that encourages water within the Subbasin to be dedicated to beneficial uses of water in a manner designed to achieve the sustainability goals and to protect against undesirable results.

8.5.1.1.8.8.1.1.1.1. Estimated Costs and Funding Plan
The Voluntary Conservation and/or Land Fallowing program is in preliminary stages of discussion and possible consideration. Therefore, no costs have been estimated for its development and implementation. Such costs would be developed should the Modesto Subbasin GSAs decide to pursue a program in the future. Separately, multiple funding programs exist as a potential source of revenue for individual landowners looking at options for land repurposing, including (EDF, 2021):
• Mitigation or Conservation Banks
• Conservation Easements
• Solar Rentals
• Grazing Leases
• Converting to Low Water Intensity Crops
• Federal and State Grant Funding Programs

8.5.1.1.9. Management of Groundwater Extractions and Recharge This MA encourages the conservation of water; this will be applicable during both drought and non-drought conditions.

8.5.1.2. Conservation Practices (Management Action 2)

8.5.1.2.1.8.1.1.1.1. Management Action Description
This MA would create a program to support the use of conservation practices in both urban and agricultural sectors.

Urban water suppliers are already obligated to consider demand reduction and conservation efforts during dry periods. These demand MAs are described in their respective Urban Water Management Plans (UWMPs). These include:
• City of Modesto Urban Water Management Plan (West Yost Associates, 2016b) o https://www.modestogov.com/860/Urban-Water-Management-Plan • Modesto Irrigation District Urban Water Management Plan (West Yost Associates, 2021)
o https://wuedata.water.ca.gov/public/uwmp_attachments/2173444449/R% 20-%20418%20-%20City%20of%20Modesto_MID%20-
%20Final%202020%20UWMP%20%20-%2006-23-21.pdf
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• City of Riverbank Urban Water Management Plan (KSN Inc, 2016) o https://www.riverbank.org/610/Urban-Water-Management-Plan-WSCP • City of Oakdale Urban Water Management Plan (MCR Engineering, 2015) o https://cadwr.app.box.com/s/hg3k8bc9vuka689jkh1x4f9i1n58ey9a/file/521 558561581
• City of Waterford (covered under City of Modesto 2015 UWMP) In addition, SB 606 and AB 1668, both signed into law in May 2018, are laws that introduce conservation mandates that will cap indoor residential use and set a target for efficient outdoor landscape irrigation based on local climate and size of landscaped areas. Urban water suppliers will be required to report on progress to meeting urban water use objectives beginning in 2023 and comply with them beginning in 2028.

In addition to meeting urban water use objectives, this MA could include changing standards for storm drainage so that storm flows do not discharge straight to a river, creek, or canal, as contemplated by the City of Modesto as a potential Group 3 Project. This would help increase the sustainability footprint of the City of Modesto as it grows. Currently approximately 36% of the City of Modesto area drains to a river or canal, while approximately 64% is captured for local recharge. If the City of Modesto adopts new Storm Drain Standards, 100% of runoff from newly developed areas would reach a retention system and contribute to recharge.

In addition to urban conservation, agricultural water suppliers serving more than 25,000 irrigated acres must adopt an Agricultural Water Management Plan (AWMP) that include reports on the implementation status of specific Efficient Water Management Practices required by the Water Conservation Act of 2009 (SB X7-7). Agencies that have developed AWMPs include:
• Modesto Irrigation District Agricultural Water Management Plan o https://www.mid.org/water/awmp/default.html
• Oakdale Irrigation District Agricultural Water Management Plan o https://wuedata.water.ca.gov/public/awmp_attachments/3350354850/OID %202020%20AWMP%20FINAL%20210323.pdf
The Modesto Subbasin GSAs may choose to evaluate the existing UWMPs and AWMPs in the Subbasin and either expand upon minimum requirements to increase the impact of such programs or implement similar conservation practice programs in other areas of the Subbasin that may not be covered under an UWMP or AWMP.

Notably, conservation practices must be considered in the greater context of the Subbasin water budget, especially at the nexus between on-farm water use and groundwater sustainability. In areas where groundwater is the primary or sole water supply, conservation practices that reduce water demand may also reduce groundwater consumption, but #25..
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conservation practices may also have unintended consequences that impede water conservation and sustainable groundwater management. Some of these consequences directly result from irrigation efficiency improvements: applying less water to an area and reducing the gap between irrigation and consumptive use also reduces deep percolation and seepage to the groundwater system. Other consequences may stem from behavioral responses and changes in irrigation resulting from these technologies and policies. If less water can be used to produce the same amount of a crop product, growers may be inclined to use the same amount of water and produce more (Lankford, et al., 2020). Additional considerations on the promises, pitfalls, and paradoxes of irrigation efficiency in water management planning are described by Lankford et al. (2020).

Further details on any expansion of the Conservation Practices program are preliminary as of the time of publishing and would need to be developed and refined further during GSP implementation.

8.5.1.2.2. Public Noticing
The Modesto Subbasin GSAs anticipates that public outreach and education on the potential structure of the Conservation Practices program, as well as feasible monitoring and enforcement mechanisms, would be necessary to enable a successful program. Outreach may include public notices, meetings, potential website presence and email announcements. Initial program implementation would likely focus on voluntary compliance while the GSAs or GSAs member agencies consider the necessary elements to begin enforcing the program potentially by 2027 (five years after adopting and submitting the GSP). This date is contingent upon monitoring results and achievement of Interim Milestones.

8.5.1.2.3.8.1.1.1.1. Permitting and Regulatory Process
Development of a Conservation Practices program is not a Project as defined by the California Environmental Quality Act (CEQA) and National Environmental Policy Act (NEPA) and would therefore not trigger either.

8.5.1.2.4.8.1.1.1.1. Expected Benefits
Benefits to Sustainability Indicators
Sustainability indicators benefitting from Conservation Practices include:
• Chronic lowering of groundwater levels – By reducing groundwater demand, this MA would reduce pumping and pumping-related contributions to chronic lowering of groundwater levels.
• Reduction of groundwater in storage – Reduced pumping throughout the Subbasin contributes to a smaller rate of reduction of groundwater in storage. • Degraded water quality – This MA does not address this sustainability indicator. #25..
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• Land subsidence – Depending on the location of Conservation Practices, reduced pumping stress on local aquifer(s) may reduce the potential for subsidence. • Depletion of interconnected surface water – Reduced pumping would reduce the potential for negative impacts to surface water flows associated with lowering groundwater levels.

Benefits to Disadvantaged Communities
Benefits to disadvantaged communities overlap with the benefits described above for sustainability indicators. Depending on how they’re structured, urban conservation programs may also provide a financial benefit to individual users who reduce their water consumption, either via a lower water bill or reduced demand on a domestic well.