The USGS used a binary texture classification of either “coarse grained” (100 percent coarse) or “fine grained” (0 percent coarse) to categorize each interval on the well logs. Coarsegrained texture was defined as consisting primarily of sand or gravel while fine grained texture was defined as consisting primarily of silt or clay (Burow et al., 2004). Once this binary texture classification was complete, the coarse-grained percentage was averaged at 1-meter intervals along the depth of the well. This simplification of the lithology on a well basis allows identification of regions and/or depths of the groundwater basin that contain higher percentages of sand-rich zones, likely representing more permeable aquifers and large quantities of groundwater in storage.
The cross sections were created using the ESRI ArcHydro module for ArcGIS. The ArcHydro module allows import and three-dimensional plotting of geologic data from boreholes and topological surfaces. ArcHydro analysis tools include projection of borehole and surface data along cross-sections at selected orientations for analysis and geologic correlation.
DWR Well Completion Reports were available for most USGS texture database wells on the cross sections. The lithologic descriptions on the Well Completion Reports were used to define marker beds, such as black sands (Mehrten Formation) or blue clays (Corcoran Clay).
The Well Completion Reports were also used to identify the screened intervals in the wells.
Where USGS texture data were not available, Well Completion Reports were used to interpret the lithology. Without the binary method used by USGS, the texture categories from the Well Completion Reports were defined on the cross sections at the same depth and thickness for which they were described on the Well Completion Reports. In this manner, the texture detail on each Well Completion Report is preserved. In areas with several closely-spaced wells, only higher-quality Well Completion Reports (i.e., most detailed data) were used.
The cross sections honor the texture information from the USGS and Well Completion Reports at well locations. Between well locations, the coarse-grained units were generally correlated based on elevation and thickness. Thick sand lenses were assumed to be more continuous and more likely to be interconnected than thinner sand lenses. The surficial #25..
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geologic map (Wagner et al., 1991) presented as Figure 3-1 was used to estimate surface contacts of the geologic formations on the cross sections when appropriate.
3.1.4.2. Cross Sections
Interpretations and observations for each of the five cross sections are described below.
Cross Section A-A’
Cross section A-A’, shown on Figure 3-12, illustrates the lithology through the center of the Subbasin from southwest to northeast. The lithology is based on data from 61 wells and incorporates a local cross section (H-H’) developed for the City of Modesto associated with a previous hydrogeologic study (Todd, 2016). The local cross section is incorporated into A-A’ immediately east of cross section B-B’ and extends for about 3 to 4 miles (see H-H’ on Figure 3-12).
The Corcoran Clay extends from the western edge of A-A’ and extends almost to the intersection of B-B’. Its extent agrees with that mapped by USGS (Burow et al., 2004). The top of the Corcoran Clay is approximately 150 feet below ground surface (bgs) at its eastern extent and dips to the west to a depth of approximately 220 feet bgs (equivalent to elevations of approximately -80 feet msl to -185 feet msl. The Corcoran Clay generally thickens to the west, ranging in thickness from about 10 feet in the east to about 70 feet in the west. The depth and thickness of the Corcoran Clay generally agrees with the Corcoran Clay in the USGS MERSTAN model (Phillips et al., 2015) and with the data incorporated into the Modesto Subbasin C2VSim model (Figures 3-9 and 3-10).
The top of the Mehrten Formation is estimated on the cross section based on the presence of black sands, which are colored orange on Figure 3-12. The Mehrten Formation crops out in the eastern Subbasin and is generally consistent with the geologic map illustrated on Figure 3-1. Black sands were not identified in the central and western Subbasin because not many wells extend deep enough to intersect the Mehrten Formation in that area. Based on the interpolated dip of the black sands, the top of the Mehrten Formation is approximately 400 feet below the City of Modesto (H-H’ on Figure 3-12), east of where cross section B-B’ crosses A-A’ (Figure 3-12).
An offset in the top of the black sands was observed during construction of cross section EE’, located north of and parallel to cross section A-A’. As described in more detail for cross section E-E’, this offset suggests vertical movement caused by a geologic fault. An offset in the black sands is also suggested by the data in a similar location on cross section A-A’, east of the intersection with cross section C-C’ (Figure 3-12). The vertical movement – downdropped eastern block relative to the western block – is also consistent with offset observed on cross section E-E’. The estimated location of the fault plane is shown on cross section AA’.
Cross section A’A’ also illustrates the presence of thick coarse-grained units both above and below the Corcoran Clay, at the western edge of the Corcoran Clay. Thick sand units are also noted in the eastern Subbasin within the Mehrten Formation. Note that the lithology #25..
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shown below the Corcoran Clay is only based on a few wells and is less certain than other areas with more wells. Wells in the western Subbasin are primarily screened either immediately above or immediately below the Corcoran Clay with some wells screened in both aquifers. Most of the wells in the eastern Subbasin are screened within the black sands of the Mehrten Formation.
Cross Section B-B’
Cross section B-B’, shown on Figure 3-13, illustrates the lithology from the northern to the southern Subbasin boundary in the western Subbasin, through the City of Modesto. The lithology is based on texture information from 38 wells and incorporates a local cross section (D-D’) developed in the City of Modesto from a previous study (Todd, 2016). The local cross section extends from north of the intersections with A-A’ to the southern edge of the cross section (at B’, Figure 3-13).
The Corcoran Clay extends from the southern edge of the cross section to slightly north of the Tuolumne River. At the Subbasin boundary, the top of the Corcoran Clay is at a depth of about 130 feet bgs (about -65 feet msl) and is about 65 feet thick. As shown on the cross section location map (Figure 3-11), the edge of the Corcoran Clay is oriented northwest to southeast and only intersects the southern portion of section B-B’. However, the Corcoran Clay does not extend as far east in this area as mapped by USGS (compare the edge of the Corcoran Clay on cross section B-B’ to the Corcoran Clay extent mapped by USGS and shown on Figure 3-11). This could indicate that the extent is more irregular than previously mapped or extends farther than indicated by well data on this section. Because the cross section interpretation is based only on a few logs, the unit may have been too thin to be identified (or not recorded) on the Well Completion Reports.
Wells present in the southern region of the cross section are screened both above and below the Corcoran Clay. To the north of the Corcoran Clay, wells tend to have long screened intervals that intersect multiple coarse-grained units. The thickest coarse-grained units on cross section B-B’ are present along the edge of the Corcoran Clay.
The wells on cross section B-B’ are not deep enough to penetrate the Mehrten Formation.
Based on where B-B’ intersects A-A’, the Mehrten Formation is at an elevation of approximately -370 feet msl in this area of the Subbasin (near the bottom of B-B’ on Figure 3-13). The deepest wells on cross section B-B’ extend to about -300 feet msl.
Cross Section C-C’
Cross section C-C’, illustrated on Figure 3-14, depicts the lithology in the central Subbasin, east of the Corcoran Clay between Riverbank and Oakdale. The cross section is based on geologic information from 43 wells.
Most of the wells on cross section C-C’ section are too shallow to encounter the Mehrten Formation. However, a few wells are several hundred feet deep and have sufficiently long #25..
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screens that intercept the Mehrten Formation black sands. These wells allow the top of the Mehrten Formation to be approximated on the cross section (Figure 3-14).
As shown on C’C’, the top of the Mehrten Formation is present at an elevation between - 100 and -200 feet msl, shallower than in cross section B-B’ due to its westward dip. The elevation of the top of the Mehrten Formation dips gently to the south along this cross section, with elevations ranging from approximately -125 feet msl along the northern Subbasin boundary to approximately -220 feet msl at the southern Subbasin boundary. The depth to the Mehrten Formation from the edge of the river channels at the Subbasin boundaries range from about 285 feet bgs in the north to 325 feet in the south. The Mehrten is likely shallower in the northern section because it crops out over a larger area in the northern part of the Subbasin (see Figure 3-14).
The thickest and most continuous coarse-grained units on the section are in the center of the Subbasin. Coarse-grained units appear to be thicker and more continuous in the southern Subbasin near Dry Creek and the Tuolumne River than along the northern Subbasin boundary.
Cross Section D-D’
Cross section D-D’ (Figure 3-15) illustrates the lithology in the eastern Subbasin. The cross section extends from the Stanislaus River to the Tuolumne River and crosses Dry Creek and the Modesto Reservoir. The cross section is based on lithology from 27 wells. Due to the lack of USGS texture data in the eastern Subbasin, most of the lithologic information on this cross section is from DWR Well Completion Reports.
The cross section shows that the Mehrten Formation is shallow or crops out as remnant hills in the eastern Subbasin. The delineation of Mehrten Formation outcrop is based on the presence of black sands and the geologic map (Figure 3-1). The cross section is dominated by coarse-grained material and black sands. It should be noted that some Well Completion Reports do not indicate the color of the textures and much of the yellow color on the section may, in fact, also represent black sands.
The cross section shows that most of the wells are hundreds of feet deep and screened within or across the black sands. The black sands and coarse-grained material appear to be thicker and more extensive in the northern half of the Subbasin.
Cross Section E-E’
Cross section E-E’, illustrated on Figure 3-16, is a local cross section in the northeast Subbasin oriented from southwest to northeast, parallel to cross section A-A’. The cross section is along the northern Subbasin boundary and extends from cross section C-C’, through Oakdale, to east of cross section D-D’. The cross section approximately follows the Stanislaus River channel, crossing it in two places, and is based on lithology from 62 wells.
Due to the high density of wells on the cross section, well numbers are shown on a separate expanded-scale version of this section, provided as Figure 3-17. #25..
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The Mehrten Formation is shallow throughout most of the cross section and crops out in the eastern region of the section. Similar to cross section D-D’, the delineation of the Mehrten Formation outcrop is based on the presence of black sands and the geologic map (Figure 3- 1). The Mehrten Formation crops out as remnant hills with the erosional surface roughly corresponding to the ground surface elevation on the cross section. The dip of the Mehrten Formation is visible because the transect is roughly parallel to the dip direction. The coarsegrained material and black sands appear to be the thickest and most continuous at depth, but this interpretation is based on only a few deep wells.
There was some irregularity in the elevation of the top of the black sands in wells in the western region of the section. It appears that the black sands on the western side of this fault are at a significantly higher elevation than on the east side of the fault, suggesting vertical movement possibly associated with a geologic fault as interpreted on E-E’. The eastern block is down-dropped relative to the western block.
The USGS (Marchand, 1980) mapped multiple surface lineaments (trending northwest to southeast) south of the Modesto Subbasin, within the Turlock Subbasin. This mapping included folds and faults with approximately northwest to southeast trends. The faulting, which occurred post-deposition, resulted in a down-dropped eastern block relative to the western block, showing reverse offset because of compressive stresses. The evidence of a fault in the Modesto Subbasin has a similar pattern of offset and trend as the faults mapped in the Turlock Subbasin.
Cross Section A-A’ with Hydrogeologic Framework
Cross section A-A’ is repeated on Figure 3-18 with a focus on formations and the geometry of the Principal Aquifers rather than textures. The cross section depicts the formation boundaries and the base of fresh water from C2VSim through the center of the Subbasin from southwest to northeast (Figure 3-11). The boundary between the base of the undifferentiated Modesto, Riverbank, and Turlock Lake Formations and the top of the Mehrten Formation is the same as shown on cross section A-A’ and is based on the geologic texture data. The base of the Mehrten Formation was approximated from geophysical logs at 13 deep oil and gas wells available from the California Department of Oil, Gas and Geothermal Resources (DOGGR). (The location of the DOGGR geophysical logs is shown on Figure 3-11).
The cross section shows the westward dip of the formations and offsets caused by two faults in the central and eastern Subbasin. The fault east of intersection with C-C’ was identified based on offset of Mehrten Formation black sands. The fault identified west of intersection with C-C’ is based on offset of the base of the Mehrten Formation identified from DOGGR geophysical logs. The fault west of C-C’ is not shown on Figure 3-12 because the wells in this area are not deep enough to intersect the black sands of the Mehrten Formation, and therefore offset could not be identified.
The base of fresh water surface from C2VSim, which represents the bottom of the Subbasin, is overlaid onto the conceptual cross section. The base of fresh water undulates throughout #25..
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the Subbasin. It is highest in the eastern Subbasin, at an elevation of approximately -550 feet msl, and deepest in the central Subbasin, at an elevation of approximately -1,000 feet msl. In the eastern Subbasin, the base of fresh water is below the Mehrten Formation, within the undifferentiated continental and marine sediments. In the central Subbasin it rises into the base of the Mehrten Formation. The undulations approximately correspond with the locations of the faults.
The conceptual cross section also illustrates the three principal aquifers: the Western Upper Principal Aquifer above the Corcoran Clay, the Western Lower Principal Aquifer below the Corcoran Clay and above the base of fresh water, and the Eastern Principal Aquifer east of the Corcoran Clay and above the base of fresh water.
3.1.4.3. Aquifer Properties
The USGS compiled aquifer property data for the Modesto and Turlock subbasins (Burow et al., 2004). The USGS reported hydraulic conductivity above the Corcoran Clay, in the Western Upper Principal Aquifer, to range from 27 to 54 feet per day (ft/day) (Page, 1977 in Burow et al., 2004). The C2VSim Modesto Model has an average hydraulic conductivity above the Corcoran Clay of 42 ft/day, which is within this published range.
The hydraulic conductivities in the Mehrten Formation, at the base of both the Eastern Principal Aquifer and Western Lower Principal Aquifer, ranged from 0.01 to 67 ft/day (Page and Balding, 1973 in Burow et al., 2004). Average hydraulic conductivity in the lower aquifer of the C2VSIM Modesto Model, which includes the Mehrten Formation, is 25 ft/day, which is within this published range.
In the Eastern Principal Aquifer, the transmissivity (T) in the shallow unconsolidated sediments is estimated to be 9,100 ft2
/day (68,068 gpd/ft). The T in the deeper, partly
consolidated sediments of both the Eastern Principal Aquifer and Western Lower Principal Aquifer was lower, approximately 8,000 ft2
/day (59,840 gpd/ft) (Page and Balding, 1973 in
Burow et al., 2004).
3.1.5. Hydrogeologic Conceptual Model Representation in Modesto C2VSim Model The hydrogeologic conceptual model was compared with the Modesto C2VSim Model to ensure that the hydrogeologic system is well represented in the model.
As discussed previously in Section 3.1.4, the original Corcoran Clay surface that was in the model was replaced with the Corcoran Clay surface from the USGS MERSTAN Model (Phillips et al., 2015). This was because an anomaly in the original surface was discovered while comparing the cross sections and well logs to the model. The Corcoran Clay surface in the USGS MERSTAN Model is the most detailed mapping of the Corcoran Clay in the Modesto Subbasin. The depth, thickness and extent of the Corcoran Clay shown on the cross sections generally agrees with the USGS MERSTAN Model, and consequently, with the revised surface in the Modesto C2VSim Model.
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The model layers are a good representation of the Principal Aquifers. The primary shallow pumping layer of the model contains most of the pumping wells. As mentioned in the previous section, the average hydraulic conductivity in the model in the Western Upper Principal Aquifer and within the Mehrten Formation were within the range published in the literature.
The hydrogeologic conceptual model is well represented in the Modesto C2VSim Model.
Because of this, the model is an effective tool for estimating water levels in areas lacking water level data, such as within the Western Lower Principal Aquifer and in the eastern Subbasin. The model is also an effective tool for developing water budgets, which will be presented in Section 4.
3.1.6. Data Gaps and Uncertainties in the Hydrogeologic Conceptual Model This section will summarize hydrogeologic data gaps that affect implementation of the Plan and are related to the GSAs ability to sustainably manage groundwater. The Plan Implementation section, when developed, will describe how these data gaps will be addressed in future GSP actions. A summary of the data gaps for the Hydrogeologic Conceptual Model is shown in the table below.
Table 3-1: Data Gaps for the Hydrogeologic Conceptual Model Issue Area Impacts on Groundwater
Management Actions to Address
Eastern
Subbasin
Aquifers
East and
Northeast
of
Modesto
Reservoir
Sparse number of wells
in this area of the
Subbasin means more
uncertainty regarding the
Eastern Principal Aquifer.
• Collect relevant data from
landowners, as available.
• Install additional monitoring
wells.
• Examine lithologic logs and
other well data when new
wells are drilled in this area.
Mehrten
Formation
Central
and
Western
Subbasin
Depth to top of Mehrten
Formation not well
understood in central
and western Subbasin
due to shallow wells.
Impacts understanding of
aquifer properties and
geometry.
• Examine lithologic logs and
other well information as
additional deep wells are
drilled in central and western
Subbasin.
• Add testing program, such as
geophysical logs, to proposed
deep wells where needed.
Exact
Base of
Fresh
Water
Entire
Subbasin
Uncertainty in Subbasin
geometry, fresh
groundwater in storage,
and water quality with
depth.
Compile TDS data for wells with
known screen intervals. Test
water quality in all new Subbasin
wells.
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3.2. GROUNDWATER CONDITIONS
An evaluation of groundwater conditions in the Modesto Subbasin was conducted using water level data obtained from numerous sources, including the DWR Water Data Library (which includes CASGEM data), USGS, MID, OID, and the municipalities and urban communities. There are more than 600 wells in the Subbasin with measured water levels between 1918 and 2018, with most measurements occurring after 1970. The locations of these wells are shown on Figure 3-19. As shown on the figure, most water level data are from wells in the western and central Subbasin, with limited data in the eastern Subbasin.
The groundwater analysis focused on data from 1990 to 2018; this water level study period overlaps the water budget study period (WY 1991 – WY 2015, see Section 3.1.2.1) while including more recent data to examine current groundwater conditions. During this period, water levels were measured at approximately 450 of these wells.
3.2.1. Groundwater Occurrence
As summarized in Section 3.1.4, groundwater is present in unconfined to semi-confined aquifers above and east of the Corcoran Clay and in confined aquifers below the Corcoran Clay. Groundwater is also present in the shallow alluvial unconsolidated to semiconsolidated deposits as well as the underlying consolidated sediments; however, groundwater conditions are not well defined in the deeper aquifers due to a lack of data.
3.2.2. Water Levels and Trends
To examine water level trends over the study period, working hydrographs were constructed for each of the approximately 450 wells with water level measurements since 1990. Representative hydrographs were chosen for discussion from wells in each principal aquifer based on data availability and on levels, fluctuations, and trends consistent with other hydrographs in a certain area. The locations of selected wells with representative hydrographs are shown on Figure 3-20 and are color-coded based on the principal aquifer in which they are screened.
Representative hydrographs are presented on Figures 3-21 through 3-25. These hydrographs have consistent horizontal scales (1990 to 2018) and vertical scales (0 to 160 feet msl) to facilitate comparisons across the Subbasin. The ground surface elevation is shown as a black line on the hydrographs unless it is greater than 160 ft msl, in which case it is noted at the top of the hydrograph. If known, the depth of the screened intervals for each well are noted on the hydrograph. Representative hydrographs include data measured at MID wells, City of Modesto wells, City of Oakdale wells, CASGEM wells and DWR Water Data Library wells.
Eight representative hydrographs from the Western Upper Principal Aquifer are illustrated on Figures 3-21 and 3-22. As shown on Figure 3-21, groundwater elevations in the western and central regions of the Western Upper principal aquifer are shallow. Depth to water in the northwest Subbasin (hydrograph 1) is within ten feet of ground surface and deepens to #25..
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the south (hydrograph 2) and east (hydrographs 3, 4 and 5). Water levels are relatively stable, especially along the western Subbasin boundary near the San Joaquin River (hydrographs 1 and 2). Water levels fluctuate more to the east. Hydrographs 3, 4 and 5 show slightly more pronounced water level declines during the recent drought. The declines are greater in the center of the Subbasin (hydrograph 4, approximately 13 feet) than near the rivers (hydrographs 3 and 5, approximately 5 or less feet).
Three hydrographs from the eastern edge of the Western Upper Principal Aquifer are shown on Figure 3-22 and illustrate a similar historical water level trend. Water levels between 1990 and 1995 are relatively low and rise after 1995 when the City of Modesto began receiving water from the Modesto Regional Water Treatment Plant (MRWTP) and pumping less groundwater. Water levels were relatively steady from 2000 to the recent drought, when declines up to 10 feet (hydrograph 7) and 15 feet (hydrograph 6) occurred. Water levels have recovered slightly since the end of the drought.
Hydrograph 8 illustrates water levels from a City of Modesto pumping well (Well 17). In 1994, shortly before the City of Modesto began receiving water from the MRWTP, water levels were the lowest of the study period. Between 1995 and 2000, after the City began receiving water from the MRWTP, water levels rose almost 50 feet. Since 2000, water levels indicate significant seasonal pumping variation, but overall have remained relatively steady.
Three hydrographs from the Western Lower Principal Aquifer are shown on Figure 3-23.
Each of these hydrographs are from City of Modesto pumping wells (Well 290, Well 313 and Well 56). Each of these hydrographs illustrate significant seasonal pumping variations.
When compared to Well 17, in the Western Upper Principal Aquifer (hydrograph 8 on Figure 3-22), it appears that the water level variation below the Corcoran Clay is more significant than above the Corcoran Clay, consistent with pumping in a confined aquifer. Water levels in City of Modesto Well 56 (hydrograph 11) depict the historical trend of water level recovery between 1995 and 2000 followed by relatively stable water levels with seasonal pumping fluctuations. In general, water levels appear to be relatively stable, with small declines during drought (about 10 to 20 feet) followed by recovery in post-drought years.
Representative hydrographs from ten wells east of the edge of the Corcoran Clay in the Eastern Principal Aquifer are illustrated on Figures 3-24 and 3-25. Hydrographs from wells in the western side of the Eastern Principal Aquifer are shown on Figure 3-24 and include three MID wells, one City of Modesto well and one well from the DWR WDL. These hydrographs indicate a deeper water table as ground surface elevations rise to the east.
Hydrographs illustrate depths to water ranging from approximately 40 feet bgs in MID-208 to more than 80 feet bgs in MID-197 (Figure 3-24). The water levels in the MID wells are relatively steady until declines during the most recent drought. Those declines increase to the east, ranging from about 12 feet in MID-208 to 27 feet in MID-214. Some recovery occurred after the drought, but water levels remain approximately 20 feet below predrought levels in the two easternmost wells, MID-214 and MID-197.
The City of Modesto well 37 (hydrograph 13), located in the center of the Subbasin close to the edge of the Corcoran Clay, has a similar water level pattern to other City of Modesto #25..
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wells in the western principal aquifers. The water level in City of Modesto Well 37 rose approximately 50 feet between 1995 and 2000 and remained relatively steady, with pumping cycles, since then. There is a slight downward water level trend since about 2005 that was less pronounced in the City of Modesto wells in the western principal aquifers.
Five hydrographs from the eastern region of the Eastern Principal Aquifer are illustrated on Figure 3-25. These hydrographs are from a City of Oakdale well (Well 5), two MID wells and two wells from the DWR WDL. Although the City of Oakdale Well 5 (hydrograph 17) has missing data between 1995 and 2009, the measured record illustrates up to 40 feet of seasonal pumping variations and an overall slightly declining trend. The other four hydrographs show historical declining trends since about the mid-2000s. For example, water levels in MID-228 (hydrograph 19, near the Tuolumne River), declined approximately 30 feet from the late 1990s to present. Most of the declines occur during the recent drought (2013 – 2016) and appear most significant in the eastern Subbasin. Water levels during the drought declined approximately 25 feet in MID-228 (hydrograph 19) and MID223 (hydrograph 21) and about 40 feet in the DWR WDL well 02S12E32P01M (hydrograph 18), north of Modesto Reservoir. In that well, recent water levels have not recovered or stabilized substantially, even during the wet year of 2017.
In general, hydrographs in the Eastern Principal Aquifer indicate that water levels in the eastern Subbasin have declined since about 2000 and have significant declines during the most recent drought. The historical declining trends and the magnitude of decline during the recent drought are most pronounced in the eastern region of the Eastern Principal Aquifer. In the eastern Subbasin, long-term rates of decline are up to about 2.7 feet/year and rates of decline during drought are up to 6 feet/year. Due to a lack of data, water level trends east of the Modesto Reservoir and in the northeastern region of the Subbasin are not known.
3.2.3. Groundwater Flow
3.2.3.1. Groundwater Elevation Contour Maps
Groundwater elevation contour maps were developed at three different times within the study period: the wettest year (1998), a dry year during the recent drought (2015), and the most recent year with a sufficient set of measured data (2017). These contour maps are shown on Figures 3-26, 3-27a, and 3-28. Each groundwater elevation contour map includes water levels measured in the unconfined Western Upper Principal Aquifer and unconfined to semi-confined Eastern Principal Aquifer. Water levels from these two principal aquifers are shown and contoured on the same map as representative of water table conditions. In addition, simulated groundwater elevation contours from September 2015 in the Unconfined Aquifer are shown on Figure 3-27b.
Maps illustrating the available water level data in the Western Lower Principal Aquifer were developed for each time period and are shown on Figures 3-29, 3-30a and 3-31. Water levels in the Western Lower Principal Aquifer cannot be contoured due to limited data.
Although many wells in the western Subbasin were drilled below the Corcoran Clay, most have screened intervals both above and below the clay. Wells shown on these figures are #25..
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screened only below the Corcoran Clay. Simulated groundwater elevation contours from the groundwater model provide a more complete representation of water levels in the Western Lower Principal Aquifer than could be developed with current data. A simulated groundwater elevation contour map for the Confined Aquifer in September 2015 is shown on Figure 3-30b.
Groundwater Flow in Spring 1998 (March and April)
Groundwater elevations measured in spring 1998 are illustrated on Figure 3-26. As shown on Figure 3-2, water year 1998 is the wettest year between 1990 and 2017. With almost 25 inches of rain, precipitation during water year 1998 was almost double the long term average (12.6 inches) and study period average (12.8 inches). As shown on the hydrographs, water levels throughout most of the Subbasin rebounded between 1995 and 2000 in response to the reduction of groundwater pumping within the City of Modesto as a result of the delivery of water from the MRWTP. For this and other reasons, 1998 water levels do not always represent the highest water levels in all parts of the Subbasin.
Groundwater elevations in spring 1998 ranged from about 150 feet msl near the Modesto Reservoir to approximately 35 feet msl in the western Subbasin. The lowest groundwater elevations occurred along the western edge of the Subbasin and within the City of Modesto along the Tuolumne River. Groundwater flow is generally to the southwest with flatter hydraulic gradients in the west. There is a southerly component of flow towards the Tuolumne River in the western Subbasin caused by a pumping depression in the City of Modesto. Groundwater elevations in this region are between about 30 and 40 feet msl, which is similar to the groundwater elevations along the western edge of the Subbasin next to the San Joaquin River. There is a general area of higher groundwater elevations in the central Subbasin, with elevations slightly over 100 feet msl. Additional localized areas of higher or lower groundwater elevations also occur in the Subbasin. As illustrated on Figure 3-26, there is a lack of measured water level data in the eastern Subbasin.
Groundwater elevations in the Western Lower Principal Aquifer are available in only two wells during spring 1998 (Figure 3-29). The wells are along the eastern edge of the aquifer and have similar water levels (41 and 44 ft msl); levels are also similar to water levels in the Western Upper Principal Aquifer.
Groundwater Flow in October 2015
Figure 3-27a illustrates groundwater elevations measured in October 2015. Water year 2015 was the third consecutive critically dry year during the recent drought and water levels reached historical lows in many areas of the Subbasin. January 2015 is defined in the Water Code as the SGMA baseline, so this map generally represents baseline conditions for the Subbasin.
As shown on Figure 3-27a, groundwater elevations ranged from approximately 130 feet msl in the eastern Subbasin to 14 feet msl in the western Subbasin along the Tuolumne River in Modesto. In October 2015, more water level data are available in the eastern Subbasin than #25..
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in spring 1998 and the highest water level (132 feet msl) was measured in the northeastern Subbasin.
Groundwater flow patterns in October 2015 are similar to spring 1998, with groundwater flow to the southwest, with a southerly component towards the Tuolumne River, especially within the City of Modesto. Hydraulic gradients are steeper in the eastern Subbasin and become flatter to the west. Even though flow directions are the same as 1998, groundwater levels in October 2015 are generally lower throughout the Subbasin.
Increased municipal pumping during the drought has created a pumping depression within the City of Modesto, with water levels approximately 20 feet lower than in spring 1998.
Similarly, increased irrigation pumping has created a pumping depression east of the City of Modesto in the central Subbasin, with water levels approximately 20 to 30 feet lower than in spring 1998. Water levels in the Western Upper Principal Aquifer appear to have the least amount of decline, on the order of 10 to 20 feet lower than in spring 1998. The magnitude of water level declines between these two time periods is larger in the east. For example, water levels in October 2015 near the Modesto Reservoir are approximately 30 to 40 feet lower than they were in spring 1998.
Simulated groundwater elevation contours in the unconfined aquifer from September 2015 are shown on Figure 3-27b. This figure shows that there is general agreement between simulated groundwater elevations from the model and measured groundwater elevations (see Figure 3-27a). Simulated groundwater elevations in the Western Upper Principal Aquifer range from approximately 20 to 40 feet msl, similar to measured data. Simulated groundwater elevations gradually increase to the east, with the 120 foot simulated contour in a similar location in the eastern Subbasin as depicted on the measured contour map. The simulated groundwater elevation contours in the central Subbasin are smoother than the contours based on measured data. This is because there is more well-by-well variability in the measured data based on localized pumping.
Groundwater elevations are available in four wells in the Western Lower Principal Aquifer for October 2015 (Figure 3-30a). The wells, located along the eastern edge of the aquifer, have elevations ranging from 26 to 41 feet msl; although there are more wells with 2015 data, elevations for the same wells are between 3 feet and 10 feet lower than in spring 1998. Simulated groundwater elevations in September 2015 provide a more complete representation of groundwater conditions in the Western Lower Principal Aquifer (Figure 3- 30b). Simulated contours show flow to the northeast, with groundwater elevations ranging from over 30 to under 20. The simulated contours are in general agreement with the limited measured data shown on Figure 3-30a.
Groundwater Flow in Spring 2017 (February through May)
Groundwater elevations measured in spring 2017 are illustrated on Figures 3-28 and 3-31.
Water year 2017 was a wet year with above average precipitation; as such, water levels are higher throughout the Subbasin than in October 2015.
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As shown on Figure 3-28, groundwater elevations range from 110 feet msl north of the Modesto Reservoir to about 20 feet msl within the City of Modesto near the Tuolumne River. Groundwater flow patterns are similar to spring 1998 and October 2015. Flow is to the southwest with a southerly component towards the Tuolumne River, most notably in the vicinity of the City of Modesto, but also in other areas.
Groundwater elevations have recovered more in the western Subbasin than they have in the eastern Subbasin. For example, water levels within the City of Modesto are about 10 to 20 feet higher than in October 2015. Groundwater elevations in the central Eastern Principal Aquifer are less than 10 feet higher than in October 2015. Although data are limited, it appears that water levels have continued to decline further to the east. Two wells north of the Modesto Reservoir show water level declines of 13 feet (from 118 to 105 feet msl) and 3 feet (from 113 to 110 feet msl) since October 2015.
Water levels at four wells in the Western Lower Principal aquifer are shown on Figure 3-31.
As in 1998 and 2015, the wells are along the eastern edge of the aquifer. Groundwater elevations are higher than they were in October 2015, ranging from 44 to 53 feet msl.
3.2.3.2. Vertical Groundwater Flow
The USGS has found that vertical groundwater movement within the extent of the Corcoran Clay is downward, from the Western Upper Principal Aquifer to the Western Lower Principal Aquifer (Burow et al., 2004). An analysis of groundwater elevation data in the Modesto Subbasin supports this.
The analysis of vertical gradients is based on water levels from a USGS well cluster and a group of nearby wells that are screened above and below the Corcoran Clay. The location of these wells is shown on Figure 3-32 and hydrographs are shown on Figures 3-33 and 3-34.
The extent of the Corcoran Clay, as defined by the USGS (Burow et al., 2004), is shown on Figure 3-32.
In 2004, USGS installed a cluster (MRWA) of three wells in the southwestern Subbasin. Two of the wells are screened above the Corcoran Clay (MRWA-1 and MRWA-2) and one is screened below the Corcoran Clay (MRWA-3). MRWA-1 is screened at a depth of 25 to 30 feet bgs (37 to 32 feet msl), in the shallow portion of the Western Upper Principal Aquifer.
MRWA-2 is screened in the deeper portion of the Western Principal Aquifer just above the Corcoran Clay, at a depth of 174 to 179 feet bgs (-112 to -117 feet msl). MRWA-3 is screened in the Western Lower Principal Aquifer, at a depth of 269 to 274 feet bgs (-207 to - 212 feet msl). According to data provided by the USGS, the Corcoran Clay was encountered from 195 to 240 feet bgs (-133 to -178 feet msl) at this location. The USGS collected water levels from these wells between 2004 and 2006 and again in 2009. These water levels are shown on Figure 3-33.
Water levels measured in the MRWA cluster show that groundwater elevations are higher in the Western Upper Principal Aquifer than the Western Lower Principal Aquifer.
Groundwater elevations above the Corcoran Clay in MRWA-1 and MRWA-2 are similar to one another and are between about 1.5 and 6 feet higher than in MRWA-3, below the #25..
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Corcoran Clay. Therefore, groundwater flow is downward from the Western Upper Principal Aquifer to the Western Lower Principal Aquifer (Figure 3-33).
Groundwater elevations in the shallow and deep regions of the Western Upper Principal Aquifer (MRWA-1 and MRWA-2) are similar except when steep declines occur below the Corcoran Clay. These declines are likely associated with pumping increases below the Corcoran Clay. The shallow unconfined aquifer does not appear to be affected (MRWA-1).
The water levels show consistent downward groundwater flow from the Western Upper Principal Aquifer to the Western Lower Principal Aquifer, which is increased with pumping in the Western Lower Principal Aquifer (Figure 3-33).
The second set of wells used for the vertical groundwater flow analysis includes one MID well (MID-103), screened above the Corcoran Clay from 53 to 81 feet bgs, and two City of Modesto wells (MOD-63 and MOD-313), screened below the Corcoran Clay at multiple intervals ranging from 171 to 456 feet bgs. Well depths in relation to the Corcoran Clay were verified with the cross sections and the base elevation of the Corcoran Clay in the model. These wells, shown on Figure 3-32, are in close proximity to one another near the eastern edge of the Corcoran Clay.
Hydrographs for these three wells are shown on Figure 3-34. The City of Modesto wells show cyclic seasonal pumping fluctuations of up to 30 feet, while the MID well is relatively steady, with fluctuations of 10 or less feet. Groundwater elevations below the Corcoran Clay in the two City of Modesto wells are very similar to one another and consistently lower than the elevations in the MID well above the Corcoran Clay. Groundwater elevations above the Corcoran Clay are about 10 to 40 feet higher than below the Corcoran Clay. The biggest differences occurred during the recent drought (2014 to 2016) due to increased pumping. Water levels in this group of wells indicate consistent downward groundwater flow from the Western Upper Principal Aquifer to the Western Lower Principal Aquifer in this area of the Subbasin.
3.2.4. Changes of Groundwater in Storage
In Bulletin 118 (DWR, 2003), DWR estimates that there is 6.5 million acre feet (MAF) of fresh groundwater in storage to a depth of 300 feet in the Modesto Subbasin. However, as shown on the cross section on Figure 3-18, the depth to the base of fresh water is deeper than 300 feet, and therefore, the DWR estimate is likely too low. In 1961, it was estimated that 14 MAF of stored groundwater is present in the Subbasin to depths of up to 1,000 feet, a more reasonable estimate given the current understanding of subbasin geometry (DWR, 2003).
Since 1961, based on declining water levels trends and fluctuations observed throughout the Subbasin, depletions in groundwater in storage has occurred in the Modesto Subbasin.
Water level trends are described in Section 3.2.2.
One accepted method of estimating current groundwater in storage changes is to construct groundwater elevation contour maps during seasonal highs for various water years and develop change in water level maps between them. By applying storage parameters to these water level changes, a change in groundwater in storage can be estimated. However, #25..
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these maps cannot be developed over the entire Modesto Subbasin with the desired level of certainty due to significant data gaps for water levels both within certain areas of the Subbasin as well as for one of the three Principal Aquifers. Consequently, the C2VSimTM model was used to develop GSP water budget analyses.
Results from the C2VSimTM model, which is well-calibrated and has reliable water budget data, provide an alternative method for estimating changes in groundwater in storage. The model also has the advantage of providing this information over the entire Subbasin, even where water level data are lacking. Selection, refinements, and calibration of the C2VSimTM model are provided in Appendix D. Water budgets, including change in groundwater in storage over a 25-year Study Period have been developed and are summarized in Chapter 5 of this GSP. Those model results represent the best technical data available for determining changes in groundwater in storage over time.
The historical water budget is described in Section 5.1.4.1. As shown on Table 5-8, about 43,000 AFY has been depleted from groundwater in storage during the historical study period, from WY 1991 to 2015. This is equivalent to a cumulative depletion of approximately 1.07 MAF. The annual and cumulative change in storage is illustrated on Figure 5-20. Given that much of the groundwater level declines have occurred during the historical study period (primarily due to increased agricultural water demand), remaining groundwater in storage can be approximated at about 13 MAF.
As summarized on Table 5-8, the historical water budget estimates groundwater production of approximately 311,000 AFY. Given the average depletion of groundwater in storage is 43,000 AFY, a sustainable yield of approximately 268,000 AFY can be estimated for the historical study period. This is a simplistic estimate and does not take into account other important components of the water budget, such as interconnected surface water.
Accordingly, this estimate cannot be projected for future conditions in the Subbasin. A more technically defensible sustainable yield estimate was developed for projected future conditions using the C2VSimTM as described in Section 5.3.
3.2.5. Groundwater Quality
Historical and current groundwater quality conditions of the Modesto Subbasin have been reviewed to characterize groundwater quality of the principal aquifers including an analysis of any constituents of concern. In particular, the analysis allows identification of groundwater quality issues that may affect the supply and beneficial uses of groundwater, including possible plumes of groundwater contamination. The compilation and analysis of historical and current data is described in the following sections, including the sources of data, screening procedures and quality assurance of the data, selection of constituents to analyze, and characteristics of the resulting data sets. Statistical summaries are also presented for select constituents.
3.2.5.1. Regional Groundwater Quality
Groundwater quality in the San Joaquin Valley is highly variable and reliant on the quality of the water recharging the aquifer, the chemical changes that occur as surface water #25..
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percolates to groundwater, and chemical changes that occur within the aquifer (Dale et al., 1966). USGS has categorized regional groundwater quality in the San Joaquin Valley into three groups based on geography: east side, west side, and axial trough (Dale et al., 1966).
East side groundwater quality is of the bicarbonate type with low total dissolved solids (TDS). This groundwater is characteristic of the surface waters that drain the granitic Sierra Nevada Range to the east of the San Joaquin Valley groundwater basin (Dale et al., 1966).
Groundwater quality in the east side reflects the quality of the quality of the local surface water including the Stanislaus and Tuolumne rivers, the primary sources of recharge to the Modesto Subbasin aquifers.
3.2.5.2. Local Groundwater Quality
Publicly available groundwater quality data for the Modesto Subbasin were used in this analysis. These data sources include STRGBA GSA member agencies (City of Modesto, City of Riverbank, City of Waterford, and Modesto Irrigation District), Eastern San Joaquin Water Quality Coalition, Central Valley Salinity Alternatives for Long-term Sustainability (CV-SALTS), and the California State Water Resources Control Board GeoTracker-GAMA and GAMA database. Water quality data from other STRGBA GSA member agencies, such as City of Oakdale, Oakdale Irrigation District, Stanislaus County, and Tuolumne County, were either not available or associated with constituents that were not included in this water quality analysis, such as total coliform and E. Coli coliform. The City of Modesto dataset includes >76,000 water quality records consisting of >30 different constituents collected between 1938 and 2018. The Eastern San Joaquin Water Quality Coalition dataset includes 50,696 records of nitrate analyses between 1902 and 2013, and 19,923 records of total dissolved solids (TDS) analyses between 1925 and 2013. The CV-SALTS database includes nitrate and TDS that were collected between 1934 to 2014 from the following five original collection agencies or sources: Regional Water Quality Control Board (RWQCB) Waste Discharge Requirements (WDR) data per the Dairy CARES program (Dairy); California Department of Public Health (CDPH); Department of Water Resources (DWR); the (USGS) National Water Information System (NWIS) program; and GeoTracker Groundwater Ambient Monitoring and Assessment (GAMA) program.
The data compiled here includes all well types, including domestic, public supply, industrial, monitoring, irrigation, and stock wells, and from all local groundwater quality monitoring programs in the Modesto Subbasin. Using these data, a Microsoft Access database was built that includes over 118,203 groundwater quality records that were collected from 1,339 wells between the start of water year 1995 (October 1, 1994) to 2019. The database includes 260 unique water quality constituents. However, only the most relevant water quality constituents for the Modesto Subbasin are analyzed here. Prior to analysis, quality assurance/quality control (QA/QC) steps were performed on the data, including the identification and removal of duplicate samples and cross-checking the correct well location.
3.2.5.3. Constituents of Concern
A list of potential constituents of concern was developed by the technical team based on a preliminary data review, and review of previous water quality analyses developed in the Subbasin. The constituent list was reviewed at two public STRGBA GSA TAC meetings – April #25..
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and July 2019. Based on input from TAC members, nine potential constituents of concern were identified for the analysis as listed in the following table.
Table 3-2: Potential Constituents of Concern
Nitrate (as N) Boron Dibromo-3-chloropropane (DBCP)
Total Dissolved Solids (TDS) Uranium Tetrachloroethene (PCE) Arsenic Gross Alpha, 1,2- 1,2,3-Trichloropropane (TCP)
The following is a summary of groundwater quality conditions in the Modesto Subbasin during historical (water year 1995 to 2014) and present (2015 to 2019) periods, emphasizing these potential constituents of concern (COCs). Based on a review of water quality and input from the TAC, these COCs are the most likely to affect groundwater quality from irrigated agriculture (i.e., nitrate, TDS, and DBCP), which is the dominant land use across the Modesto Subbasin, from other human point sources (i.e., PCE) and from natural geogenic sources (i.e., arsenic, boron, uranium, and Gross Alpha) in the Subbasin. Nitrate is reported here as nitrate (as N); nitrate values reported in the original data sources as nitrate (as NO3 -
)
were converted to nitrate (as N) prior to analysis.
Nitrate
Nitrate is the most common soluble form of nitrogen in natural groundwater and originates from natural and anthropogenic sources. In general, naturally occurring nitrate is found in low concentrations in groundwater and is derived from precipitation, atmospheric deposition, and natural biogeochemical cycling processes in soils, including the decomposition of organic matter. The most common anthropogenic source of nitrate is the application of nitrogen fertilizers, particularly on irrigated agricultural lands (Gurdak and Qi, 2012). As a result, nitrate is the most ubiquitous nonpoint-source COC of groundwater resources worldwide, including the Central Valley in California (Gurdak and Qi, 2012).
Point sources of nitrate in groundwater include feedlot and dairy drainage, leaching from septic systems, wastewater percolation, industrial wastewater, aerospace activities, and food processing waters (Viers et al., 2012). Denitrification is the only natural process that attenuates nitrate concentrations in groundwater. Previous studies have shown that denitrification is promoted in groundwater with anoxic conditions (dissolved oxygen (DO) <
0.5 mg/L) and large amounts of organic carbon (Gurdak and Qi, 2012). However, there are too few measurements of DO (N = 29) in the database to evaluate if oxic or anoxic conditions exist and the potential for denitrification. All of the DO samples except for two have concentrations in the oxic range (>0.5 milligrams per liter (mg/L)), which indicates a limited potential for denitrification. Future groundwater quality monitoring that includes measurements of DO could help characterize the potential for denitrification and explain the vulnerability of groundwater in the Modesto Subbasin to nitrate contamination.
Nitrate in groundwater from municipal wells in the Modesto Subbasin has been detected in concentrations that approach and, in some cases, exceed the MCL for drinking water (JJ&A #25..
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and Formation Environmental, 2019). Currently, six municipal wells in the City of Modesto have been taken off-line due to elevated nitrate concentrations (JJ&A and Formation Environmental, 2019). Blending of water is being used to reduce nitrate concentrations at other municipal wells. Nitrate is present in the City of Modesto’s drinking water aquifers because of historical agricultural and wastewater management activities. Nitrate is often detected in the shallow aquifer system, but in some cases, can be drawn down into the deeper aquifer by pumping or through wells with long screened or perforated intervals (Jurgens et al., 2008). Nitrate migration is influenced by downward hydraulic gradients created by municipal pumping, and elevated nitrate concentrations are being drawn deeper in the aquifer near local cones of depression (JJ&A and Formation Environmental, 2019).
A total of 41,898 groundwater samples in the Modesto Subbasin have nitrate analyses and an average concentration of 5.3 mg/L (as N) and generally meet drinking water quality standards (Table 3-3). The median value (5.0 mg/L) is approximately double of the range of nitrate concentrations (2 to 3 mg/L) that have been established by previous studies as representing relative background concentrations from natural processes (Gurdak and Qi, 2012). Although isotopic analysis on the nitrate is needed to identify the source, the median value of 5.0 mg/L indicates that more than half of the samples are above the relative background concentration and thus have a nitrogen input from mostly human sources, such as fertilizers. The majority (93%) of the nitrate analyses have concentrations that are below the MCL of 10 mg/L (as N) (Table 3-3). However, 7% of the nitrate samples have concentrations that exceed the MCL (Table 3-3).
The average and maximum concentrations of nitrate in groundwater from wells in the Modesto Subbasin during the period of water year 1995 to 2019 are shown in Figures 3-35 and 3-36. Nitrate concentrations are illustrated as green circles (less than 5 mg/L), yellow circles (between 5 mg/L and the MCL of 10 mg/L), orange circles (between 10 and 15 mg/L), and red circles (greater than 15 mg/L). Wells with average nitrate concentrations below the MCL of 10 mg/L (as N) tend to be located within the central part of the Subbasin, especially within the urban areas surrounding Modesto, Oakdale, Riverbank, and Waterford (Figure 3- 35). The wells that have average nitrate concentrations that exceed the MCL of 10 mg/L (as N) are mostly located within the agricultural lands to the west and east of Modesto, but there are also clusters of exceedances within the City of Modesto (Figure 3-35). The spatial pattern of maximum nitrate concentrations is similar to the spatial pattern of average nitrate concentrations; most wells with maximum nitrate concentrations below the MCL tend to be in urban areas and the maximum nitrate concentrations above the MCL tend to be in the agricultural lands (Figure 3-36). However, there are several wells in Modesto and other urban areas of the Subbasin that have maximum nitrate concentrations above the MCL. The spatial patterns in the average and maximum nitrate concentrations are apparently influenced by the general land-use pattern of the Subbasin. #25..
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Table 3-3: Summary Statistics of Select Groundwater Quality Constituents Summary statistics of nitrate concentrations in groundwater from the Eastern Principal Aquifer, Western Upper Principal Aquifer, and Western Lower Principal Aquifer are shown in Tables 3-4, 3-5, and 3-6, respectively. The average nitrate concentrations are similar (5.6,
5.9, and 5.8 mg/L) in the Eastern, Western Upper, and Western Lower Principal Aquifers.
The percentage of samples that exceed the 10 mg/L MCL in the Western Upper (13%) and Western Lower (22%) is greater than in the Eastern Principal Aquifer (3%). The data indicate that groundwater quality is relatively similar above and below the Corcoran Clay. <0.5MCL
>0.5MCL
to MCL
>MCL Min. Median Avg. Max.
Nutrients
Nitrate (as N), mg/L 10 mg/L1 41,898 50% 42% 7% 0.0 5.0 5.3 490 Pesticides
DBCP, µg/L 0.2 µg/L1 9,636 74% 12% 14% 0.0 0.0 0.1 18
TCP, µg/L 0.005 µg/L1 5,004 96% 0% 4% 0.000 0.000 0.008 12 Radionuclides
Gross Alpha, pCi/L 15 pCi/L1 1,369 65% 20% 15% -0.6 4.1 6.9 47 Uranium, pCi/L 20 pCi/L1 3,326 71% 20% 8% 0.0 4.9 7.4 65 Secondary Maxiumum Contaminant Level Constituents
Total dissolved solids, mg/L 1,000 mg/L2 16,288 55% 30% 14% 0.0 450.0 703.2 20,000 Trace Elements
Arsenic, µg/L 10 µg/L1 5,993 72% 20% 7% 0.0 2.9 4.8 300 Boron, mg/L 1 mg/L* 841 98% 1% 1% 0.0 0.0 1.9 200
Volatile Organic Compounds (VOC)
PCE, µg/L 5 µg/L1 8,262 87% 4% 8% 0.0 0.0 10.4 8,860
Notes:
1 MCL: California drinking water Maximum Contaminant Level 2 SMCL: California drinking water Secondary Maximum Contaminant Level <0.5MCL: percentage of samples with concentrations less than one-half the MCL. >0.05MCL to MCL: percentage of samples with concentrations between one-half of the MCL to the MCL. >MCL: percentage of samples with concentrations greater than the MCL. *California State Notification Level (CA-NL). Boron does not have an MCL. min.: minimum concentration
avg.: average concentration
max.: maximum concentration
Water Quality Constituent
California Percentage of Samples
MCL1
or
SMCL2
Number
of
Samples
Concentrations
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Table 3-4: Summary Statistics of Select Groundwater Quality Constituents for the Eastern Principal Aquifer
<0.5MCL
>0.5MCL
to MCL
>MCL Min. Median Avg. Max.
Nutrients
Nitrate (as N), mg/L 10 mg/L1 25,425 39% 58% 3% 0.0 5.7 5.6 490 Pesticides
DBCP, µg/L 0.2 µg/L1 8,518 71% 14% 15% 0.0 0.0 0.1 18
TCP, µg/L 0.005 µg/L1 4,568 96% 0% 4% 0.000 0.000 0.008 12 Radionuclides
Gross Alpha, pCi/L 15 pCi/L1 920 72% 17% 12% -0.6 3.6 5.7 31 Uranium, pCi/L 20 pCi/L1 2,285 81% 14% 5% 0.0 4.0 5.9 52 Secondary Maxiumum Contaminant Level Constituents
Total dissolved solids, mg/L 1,000 mg/L2 6,963 74% 25% 1% 0.0 380 389 3,000 Trace Elements
Arsenic, µg/L 10 µg/L1 4,245 86% 11% 3% 0.0 2.2 3.1 130 Boron, mg/L 1 mg/L* 606 97% 1% 2% 0.0 0.0 2.6 200
Volatile Organic Compounds (VOC)
PCE, µg/L 5 µg/L1 5,983 86% 5% 9% 0.0 0.0 6.3 8,860
Notes:
1 MCL: California drinking water Maximum Contaminant Level 2 SMCL: California drinking water Secondary Maximum Contaminant Level <0.5MCL: percentage of samples with concentrations less than one-half the MCL. >0.05MCL to MCL: percentage of samples with concentrations between one-half of the MCL to the MCL. >MCL: percentage of samples with concentrations greater than the MCL. *California State Notification Level (CA-NL). Boron does not have an MCL. min.: minimum concentration
avg.: average concentration
max.: maximum concentration
Water Quality Constituent
California Percentage of Samples
MCL1
or
SMCL2
Number
of
Samples
Concentrations
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Table 3-5: Summary Statistics of Select Groundwater Quality Constituents for the Western Upper Principal Aquifer
<0.5MCL
>0.5MCL
to MCL
>MCL Min. Median Avg. Max.
Nutrients
Nitrate (as NO3), mg/L 10 mg/L1 2,326 47% 40% 13% 0.0 5.3 5.9 52 Pesticides
DBCP, µg/L 0.2 µg/L1 434 75% 2% 23% 0.0 0.0 0.1 1.5
TCP, µg/L 0.005 µg/L1 118 100% 0% 0% 0.0 0.000 0.000 0.000 Radionuclides
Gross Alpha, pCi/L 15 pCi/L1 153 33% 33% 33% 0.0 11.4 12.4 47.2 Uranium, pCi/L 20 pCi/L1 433 29% 52% 20% 0.0 13.0 13.6 32 Secondary Maxiumum Contaminant Level Constituents
Total dissolved solids, mg/L 1,000 mg/L2 1,215 46% 41% 13% 0.0 530 733 20,000 Trace Elements
Arsenic, µg/L 10 µg/L1 1,108 42% 41% 17% 0.0 5.4 9.5 300 Boron, mg/L 1 mg/L* 139 100% 0% 0% 0.0 0.2 0.1 0.3
Volatile Organic Compounds (VOC)
PCE, µg/L 5 µg/L1 1,014 93% 1% 7% 0.0 0.0 0.9 250
Notes:
1 MCL: California drinking water Maximum Contaminant Level 2 SMCL: California drinking water Secondary Maximum Contaminant Level <0.5MCL: percentage of samples with concentrations less than one-half the MCL. >0.05MCL to MCL: percentage of samples with concentrations between one-half of the MCL to the MCL. >MCL: percentage of samples with concentrations greater than the MCL. *California State Notification Level (CA-NL). Boron does not have an MCL. min.: minimum concentration
avg.: average concentration
max.: maximum concentration
Water Quality Constituent
California Percentage of Samples
MCL1
or
SMCL2
Number
of
Samples
Concentrations
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Table 3-6: Summary Statistics of Select Groundwater Quality Constituents for the Western Lower Principal Aquifer
Total Dissolved Solids
Total dissolved solids (TDS) represent the total concentration of anions and cations in water and is a useful indicator of mineralization, salt content, and overall groundwater quality. The TDS concentrations in groundwater of the Modesto Subbasin generally meet drinking water quality standards (Table 3-3) and some irrigation requirements. A total of 16,288 groundwater samples in the Modesto Subbasin have TDS analyses and only 14% of those samples exceed the California Secondary Maximum Contaminant Level (SMCL) of 1,000 mg/L (Table 3-3).
TDS can also be used to characterize the salinity of irrigation water, which can affect crop health and yield (Grattan, 2002). It is recommended that TDS concentrations should be below about 450 mg/L for irrigation of salt sensitive crops, and TDS concentrations between about 450 and 1,000 mg/L can represent a salinity hazard for plants if used as irrigation water (Bauder et al., 2014). About half (49%) of the samples have TDS concentrations less <0.5MCL
>0.5MCL
to MCL
>MCL Min. Median Avg. Max.
Nutrients
Nitrate (as N), mg/L 10 mg/L1 445 50% 28% 22% 0.0 4.8 5.8 17 Pesticides
DBCP, µg/L 0.2 µg/L1 110 100% 0% 0% 0.0 0.0 0.0 0
TCP, µg/L 0.005 µg/L1 133 95% 0% 5% 0.000 0.000 0.000 0 Radionuclides
Gross Alpha, pCi/L 15 pCi/L1 30 93% 7% 0% 0.0 0.0 1.7 14 Uranium, pCi/L 20 pCi/L1 92 97% 3% 0% 0.0 1.0 1.4 13
Secondary Maxiumum Contaminant Level Constituents
Total dissolved solids, mg/L 1,000 mg/L2 66 100% 0% 0% 45.0 188 192 468 Trace Elements
Arsenic, µg/L 10 µg/L1 222 9% 74% 17% 0.0 9.0 8.3 14
Boron, mg/L 1 mg/L* 13 100% 0% 0% 0.0 0.1 0.1 0
Volatile Organic Compounds (VOC)
PCE, µg/L 5 µg/L1 438 100% 0% 0% 0.0 0.0 0.0 1
Notes:
1 MCL: California drinking water Maximum Contaminant Level 2 SMCL: California drinking water Secondary Maximum Contaminant Level <0.5MCL: percentage of samples with concentrations less than one-half the MCL. >0.05MCL to MCL: percentage of samples with concentrations between one-half of the MCL to the MCL. >MCL: percentage of samples with concentrations greater than the MCL. *California State Notification Level (CA-NL). Boron does not have an MCL. min.: minimum concentration
avg.: average concentration
max.: maximum concentration
Water Quality Constituent
California Percentage of Samples
MCL1
or
SMCL2
Number
of
Samples
Concentrations
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than 450 mg/L and would not cause plant stress. However, 36% of samples are between 450 and 1,000 mg/L and 14% of samples are greater than 1,000 mg/L. Therefore, about 51% of groundwater samples have TDS concentrations that could result in plant stress and salinity hazard as irrigation water.
To identify any areas of concern, the average and maximum TDS concentrations in groundwater from wells within the Modesto Subbasin during the period of water year 1995 to 2019 are shown in Figures 3-37 and 3-38. TDS concentrations are illustrated as green circles (below 500 mg/L), yellow circles (between 500 and 1,000 mg/L), orange circles (between 1,000 and 1,500 mg/L), and red circles (above 1,500 mg/L). The median and maximum TDS concentrations in groundwater throughout most of the Modesto are below 1,000 mg/L (Figures 3-37 and 3-38). Concentrations of TDS are generally lowest (less than 500 mg/L) in the central part of the Subbasin, especially within the urban areas surrounding Modesto, Oakdale, Riverbank, and Waterford (Figure 3-37 and 3-38). Concentrations of TDS above the MCL are generally found in wells located in the San Joaquin River National Wildlife Refuge on the western extent of the Subbasin, in southwest Modesto, and to the southeast of Modesto (Figure 3-37 and 3-38).
Summary statistics of TDS concentrations in groundwater from the Eastern Principal Aquifer, Western Upper Principal Aquifer, and Western Lower Principal Aquifer are shown in Tables 3-4, 3-5, and 3-6, respectively. The average TDS concentrations are similar (389 and 192 mg/L) in the Eastern and Western Lower Principal Aquifers. However, the average TDS in the Western Upper Principal Aquifer (733 mg/L) is much higher than in the other two Principal Aquifers. Similarly, 13% of TDS samples from the Western Upper Principal Aquifer exceed the MCL, while only 1 and 0% of the samples from the Eastern and Western Lower exceed the MCL. These results, along with the 20,000 mg/L maximum concentration may indicate a point source affecting TDS concentrations in the Western Upper Principal Aquifer (Table 3- 5).
Arsenic
Arsenic is a naturally occurring trace element in rocks, soils, and groundwater in some areas of the Central Valley aquifer (Burton et al., 2012). In the Modesto Subbasin, arsenic in groundwater is generally naturally occurring and is largely derived from the Sierran sediments that were transported to the eastern San Joaquin Valley by glacial and fluvial processes (Jurgens et al., 2008). Previous studies of arsenic in the San Joaquin Valley (Belitz et al., 2003; Welch et al., 2006; Izbicki et al., 2008; and Burton et al., 2012) and a literature review of arsenic (Welch et al., 2000) have identified two dominant mechanisms for elevated arsenic in groundwater. The first mechanism is the reductive dissolution of arsenopyrite or other iron or manganese oxyhydroxides under iron- or manganese-reducing conditions. The second mechanism is the pH-dependent desorption of arsenic from aquifer sediments under oxic conditions, which tends to occur in groundwater with pH above 7.5 (Stollenwerk, 2003). Given the general oxic nature of groundwater in the Subbasin, sorption and desorption on iron oxyhydroxides at pH above 7.5 is expected to be the most significant control on arsenic groundwater mobility. Another mechanism that has been identified is the decreased resorption due to increasing pH, competing species, or lack of sorption sites #25..
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(Jurgens et al., 2008; Jurgens et al., 2009). Arsenic can also be mobilized from aquitards by dewatering (Smith et al., 2018). The USGS (2008) indicate that migration of arsenic in groundwater in the study area can be facilitated by lateral and vertical gradients created by municipal pumping and by vertical movement through wells with long screened or perforated intervals. Additionally, it has been proposed that geochemical changes in modern recharge water, such as relatively high dissolved organic carbon concentrations could contribute to mobilization of arsenic in the aquifer (JJ&A and Formation Environmental, 2019). Anthropogenic sources of arsenic in groundwater can include the use of wood preservatives, paints and dyes, and from some mining and oilfield operations (Welch et al., 2000).
Groundwater arsenic concentrations in the Subbasin are generally higher in older and deeper groundwater samples (Jurgens et al., 2009). Arsenic in groundwater from municipal wells has been detected in concentrations that approach and, in some cases, exceed the MCL for drinking water (JJ&A and Formation Environmental, 2019). Several municipal wells from the City of Modesto have been taken off-line due to elevated arsenic concentrations (JJ&A and Formation Environmental, 2019).
The concentrations of arsenic are generally low in groundwater of the Modesto Subbasin as compared to the MCL (Table 3-3). A total of 5,993 groundwater samples have arsenic analyses and only 7% of those analyses exceed the California MCL of 10 µg/L (Table 3-3).
The wells with average concentrations of arsenic that exceed the MCL are generally located in the urban area of Modesto and in wells on the western extent of the Subbasin (Figures 3- 39). Wells with maximum concentrations of arsenic that exceed the MCL are also generally located in the urban areas of Modesto and Riverbank, and wells on the western extent of the Subbasin (Figure 3-40).
Summary statistics of arsenic concentrations in groundwater from the Eastern Principal Aquifer, Western Upper Principal Aquifer, and Western Lower Principal Aquifer are shown in Tables 3-4, 3-5, and 3-6, respectively. The average arsenic concentrations in the Western Upper (9.5 µg/L) and Western Lower (8.3 µg/L) Principal Aquifers are more than double the
3.1 µg/L average concentration in the Eastern Principal Aquifer. Similarly, 17% of the arsenic samples in both the Western Upper and Western Lower exceed the MCL, as compared to only 3% of samples in the Eastern Principal Aquifer. These data indicate important differences may exist in the source(s) and geochemical conditions that control arsenic in groundwater of the Western Upper and Lower Principal Aquifers as compared to the Eastern Principal Aquifer.
Uranium
Uranium in groundwater in the Modesto Subbasin is generally naturally occurring and is largely derived from granitic rocks in the Sierra Nevada rather than sources at land surface (Jurgens et al., 2008). The uranium was weathered from these rocks and oxidized and adsorbed to sediments that were transported to the eastern San Joaquin Valley by glacial and fluvial processes and deposited in the alluvial fans that now make up the Modesto Subbasin (Jurgens et al., 2008). Uranium is a relatively prevalent contaminant in shallow #25..
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Revised July 2024
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and intermediate depth aquifers in the study area, including beneath the City of Modesto (JJ&A and Formation Environmental, 2019). The mobilization of uranium in the shallow and intermediate aquifer is likely influenced by elevated bicarbonate concentrations in modern and oxic recharge water resulting from agricultural activities (Jurgens et al., 2009). Irrigation return flow that recharges the aquifer can be relatively elevated in bicarbonate concentrations because of the rich and active biomes of the agricultural soils that create elevated carbon dioxide and relatively high partial pressures of carbon dioxide that often result in bicarbonate water type of modern recharge. The uranium is mobilized from the natural sediments when the bicarbonate-rich water flow downward through the aquifer and replaces older groundwater that has relatively lower bicarbonate concentrations (Jurgens et al., 2009). Uranium concentrations have also been observed to be negatively correlated with pH (Burton et al., 2012). Therefore, uranium concentrations are generally higher near the water table and in shallow groundwater and decrease with depth (Jurgens et al., 2008).
Uranium has been detected in municipal wells at concentrations that approach and, in some cases, exceed the MCL for drinking water (JJ&A and Formation Environmental, 2019).
Currently, nine municipal wells in the City of Modesto have been taken off-line due to elevated uranium concentrations (JJ&A and Formation Environmental, 2019).
The concentrations of uranium are generally low in groundwater across much of the Modesto Subbasin as compared to the MCL (Table 3-3). A total of 3,326 groundwater samples have uranium analyses and 8% of those analyses exceed the California MCL of 20 pCi/L (Table 3-3). Most of the uranium samples were collected from supply wells within the urban areas of Modesto, Oakdale, Riverbank, and Waterford. The wells with average (Figure 3-41) and maximum (Figure 3-42) uranium concentrations that exceed the MCL tend to be located in the City of Modesto.
Summary statistics of uranium concentrations in groundwater from the Eastern Principal Aquifer, Western Upper Principal Aquifer, and Western Lower Principal Aquifer are shown in Tables 3-4, 3-5, and 3-6, respectively. The uranium concentrations in groundwater are much greater in the Western Upper Principal Aquifer, as compared to the Eastern or Western Lower Principal Aquifers. A total of 20% of uranium samples in the Western Upper exceed the MCL, while only 5 and 0% in the Eastern and Western Lower, respectively, exceed the MCL. These differences in uranium concentration among groundwater of the Principal Aquifers are consistent with the processes of the oxic and bicarbonate rich irrigation return flow that mobilizes uranium in the shallow and intermediate aquifer.
Gross Alpha
Alpha particles (α-particles) are a type of radiation emitted by some radionuclides. The alpha particles consist of two protons and two neutrons. Their travel range is only a few centimeters. Once alpha particles lose energy, they pick up electrons and become helium.
Alpha emitting radionuclides are naturally occurring elements, and include radium-226, uranium-238, radium-226, and radon-222. Radium-226 and radon-222 are generally the alpha emitters of greatest interest to drinking water because they are groundwater #25..
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Revised July 2024
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contaminants widely distributed in the U.S. and associated with granitic rock, including the Sierra Nevada. The California MCL for gross alpha in drinking water is 15 pCi/L.
The concentrations of gross alpha are relatively low in groundwater across much of the Modesto Subbasin as compared to the MCL (Table 3-3). A total of 1,369 groundwater samples have gross alpha analyses and 85% of those analyses have concentrations that are less than the California MCL of 15 pCi/L. A total of 15% of the groundwater samples exceed the gross alpha MCL, which is a higher percentage than uranium samples exceeding the MCL (Table 3-3). Similar to the uranium samples, most of the gross alpha samples were collected from supply wells within the urban areas of Modesto, Oakdale, Riverbank, and Waterford.
The wells with average (Figure 3-43) and maximum (Figure 3-44) uranium concentrations that exceed the MCL tend to be located in the City of Modesto, especially in the southwest part of Modesto.
Summary statistics of gross alpha in groundwater from the Eastern Principal Aquifer, Western Upper Principal Aquifer, and Western Lower Principal Aquifer are shown in Tables 3-4, 3-5, and 3-6, respectively. Similar to the pattern of uranium, the gross alpha in groundwater is much greater in the Western Upper Principal Aquifer, as compared to the Eastern or Western Lower Principal Aquifers. A total of 20% of uranium samples in the Western Upper exceed the MCL, while only 5 and 0% in the Eastern and Western Lower, respectively, exceed the MCL. Similar to uranium, these differences in gross alpha among groundwater of the Principal Aquifers are consistent with the processes of the oxic and bicarbonate rich irrigation return flow that mobilizes uranium in the shallow and intermediate aquifer.
Boron
Boron is a naturally occurring trace element in many minerals and rocks, including igneous rocks such as granite and pegmatite, and some evaporite minerals. Borax is a boroncontaining evaporite mineral that is mined in California and is used as a cleaning agent and therefore may be present in sewage and industrial wastes (Burton et al., 2012). There is no MCL for boron. However, California has a Notification Level (NL) of 1 mg/L. Boron is an essential element for plant growth in relatively small concentrations. However, for many crops, boron concentrations greater than 1 to 2 mg/L may be toxic (Ayers and Westcot, 1994).
The concentrations of boron are generally very low in groundwater in the Modesto Subbasin as compared to the NL (Table 3-3). A total of 841 groundwater samples have boron analyses and 99% of those analyses have concentrations that are less than the California NL of 1.0 mg/L and 1% have concentrations that exceed the NL (Table 3-3). The average (Figures 3-45) and maximum (Figures 3-46) boron concentrations of groundwater in wells that exceed the NL are generally located in Waterford, which may indicate a potential point-source contamination issue. 98% of the boron analyses have concentrations below 0.5 mg/L (Table 3-3), and thus the boron concentrations in groundwater of the Modesto Subbasin are well below toxic levels for plants.
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Revised July 2024
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Summary statistics of boron concentrations in groundwater from the Eastern Principal Aquifer, Western Upper Principal Aquifer, and Western Lower Principal Aquifer are shown in Tables 3-4, 3-5, and 3-6, respectively. There are no major differences in boron concentration or percentage of samples that exceed the NL among the three Principal Aquifers.
Pesticides
Pesticides in groundwater can result from the over-application on agricultural lands or from point-source contamination and preferential flow down improperly constructed wells. While pesticides are typically soluble in water, many can be highly sorptive to soils, which can slow their transport to the water table. The analysis is focused on the two widely detected pesticides Dibromochloropropane (DBCP) and 1,2,3-Trichloropropane (TCP).
Dibromochloropropane (DBCP)
Dibromochloropropane (DBCP) was a widely used agricultural nematocide and soil fumigant in parts of the Central Valley that was first detected in California drinking water in 1979 and later banned in the late 1970s. In 1983, a statewide drinking water source monitoring program was initiated and found DBCP to be the most commonly detected pesticide in groundwater (CA Department of Health Services, 1999). DBCP is relatively mobile when dissolved in water and free DBCP may occur as a dense non-aqueous phase liquid (DNAPL).
DBCP is toxic to humans at low concentrations, and thus has presented a local concern (JJ&A and Formation Environmental, 2019). The Federal and California MCL for DBCP is 0.2 μg/L.
DBCP was detected in at least seven municipal wells in the City of Modesto at concentrations above the MCL that warranted the use of wellhead treatment using granular activated carbon (Jurgens et al., 2008). DBCP has also been detected at lower concentrations below the MCL in water from at least seven municipal wells from the City of Modesto (JJ&A and Formation Environmental, 2019).
The concentrations of DBCP are generally low in groundwater of the Modesto Subbasin as compared to the MCL (Table 3-3). A total of 9,636 groundwater samples have DBCP analyses and 86% of those analyses and below the California MCL of 0.2 μg/L (Table 3-3). The remaining 14% of samples with DBCP concentrations above the MCL are from wells that are generally located to the north, west, and southeast of the City of Modesto (Figures 3-47 and 3-48).
Summary statistics of DBCP concentrations in groundwater from the Eastern Principal Aquifer, Western Upper Principal Aquifer, and Western Lower Principal Aquifer are shown in Tables 3-4, 3-5, and 3-6, respectively. The percentage of DBCP samples that exceed the MCL are somewhat similar (15 and 23%) in the Eastern and Western Upper and greater than in the Western Lower (0%) Principal Aquifer. Unlike nitrate concentrations that were somewhat similar above and below the Corcoran Clay, relatively higher concentrations of DBCP appears to be more frequently detected in only the Western Upper Principal Aquifer.
The relatively longer flow paths and travel times for groundwater below the Corcoran Clay may help to limit DBCP concentrations in the Western Lower Principal Aquifer. #25..
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1,2,3-Trichloropropane (TCP)
1,2,3-Trichloropropane (TCP) is a chlorinated hydrocarbon with high chemical stability that often occurs as an intermediate in chemical manufacturing. It is a manmade chemical that is often found at industrial or hazardous waste sites, used as a cleaning and degreasing solvent, and associated with pesticide products (SWRCB, 2019). TCP may be produced as a byproduct of processes used to produce soil fumigant chemicals. TCP is also a major and minor component of several soil fumigants that were used historically in California through most of the 1980s (Burton et al., 2012). Although TCP was banned from pesticides in the 1990s, it has been detected in groundwater beneath agricultural areas of the Central Valley as part of the GAMA sampling program (Shelton et al., 2008). TCP is an emerging contaminant of concern because it is widely detected and is a probable carcinogen to humans (SWRCB, 2019). In 2017, California adopted an MCL of 0.005 μg/L for drinking water, and now many water supply systems are being monitored for TCP. TCP has been detected in several wells throughout the Subbasin at concentrations above the MCL (JJ&A and Formation Environmental, 2019).
The concentrations of TCP in groundwater in the Modesto Subbasin as compared to the MCL are shown in Table 3-3. A total of 5,004 groundwater samples have TCP analyses and 4% of those analyses are above the California MCL of 0.005 μg/L (Table 3-3). The wells with average (Figures 3-49) and maximum (Figures 3-50) TCP concentrations that exceed the MCL are located primarily in the urban areas of Modesto, Riverbank and Waterford. As discussed below in the section on historical and present trends, the wells with elevated TCP tend to have concentrations that are sometimes two to three orders of magnitude greater than the MCL. Such high concentrations of TCP may indicate locations of point-source contamination.
Summary statistics of TCP concentrations in groundwater from the Eastern Principal Aquifer, Western Upper Principal Aquifer, and Western Lower Principal Aquifer are shown in Tables 3-4, 3-5, and 3-6, respectively. TCP exceedances of the MCL occur in 15% of Eastern Principal Aquifer samples, 23% of Western Upper Principal Aquifer samples, and 0% of Western Lower Principal Aquifer samples. These data suggest that relatively lower concentrations of TCP are below the Corcoran Clay.
Tetrachloroethylene (PCE)
Volatile organic compounds (VOCs) have been detected in several wells in and around the City of Modesto and in Oakdale (JJ&A and Formation Environmental, 2019). The source of the VOCs is largely attributed to historical dry-cleaning operations. At least seven City of Modesto wells are currently receiving treatment to remove PCE, trichloroethylene, and (or) Freon-113 (JJ&A and Formation Environmental, 2019). There have been a number of response actions in the Modesto area to the PCE contamination, including site investigations, groundwater extraction to address shallow groundwater contamination, and soil vapor extraction to address source removal and potential vapor intrusion into buildings (JJ&A and Formation Environmental, 2019). Therefore, the VOC analysis here is focused on PCE.
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Tetrachloroethylene (PCE) is a manufactured chemical and does not occur naturally in the environment. It is a regulated contaminant with a Federal and California MCL of 5 μg/L (SWRCB, 2017). Common sources of PCE include dry cleaning operations, textile operations, and metal degreasing processes. It was also widely used in the production of CFC-113 and other fluorocarbons. PCE is also used in rubber coatings, solvent soaps, printing inks, adhesives and glues, sealants, polishes, lubricants, and pesticides. PCE is a DNAPL and has moderate to high mobility.
The concentrations of PCE are generally low in groundwater in the Modesto Subbasin as compared to the MCL (Table 3-3). A total of 8,262 groundwater samples have PCE analyses and 92% of those analyses are below the California MCL of 5 μg/L (Table 3-3). Most PCE concentrations above the MCL are from wells located in Modesto and Oakdale, which are likely impacted by historical dry-cleaning operations (Figures 3-51 and 3-52).
Summary statistics of PCE concentrations in groundwater from the Eastern Principal Aquifer, Western Upper Principal Aquifer, and Western Lower Principal Aquifer are shown in Tables 3-4, 3-5, and 3-6, respectively. The percentage of PCE samples that exceed the MCL are somewhat similar (9% and 7%) in the Eastern and Western Upper and greater than in the Western Lower (0%) Principal Aquifer. Similar to patterns in DBCP and TCP concentrations, relatively lower concentrations of PCE appear to be detected below the Corcoran Clay in the Western Lower Principal Aquifer. The low permeability of the clay associated with relatively longer flow paths and travel times for groundwater below the Corcoran Clay may help to limit PCE concentrations in the Western Lower Principal Aquifer.
3.2.5.4. Trends in Historical and Present Groundwater Quality Statistical tests were used to evaluate if the concentrations of groundwater quality constituents are statistically similar or different between historical (water year 1995 to 2014) and present (2015 to 2019) periods. This analysis will help identify processes that may affect the temporal trends in the groundwater quality of the Modesto Subbasin.
First, the Shapiro-Wilk test for normality was used to test the null hypothesis that the groundwater quality constituents come from a normal distribution. Results of the ShapiroWilk test support a rejection of the null hypothesis (α-level = 0.05) and indicate that nitrate, DBCP, TCP, Gross Alpha, Uranium, TDS, arsenic, boron, and PCE all have a non-normal distribution.
Based on the results of the Shapiro-Wilk tests, the nonparametric Wilcoxon rank-sum test was used to test the null hypothesis that the groundwater quality constituents sampled between the historical and present period come from populations that have the same distribution and thus are statistically similar. Results of the Wilcoxon rank-sum test support the decision to fail to reject the null hypothesis (α-level = 0.05) for TCP (p-value = 0.767), gross alpha (p-value = 0.212), and PCE (p-value = 0.981) (Figure 3-53), which indicates that these groundwater quality constituents have statistically similar median concentrations during the historical and present periods. However, the results of the Wilcoxon rank-sum test for nitrate (p-value = <0.001), DBCP (p-value = <0.001), uranium (p-value = <0.001), TDS (p-value = 0.001), arsenic (p-value = <0.001), and boron (p-value = <0.001) support the #25..
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decision to reject the null hypothesis (Figure 3-54), which indicates that these groundwater quality constituents have statistically different median concentrations during the historical and present periods. The median concentrations of nitrate, DBCP, arsenic, and boron are statistically lower in the present period than the historical period (Figure 3-54). Conversely, the median concentrations for uranium and TDS are statistically higher in the present period than the historical period (Figure 3-54).
The temporal linear trends in groundwater quality constituents are evaluated in Figures 3- 55 and 3-56. Results of the trend analysis indicate statistically significant (α-level = 0.05) increasing trends for TCP (p-value = <0.001) and gross alpha (p-value = <0.001) concentrations, but no statistically significant temporal trend for PCE (p-value = 0.141) (Figure 3-55). Results of the trend analysis indicate statistically significant (α-level = 0.05) increasing trends for TDS (p-value = <0.001), nitrate (p-value = <0.001), and uranium (pvalue = <0.001) concentrations (Figure 3-56). Conversely, there are decreasing trends for DBCP (p-value = <0.001) and arsenic (p-value = 0.002), but no statistically significant trend for boron (p-value = 0.232) (Figure 3-56).
These findings indicate that TCP, gross alpha, TDS, nitrate, and uranium concentrations are increasing over time in the Modesto Subbasin, while DBCP and arsenic concentrations are decreasing over time in the Modesto Subbasin.
3.2.5.5. Contamination Sites from GeoTracker
The State Water Resources Control Board (SWRCB) GeoTracker online database was accessed to identify active and former contamination cleanup sites within the Subbasin. As of November 2021, 320 cleanup sites are documented on GeoTracker in the Modesto Subbasin. Less than 10 percent of these (28 sites) are open, and the remaining (292 sites) are closed. Active remediation or monitoring is still occurring at the open sites. The open cases include 2 Leaking Underground Storage (LUST) sites, 24 Cleanup Program sites, and 2 Military sites.
The contamination sites from GeoTracker are presented on Figure 3-57, and the number of each site (open and closed) is shown in the legend of this figure. Most of the sites are in the cities of Modesto, Riverbank, Oakdale and Waterford. Available data uploaded to GeoTracker from these sites will be considered in the annual analysis of groundwater quality to be conducted by the GSAs as part of GSP implementation (see Section 6.6).
3.2.6. Land Subsidence
The overdraft conditions exacerbated by the recent drought resulted in lowered groundwater levels – a condition that can contribute to subsidence of the ground surface.
As water levels decline in the subsurface, dewatering and compaction of predominantly fine-grained deposits, such as clay and silt, can cause the overlying ground surface to subside.
This process is illustrated by two conceptual diagrams shown on Figure 3-58. The upper diagram depicts an alluvial groundwater basin with a regional clay layer and numerous #25..
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smaller discontinuous clay layers. Water level declines associated with pumping cause a decrease in water pressure in the pore space (pore pressure) of the aquifer system (Galloway, et al., 1999). Because the water pressure in the pores helps support the weight of the overlying aquifer, the pore pressure decrease causes more weight of the overlying aquifer to be transferred to the grains within the structure of the sediment layer. The difference between the water pressure in the pores and the weight of the overlying aquifer is the effective stress. If the effective stress borne by the sediment grains exceeds the structural strength of the sediment layer, then the aquifer system begins to deform. This deformation consists of rearrangement and compaction of fine-grained units7 , as illustrated
on the lower diagram of Figure 3-58. The tabular nature of the fine-grained sediments allows for preferred alignment and compaction. As the sediments compact, the ground surface can sink, as illustrated by the 2nd column on the lower diagram of Figure 3-58.
Land subsidence due to groundwater withdrawals can be temporary (elastic) or permanent (inelastic).
Elastic deformation occurs when sediments compress as pore pressures decrease but expand by an equal amount as pore pressures increase. A decrease in water levels from groundwater pumping causes a small elastic compaction in both coarse- and fine-grained sediments; however, this compaction recovers as the effective stress returns to its initial value. Because elastic deformation is relatively minor and fully recoverable, it is not considered an impact.
Inelastic deformation occurs when the magnitude of the greatest pressure that has acted on the clay layer since its deposition (preconsolidation stress) is exceeded. This occurs when groundwater levels in the aquifer reach a historically low level. During inelastic deformation, or compaction, the sediment grains rearrange into a tighter configuration as pore pressures are reduced. This causes the volume of the sediment layer to reduce, which causes the land surface to subside. Inelastic deformation is permanent because it does not recover as pore pressures increase. Clay particles are often planar in form and more subject to permanent realignment (and inelastic subsidence). In general, coarse-grained deposits (e.g., sand and gravels) have sufficient intergranular strength and do not undergo inelastic deformation within the range of pore pressure changes encountered from groundwater pumping.
The volume of compaction is equal to the volume of groundwater that is expelled from the pore space, resulting in a loss of storage capacity. This loss of storage capacity is permanent but may not be substantial because clay layers do not typically store significant amounts of usable groundwater (LSCE, et al., 2014). Inelastic compaction, however, may decrease the vertical permeability of the clay resulting in minor changes in vertical flow. 7 Although extraction of groundwater by pumping wells causes a more complex deformation of the aquifer system than discussed herein, the simplistic concept of vertical compaction is often used to illustrate the land subsidence process (Galloway, et al., 1999; LSCE et al., 2014). #25..
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The following potential impacts can be associated with land subsidence due to groundwater withdrawals (modified from LSCE, et al., 2014):
• Damage to infrastructure including foundations, roads, bridges, or pipelines; • Loss of conveyance in canals, streams, or channels;
• Diminished effectiveness of levees;
• Collapsed or damaged well casings; and
• Land fissures.
Land subsidence in the San Joaquin Valley has been documented for more than 90 years and recent investigations using satellite imagery indicate continuing problems in some areas.
However, subsidence is not a significant issue in Modesto Subbasin. Figure 3-59 illustrates the results of a subsidence study conducted by the USGS (Faunt et al., 2015) in the San Joaquin Valley from 2008 to 2010. This study shows that subsidence did not occur within Modesto Subbasin during this time period.
Beginning in June 2015, vertical displacement was estimated throughout many California groundwater basins using Interferometric Synthetic Aperture Radar (InSAR) data. The InSAR data are collected by the European Space Agency (ESA) Sentinel-1A satellite and processed by TRE ALTAMIRA Inc. (TRE), under contract with DWR as part of DWR's SGMA technical assistance. Figure 3-60 illustrates vertical displacement (in feet) for the Modesto Subbasin from June 2015 to October 2020, a period of approximately five years. Most of the Subbasin is shaded grey on this figure, indicating an absence of land subsidence. Negative vertical displacement (subsidence), shown by yellow to light brown colors, is indicated in the central and eastern Subbasin, within the Eastern Principal Aquifer (east of the Corcoran Clay), and also in the northwest corner of the Subbasin and in a thin strip along the lower reach of the Stanislaus River. Most of the eastern Subbasin indicates vertical displacement between 0 and 0.05 feet (0.6 inches), as shown by the yellow shading. This equates to a rate of approximately 0.12 inches per year over the five year period. There are two small areas in the eastern Subbasin where a larger rate of subsidence is indicated. The maximum measured subsidence, shown by the small brown shaded area, is 0.15 feet (1.8 inches). This is a minimal amount of measured subsidence and could possibly be due, in part, to the abundance of clay surficial soils (see black shading on Figure 3-6) that have the potential to shrink. Also, there are restrictive layers in the soil in the eastern part of the Subbasin that, if disturbed by agricultural operations, could alter the ground surface elevation. This type of vertical displacement is not likely related to groundwater extraction. This subsidence is not likely to impact critical infrastructure in this area. The measured subsidence in the northwest Subbasin is mostly between 0 and 0.5 feet (0.6 inches) over the five year period (yellow shading), with maximum measured subsidence on the order of 0.1 feet (1.2 inches, orange shading) over the five year period. There is a higher potential for subsidence in the western Modesto Subbasin if groundwater levels are lowered below the Corcoran Clay.
A recent study conducted by Towill, Inc. and TRE Altamira, Inc., under contract with DWR, showed that InSAR vertical displacement data is highly accurate in most areas. The study #25..
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compared vertical displacement ground surface elevation data from InSAR to continuously operating global positioning system (CGPS) base stations (Towill, 2021). The study found that the two data sets had a high degree of correlation, with only a very small state-wide absolute difference of 8.86 mm. The study concludes that InSAR data accurately measured vertical displacement in California’s ground surface to within 18 mm (0.7 inches) between January 1, 2015, and October 1, 2020. The InSAR data cover the full extent of the Subbasin and provide a reasonable dataset to use as a screening tool to evaluate subsidence in the Modesto Subbasin. The InSAR data will be updated annually and discussed in the GSP annual reports.
In addition to the InSAR data, there are four GPS stations in the Subbasin. As shown on Figure 3-60, three of these stations are along the Highway 99 corridor in Salida and Modesto, and one is in the northeastern corner of the Subbasin. These GPS stations indicate zero to low rates of vertical displacement. Stations P260, CMOD and P306 showed no subsidence, while P781 indicated land subsidence of about 0.048 inches per year. The data from these stations shows a cyclic pattern to ground surface elevation, demonstrating the effects of inelastic land subsidence.
3.2.7. Interconnected Surface Water
The Tuolumne, Stanislaus, and San Joaquin rivers are all interconnected surface water as defined by SGMA. These three rivers flow for approximately 122 miles along three of the four Subbasin boundaries. The Stanislaus River is approximately 59 miles long along the northern Subbasin boundary, the Tuolumne River approximately 47 miles along the southern boundary and the San Joaquin River approximately 16 miles along the western boundary.
The segment of the San Joaquin River along the Modesto Subbasin can be characterized as a net gaining reach, historically and also based on future projected conditions. The Tuolumne and Stanislaus river systems are more dynamic, with recharge and baseflow varying along segments of the rivers both seasonally and over time. This dynamic system is a result of both natural conditions and managed operations. Both rivers are actively managed to provide critical water supplies for the Modesto, Turlock, and Eastern San Joaquin subbasins.
As described in more detail in Chapter 5 (see Section 5.1.4), total stream inflows into the Subbasin during the historical study period are approximately 2.5 MAF. Approximately half of this inflow (1.3 MAF) is from the San Joaquin River, with the other half split between the Stanislaus River (0.5 MAF) and the Tuolumne River (0.7 MAF). The Stanislaus River and Tuolumne River drain into the San Joaquin River, and the outflow from the San Joaquin River out of the Subbasin is approximately 2.8 MAF during the historical study period.
The location, quantity, and timing of deletions of these interconnected rivers were analyzed using the integrated surface water-groundwater model C2VSimTM. Development of the model and model calibration is described in Appendix D (see Appendix D Sections 2.1.2,
3.4, and 4.3.2). Analysis of interconnected surface water and surface water budgets under historical, current, and future projected conditions is provided in Chapter 5. #25..
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STRGBA GSA/Tuolumne GSA 3-45
Revised July 2024
TODD GROUNDWATER
Data tables in Chapter 5 provide details for estimating average gaining or losing conditions along each river. As shown on Table 5-2, during the historical period (WY 1991 – WY 2015), the Tuolumne, Stanislaus, and San Joaquin rivers were all net gaining rivers in the Modesto Subbasin. During that period, net gains from the groundwater system (baseflow) to the Tuolumne, Stanislaus, and San Joaquin rivers were 31,000 AFY, 16,000 AFY, and 14,000 AFY, respectively.
The model predicts that under the 50-year projected conditions the San Joaquin River will remain a net gaining river into the future with a net gain of 9,000 AFY. The Tuolumne and Stanislaus rivers are predicted to transition to overall net losing rivers, with average net losses of 11,000 and 24,000 AFY, respectively (Table 5-2). An increase in stream seepage to groundwater (streamflow depletion) was predicted for all rivers if current land and water use remain the same without additional water supplies.
To illustrate the variability of losing/gaining reaches along each river, the C2VSimTM was used to analyze each river node in the model as predominantly gaining, losing, or mixed conditions for both historical and projected future conditions. This nodal analysis is presented on Figure 3-61. Model nodes are represented as small circles along each of the rivers.
For illustration purposes, the model nodes are color coded with respect to net gaining or losing conditions for the two different simulation periods. Although conditions are highly dynamic at each node, the predominant condition (occurring in 85 percent of the model months represented) is highlighted. If conditions at the node are predominantly gaining, the node is blue; predominantly losing nodes are orange, and nodes that are not predominantly losing or gaining are labeled “mixed” and colored green. The node color does not represent quantity and does not account for seasonal or annualized volumes of water (Figure 3-61).
A comparison between the historical simulation and the projected future simulation shows locations where predominantly gaining reaches (blue) transition to predominantly losing reaches (orange) or mixed conditions (green) over time (Figure 3-61). On the Stanislaus River, this transition occurs over most of the river but is most pronounced downstream of Oakdale. On the Tuolumne River, most of the change occurs in the eastern two-thirds of the river, upstream of the City of Modesto. Along the short segment of the San Joaquin River that defines the Modesto Subbasin, conditions are either gaining or mixed with less change predicted from historical to future conditions (Figure 3-61).
Although the model indicates that all reaches of the rivers remain connected through historical and future projected conditions, increases in streamflow depletion over time are indicated by the model water budgets and illustrated by the nodal analysis. The nodal analysis correlated strongly with predicted changes in groundwater elevations. This correlation indicates that streamflow depletions are primarily associated with groundwater extractions. The correlation further suggests that if water level declines associated with local overdraft conditions are arrested, predicted increases in streamflow depletions can be reduced. Additional modeling supports this conclusion (Sections 5.3 and 8.5.1).This indication highlights the need for water level monitoring (Chapter 7). These conditions also #25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 3-46
Revised July 2024
TODD GROUNDWATER
guided the selection of sustainable management criteria (Chapter 6) for interconnected surface water and the development of GSP projects and management actions to arrest local water level declines (Chapter 8). Additional details on the water budget analysis of surface water are provided in Chapter 5.
3.2.8. Groundwater Dependent Ecosystems
A groundwater dependent ecosystem (GDE) is defined under SGMA as “ecological communities of species that depend on groundwater emerging from aquifers or on groundwater occurring near the ground surface” (23 CCR § 351(m)).
To support identification of groundwater dependent ecosystems (GDEs), DWR created the Natural Communities Commonly Associated with Groundwater (NCCAG) dataset. This Natural Communities dataset is a compilation of 48 publicly available State and federal agency datasets that map vegetation, wetlands, springs, and seeps in California. The resultant mapping of natural vegetation communities and wetlands commonly associated with groundwater has been reviewed by DWR, California Department of Fish and Wildlife (CDFW), and The Nature Conservancy (TNC) and provided online for California groundwater basins. The data included in the Natural Communities dataset do not necessarily represent GDEs but can be used as a starting point in identifying GDEs within a groundwater basin.
The NCCAG dataset includes two sets of polygons that represent different habitat classes.
The first class is wetland features commonly associated with the surface expression of groundwater under natural, unmodified conditions. The second class is vegetation types that are commonly associated with the sub-surface presence of groundwater (phreatophytes) (DWR, 2018d). The presence of wetland or vegetation polygons in the NCCAG dataset, however, does not necessarily indicate the presence of a GDE. Rather, the NCCAG dataset provides a starting point for identifying potential GDEs.
The vegetation and wetlands polygons from the NCCAG dataset within the Modesto Subbasin are illustrated on Figure 3-62. There are approximately 1,800 NCCAG polygons (768 wetlands and 1,027 vegetative) in the Modesto Subbasin. Most of the wetlands and vegetation polygons are present along the three major rivers (Stanislaus, Tuolumne and San Joaquin rivers), along Dry Creek, between Dry Creek and the Tuolumne River, scattered in the eastern Subbasin, and along the western Subbasin boundary, within the San Joaquin River Natural Wildlife Refuge.
Given the large number of NCCAG polygons, it was not feasible to investigate the details of each polygon in the Subbasin. However, a depth to water analysis was conducted as a first approximation to identify wetlands and vegetation polygons in areas where depth to water exceeds rooting depths, in accordance with The Nature Conservancy’s guidance (The Nature Conservancy, 2018).
Groundwater elevations were used to estimate depth to water during the wettest year of the GSP Study Period (Spring 1998) and at the end of the GSP Study Period, during a critically dry year (Fall 2015). These two years generally represent periods of high (1998) #25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 3-47
Revised July 2024
TODD GROUNDWATER
and low (2015) water levels over average hydrologic conditions. Using ArcGIS, a groundwater elevation surface was developed from simulated groundwater elevations from the C2VSim-TM model for each of the two years. This surface was subtracted from a digital elevation map (DEM) of ground surface elevations to develop depth to water maps.
The areas within the Modesto Subbasin with a depth to water within 30 feet in Spring 1998 are shown on Figure 3-63. In general, depth to water is within 30 feet along the river boundaries, along Dry Creek, and in the western Subbasin. The NCCAG polygons were then overlaid onto the depth to water map and polygons were removed from the map in areas where depth to water exceeded 30 feet. It is assumed that the vegetation and wetlands do not have access to groundwater when depth to water is deeper than 30 feet.
The map showing wetland and vegetation polygons in areas with depth to water within 30 feet in Spring 1998 is illustrated on Figure 3-64. This map has 1,525 polygons (567 wetland and 958 vegetative), an approximate 15 percent decrease from the original NCCAG dataset.
Potential GDEs are present along the river boundaries, along Dry Creek and in the western Subbasin. Potential GDEs were eliminated in the eastern Subbasin, and away from the rivers and Dry Creek. Figure 3-64 represents the potential GDEs that were present in Spring 1998. Since this was the wettest period within the GSP study period, with the highest water levels in many parts of the Subbasin, this map represents the potential GDEs that could have been present in the Modesto Subbasin during the GSP Study Period (WY 1990 – WY 2015).
A similar analysis was conducted for water levels in Fall 2015. The areas of the Modesto Subbasin with a depth to water within 30 feet are illustrated on Figure 3-65. Depth to water is within 30 feet within a thin band along the river boundaries, the western stretch of Dry Creek and along the western edge of the Subbasin. The wetland and vegetative polygons in areas where depth to water is within 30 feet are shown on Figure 3-66. As compared to the 1998 map (Figure 3-64), potential GDEs were eliminated along most of Dry Creek. This map has 1,285 polygons (462 wetland and 823 vegetative), an approximate 28 percent decrease from the original NCCAG dataset.
SGMA legislation requires the Subbasin GSAs to be responsible for GDEs that are present at the end of the GSP Study Period (WY 2015). Therefore, the polygons shown on Figure 3-66 are potential GDEs that will be further evaluated following GSP adoption.
In 2021, Moore Biological Consultants reviewed the potential GDEs identified in Fall 2015 (Figure 3-66) within Mapes Ranch, a private property near the San Joaquin River. Moore Biological Consultants conducted a desktop study and a field survey and concluded that 56 potential GDE polygons (46 wetland and 10 vegetative) identified within the Mapes Ranch property are not GDEs. This study is provided in Appendix E. These polygons were removed from the Fall 2015 map of potential GDEs, as shown on Figure 3-67.
Based on the Fall 2015 depth to water analysis and the study conducted by Moore Biological Consultants, there are 1,229 potential GDE polygons (416 wetland and 813 vegetative) in the Modesto Subbasin (Figure 3-67). This is an approximate 31 percent decrease from the #25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 3-48
Revised July 2024
TODD GROUNDWATER
original NCCAG dataset. These potential GDEs occur along the river boundaries, the downstream reach of Dry Creek and along the western Subbasin boundary.
The GSAs plan to further investigate the potential GDEs during GSP implementation.
3.2.9. Data Gaps and Uncertainties for Groundwater Conditions This section will summarize groundwater condition data gaps that affect implementation of the Plan and are related to the GSAs ability to sustainably manage groundwater. The Plan Implementation section, when developed, will describe how these data gaps will be addressed in future GSP actions. A summary of data gaps identified for the Groundwater Conditions analysis in the Modesto Subbasin is summarized in the following table.
Table 3-7: Data Gaps for the Groundwater Conditions
Issue Area
Impacts on
Groundwater
Management
Actions to Address
Water Levels
in Western
Lower
Principal
Aquifer
Western
Lower
Principal
Aquifer
Groundwater levels
and flow; vertical
gradients; evaluation
for potential future
land subsidence;
insufficient wells for
groundwater elevation
mapping.
• Install monitoring wells
screened solely in the
Western Lower Principal
Aquifer.
• Locate existing wells to
incorporate into monitoring
program, if available.
Groundwater
Conditions in
Eastern
Subbasin
East of the
OakdaleWaterford
Highway
Groundwater flow and
quality of Eastern
Principal Aquifer
• Install monitoring wells in
eastern Subbasin.
• Obtain water level data
from landowners.
Interconnected
Surface Water
River
boundaries
Groundwater levels
and flow, surface water
availability, water
budgets
• Continued analysis with
C2VSimTM Model.
• Improve monitoring.
GDEs River
boundaries
Groundwater levels
and flow
Verify presence of GDEs based
on NCCAG dataset.
#25..
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(
"N
0 4
Scale in Miles
April 2020
Figure 3-1
Regional Geology
Stanislaus River
Tuol umne River
San Joaqu in River
Qm
Qtl
Qr
Tm
Qdp
Tvs
Ei
Pl
Mtm
Geologic Units
t
Pl
Q
Qm
Qr
Qtl
Qdp
Mtm
Tm
Tvs
Ei
Legend
River
Modesto Subbasin
Source: Wagner, D.L., Bortugno, E.J., and McJunking, R.D., 1991, Geologic Map of the San Francisco-San Jose Quadrangle,
California, 1:250,000.
Dredge tailings
Laguna Formation
Alluvium
Modesto Formation
Riverbank Formation
Turlock Lake Formation
Dos Palos Alluvium
Table Mountain Latite
Mehrten Formation
Valley Springs Formation
Ione Formation
#25..
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Path: C:\Projects\Modesto GSP 74305\GRAPHICS\Figure 3-2 Precipitation.gpj April 2020 Figure 3-2
Annual Precipitation
Water Year
Wet
Above Normal
Below Normal
Dry
Critically Dry
0 5
10 15
20 25
Water Year Precipitation (inches)
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 Water Year
* Long Term Average (1961-2015) = 12.6 inches
Study Period Average (WY 1991-2015) = 12.8 inches
Average*
Notes:
Source - MID weather station (Modesto CA).
Water Year - October 1 through September.
#25..
- 1761 -
MID Weather Station
Figure 3-3
Average Annual
Precipitation
Legend
Weather Station
River
Modesto Subbasin
Annual Precipitation (inches)
High : 21
Low : 11
(
"N
0 4
Scale in Miles
April 2020
Stanislaus River
Tuol umne River
San Joaqu in River
Source: PRISM, average from 1981 to 2010
#25..
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Legend
River
Modesto Subbasin
Ground Surface Elevation
(ft msl)
> 600
550 - 600
500 - 550
450 - 500
400 - 450
350 - 400
300 - 350
250 - 300
200 - 250
150 - 200
100 - 150
50 - 100
> 50
(
"N
0 4
Scale in Miles
April 2020 Figure 3-4
Ground Surface
Elevations
Stanislaus River
Tuol umne River
San Joaqu in River
Dry Creek
#25..
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1 1'
2' 3'
4'
2 3
4 Legend
Section Line
River
Modesto Subbasin
April 2020
Figure 3-5
Topographic Profiles
(
"N
0 5
Scale in Miles
Stanislaus River
Tuolu mne River
San Joaquin River
Dry Creek
#25..
- 1764 -
No Soils
Data
(
"N
0 4
Scale in Miles
November 2021 Figure 3-6
Soils and Surficial
Restrictive Layers
Stanislaus River
Tuol umne River
San Joaqu in River
Legend
Soil Restrictive Layer
Soil Classification
Not Classified
Clay
Clay and Silt
Silt and Silty Sands
Clayey Sands
Clayey, Silty Sands
Silty Sands
Poorly graded Sand and Silt
Poorly Graded Sand
Clayey Gravels
Clayey, Silty Gravels
Silty Gravels
Gravel and Sand
Source: SSURGO, 2003
#25..
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-800
-750
-700
-650
-850
-600
-900
-950
-550
-500
-1000
-1050
-450
-1100
-400
-1150
-1200
-1250
-1300
-350
-900
-750
-1000
-1300
-650
-750
-700
-700
-900
-800
-650
-550
-800
-600
-850
-600
-900
-800
-1050
-850
-750
-950
-850
-600
-700
-1000
-1100
-600
-400
-800
-800
-400
-600
(
"N
0 4
Scale in Miles
April 2020
Figure 3-7
Base of Fresh Water
Stanislaus River
Tuol umne River
San Joaqu in River
Legend
USGS Base of Fresh Water Elevation (ft-msl)
C2VSIM Base of Fresh Water Elevation (ft-msl)
River
Modesto Subbasin
#25..
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L4
L1 A2
L2
L3
Base of fresh water
L1: Aquifer ranging from the ground surface to the top of the pumping layer A2: Corcoran Clay
L2: Primary shallow pumping layer
L3: Deeper pumping layer (bottom of layer is the base of fresh water) L4: Saline aquifer (bottom of layer is the base of continental deposits) April 2020 Figure 3-8
C2VSIM
Model Layers
#25..
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-160
-90
-140
-120
-110
-180
-80
-150
-130
-100
-150
-80
-90
-130 -100
(
"N
0 4
Scale in Miles
May 2020 Figure 3-9
CV2SIM Top of
Corcoran Clay
Stanislaus River
Tuol umne River
San Joaqu in River
Legend
River
Modesto Subbasin
Top of Corcoran Clay Elevation (ft msl)
Corcoran Clay Extent (Burow et al., 2004)
Base of Western Upper
Principal Aquifer
#25..
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-140
-150
-120
-220
-160
-190
-180
-110
-170
-130
-190
-210
-220
-190
-160
-180
-130
(
"N
0 4
Scale in Miles
May 2020 Figure 3-10
CV2SIM Base of
Corcoran Clay
Stanislaus River
Tuol umne River
San Joaqu in River
Legend
River
Bottom of Corcoran Clay Elevation (ft msl)
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
Top of Western Lower
Principal Aquifer
#25..
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E'
A'
B'
C'
D'
B E
C
D
A
Sources: Esri, HERE, Garmin, Intermap, increment P Corp., GEBCO, USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance Survey, Esri Japan, METI, Esri China (Hong Kong), (c) OpenStreetMap contributors, and the GIS User Community April 2020 Figure 3-11
Cross Section
Locations
Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-11 Cross-Section Lines.mxd Legend
!( USGS Texture Data Wells
!( Cross Section Well
!( DOGGR Geophysical Log
Section Line
Rivers
Basin Boundary
Corcoran Clay Extent (Burow et al., 2004)
County Line
(
N
0 1.5 3
Miles
#25..
- 1770 -
ZZ
Mehrten Formation
Corcoran Clay
| |
City of Modesto
H-H'
?
?
?
Southwest Northeast
Western Upper
Principal Aquifer
Western Lower
Principal Aquifer
Eastern
Principal Aquifer
E0114877
52129
374481799940
e0223215
145649
476389
22677
325398
426391
E0253458
476388
946031
40469
83085
994562
1665629043
96157
129575
53695
129606
25 33
75791
26580
168904
168954
326897381772
374447370469340961547523516492426314488254304030
718322
781541
46452
242804
409168370429529989
955618
582961
791023
E0126128
582970
912374
994014
926300
495243
965680
789376
994502
515495117773
264
58 259
54 71022
260
Eastern Basin
Boundary
Western Basin
Boundary
B-B' C-C' D-D'
San Joaquin
River
A A' 101112131415161718192021222324252627282930313233343536 0123456789
-500
-400
-300
-200
-100
0 100
200
300
400
500
-600
May 2020
Figure 3-12
Cross Section A-A'
Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-12 Cross-Section A-A'.mxd Miles
Elevation (feet msl)
Approximate Fault Location
ZInferred Fault Motion
Estimated Top of Mehrten Formation
Coarse-Grained Material
Blue Clay
Black Sands
Fine-Grained Material
#25..
- 1771 -
433926
129692
4811539559104882641540965691864649649517089356468017704838119261723423476378 50 241617111472142800441077395171401149357217
109756829422622
428003
149527381805220634
A-A'
Southern Basin
Boundary
Northern Basin
Boundary
Stanislaus
River
Tuolumne
River
B B'
0 1 2 3 4 5 6 7 8 9 10
-300
-200
-100
0 100
200
-400
May 2020
Figure 3-13
Cross Section B-B'
Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-13 Cross-Section B-B'.mxd Miles
Elevation (feet msl)
Legend
Fine-Grained Material
Coarse-Grained Material
Blue Clay
| City of Modesto |
D-D'
Northwest Southeast
Western Upper
Principal Aquifer
Western Lower
Principal Aquifer
Eastern
Principal
Aquifer
#25..
- 1772 -
Mehrten Formation
?
121275
158153164761306538
54431516475732639
306540
718677418181540885253416086850198151174287439229624982139253852530140913553690938286246493039612480749735410970515802131688411286152934589332374457243220 37036
6086419115936087251138
250564
544404226854
A-A'
Southern Basin
Boundary
Northern Basin
Boundary
Stanislaus
River
Dry
Creek
Tuolumne
River
C C'
-500
-400
-300
-200
-100
0 100
0 1 2 3 4 5 6 7 8 9 10 11 12
May 2020
Figure 3-14
Cross Section C-C'
Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-14 Cross-Section C-C'.mxd Legend
Esimated Top of Mehrten Formation
Fine-Grained Material
Coarse-Grained Material
Black Sands
Miles
Elevation (feet msl)
Northwest Southeast
Eastern
Principal Aquifer
#25..
- 1773 -
Mehrten Formation 465239 96056
40471
111238
290064
733016
515495
117773
e0162496995114
956988
706223
955556
76517
111237
e0147907
994068994067
993831
947882947881
1097538
994356
190808
e0250577
e0256752
e0187445
Modesto
Reservoir
D D'
A-A'
Southern
Basin
Boundary
Northern
Basin
Boundary
Dry
Creek
Stanislaus
River
Tuolumne
River
0123456789 10 11 12 13 14
-400
-300
-200
-1000
100
200
300
400
500
May 2020
Figure 3-15
Cross Section D-D'
Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-15 Cross-Section D-D'.mxd Miles
Elevation (feet msl)
Legend
Estimated Top of Mehrten Formation
Fine-Grained Material
Coarse-Grained Material
Black Sands
Northwest Southeast
Eastern
Principal Aquifer
#25..
- 1774 -
ZZ
Mehrten
Formation
Mehrten
Formation
400
-500
-400
-300
-200
-100
0 100
300
200
0 1 2 3 4 5 6 7 8 9 10 11
D-D'
C-C'
Stanislaus
River
E E'
May 2020
Figure 3-16
Cross Section E-E'
Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-16 Cross-Section E-E'.mxd Legend
Fine-Grained Material
Coarse-Grained Material
Black Sands
Approximate Fault Location
ZInferred Fault Motion
Miles
Elevation (feet msl)
Southwest Northeast
Eastern
Principal
Aquifer
Estimated Top of
Mehrten Formation
#25..
- 1775 -
803712
999241603
532935
774024
25159
769562
811818
782928
718677
344796
246486
197525
90554
149374
21348
287406
22712
22725
29026
29032
29046
41802
60854
70276
76533
111223
125851
125894
129552
129557
129605
131078
131079
189390
191501
219486
275128
287087
301082
306540
334937
377463
422605
484065
516486
533189
548965
717223
815341
945057946077
1097042
769556484031
777459
29027
29039
60884
189372
252942
488754
497397
400
100
300
200
D-D'
May 2020 Figure 3-17
Cross Section E-E'
Alternative Scale
Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-17 Cross Section E-E' well labels.mxd Legend
Fine-Grained Material
Coarse-Grained Material
Black Sands
Approximate Fault Location
ZInferred Fault Motion Elevation (feet msl) E E'
Southwest Northeast
Eastern
Principal
Aquifer
Estimated Top of
Mehrten Formation
#25..
- 1776 -
ZZ
ZZ
>
>
>
>
>
>
>
>
>
>
>
>
>
#
Mehrten Formation
Corcoran Clay
Undifferentiated
Modesto, Riverbank,
and Turlock Lake
Formations
Mehrten Formation
Mehrten Formation
Undifferentiated
Continental and
Marine Sediments
Undifferentiated
Continental and
Marine Sediments
Undifferentiated
Continental and
Marine Sediments
Western Upper
Principal Aquifer
Western Lower
Principal Aquifer Eastern
Principal
Aquifer
71022
264
259
260
54 955618
912374
515495
994014
516492
718322
370429
168904
96157
129575
16656
3325
40469
253458
83085
994562
946031
426391
22677
52129
114877
374481
799940223215145649
476388476389325398
26580
46452
29043
75791
129606
168954
242804
326897
374447
370469340961304030
409168
781541547523426314
488254
53695
381772
582970791023
529989
126128582961
965680
495243
926300
789376994502
117773
58 Eastern Basin
Boundary
Western Basin
Boundary
B-B' C-C' D-D'
San Joaquin
River
10 11
12131415161718192021222324252627282930313233343536
0123456789
A A'
-1400
-1300
600
500
400
300
200
100
0 -100
-200
-300
-400
-500
-600
-700
-800
-900
-1000
-1100
-1200
Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-18 Cross-Section A-A' r2.mxd May 2020 Figure 3-18
Hydrogeologic
Framework
Cross Section A-A'
Approximate Fault Location
ZInferred Fault Motion
Approximate Formation Boundary
>
Approximate Base of Mehrten Formation Inferred from DOGGR Geophysical Log Groundwater Elevation October 2015
Southwest Northeast
C2VSIM Base of Fresh Water
#25..
- 1777 -
!( !(
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!(
!(!(!(!(!( D D !(!(
D D
D D
D D D
D D D D D D
D
D D D D D D
D DD D D D
D D
D D
D D D D D
D D D D D D
D D D D D D D
D
D D D
#
# # # # # # # # # # # # # # # # # #
# ## # # #
#
# #
## # # ## # #
# # #
# # # # # # #
# #
# #
# #
# #
# # ### # #
^_
# #
#
# # # # # # # # # # # # #
^_
#
# # # # #
#
#
# #
#
# # # # # # #
# # ## # ## # #
# ## #
#
#
# # # # # # # # # # # # # # # # #
#
# # # # # ## # # # # # ## # # # # # # # # #
^_
# # # #
#
# # ### # # #
#
### ## # # #
# # ## # # # # ## # # # # # # ## # ## # # # # # # ## # # #
#
#
# # #
# #
# # # # # # #
# # # # ## # # # # # ## # # ## # #
# # # #
# # # # # # # # # # # # ## # ## # # # # # # # # # #
# # ## ## ### # # # ## ## # #### # ## ## # # #
# # ##
# #
# # # # # # #### # ## # #
#
^_
^_
^_ ^_
^_
^_
^_
^_
Legend
Source
!( MID
!( Modesto
!( OID
!( Oakdale
!( Riverbank
!( Waterford
D CASGEM
# DWR WDL
^_ USGS
Modesto Subbasin
(
"N
0 4
Scale in Miles
April 2020 Figure 3-19
Wells with Water
Level Data
Stanislaus River
Tuol umne River
San Joaqu in River
#25..
- 1778 -
!(
!( !(
!(
!(
!(
!(
!(
!(
!(
!(
!(
!(
!( !(
!(
!(
!(
!(
!( !(
OAK-005
Mod-037
MID-223
MID-214
MID-208
MID-197
02S11E29M001M
02S12E32P001M
Mod-313
Mod-056
Mod-017 MID-109
MID-103
MID-037
04S08E02H001M
MID-228
02S09E28H001M
Mod-290
MID-082
02S08E33F001M
04S08E06L001M
Legend
!( Above Corcoran
!( Below Corcoran
!( East of Corcoran
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin (
"N
0 4
Scale in Miles
April 2020 Figure 3-20
Representative
Hydrograph Locations
Stanislaus River
Tuol umne River
San Joaqu in River
#25..
- 1779 -
!( !(
!(
!(
!(
1 3
4 5 2
Legend
!( Above Corcoran
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
(
"N
0 4
Scale in Miles
May 2020 Figure 3-21
Representative Hydrographs
Western Upper
Principal Aquifer (1 of 2)
Stanislaus River
Tuol umne River
San Joaqu in River
1 2 3
4 5
#25..
- 1780 -
!(
!(
!(
8 7
6 Legend
!( Above Corcoran
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
(
"N
0 4
Scale in Miles
May 2020 Figure 3-22
Representative Hydrographs
Western Upper
Principal Aquifer (2 of 2)
Stanislaus River
Tuol umne River
San Joaqu in River
6 7 8
#25..
- 1781 -
!(
!(
!(
9 11
10 Legend
!( Below Corcoran
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
(
"N
0 4
Scale in Miles
May 2020 Figure 3-23
Representative Hydrographs
Western Lower
Principal Aquifer
Stanislaus River
Tuol umne River
San Joaqu in River
9 10 11
#25..
- 1782 -
!(
!(
!(
!(
!( 13
15 12
16 14
Legend
!( East of Corcoran
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
(
"N
0 4
Scale in Miles
May 2020 Figure 3-24
Representative Hydrographs
Eastern Principal
Aquifer (1 of 2)
Stanislaus River
Tuol umne River
San Joaqu in River
13 14
15 16
12 #25..
- 1783 -
!(
!(
!(
!(
!(
17 19
21 18
20 Legend
!( East of Corcoran
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
(
"N
0 4
Scale in Miles
May 2020 Figure 3-25
Representative Hydrographs
Eastern Principal
Aquifer (2 of 2)
Stanislaus River
Tuol umne River
San Joaqu in River
18 19
20 21
17 #25..
- 1784 -
60 80
100
40 140
120
60 40
100
80 100
40 120
97 74 91
88 54 88
51 97
82 87 96
54 59 95
94 98
99 90 52 97 58 47
42 50 89 48 86 71 42 95
52 66
48 65
60 79
49 49
42 50
36 37
34 36
67 65
43 38
63 61 60
55 57 62
37 37
43 45 42
34 71
70 42
50 64
67 48
55 60
59 37
57 54
52 47
46 49
41 42
73 81
71 81
73 69
74 90
48 50
91 91
90 94
93 96 94 97
63 76
63 62
72 66
40 86
74 64
45 60
90 38
41 42
48 41
43 42
44 72
43 38
48 44
49 48
51 91
89 41
44 45 56
42 44
49 55
44 46
47 45
53 41
53 51
53 46
45 46
127
117 119 110 110 100 108 105 101 111
101
107 102
149 101 101 102
105
100 102 106 100
101
122
149
139
124
136
Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community Legend
Groundwater Elevation (ft msl)
<=20
20.1 - 40
40.1 - 60
60.1 - 80
80.1 - 100
100.1 - 120
120.1 - 140
>140
Groundwater Elevation Contour
Groundwater Elevation Contour, Estimated
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
May 2020 Figure 3-26
Groundwater Elevation
Contours, Spring 1998
Unconfined Aquifer
Document Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-26 Groundwater_Elevation_Contours_Spring 1998 Unconfined.mxd (
"N
0 3
Scale in Miles
Note: Groundwater Elevations are from the Western Upper Principal Aquifer and Eastern Principal Aquifer #25..
- 1785 -
80 40
60 100
120
60 80
40 120
100
40 40
60 40
40 20
20 9
74 99
70 69 64 76
76 78 79
70 75
86 70
65 56
59 87
81 33
39 33
21 29
39 43
31 11
49 39
55 22
40 28
50 40 40
37 44
28 36
41 35
29 14
35 32
40 35
44 30
28 33
24 49
47 29
37 23
27 41
44 37
40 31
35 42
41 38
40 39
42 38
40 35
31 71
78 42
83 86
70 74
77 75
49 48 37
57 58 51 59 54
49 57 62
64 63
64 60
71 87
56 61 76
90 57
32 43
50 59
65 34
18 62
60 62 62 71 55
59 97
59 89
99 42
57 54
64 41
48 52
48 36
38 52
54 36
28 42
40 66
31 36
34 33
35 35
37 33
36 27
33 28
39 32
36 53
49 26
35 36 38
33 28
39 34
36 28
15 36
36 31
28 34
34 35
33 114
116
122
111
104
132
113
118
Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community Legend
Groundwater Elevation (ft msl)
<= 20
20.1 - 40
40.1 - 60
60.1 - 80
80.1 - 100
100.1 - 120
120.1 - 140
> 140
Groundwater Elevation Contour
Groundwater Elevation Contour, Estimated
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
October 2021 Figure 3-27a
Groundwater Elevation
Contours, October 2015
Unconfined Aquifer Note: Groundwater Elevations are from the Western Upper Principal Aquifer and Eastern Principal Aquifer #25..
- 1786 -
40 40
40 200
180
160
150
140
130
120
110
90 80
70 60
100
190
170
180
150
160
30 20
10 50
140
Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community Legend
Modesto Subbasin
Corcoran Clay Extent (Burow et al., 2004)
Model Layer 1 Groundwater Elevation Contour
October 2021 Figure 3-27b
Simulated Groundwater Elevation
Contours, September 2015
Unconfined Aquifer Note: Groundwater Elevations are from the Western Upper Principal Aquifer and Eastern Principal Aquifer #25..
- 1787 -
60 80
80 40
100
40 60
60 40
40 20
90 87 66 85 60
72 39
83 59
52 47 77
68 38 81 65 72
36 30
49 38
28 67
67 39
53 78
83 87
50 74
79 48 45 45
52 43
39 37
47 43
45 22 21
57 46
60 31
46 44
57 48 47
43 55
49 48
44 39 37 34
43 40
48 52
56 49
41 48
37 59
57 48
52 50
40 51
41 54
44 54
50 52
48 48
43 48
49 41
40 65
68 71
70 70
90 68
78 82
93 62
42 48
60 65
77 39
42 70
64 67 67
60 67
53 65
64 61
54 53
61 55
43 42
59 42
33 53
46 34
38 36
35 39
37 41
36 39
34 35
34 42
37 37
60 51
34 39
37 40
38 34
41 36
39 31
19 38
40 36
30 36
35 38
105
110
105
101
80?
Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community Legend
Groundwater Elevation (ft msl)
<= 20
20.1 - 40
40.1 - 60
60.1 - 80
80.1 - 100
100.1 - 120
120.1 - 140
> 140
Groundwater Elevation Contour
Groundwater Elevation Contour, Estimated
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
May 2020 Figure 3-28
Groundwater Elevation
Contours, Spring 2017
Unconfined Aquifer
Document Path: C:\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-28 Groundwater_Elevation_Contours_Spring 2017 Unconfined.mxd (
"N
0 3
Scale in Miles
Note: Groundwater Elevations are from the Western Upper Principal Aquifer and Eastern Principal Aquifer #25..
- 1788 -
41 44
Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community Legend
Groundwater Elevation (ft msl)
<=20
20.1-40
40.1-60
60.1-80
80.1-100
100.1-120
120.1-140
>140
Corcoran Clay Extent (Burow et al., 2004)
Modesto_GSP
May 2020 Figure 3-29
Groundwater Elevations
Spring 1998
Confined Aquifer Note: Groundwater Elevations are from the Western Lower Principal Aquifer #25..
- 1789 -
26 31
26 41
Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community Legend
Groundwater Elevation (ft msl)
<=20
20.1-40
40.1-60
60.1-80
80.1-100
100.1-120
120.1-140
>140
Corcoran Clay Extent (Burow et al., 2004)
Modesto_GSP
October 2021 Figure 3-30a
Groundwater Elevations
October 2015
Confined Aquifer Note: Groundwater Elevations are from the Western Lower Principal Aquifer #25..
- 1790 -
30 20
20 Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community Legend
Modesto Subbasin
Corcoran Clay Extent (Burow et al., 2004)
Model Layer 2 Groundwater Elevation Contour (feet msl)
December 2021 Figure 3-30b
Simulated Groundwater Elevation
Contours, September 2015
Confined Aquifer Note: Groundwater Elevations are from the Western Upper Principal Aquifer and Eastern Principal Aquifer #25..
- 1791 -
45 46
44 53
Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community Legend
Groundwater Elevation (ft msl)
<=20
20.1-40
40.1-60
60.1-80
80.1-100
100.1-120
120.1-140
>140
Corcoran Clay Extent (Burow et al., 2004)
Modesto_GSP
May 2020 Figure 3-31
Groundwater Elevations
Spring 2017
Confined Aquifer Note: Groundwater Elevations are from the Western Lower Principal Aquifer #25..
- 1792 -
MRWA-2
MID-103
Mod-313
Mod-063
Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community Legend
Corcoran Clay Extent (Burow et al., 2004)
Modesto Subbasin
April 2020 Figure 3-32
Wells Used for
Vertical Flow Analysis
(
"N
0 2
Scale in Miles
#25..
- 1793 -
April 2020 Figure 3-33
Vertical Flow
Hydrographs,
USGS Well Cluster
32 34
36 38
40 42
44 46
Groundwater Elevation (ft msl)
2004 2005 2006 2007 2008 2009 2010
MRWA-1 (Above Corcoran screen 25-30 ft)
MRWA-2 (Above Corcoran screen 174-179 ft)
MRWA-3 (Below Corcoran screen 269-274 ft)
#25..
- 1794 -
April 2020 Figure 3-34
Vertical Flow
Hydrographs, MID and
City of Modesto Wells
0 10
20 30
40 50
60 70
Groundwater Elevation (ft msl)
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 MID-103 (Above Corcoran screen 53-81 ft)
MOD-313 (Below Corcoran screen 171-187, 225-235, 307-322 ft) MOD-63 (Below Corcoran screen 200-266, 364-396, 422-456 ft) #25..
- 1795 -
Legend
Nitrate (as N) Average Concentration
< 5 mg/L
5 to 10 mg/L
10 to 15 mg/L
> 15 mg/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
May 2020 Figure 3-35
Nitrate (as N)
Average Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuolu mne River
San Joaquin River
Dry Creek
#25..
- 1796 -
Legend
Nitrate (as N) Maximum Concentration
< 5 mg/L
5 to 10 mg/L
10 to 15 mg/L
> 15 mg/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
May 2020 Figure 3-36
Nitrate (as N)
Maximum Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1797 -
Legend
TDS Average Concentration
< 500 mg/L
500 to 1000 mg/L
1000 to 1500 mg/L
> 1500 mg/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-37
TDS Average
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1798 -
Legend
TDS Maximum Concentration
< 500 mg/L
500 to 1000 mg/L
1000 to 1500 mg/L
> 1500 mg/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-38
TDS Maximum
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1799 -
Legend
Arsenic Average Concentration
> 15 ug/L
10 to 15 ug/L
5 to 10 ug/L
< 5 ug/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-39
Arsenic Average
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuolu mne River
San Joaquin River
Dry Creek
#25..
- 1800 -
Legend
Arsenic Maximum Concentration
> 15 ug/L
10 to 15 ug/L
5 to 10 ug/L
< 5 ug/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-40
Arsenic Maximum
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1801 -
Legend
Uranium Average Concentration
< 10 pCi/L
10 to 20 pCi/L
20 to 30 pCi/L
> 30 pCi/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-41
Uranium Average
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1802 -
Legend
Uranium Maximum Concentration
< 10 pCi/L
10 to 20 pCi/L
20 to 30 pCi/L
> 30 pCi/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-42
Uranium Maximum
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1803 -
Legend
Gross Alpha Average Concentration
> 22.5 pCi/L
15 to 22.5 pCi/L
7.5 to 15 pCi/L
< 7.5 pCi/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-43
Gross Alpha Average
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1804 -
Legend
Gross Alpha Maximum Concentration
> 22.5 pCi/L
15 to 22.5 pCi/L
7.5 to 15 pCi/L
< 7.5 pCi/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-44
Gross Alpha Maximum
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuolu mne River
San Joaquin River
Dry Creek
#25..
- 1805 -
Legend
Boron Average Concentration
> 1.5 mg/L
1.0 to 1.5 mg/L
0.5 to 1.0 mg/L
< 0.5 mg/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-45
Boron Average
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1806 -
Legend
Boron Maximum Concentration
> 1.5 mg/L
1.0 to 1.5 mg/L
0.5 to 1.0 mg/L
< 0.5 mg/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-46
Boron Maximum
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1807 -
Legend
DBCP Average Concentration
> 0.3 ug/L
0.2 to 0.3 ug/L
0.1 to 0.2 ug/L
< 0.1 ug/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-47
DBCP Average
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1808 -
Legend
DBCP Maximum Concentration
> 0.3 ug/L
0.2 to 0.3 ug/L
0.1 to 0.2 ug/L
< 0.1 ug/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-48
DBCP Maximum
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1809 -
Legend
TCP Average Concentration
> 0.0075 ug/L
0.005 to 0.0075 ug/L
0.0025 to 0.005 ug/L
< 0.0025 ug/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-49
TCP Average
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1810 -
Legend
TCP Maximum Concentration
> 0.0075 ug/L
0.005 to 0.0075 ug/L
0.0025 to 0.005 ug/L
< 0.0025 ug/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-50
TCP Maximum
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1811 -
Legend
PCE Average Concentration
> 7.5 ug/L
5 to 7.5 ug/L
2.5 - 5 ug/L
<2.5 ug/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-51
PCE Average
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1812 -
Legend
PCE Maximum Concentration
> 7.5 ug/L
5 to 7.5 ug/L
2.5 - 5 ug/L
<2.5 ug/L
Corcoran Clay Extent (Burow et al., 2004)
Rivers
Modesto Subbasin
April 2020 Figure 3-52
PCE Maximum
Concentration
(
N
0 3
Scale in Miles
Stanislaus River
Tuo lu mne River
San Joaquin River
Dry Creek
#25..
- 1813 -
Historical Present
Historical Present Historical Present
PCE (
µg/L)
5.5 5.0
4.5 4.0
3.5 3.0
2.5 2.0
1.5 1.0
0.5 0
TCP (
µg/L)
10 1.0
0.1 0.01
0.001
MCL
Gross Alpha (pCi/L)
50 45
40 35
30 25
20 15
10 5
0 (a) (b)
(c)
MCL
values outside graph:
>5.5 to 460 ug/L
75th percentile + (1.5 x IQR)
75th percentile
median (average)
25th percentile
25th percentile - (1.5 x IQR)
outlier
Explanation
MCL or SMCL
MCL
values outside graph:
>5.5 to 97 ug/L
April 2020
Figure 3-53
Box Plots (1 of 2)
Note:
Concentrations of (a) TCP, (b) Gross Alpha, and (c) PCE under historical (water year 1995 to 2014) and present (2015 to 2019) periods, as compared to their respective Maximum Contaminant Level (MCL) or Secondary Maximum Contaminant Level (SMCL). #25..
- 1814 -
Arsenic (
µg/L)
Historical Present
Historical Present Historical Present
Historical Present Historical Present
80 60
40 20
0 MCL
DBCP (
µg/L)
2 1
0 Total dissolved solids (mg/L)6,000
5,000
4,000
3,000
2,000
1,000
0 SMCL
Nitrate (mg/L as N)
100
80 60
40 20
0 MCL
Uranium (
µg/L)
70 60
50 40
30 20
10 0
MCL
(a) (b)
(c) (d)
(e) (f)
values outside
graph: 490 mg/L values outside
graph: 20,000 mg/L
values outside
graph:
80 to 300 ug/L
values outside
graph: 84 to 85 ug/L
MCL
values outside graph:
3.6 to 18.3 ug/L
Historical Present
Boron (mg/L)
1.1 1.0
0.9 0.8
0.7 0.6
0.5 0.4
0.3 0.2
0.1 0
MCL
values outside
graph:
100 to 200 mg/L
April 2020
Figure 3-54
Box Plots (2 of 2)
Note:
Concentrations of (a) total dissolved solids, (b) nitrate (as N), (c) DBCP, (d) uranium, (e) arsenic, and (f) boron under historical (water year 1995 to 2014) and present (2015 to 2019) periods, as compared to their respective Maximum Contaminant Level (MCL) or Secondary Maximum Contaminant Level (SMCL).
#25..
- 1815 -
01/01/1995 01/01/2001 01/01/2004 01/01/2007 01/01/2010 01/01/2013 01/01/2016 1/1/1995 1/1/2001 1/1/2007 1/1/2013 1/1/2019
PCE (
µg/L)
500
400
300
200
100
0 TCP (
µg/L)
2 1
0 p-value = <0.001
Gross Alpha (pCi/L)
50 45
40 35
30 25
20 15
10 5
0 (a) (b)
(c)
p-value = <0.001
1/1/1995 1/1/2001 1/1/2007 1/1/2013 1/1/2019
Explanation
linear trend
1/1/1995 1/1/2001 1/1/2007 1/1/2013 1/1/2019
p-value = 0.141
values outside graph:
12.3 ug/L
April 2020 Figure 3-55
Linear Temporal
Trends (1 of 2)
Note:
Linear temporal trends of (a) TCP, (b) Gross Alpha, and (c) PCE.
The p-values less than the alpha-level of 0.05 are statistically significant trends.
#25..
- 1816 -
Arsenic (
µg/L)
80 60
40 20
0 DBCP (
µg/L)
2 1
0 Total dissolved solids (mg/L)6,000
5,000
4,000
3,000
2,000
1,000
0 Nitrate (mg/L as N)
100
80 60
40 20
0 Uranium (
µg/L)
70 60
50 40
30 20
10 0
(a) (b)
(c) (d)
(e) (f) Boron (mg/L)
1/1/1995 1/1/2001 1/1/2007 1/1/2013 1/1/2019
p-value = <0.001 p-value = <0.001
1/1/1995 1/1/2001 1/1/2007 1/1/2013 1/1/2019
p-value = <0.001
1/1/1995 1/1/2001 1/1/2007 1/1/2013 1/1/2019
p-value = 0.002
1/1/1995 1/1/2001 1/1/2007 1/1/2013 1/1/2019
p-value = 0.232
1/1/1995 1/1/2001 1/1/2007 1/1/2013 1/1/2019
200
180
160
140
120
100
80 60
40 20
0 values outside
graph: 490 mg/L
values outside
graph: 20,000 mg/L
values outside
graph:
80 to 300 ug/L
values outside graph:
3.6 to 18.3 ug/L p-value = <0.001
1/1/1995 1/1/2001 1/1/2007 1/1/2013 1/1/2019
April 2020 Figure 3-56
Linear Temporal
Trends (2 of 2)
Note:
Linear temporal trends of (a) total dissolved solids, (b) nitrate (as N), (c) DBCP, (d) uranium, (e) arsenic, and (f) boron . The p-values less than the alpha-level of 0.05 are statistically significant trends.
#25..
- 1817 -
B
B
B
B
B
B
B
B B
B
B B B
B
B
B
B
B
B
B
B
B
B
B
B
B B B
BB
B
B
B
B
B
B
B B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
BB
B
B
B
B
B
B
B B
B
B
B
B BB
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B B
B
B B
B
B B B
B B
B
B
B
B
B
B
B B B
B
B
B
B B
B
B
B B
B
B
B
B
B
B B
B B
B
B B
B
B
B B
B
B
BB
B
B
B
BB
B
B BB
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B B
B
B
B
B
B
B
B
B
B
B
B
B
B
B BB B B
B
B B
B
B
B
B
B
B
B
BBB
B B B B
B
B
B
B
B
B
BB
B
B B
B
B
B
B
BB B
B
B
B
BB
B
B
B
B
B
B
BB B #
#
*
*
Stanislaus River
Tuol umne River
San Joaqu in River
Site Type and Case Status
Cleanup Program Site - Case Open (24)
Cleanup Program Site - Case Closed (45)
#* LUST Cleanup Site - Case Open (2)
B LUST Cleanup Site - Case Closed (246)
Military Cleanup Site - Case Open (2)
Military Cleanup Site - Case Closed (1)
Modesto Subbasin
November 2021
Figure 3-57
Contamination Sites
(
"N
0 4
Scale in Miles
Note: Contamination cleanup sites accessed from the SWRCB GeoTracker online database. #25..
- 1818 -
November 2021 Figure 3-58
Concepts of
Land Subsidence
Source: Galloway et al., 1999.
After LSCE et al., 2014.
Area of
Largest Subsidence
Land
Subsidence
Regional clay layer Discontinuous
clay interbeds
#25..
- 1819 -
November 2021 Figure 3-59
San Joaquin Valley
Subsidence, 2008-2010
(
"N
0 10
Scale in Miles
Legend
Modesto Subbasin
Source: Faunt et al., 2015
#25..
- 1820 -
#*
#*
#*
#*
¬«4
CMOD
P260
P306
P781
Figure 3-60
Vertical Displacement
from InSAR Data,
June 2015 – October 2020
Path: \\todd-file\data\Projects\Modesto GSP 74305\GIS\Figures\Section 3\Figure 3-60 Subsidence.mxd Legend
Vertical Displacement (feet)
-0.4 to- -0.35
-0.35 to -0.3
-0.3 to -0.25
-0.25 to -0.2
-0.2 to -0.15
-0.15 to -0.1
-0.1 to -0.05
-0.05 - 0
0 - 0.05
0.05 - 0.1
Basins_Boundaries
County Line
Extent of Corcoran Clay
GPS Station #* Vertical Displacement: < 0 to -1 feet #* Vertical Displacement: > 0 feet (N
0 1.53
Miles
Source: DWR SGMA Data Viewer, TRE Altamira InSAR Data
November 2021
#25..
- 1821 -
January 2022
Figure 3-61
Interconnected
Surface Water Conditions
Historical Conditions Simulation WY 1991-WY 2015 Projected Future Conditions Simulation Years 1-50 San Joaquin River Tuolumne River
Stanislaus River
San Joaquin River Tuolumne River
Stanislaus River
#25..
- 1822 -
(
"N
0 4
Scale in Miles
January 2022 Figure 3-62
Vegetation Commonly
Associated with
Groundwater and Wetlands
Stanislaus River
Tuol umne River
San Joaqu in River
Source; DWR Natural Communities
Commonly Associated with Groundwater
Legend
Wetlands (768)
Vegetation commonly associated with groundwater (1,027) Modesto Subbasin
River
#25..
- 1823 -
(
"N
0 4
Scale in Miles
January 2022 Figure 3-63
Areas with Depth to
Water within 30 feet
in Spring 1998
Stanislaus River
Tuol umne River
San Joaqu in River
Legend
Areas with Depth to Water within 30 feet in Spring 1998 River
Modesto Subbasin
Source; DWR Natural Communities
Commonly Associated with Groundwater
#25..
- 1824 -
(
"N
0 4
Scale in Miles
Figure 3-64
Potential Vegetation and
Wetland GDEs in Spring 1998,
Based on Depth to Water
Stanislaus River
Tuol umne River
San Joaqu in River
Legend
Potential Wetland GDEs (567)
Potential Vegetation GDEs (958)
Modesto Subbasin
River
Source; DWR Natural Communities
Commonly Associated with Groundwater
January 2022
#25..
- 1825 -
(
"N
0 4
Scale in Miles
Figure 3-65
Areas with Depth to
Water within 30 feet
in Fall 2015
Stanislaus River
Tuol umne River
San Joaqu in River
Legend
Areas with Depth to Water within 30 feet in Fall 2015
Modesto Subbasin
River
Source; DWR Natural Communities
Commonly Associated with Groundwater
January 2022
#25..
- 1826 -
(
"N
0 4
Scale in Miles
Figure 3-66
Potential Vegetation and
Wetland GDEs in Fall 2015,
Based on Depth to Water
Stanislaus River
Tuol umne River
San Joaqu in River
Legend
Potential Wetland GDEs (462)
Potential Vegetation GDEs (823)
Modesto Subbasin
River
Source; DWR Natural Communities
Commonly Associated with Groundwater
January 2022
#25..
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(
"N
0 4
Scale in Miles
Figure 3-67
Potential Vegetation
and Wetland GDEs
in Fall 2015
Stanislaus River
Tuol umne River
San Joaqu in River
Legend
Potential Wetland GDEs (416)
Potential Vegetation GDEs (813)
Modesto Subbasin
River
January 2022
Note:
Some potential GDEs on Figure 3-66 are not
shown on this figure because they were
determined not to be GDEs based on a study
conducted by Moore Biological Consultants in
November 2021.
Source;
DWR Natural Communities Commonly
Associated with Groundwater
#25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 4-1
Revised July 2024
TODD GROUNDWATER
4. NOTICE AND COMMUNICATION
The GSAs in the Modesto Subbasin conducted a number of activities to engage beneficial users of groundwater, interested parties, and the general public in the development of the GSP. The STRGBA GSA and Tuolumne GSA were responsible for conducting outreach and engagement related to the SGMA for the portions of the Subbasin located within their respective service areas. The STRGBA GSA, which covers almost all of the Subbasin, took the lead in outreach with Tuolumne GSA coordinating through an agreement with Stanislaus County (Appendix A).
4.1. DECISION MAKING PROCESS
As described in Chapter 1 of this GSP, the Stanislaus and Tuolumne Rivers Groundwater Basin Association (STRGBA) agencies entered into a Memorandum of Understanding (MOU) to form the STRGBA GSA in February 2017. The STRGBA GSA is governed by a committee tasked with overseeing activities to achieve the objectives of SGMA applicable within the Modesto Subbasin (Committee). Each member agency designates one staff person and one or more alternates to serve on the Committee. Stanislaus County participates in the Committee on behalf of the Tuolumne GSA.
Each calendar year, the Committee elects a chair and vice chair from its members. The chair is responsible for presiding over and notifying members of Committee meetings. Except for actions for which a different approval standard is set forth in the MOU, all actions of the Committee are approved by a majority vote carried by of the members present.
To provide a venue for discussion of technical topics related to the development of the GSP, the STRGBA GSA also formed a Technical Advisory Committee (TAC). TAC membership is not defined in the MOU, but it generally includes one participating representative from each of the STRGBA GSA member agencies. Stanislaus County, a STRGBA GSA member agency, represented both itself as well as the Tuolumne GSA in these TAC meetings.
Both Committee and TAC meetings are open to the public and held in accordance with the Ralph M. Brown Act (California Government Code section 5490 et sq.). These meetings are further described in Section 3.4.1.
4.2. GROUNDWATER BENEFICIAL USES AND USERS
Beneficial users and uses of groundwater were identified and engaged by the GSAs based on the place- and interest-based categories described in SGMA and codified in Water Code Section 10723.2:
(a) Holders of overlying groundwater rights, including:
(1) Agricultural users, including farmers, ranchers, and dairy professionals (2) Domestic well owners
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Revised July 2024
TODD GROUNDWATER
(b) Municipal well owners
(c) Public water systems
(d) Local land use planning agencies
(e) Environmental users of groundwater
(f) Surface water users, if there is a hydrologic connection between surface and groundwater bodies
(g) The federal government, including, but not limited to, the military and managers of federal lands
(h) California Native American tribes
(i) Disadvantaged communities, including, but not limited to, those served by private domestic wells or small community water systems
(j) Entities listed in Section 10927 that are monitoring and reporting groundwater elevations in all or a part of a groundwater basin managed by the groundwater sustainability agency
Beneficial users and uses representing these categories and nature of consultation with these users are further described below and identified in Table 4-1. #25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 4-3
Revised July 2024
TODD GROUNDWATER
Table 4-1: Nature of Consultation with Beneficial Users KEY: GSA = Groundwater Sustainability Agency, MID = Modesto Irrigation District, OID = Oakdale Irrigation District, STRGBA = Stanislaus and Tuolumne Rivers Groundwater Basin Association Participation in Stakeholder AssessmentMembership on STRGBA GSA and/or Technical Advisory Committee or Tuolumne County GSAInterested Parties DatabasePublic Meetings, Workshops, and Subbasin Office HoursTargeted Outreach to Representatives of Beneficial Users Agricultural water providers - MID, OID X X X X X
Individual agricultural water users, including dairies, farmers, and ranchers X X X X
Domestic Well Owners Domestic well owners X X X X
City of Modesto X X X X X
City of Oakdale X X X X X
City of Riverbank X X X X X
City of Waterford X X X X X
Municipal supply wells owners X X X X X
MID X X X X X
OID X X X X X
Public Water Systems [See Section 2, Table 2-1 for the list of public water systems in the Subbasin] X X
City of Modesto Planning Commission X X
City of Oakdale Planning Commission X X
City of Riverbank Planning Commission X X
City of Waterford Planning Commission X X
Stanislaus County Local Agency Formation
Commission X X
Stanislaus County Planning Commission X X X
Tuolumne County Local Agency Formation
Commission X
Tuolumne County Local Planning Commission X X X
California Department of Fish and Wildlife X X
Tuolumne River Trust X X X
U.S. Fish and Wildlife Service X X
Individual landowners X X
MID X X X X
OID X X X X
Tuolumne River Trust X X X
Federal Government U.S. Fish and Wildlife Service X
California Native American
Tribes
[There are no tribal lands are documented in the DWR
Water Management Planning Tool or are known to
exist in the Modesto Subbasin.]
Airport X X
City of Oakdale X X X X X
City of Waterford X X X X X
Empire X X X
Rouse X X
West Modesto X X
Groundwater Monitoring
and Reporting Entities STRGBA X X X X X
Disadvantaged Communities
(Census Designated Tracts)
Surface Water Users
Environmental Users of
Groundwater
Nature of Consultation
Agricultural Users
Municipal and Industrial
Well Owners
Local Land Use Planning
Agencies
Beneficial User Category Beneficial Users
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4.2.1. Agricultural Users (§10723.2(a)(1))
The Modesto Subbasin is largely agricultural. In 2017, approximately 64 percent of the Subbasin was defined as irrigated agriculture (Stanislaus Land Use dataset, 2017). Irrigated agriculture covers about 157,911 acres. Approximately 23 percent of the Subbasin (about 56,777 acres) consists of non-agriculture, non-irrigated agriculture (e.g., rangeland), undeveloped land, and surface water.
Agricultural groundwater users include growers, ranchers, and dairies. Water for agricultural purposes is primarily provided through groundwater extracted from the Subbasin, as well as surface water supplies provided by the Modesto Irrigation District (MID) and Oakdale Irrigation District (OID). MID and OID each operate groundwater wells to supplement surface water deliveries and manage the water table.
Agricultural interests are represented on the Committee by MID and OID; in addition, the elected boards and councils of the STRGBA GSA member agencies provide broad agricultural representation. Individuals representing agricultural water users were also part of the initial stakeholder assessment conducted to develop the Communication and Engagement Plan; and actively participated in monthly Committee and TAC meetings, public workshops, and GSP chapter public comment processes.
During development of the GSP, MID and OID conducted outreach on groundwater management practice and SGMA to their agricultural customers. Information was provided at MID and OID grower meetings, in newsletters, and during presentations to the MID and OID Boards of Directors. Agricultural groundwater users also participated in the Subbasin stakeholder assessment, which is described in the Communication and Engagement Plan.
4.2.2. Domestic Well Owners (§10723.2(a)(2))
Domestic wells are present throughout the Subbasin, but the highest density occurs in the central region of the Subbasin, along the Stanislaus and Tuolumne Rivers, and west of the City of Modesto. OID also provides domestic water from District-owned wells for its rural water system and serves as the trustee of six improvement districts. A density of domestic wells is illustrated on Figure 2-14 in Chapter 2.
Domestic well owners are represented on the Committee by OID and Stanislaus County and had the opportunity to consult in development of the GSP through monthly public meetings, workshops, and GSP public comment processes. An informational postcard was distributed to over 350 landowners in the eastern part of the Subbasin with a high density of domestic wells to inform them about development of the GSP. The STRGBA GSA also engaged the Municipal Advisory Councils for the communities of Airport, West Modesto, and Empire, located in unincorporated Stanislaus County, to inform them about development of the GSP and solicit input on locations for new groundwater monitoring wells. #25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 4-5
Revised July 2024
TODD GROUNDWATER
4.2.3. Municipal & Industrial Well Owners (§10723.2(b)) There are approximately 150 municipal supply wells in the Subbasin, as shown in Chapter 2, Figure 2-13. The highest concentration of municipal supply wells is located within the City of Modesto. There are also public supply wells located in the Cities of Oakdale, Riverbank, and Waterford; and unincorporated areas of Stanislaus County. The Cities of Modesto, Oakdale, Riverbank, and Waterford pump groundwater for municipal and industrial water supply.
MID and OID also operate groundwater wells to supplement surface water supplies and manage the water table.
All four cities, Stanislaus County, MID, and OID are member agencies of the STRGBA GSA and represent municipal and industrial well owners. Member agency staff provided periodic updates to their respective governing bodies informing them about progress developing the GSP and consulting on key groundwater management decisions. STRGBA GSA staff also provided presentations on SGMA and the GSP at meetings of the Manufacturer’s Council of Central Valley. In addition, municipal and industrial well owners participated in the stakeholder assessment.
4.2.4. Public Water Systems (§10723.2(c))
Public water systems in the Subbasin include the Cities of Modesto, Oakdale, Riverbank, and Waterford, as well as small community water supply systems operated by the respective community and regulated by Stanislaus County. There are approximately 77 water systems in the Subbasin that are not municipal or irrigation districts. A majority of these systems are very small. A summary of these non-municipal and non-irrigation systems is provided in Chapter 2, Table 2-1 of the GSP.
The Cities of Modesto, Oakdale, Riverbank, and Waterford are all represented on the STRGBA Committee. Small community water systems were represented in development of the GSP by Stanislaus County.
4.2.5. Local Land Use Planning Agencies
Local land use planning agencies in the Modesto Subbasin include the planning commissions of the City of Modesto, City of Oakdale, City of Riverbank, City of Waterford, Stanislaus County, and Tuolumne County, as well the Stanislaus County and Tuolumne County Local Agency Formation Commissions. These agencies are represented on the Committee by their respective STRGBA GSA representative.
4.2.6. Environmental Users of Groundwater
The GSAs used the California Department of Water Resources’ (DWR) Natural Communities Commonly Associated with Groundwater as a starting point to identify groundwater dependent ecosystems within the Modesto Subbasin. The mapping shows wetlands and vegetation along the three major rivers (Stanislaus, Tuolumne, and San Joaquin Rivers), #25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 4-6
Revised July 2024
TODD GROUNDWATER
along Dry Creek and areas between Dry Creek and the Tuolumne River, and within the San Joaquin River Natural Wildlife Refuge.
Environmental users of groundwater were invited to participate in monthly Committee and TAC meetings as well as public workshops. A representative from the Tuolumne River Trust also participated in the stakeholder assessment.
4.2.7. Surface Water Users (§10723.2(f))
The Tuolumne and Stanislaus Rivers provide the primary sources of water in the Modesto Subbasin. Surface water is used for agricultural, municipal, industrial, and environmental purposes. MID delivers surface water from the Tuolumne River for agricultural irrigation.
MID also treats and delivers surface water from the Tuolumne River to the City of Modesto for municipal and industrial use. OID diverts water from the Stanislaus River to municipal and agricultural customers. Other surface water users include individual landowners with riparian water rights.
Surface water users are represented on the Committee and TAC by MID and OID. The STRGBA GSA also coordinated with GSAs in the Turlock Subbasin regarding management of flows in the Tuolumne River. In addition, Stanislaus County represents surface water users in non-district areas.
4.2.8. Federal Government (§10723.2(g))
Federal government agencies in the Modesto Subbasin include the U.S. Fish and Wildlife Service, which runs the San Joaquin River National Wildlife Refuge. The Local Redevelopment Authority oversees the transfers, reuse, and redevelopment of the former Riverbank Army Ammunition Plant, which was previously owned by the U.S. Army. Federal agencies were invited to participate in monthly Committee and TAC meetings and public workshops.
4.2.9. California Native American Tribes (§10723.2(h))
No tribal lands are documented in the DWR Water Management Planning Tool or are known to exist in the Modesto Subbasin.
4.2.10. Disadvantaged Communities (§10723.2(i))
Data published by the U.S. Census Bureau in 2018 show six Census Designated Places within the Modesto Subbasin that meet the annual Median Household Income (MHI) criteria to be considered a disadvantaged community or severely disadvantaged community by the State:
Airport, Empire, Oakdale, Rouse, Waterford, and West Modesto. These communities are identified in Figure 4-1. The MHI for each is identified in Table 4-2. #25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 4-7
Revised July 2024
TODD GROUNDWATER
Figure 4-1: Disadvantaged and Severely Disadvantaged Communities Table 4-2: Census-Designated Places Designated as Disadvantaged Census-Designated
Place
Median Household
Income1
Population2
Airport $28,352 1,389
City of Oakdale $54,443 23,181
City of Waterford $54,886 9,120
Empire $36,774 4,202
Rouse $46,300 1,913
West Modesto $33,920 5,965
Notes;
1 Median Household Income is based on 2014–2018 American Community Survey 5-Year Estimates
2 Population is based on U.S. Census Bureau 2020 Decennial Census data These communities are represented on the Committee and TAC by the City of Modesto, City of Oakdale, City of Waterford, and Stanislaus County. Water users in these communities were notified about development of the GSP through bilingual (English-Spanish) water bill inserts; notices and information distributed through the STRGBA GSA member agencies’ #25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 4-8
Revised July 2024
TODD GROUNDWATER
existing communication platforms (e.g., websites, social media accounts, newsletters); and presentations provided at community advisory councils and other organizations.
The STRGBA GSA distributed a bilingual electronic survey in Spring and Summer 2019 to assess stakeholders’ understanding and perspectives on key SGMA topics and gather input on preferred outreach strategies. The survey was promoted via utility bill inserts, postings on the STRGBA GSA and GSA member agencies’ websites and social media pages, and a notice in the Stanislaus County Farm Bureau’s Farm News. The survey went out to all water service customers, which included the communities of West Modesto, Rouse, Airport, Empire, and the City of Modesto. The survey results were posted on the STRGBA GSA website and used to develop the Modesto Subbasin Communication and Engagement Plan.
City of Modesto staff, on behalf of the STRGBA GSA, also attended various community meetings to discuss proposed locations for new groundwater monitoring wells and inform community members about development of the GSP. This included presentations at the Airport Neighborhood Collaborative, West Modesto Community Collaborative, and Empire Municipal Advisory Council in August and September 2019. In addition, informational materials were distributed through Stanislaus County Municipal Advisory Councils.
Groundwater users in communities designated as disadvantaged also had the opportunity to participate in development of the GSP through monthly Committee and TAC meetings and public workshops.
4.2.11. Groundwater Elevation Monitoring and Reporting Entities (§10723.2(j)) STRGBA serves as the CASGEM Monitoring Entity for the Modesto Subbasin. Each municipality also monitors groundwater quality for its supply wells in compliance with state requirements.
4.3. PUBLIC ENGAGEMENT
The GSAs utilized a variety of tools and activities to encourage the active involvement of diverse social, cultural, and economic elements of the population within the Modesto Subbasin. These activities were guided by the Modesto Subbasin Communication and Engagement Plan, which is provided in Appendix F. The activities identified in the Communication and Engagement Plan were adapted in accordance with state and local social distancing requirements resulting from the COVID-19 pandemic.
To support execution of the activities identified in the plan and ensure a collaborative and inclusive GSP development process, the GSAs utilized DWR’s Facilitation Support Services.
Facilitation and outreach support was provided by Stantec Consulting Services Inc (Stantec).
4.3.1. Outreach Tools
The GSAs used several tools to support communication and engagement activities with stakeholders in the Modesto Subbasin. These tools include the following:
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 4-9
Revised July 2024
TODD GROUNDWATER
• Project Website: The STRGBA GSA member agencies have updated the STRGBA website (www.strgba.org) to provide information about SGMA and house GSA meeting and outreach materials. The Tuolumne GSA has added a SGMA-related page (https://www.tuolumnecounty.ca.gov/1292/Sustainable-GroundwaterManagement-Act-S) to the Tuolumne County website. • Interested Parties Database: Pursuant to the requirements of SGMA, the GSAs developed and maintain an Interested Party Database. The Database is used to notify stakeholders of pending meetings and workshops, opportunities for public comment, and notices of other GSA outreach actions.
• Newsletter: The STRGBA GSA distributes a semi-annual electronic newsletter to keep interested parties informed about progress in developing the GSP, opportunities for public engagement, and groundwater management issues or news of regional importance. Newsletters were distributed to the Interested Parties Database in Spring 2020, Fall 2020, and Spring 2021. Copies of the newsletter were also posted on the Subbasin website.
• Informational Materials: The Modesto Subbasin GSAs developed a suite of materials to inform beneficial users and interested parties about SGMA and topics pertaining to the GSP. This included fact sheets, frequently asked questions, presentation slides, and utility bill inserts. Many of these materials were translated into Spanish. To ensure consistent messaging across the basin, the GSAs also developed template presentation slides at different stages of GSP development to support presentations to member agency briefings and presentations to local industry and community groups.
• Postcard: The STRGBA GSA distributed an informational postcard to over 350 landowners in the non-districted area of the eastern portion of the Subbasin in September 2020 informing them about development of the GSP and inviting them to participate in the plan development process.
4.3.2. Outreach Activities
The GSAs conducted a variety of outreach activities to provide opportunities for beneficial users and other interested parties to stay informed and engaged in the development of the GSP. These activities were informed by the results of an electronic survey distributed by the STRGBA GSA and stakeholder assessment conducted by Stantec staff in Spring 2019.
Outreach activities included public STRGBA GSA and TAC meetings, GSP development workshops and office hours, member agency briefings, and presentations to organizations representing beneficial users of groundwater. Each of these activities is described in the Modesto Subbasin Communication and Engagement Plan, provided in Appendix F.
The GSAs utilized partnerships with trusted messengers in the Modesto Subbasin to broaden the dissemination of SGMA information and connect with hard-to-reach stakeholder groups. This included disseminating information through the Stanislaus County #25..
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STRGBA GSA/Tuolumne GSA 4-10
Revised July 2024
TODD GROUNDWATER
Farm Bureau, Manufacturers Council of the Central Valley, Empire Municipal Advisory Council, and local neighborhood collaboratives and community organizations. In addition, the STRGBA GSA conducted extensive public outreach to the communities of West Modesto, Rouse, Empire, Airport, and the City of Modesto regarding the locations and installation of new groundwater monitoring wells.
4.4. LIST OF PUBLIC MEETINGS
To consult beneficial users in development of the GSP and make decisions in a transparent and inclusive setting, the GSAs coordinated monthly public meetings, annual public workshops, and regular GSP office hours. In addition, the GSAs representatives provided presentations on the GSP at public meetings of their governing bodies and parties representing beneficial users. Table 4-3 provides a list of the public meetings where the GSP was discussed or considered by the GSAs. A description of the committee meetings and public workshops is provided below.
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TODD GROUNDWATER
Table 4-3: List of Public Meetings at Which the Groundwater Sustainability Plan Was Discussed
Type of Meeting Format Date(s)
Community
Presentations
Manufacturer’s Council of Central
Valley Meeting 04/18/2018
Airport Neighborhood Collaborative
Meeting 09/09/2019
West Modesto Community
Collaborative Meeting 09/11/2019
Empire Municipal Advisory Council
Meeting 08/28/2019
Manufacturer’s Council of Central
Valley Meeting 07/15/2020
Modesto Chamber of Commerce,
Government Relations Committee
Meeting
11/20/2020
Mid San Joaquin RFMP Stakeholder
Meeting 07/29/2021
Modesto Rotary 08/04/2021
Soroptimist International of Modesto 09/23/2021
Modesto Chamber of Commerce,
Government Relations Committee
Meeting
10/15/2021
Public Workshop/
Groundwater
Sustainability Plan
Office Hours
Virtual
06/01/2020
03/25/2021
05/28/2021
08/25/2021
Stanislaus and
Tuolumne Rivers
Groundwater Basin
Association
Groundwater
Sustainability Agency
Committee Meeting
In-Person and Virtual
01/18/2018 01/08/2020
02/14/2018 02/12/2020
05/09/2018 03/11/2020
06/13/2018 04/08/2020
07/11/2018 05/13/2020
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TODD GROUNDWATER
Table 4-3: List of Public Meetings at Which the Groundwater Sustainability Plan Was Discussed (contd.)
Type of Meeting Format Date(s)
08/08/2018 06/10/2020
Stanislaus and Tuolumne
Rivers Groundwater
Basin Association
Groundwater
Sustainability Agency
Committee Meeting
(contd.)
In-Person and Virtual (contd.)
09/12/2018 07/08/2020
10/10/2018 08/12/2020
01/09/2019 09/09/2020
02/13/2019 10/14/2020
03/13/2019 12/09/2020
04/10/2019 03/10/2021
05/08/2019 04/14/2021
06/12/2019 05/12/2021
07/10/2019 06/09/2021
08/14/2019 07/14/2021
09/11/2019 08/11/2021
10/09/2019 09/08/2021
11/13/2019 10/13/2021
12/11/2019 11/10/2021
12/08/2021
Stanislaus and Tuolumne
Rivers Groundwater
Basin Association
Groundwater
Sustainability Agency
Technical Advisory
Committee Meeting
In-Person and Virtual
04/10/2019 01/13/2021
07/10/2019 02/10/2021
08/14/2019 06/23/2021
11/13/2019 07/28/2021
12/11/2019 08/11/2021
05/13/2020 09/08/2021
08/12/2020 09/22/2021
10/27/2020 10/13/2021
12/9/2020 11/20/2021
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4.4.1. STRGBA Committee and Technical Advisory Committee Meetings Monthly STRGBA GSA Committee and TAC meetings served as key opportunities for beneficial users and interested parties to track the process and consult in development of the GSP. Both meetings are held and noticed in accordance with the Brown Act and are open for members of the public to listen and provide comments. Comments on items on the agenda may be provided after STRGBA GSA discussion on the item. There is also time set aside for members of the public to provide comment on items not on the agenda. Public comments are recorded in the meeting minutes, which are posted on the STRGBA GSA website. Comments were recorded and considered by the planning team when developing and revising the GSP chapters.
The meetings were initially held in-person at MID’s office at 1231 11th Street, Modesto, CA 95354 and by teleconferencing. In April 2020, the meetings were shifted to a virtual platform due to social distancing requirements and temporary changes in Brown Act requirements resulting from the COVID-19 pandemic. Members of the public were able to provide comment at the meetings via calling into the meeting or submitting comments in the virtual meeting platforms.
The GSAs noticed the meetings via a posting on the STRGBA GSA website and email distributed to the Interested Parties Database. A notice was also posted at the MID office for in-person meetings. Meeting agendas and materials were distributed to the Interested Parties Database and posted on the STRGBA GSA website prior to each meeting.
4.4.2. Public Workshops and GSP Office Hours
The GSAs held a public workshop and several Office Hours to inform beneficial users and interested parties about the GSP development process and collect input on topics central to the development of the GSP and groundwater management practices. The GSAs hosted a public workshop in June 2020 focused on SGMA and GSP development process.
The GSAs also hosted three Office Hours in March 2021, May 2021, and August 2021. The workshop topics included the draft sustainable management criteria, groundwater monitoring network, and management areas. The Office Hours are less formal than regular workshops and provide members of the public an opportunity to have a dialogue with STRGBA GSA representatives outside of the monthly meetings.
All workshops and Office Hours scheduled after April 2020 were held virtually due to local and state social distancing requirements resulting from the COVID-19 pandemic. Questions and comments submitted by members of the public was recorded by the planning and outreach staff. A summary of feedback provided by workshop participants was provided at GSP Coordination Committee and Technical Committee meetings and recorded in the workshop summaries, provided in Appendix F. Recordings of the May and August 2021 Office Hours were also made available on the STRGBA GSA website. #25..
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TODD GROUNDWATER
The GSAs noticed the workshops and GSP Office Hours via a bilingual (English-Spanish) flyer which was posted on the STRGBA GSA and member agencies’ websites and member agencies’ social media sites and was distributed to the Interested Parties Database.
4.4.3. Other Public Meetings
In addition to monthly public meetings and annual workshops, the STRGBA GSA member agency representatives also discussed the GSP at public meetings of the respective governing bodies and local community and civic organizations. Table 4-3 provides a list of other public meetings during which the GSP was discussed or considered.
4.5. GSP COMMENTS AND RESPONSES
This section describes the process the GSAs used to solicit and respond to comments on the draft GSP. The draft GSP chapters were released for public review and comment as they were developed. Public comments were collected via email. In addition, interested parties could provide verbal comments during monthly Committee and TAC meetings and public workshops. Comments that raised substantive technical or policy issues resulted in revisions to the Draft GSP and are reflected in the draft plan.
4.5.1. Public Comment Process
The GSAs used a serial public comment process to provide beneficial users and members of the public multiple opportunities to review and provide comment on the draft GSP. Draft GSP chapters were released for public review and comment as they were completed.
Members of the public were notified of the public comment period through an email distributed to the Interested Parties Database.
Comments were collected via an email to the STRGBA GSA and verbally during monthly Committee and TAC meetings. Comments provided at public meetings and workshops were recorded in the meeting minutes or workshop summary and reviewed by STRGBA GSA member agency staff. Copies of comments received on the draft GSP chapters were posted on the STRGBA GSA website.
At the close of each GSP chapter public comment period, comments received were reviewed by the STRGBA GSA member agency staff and summarized and discussed at monthly Committee and TAC meetings. Comments that raised credible technical or policy issues resulted in revisions to the draft GSP.
Pursuant to the requirements of California Water Code Section 10728.4, the GSAs also distributed a notice of intent to adopt the GSP to cities and counties within the GSP area.
The notice was jointly distributed on August 10, 2021. A copy of the notice is provided in Appendix F.
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4.6. PUBLIC INVOLVEMENT DURING GSP IMPLEMENTATION
The GSAs will keep members of the public and interested parties informed about progress implementing the GSP through emails to the Interested Parties Database, regularly scheduled public meetings, and annual workshops. The GSAs will continue to maintain the website and Interested Parties Database. Emails will be distributed to the Interested Parties Database on a regular basis to inform interested parties about upcoming meetings and public workshops, GSP implementation milestones, and the status of projects and management actions. The website will be updated on an as-needed basis to include information on and announcements pertaining to GSP implementation. The website will also serve as a repository for copies of the Modesto Subbasin Annual Reports and other materials developed during GSP implementation.
It is anticipated at that the STRGBA GSA will continue to meet on a monthly basis.
Committee meetings will be noticed on the STRGBA GSA website and via an email to the Interested Parties Database. The GSAs will also hold public workshops as needed to keep members of the public and interested parties informed about progress implementing the GSP. The GSAs will notice the workshops via posting on the website, e-blast, and targeted outreach to organizations and agencies representing beneficial users in the Subbasin. The GSAs and GSA member agencies will also continue to conduct presentations to key stakeholder organizations on an as-needed basis to inform the about implementation of the GSP and groundwater conditions.
Additional public outreach activities may be conducted to support planning, design, and construction activities related to the groundwater management projects. Such activities will be noticed on the website and via an e-blast to the Interested Parties Database.
4.7. PUBLIC ENGAGEMENT FOR REVISED GSP
During development of the revised GSP in 2024, the GSAs conducted similar outreach and public engagement as occurred during development of the original GSP, submitted January 2022. This included updates to the STRGBA GSA website, notification to stakeholders, and presentations at monthly STRGBA GSA meetings.
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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 area8
-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.
8 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 #25..
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conditions, wet periods, and normal periods. By incorporating these varied conditions into 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 D.
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..
- 1857 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-15
Revised July 2024
TODD GROUNDWATER
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..
- 1858 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-16
Revised July 2024
TODD GROUNDWATER
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 #25..
- 1859 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-17
Revised July 2024
TODD GROUNDWATER
1969-2018). The goal of the water budget analysis is to characterize the water supply and 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..
- 1860 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-18
Revised July 2024
TODD GROUNDWATER
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..
- 1861 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-19
Revised July 2024
TODD GROUNDWATER
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..
- 1862 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-20
Revised July 2024
TODD GROUNDWATER
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 performed 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..
- 1863 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-21
Revised July 2024
TODD GROUNDWATER
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 5-20. 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 5-20. Figure 5-20 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..
- 1864 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-22
Revised July 2024
TODD GROUNDWATER
Figure 5-9: Groundwater Recharge and Extraction – Modesto Subbasin Figure 5-10: Net Recharge – Modesto Subbasin
#25..
- 1865 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-23
Revised July 2024
TODD GROUNDWATER
Figure 5-11: Groundwater Recharge and Extraction – Modesto Zone Figure 5-12: Net Recharge – Modesto Zone
#25..
- 1866 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-24
Revised July 2024
TODD GROUNDWATER
Figure 5-13: Groundwater Recharge and Extraction – Oakdale South Zone Figure 5-14: Net Recharge – Oakdale South Zone
#25..
- 1867 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-25
Revised July 2024
TODD GROUNDWATER
Figure 5-15: Groundwater Recharge and Extraction – Non-District East Zone Figure 5-16: Net Recharge – Non-District East Zone
#25..
- 1868 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-26
Revised July 2024
TODD GROUNDWATER
Figure 5-17: Groundwater Recharge and Extraction – Non-District West Area Figure 5-18: Net Recharge – Non-District West Area
#25..
- 1869 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-27
Revised July 2024
TODD GROUNDWATER
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.
#25..
- 1870 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-28
Revised July 2024
TODD GROUNDWATER
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.
#25..
- 1871 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-29
Revised July 2024
TODD GROUNDWATER
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. #25..
- 1872 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-30
Revised July 2024
TODD GROUNDWATER
Figure 5-22: Current Conditions Average Annual Water Budget – Land Surface System, Modesto Subbasin
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.
#25..
- 1873 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-31
Revised July 2024
TODD GROUNDWATER
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 on 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.
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 D.
Figure 5-24 shows the water budget schematic for the Modesto Subbasin with average annual projected values for each component.
#25..
- 1874 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-32
Revised July 2024
TODD GROUNDWATER
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).
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 aquifer9 to 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 9 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.
#25..
- 1875 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-33
Revised July 2024
TODD GROUNDWATER
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 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.
#25..
- 1876 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-34
Revised July 2024
TODD GROUNDWATER
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
#25..
- 1877 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-35
Revised July 2024
TODD GROUNDWATER
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.
#25..
- 1878 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-36
Revised July 2024
TODD GROUNDWATER
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 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..
- 1879 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 5-37
Revised July 2024
TODD GROUNDWATER
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..
- 1884 -
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Revised July 2024
TODD GROUNDWATER
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
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• Desktop IWFM Tools
SGMA Data Viewer provides the location for which the climate change forecasts datasets10 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 5-11 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 5-11 is the hydrologic model used by DWR to estimate unimpaired flows 10 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
Projected
Conditions Baseline
with Climate
C2VSimTM
C2VSimTM Projected
Conditions with
Climate Change
Projected
Conditions Water
Budget
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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 5-11 is DWR’s terminology for streams whose flow is impacted by ongoing water operations and 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- 11 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.
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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.
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.) #25..
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Revised July 2024
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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 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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Revised July 2024
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Figure 5-42: Tuolumne River Hydrograph
Figure 5-43: Tuolumne River Exceedance Curve
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Revised July 2024
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Figure 5-44: Stanislaus River Hydrograph
Figure 5-45: Stanislaus River Exceedance Curve
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Revised July 2024
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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) 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 5-12.
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
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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.
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 5-13 though Table 5-15.
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.11 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 11 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 5-13: 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 5-14: 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 5-15: 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 5-16 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 #25..
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be considered for the next update of the GSP along with improvements in DWR’s climate change data.
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)
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Group 2: Groundwater Only Users
• Non-District East
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 #25..
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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 considered. The status of plans for implementing management actions related to pumping reductions is further discussed in Chapter 8 - Projects and Management Actions.
Table 5-17 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 5-17: 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 5-18 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
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Figure 5-57: Sustainable Yield Average Annual Water Budget Groundwater System – Modesto Subbasin
Figure 5-58: Sustainable Yield Water Budget Groundwater System – Modesto Subbasin
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Figure 5-59: Sustainable Yield Water Budget Groundwater Recharge and Extraction – Modesto Subbasin
Figure 5-60: Sustainable Yield Water Budget Net Recharge – Modesto Subbasin
5.3.1. 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, #25..
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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 Subbasin. 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.
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6. SUSTAINABLE MANAGEMENT CRITERIA
GSP regulations provide a framework for locally-defined and quantitative sustainable management criteria, which allow 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 summary12 of these terms – and the application of each – are provided below:
• Undesirable Results (URs13) – 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:
12 Sustainable management criteria are more fully defined in SGMA (CWC 10721(a) – (ab) and GSP regulations (§351(a) – (an)). 13 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 Group14 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;
14 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.
Chronic
Lowering
of Water
Levels
Reduction of
Groundwater
in Storage
Seawater
Intrusion
Degraded
Water
Quality
Inelastic
Land
Subsidence
Depletion of
Interconnected
Surface
Water
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 states that the GSAs are not required to address undesirable results that occurred before – #25..
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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
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 predicts future streamflow
depletions that may lead to
undesirable results. GSAs will manage
water levels to reduce future increases
in streamflow depletions.
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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 2nd 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 County15 as 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
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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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-10
Revised July 2024
TODD GROUNDWATER
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). Interim milestones are described in Section 6.3.3. Section 6.3.4 provides the approach and selection of measurable objectives (MOs).
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 5-8).
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-11
Revised July 2024
TODD GROUNDWATER
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 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.
#25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-12
Revised July 2024
TODD GROUNDWATER
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-13
Revised July 2024
TODD GROUNDWATER
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 wells16.
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 16 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..
- 1925 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-14
Revised July 2024
TODD GROUNDWATER
(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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-15
Revised July 2024
TODD GROUNDWATER
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-16
Revised July 2024
TODD GROUNDWATER
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-17
Revised July 2024
TODD GROUNDWATER
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 G.
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-18
Revised July 2024
TODD GROUNDWATER
• 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..
- 1930 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-19
Revised July 2024
TODD GROUNDWATER
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-20
Revised July 2024
TODD GROUNDWATER
• 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.
#25..
- 1932 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-21
Revised July 2024
TODD GROUNDWATER
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).
#25..
- 1933 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-22
Revised July 2024
TODD GROUNDWATER
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-23
Revised July 2024
TODD GROUNDWATER
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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Modesto Subbasin GSP
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Revised July 2024
TODD GROUNDWATER
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 6-5; 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 5-17 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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Modesto Subbasin GSP
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Revised July 2024
TODD GROUNDWATER
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-26
Revised July 2024
TODD GROUNDWATER
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-27
Revised July 2024
TODD GROUNDWATER
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%20%20 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 G.
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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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-28
Revised July 2024
TODD GROUNDWATER
The analysis includes 4,563 water supply wells with construction information in the Modesto Subbasin. 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 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
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 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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Revised July 2024
TODD GROUNDWATER
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 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 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.
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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Revised July 2024
TODD GROUNDWATER
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 signed Resolutions, provided in Appendix C. 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.
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.
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STRGBA GSA/Tuolumne GSA 6-31
Revised July 2024
TODD GROUNDWATER
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 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 #25..
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STRGBA GSA/Tuolumne GSA 6-32
Revised July 2024
TODD GROUNDWATER
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 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 #25..
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Modesto Subbasin GSP
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Revised July 2024
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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.
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. #25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 6-34
Revised July 2024
TODD GROUNDWATER
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.
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.
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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.
6.3.4. 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 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.
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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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 6-10: 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 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 #25..
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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.17 In 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.
Interim milestones are described in Section 6.4.3. Section 6.4.4 provides the approach and selection of MOs.
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 overdraft18, 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 17 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)). 18 Other causes of reduction of groundwater in storage include net subsurface outflows or contributions to baseflow in rivers or streams.
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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)
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 #25..
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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.
Table 6-11: 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
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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).
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 5-17). 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 #25..
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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 6-12: 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 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 5-20, 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).
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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 5-17). • 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 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 #25..
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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 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. #25..
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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.
6.4.4. 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.
Table 6-13: 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.
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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 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 #25..
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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 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 #25..
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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 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 #25..
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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., 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.
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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 6-14: 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
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 #25..
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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
6.6.4 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 6.6.4.
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.
Table 6-15: 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
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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)
• 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 #25..
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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 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.
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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 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. #25..
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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 used 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.
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. #25..
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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.
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.
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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 5-17). 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 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 Waterford19 – 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 19 The City of Waterford operates drinking water supply wells for the community of Hickman in the Turlock Subbasin.
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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 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 #25..
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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 H. 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 H, monitoring sites consist of municipal supply wells, monitoring wells, and domestic wells. Although most domestic wells are currently sampled for nitrate only (Appendix H), 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.
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
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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 6-16: 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 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 #25..
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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.
Interim milestones are described in Section 6.7.3. Section 6.7.4 provides the approach and selection of measurable objectives (MOs).
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.
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 InSAR20 – 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 20 InSAR refers to Interferometric Synthetic Aperture Radar data. #25..
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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 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 #25..
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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.
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. #25..
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Table 6-17: 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 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 #25..
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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 6-18: 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
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.
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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.
The map on Figure 3-59 also shows the locations of three existing global positioning system (GPS) stations21 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.
21 Installed and operated by the U.S. Bureau of Reclamation in connection with the San Joaquin River Restoration Program.
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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 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 6-5.
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.
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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 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 #25..
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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 semiannual 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 #25..
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chronic lowering of water levels are used as a proxy for the land subsidence sustainability indicator.
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 6-19: 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 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. IMs are described in Section 6.8.3. Section 6.8.4 provides the approach and selection of MOs.
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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 watergroundwater 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. #25..
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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 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.
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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.
Table 6-20: 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 #25..
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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.
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.
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Table 6-21: 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.
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 D of this GSP. #25..
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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 AFY22 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 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.
22 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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Table 6-22: 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 5.1.4.3 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).
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:
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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 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. #25..
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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- 22), 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 6-22 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 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.
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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 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.
6.8.4. 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 6.3.4, 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 #25..
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occurring between the MT (with temporary exceedances) and anticipated high water levels that occur during wet periods.
Table 6-23: 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. 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 6-24, 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 6-24 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 6-24: Sustainable Management Criteria Summary
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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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Oakdale ID
Management Area
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Management Areas
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03 Miles
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Corcoran Clay Extent
Management Area
Modesto ID
Oakdale ID
Non-District West
Non-District East
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Legend ! Representative Monitoring Well (RMW) !!
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Wells at Risk at 2027 Interim Milestone,
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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
#25..
- 2000 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 7-1
Revised July 2024
TODD GROUNDWATER
7. MONITORING NETWORK
The overall objective of the monitoring network for this Groundwater Sustainability Plan (GSP) is to yield representative information about groundwater conditions to guide and evaluate GSP implementation. Specifically, the GSP monitoring network is designed to:
• Evaluate groundwater conditions relative to sustainability indicators. • Monitor for minimum thresholds to avoid undesirable results. • Track interim milestones and measurable objectives to demonstrate progress on reaching sustainability goals for the Subbasin.
• Expand the existing monitoring network to better represent the entire Subbasin and address data gaps.
• Reduce uncertainty and provide better data to guide management actions, document the water budget, and improve understanding of the interconnection of surface water and groundwater.
• Identify and track potential impacts on beneficial uses and users of groundwater.
This GSP builds on existing monitoring programs with the intent to provide sufficient data for demonstrating short-term, seasonal, and long-term trends in groundwater levels.
Existing monitoring programs include the CASGEM monitoring program, public water supplier groundwater monitoring programs in the municipalities, agricultural water supplier groundwater monitoring programs in Modesto Irrigation District (MID) and Oakdale Irrigation District (OID), and the Irrigated Lands Regulatory Program. These existing monitoring programs are described in Section 2.4.
The following summarizes the monitoring network. Section 7.1 describes the monitoring network for each sustainability indicator. Section 7.2 provides protocols for data collection and monitoring. Section 7.3 describes how the monitoring network will be assessed and improved. Section 7.4 summarizes the data management system (DMS) for data collected from the monitoring network. Figures for Chapter 7 are provided at the end of the text to minimize interruption and facilitate multiple references to each figure.
7.1. DESCRIPTION OF MONITORING NETWORK
Groundwater level monitoring networks were developed to observe and document the chronic lowering of groundwater levels, reduction of groundwater in storage, land subsidence, and depletions of interconnected surface water. The applicability and rationale for using groundwater elevations to monitor each of these four sustainability indicators is discussed in Chapter 6, Sustainable Management Criteria. The monitoring networks are composed of representative monitoring wells that will be used to monitor sustainable management criteria for these sustainability indicators during the GSP implementation and planning horizon. Accordingly, groundwater elevations have been selected for a minimum threshold (MT) and measurable objective (MO) for each well in the monitoring network. #25..
- 2001 -
Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 7-2
Revised July 2024
TODD GROUNDWATER
The monitoring networks consist of CASGEM wells, City of Modesto monitoring wells, USGS monitoring wells and monitoring wells constructed in 2021 with Proposition 68 grant funding from DWR. The monitoring networks are illustrated on Figures 7-1 through 7-5.
The figures show locations of the wells in each monitoring network and the MT and MO for each well. Note that the current CASGEM program is being phased out and transitioned to the GSP monitoring network.
As described in Chapter 6, the monitoring network for degradation of water quality will be based on wells monitored by others and available at the State Water Resources Control Board (SWRCB) GeoTracker website. This network consists of drinking water supply wells, regulated facilities, and regional water quality programs such as the Groundwater Ambient Monitoring and Assessment (GAMA) Program. When combined with additional data from regulated water quality coalitions, this collective dataset represents a comprehensive network for tracking and evaluation of water quality with respect to the sustainable management criteria. Additional information on this monitoring network is provided in Section 7.1.4 below.
A monitoring network was not developed for the seawater intrusion sustainability indicator.
As discussed in Chapter 6, the GSAs found that seawater intrusion, as defined by GSP regulations, is not applicable to the inland Modesto Subbasin. Specifically, GSAs determined that seawater intrusion is not present in the Subbasin and is not likely to occur in the future (see Section 6.5). In accordance with GSP regulations, no sustainable management criteria have been assigned to this indicator, and no monitoring network has been established (§354.34(j)).
As described in Chapter 6, 2027 Interim Milestones (IMs) were developed for monitoring network wells in the OID and Non-District East Management Areas. The first IM occurs in 2027 with target values set below the MTs to provide a buffer to allow water levels to drop below the MT, recognizing that water levels in these wells may continue to decline after the GSP is adopted as projects are being brought online. This concept acknowledges that the aquifer response to projects and management actions will take time. 2027 IM values assume that recent water level declines will continue at similar rates between 2022 and 2027. Additional IMs are at five-year increments: the 2032 IM is the MT, the 2037 IM is halfway between the MT and the MO, and the 2042 IM is the MO. IMs provide a glide path for the Modesto Subbasin to reach its sustainability goal.
Summaries of the monitoring networks are provided in Tables 7-1 and 7-2. Well information includes the well ID, State Well Number, CASGEM identification number where applicable, well type, and Principal Aquifer and Management Area in which the well is located, location coordinates, well depth, screen interval depth, the MT and MO, a brief summary of how the MT and MO were developed, and the 2027 IM where applicable. #25..
- 2002 -
Table 7‐1: Summary of Monitoring Network, Chronic Lowering of Groundwater Levels Program Well ID State Well Number CASGEM Identification Number Well Use / Status Principal Aquifer MID OID
Non‐District East
Non‐District West Latitude (NAD 83) Longitude (NAD 83) Ground Surface Elevation (feet msl) Reference Point Elevation (feet msl) Total Well Depth (feet bgs) Screen Interval Depths (feet bgs) Minimum Threshold (MT) (feet msl) Measurable Objective (MO) (feet msl) MT/MO Note Interim Milestone (2027) (feet msl) CASGEM Albers 232 03S10E26D001M 3559 Active Irrigation Eastern x 37.651020 ‐120.847696 145.4 145.7 460 196‐288 60 76 based on measured data at the well ‐‐ CASGEM Allen OID‐01 02S10E16M001M 4430 Active Irrigation Eastern x 37.759897 ‐120.885401 145.6 145.7 415 0‐120 72 81 based on measured data at the well 61 CASGEM American 208 02S08E25P001M 3723 Active Irrigation Eastern x 37.728064 ‐121.041430 99.9 99.9 320 79‐272 48 55 based on measured data at the well ‐‐ CASGEM Bangs Ave 243 03S08E01K001M 3152 Active Irrigation Eastern x 37.703436 ‐121.038476 90.0 90.0 346 141‐251 32 46 based on measured data at the well ‐‐ CASGEM Bentley OID‐02 02S10E33J001M 4590 Active Irrigation Eastern x 37.715973 ‐120.866949 171.9 172.1 500 120‐175 71 85 based on measured data at the well 56 CASGEM Birnbaum OID‐03 02S10E15N001M 4429 Active Irrigation Eastern x 37.755921 ‐120.863872 149.4 149.8 293 55‐293 72 86 based on measured data at the well 61 CASGEM Blossom 230 03S11E30K001M 3903 Active Irrigation Eastern x 37.645614 ‐120.801537 154.8 155.0 412 179‐283 61 78 based on measured data at the well ‐‐ CASGEM Canfield 90 04S08E06L001M 26633 Active Irrigation Western Upper x 37.613113 ‐121.130799 52.0 52.3 151 40‐75 32 36 based on measured data at the well ‐‐ CASGEM Cavil 214 03S10E06G001M 27057 Active Irrigation Eastern x 37.705044 ‐120.911296 135.6 135.6 480 107‐275 53 73 based on measured data at the well ‐‐ CASGEM Claribel 206 03S09E03D001M 2093 Active Irrigation Eastern x 37.708526 ‐120.974280 114.1 114.5 650 96‐550 49 62 based on measured data at the well ‐‐ CASGEM Crane OID‐06 02S10E29E001M 29444 Active Irrigation Eastern x 37.733378 ‐120.899126 160.1 160.4 505 155‐198 66 77 based on measured data at the well 55 CASGEM Curtis #2 100 03S08E09P001M 3303 Active Irrigation Western Upper x 37.685351 ‐121.097462 63.6 63.6 124 79‐100 34 41 based on measured data at the well ‐‐ CASGEM Furtado OID‐07 02S11E32L001M 2529 Active Irrigation Eastern x 37.718381 ‐120.786289 212.0 212.5 590 200‐580 69 81 based on measured data at the well 51 CASGEM Gates Road 101 03S07E24M001M 3146 Active Irrigation Western Upper x 37.659699 ‐121.155215 44.2 44.2 64 ‐‐ 24 33 based on measured data at the well ‐‐ CASGEM Hart Road 88 03S08E08D001M 3301 Active Irrigation Western Upper x 37.694807 ‐121.122902 54.9 55.2 130 73‐85 35 40 based on measured data at the well ‐‐ CASGEM Head Lateral 3 215 03S10E17K001M 3552 Active Irrigation Eastern x 37.674398 ‐120.891430 135.8 135.6 476 116‐400 56 73 based on measured data at the well ‐‐ CASGEM Head Lateral 8 194 02S08E27N001M 38870 Active Irrigation Eastern x 37.727189 ‐121.087002 79.5 79.8 302 148‐211 40 47 based on measured data at the well ‐‐ CASGEM Jones WID 228 03S11E29J001M 38872 Active Irrigation Eastern x 37.641798 ‐120.776177 166.4 166.4 324 188‐280 55 75 based on measured data at the well ‐‐ CASGEM Katen 69 03S07E25P001M 3147 Active Irrigation Western Upper x 37.637929 ‐121.149890 45.1 45.1 160 13‐148 27 33 based on measured data at the well ‐‐ CASGEM Langdon Merle 241 02S09E28H001M 3876 Active Irrigation Eastern x 37.734908 ‐120.977526 128.4 128.5 595 160‐300 50 62 based on measured data at the well ‐‐ CASGEM Lateral one 195 03S10E32G001M 3877 Active Irrigation Eastern x 37.632523 ‐120.889283 126.0 126.0 260 141‐210 42 52 based on measured data at the well ‐‐ CASGEM Machado 23 03S08E17R001M 3864 Active Irrigation Western Upper x 37.668045 ‐121.105038 59.1 59.3 80 ‐‐ 31 40 based on measured data at the well ‐‐ CASGEM Marquis OID‐10 02S10E20C001M 29436 Active Irrigation Eastern x 37.753232 ‐120.896930 138.4 138.8 125 27‐125 85 91 based on measured data at the well 78 CASGEM North Ave 103 03S08E14B001M 3854 Active Irrigation Western Upper x 37.678393 ‐121.054335 73.9 74.6 130 53‐81 41 50 based on measured data at the well ‐‐ CASGEM Paradise 235 04S08E02L001M 2151 Active Irrigation Western Upper x 37.614186 ‐121.057863 73.7 73.9 258 96‐132 34 41 based on measured data at the well ‐‐ CASGEM Paulsell 1 OID‐11 02S12E31K001M 26187 Active Irrigation Eastern x 37.717864 ‐120.691876 195.9 197.5 815 195‐410 88 117 based on measured data at the well 53 CASGEM Paulsell 2 OID‐12 02S12E32P001M 38865 Active Irrigation Eastern x 37.710953 ‐120.676939 193.9 195.6 815 132‐815 94 123 based on measured data at the well 58 CASGEM Perley 202 03S09E14P001M 2109 Active Irrigation Eastern x 37.667719 ‐120.951955 104.9 105.4 255 76‐204 36 45 based on measured data at the well ‐‐ CASGEM Philbrick 201 04S08E02H001M 26591 Active Irrigation Western Upper x 37.619159 ‐121.050003 73.1 73.5 88 58‐74 34 41 based on measured data at the well ‐‐ CASGEM Quesenberry 223 03S12E19G001M 27424 Active Irrigation Eastern x 37.659773 ‐120.689681 197.0 197.0 380 168‐208 89 110 based on measured data at the well 72 CASGEM Riverbank OID‐13 02S09E27G001M 49463 Active Irrigation Eastern x 37.735134 ‐120.964821 132.3 134.2 560 200‐550 42 54 based on measured data at the well ‐‐ CASGEM Schmidt 227 03S11E27G003M 3897 Active Irrigation Eastern x 37.648671 ‐120.736000 192.3 192.2 248 113‐153 59 78 based on measured data at the well ‐‐ CASGEM Van Buren 43 03S08E21Q001M 3873 Active Irrigation Western Upper x 37.654644 ‐121.094887 63.3 63.5 196 76‐116 38 45 based on measured data at the well ‐‐ CASGEM Warnock 46 03S08E29K001M 4015 Active Irrigation Western Upper x 37.642900 ‐121.108575 55.1 55.1 240 ‐‐ 35 42 based on measured data at the well ‐‐ Management Area
Modesto Subbasin GSPSTRGBA GSA/Tuolumne GSA
7‐3
Revised July 2024
TODD GROUNDWATER
#25..
- 2003 -
Table 7‐1: Summary of Monitoring Network, Chronic Lowering of Groundwater Levels Program Well ID State Well Number CASGEM Identification Number Well Use / Status Principal Aquifer MID OID
Non‐District East
Non‐District West Latitude (NAD 83) Longitude (NAD 83) Ground Surface Elevation (feet msl) Reference Point Elevation (feet msl) Total Well Depth (feet bgs) Screen Interval Depths (feet bgs) Minimum Threshold (MT) (feet msl) Measurable Objective (MO) (feet msl) MT/MO Note Interim Milestone (2027) (feet msl) Management Area
CASGEM Wellsford 233 03S10E16K001M 3551 Active Irrigation Eastern x 37.673607 ‐120.875297 141.9 142.0 468 158‐358 62 77 based on measured data at the well ‐‐ CASGEM Wood 210 03S10E18P001M 3553 Active Irrigation Eastern x 37.667487 ‐120.912168 121.3 121.3 606 87‐547 52 66 based on measured data at the well ‐‐ CASGEM Young 76 04S08E04G001M 38078 Active Irrigation Western Upper x 37.618051 ‐121.094288 61.5 62.1 175 12‐152 36 42 based on measured data at the well ‐‐ City of Modesto MOD‐MWA‐2 ‐‐ not applicable Monitoring Well Eastern x 37.642986 ‐120.931770 ‐‐ 103.8 175 150‐170 30 36 MT: based on Oct 2015 contour map; MO:
based on historic high, spring 1998 contour
map
‐‐
City of Modesto MOD‐MWB‐1 ‐‐ not applicable Monitoring Well Western Upper x 37.690559 ‐121.044299 ‐‐ 78.8 177 152‐172 40 49 MT: estimated from fall 2015 contour map;
MO: historic high estimated from spring 1998
contour map
‐‐
City of Modesto MOD‐MWB‐2 ‐‐ not applicable Monitoring Well Western Lower x 37.690559 ‐121.044245 ‐‐ 78.7 250 225‐245 26 34 MT: estimated from fall 2015 contour map;
MO: historic high estimated from spring 1998
contour map
‐‐
City of Modesto MOD‐MWC‐3 ‐‐ not applicable Monitoring Well Eastern x 37.672249 ‐120.940908 ‐‐ 105.6 285 260‐280 40 50 MT: based on October 2015 contour map, MO: based on spring 1998 contour map ‐‐ City of Modesto MOD‐MWD‐1 ‐‐ not applicable Monitoring Well Western Upper x 37.649959 ‐121.048685 ‐‐ 73.3 129 104‐124 30 40 MT: estimated from fall 2015 contour map
and MT at nearby CASGEM well (McDonald);
MO: based on historic high from spring 1998
contour map
‐‐
City of Modesto MOD‐MWD‐3 ‐‐ not applicable Monitoring Well Western Lower x 37.649958 ‐121.048649 ‐‐ 73.2 243 218‐238 30 37 MT: estimated from fall 2015 measured
contour map and model contours (Layer 2);
MO: historic high estimated from spring 1998
contour map
‐‐
USGS FPA‐2 003S009E08K004M not applicable Monitoring Well Eastern x 37.686194 ‐121.000917 ‐‐ 91.0 122.2 115‐120 38 48 MT: based on October 2015 contour map;
MO: based on maximum of measured data
(higher than estimate from spring 1998contour map)
‐‐
USGS OFPB‐2 003S009E11F002M not applicable Monitoring Well Eastern x 37.690194 ‐120.951417 ‐‐ 104.0 174.5 166‐171 35 53 MT: based on fall 2015 contour map; MO:
historic high based on spring 1998 contour
map
‐‐
USGS MRWA‐2 003S008E33R002M not applicable Monitoring Well Western Upper x 37.624121 ‐121.086103 ‐‐ 64.0 183 174‐179 36 43 MT: estimated from fall 2015 contour map
and based on nearby CASGEM well (Young);
MO: historic high estimated from spring 1998
contour map and CASGEM well (Young)
‐‐
USGS MRWA‐3 003S008E33R001M not applicable Monitoring Well Western Lower x 37.624121 ‐121.086103 ‐‐ 64.0 280 269‐274 28 36 MT: estimated from model contours
September 2015 (Layer 2); MO: historic high
based on measured data
‐‐
Prop 68 MW‐1S ‐‐ not applicable Monitoring Well Western Upper x 37.707630 ‐121.087167 68.4 68.0 125 100‐120 33 43 MT: based on fall 2015 contour map; MO:
historic high based on spring 1998 contour
map
‐‐
Prop 68 MW‐1D ‐‐ not applicable Monitoring Well Western Lower x 37.707631 ‐121.087136 68.5 67.9 250 225‐245 14 27 MT: based on measured data in April 2021
(lower than fall 2015 contour map); MO:
historic high based on spring 1998 contour
map
‐‐
Modesto Subbasin GSPSTRGBA GSA/Tuolumne GSA
7‐4
Revised July 2024
TODD GROUNDWATER
#25..
- 2004 -
Table 7‐1: Summary of Monitoring Network, Chronic Lowering of Groundwater Levels Program Well ID State Well Number CASGEM Identification Number Well Use / Status Principal Aquifer MID OID
Non‐District East
Non‐District West Latitude (NAD 83) Longitude (NAD 83) Ground Surface Elevation (feet msl) Reference Point Elevation (feet msl) Total Well Depth (feet bgs) Screen Interval Depths (feet bgs) Minimum Threshold (MT) (feet msl) Measurable Objective (MO) (feet msl) MT/MO Note Interim Milestone (2027) (feet msl) Management Area
Prop 68 MW‐2S ‐‐ not applicable Monitoring Well Western Upper x 37.613886 ‐121.023442 71.1 70.7 135 110‐130 34 41 MT/MO: based on nearby CASGEM well (Philbrick) ‐‐ Prop 68 MW‐2D ‐‐ not applicable Monitoring Well Western Lower x 37.613886 ‐121.023475 71.2 71.0 281 256‐276 35 40 MT: based on fall 2015 model contour map
(Lay 2); MO: based on historic high of
measured data
‐‐
Prop 68 MW‐3S ‐‐ not applicable Monitoring Well Eastern x 37.630743 ‐120.967621 95.8 95.6 161 136‐156 25 31 MT: based on historic low at nearby MOD‐
225; MO: based on max of measured data
(slightly higher than historic high based on
spring 1998 contour map)
‐‐
Prop 68 MW‐3D ‐‐ not applicable Monitoring Well Eastern x 37.630711 ‐120.967621 95.7 95.3 283 258‐278 25 31 MT/MO: same as MW‐3S (so far, measured water level data are similar) ‐‐ Prop 68 MW‐4S ‐‐ not applicable Monitoring Well Eastern x 37.728565 ‐120.941555 136.6 136.3 165 140‐160 56 67 MT: based on fall 2015 contour map; MO:
historic high based on spring 1998 contour
map
‐‐
Prop 68 MW‐5S ‐‐ not applicable Monitoring Well Eastern x 37.763120 ‐120.825360 191.9 191.6 175 150‐170 69 89 MT: based on historic low at nearby Oak‐008;
MO: based on historic high at nearby Oak‐
008
68 Prop 68 MW‐6S ‐‐ not applicable Monitoring Well Eastern x 37.646100 ‐120.752540 171.3 170.9 179 154‐174 65 83 MT: based on fall 2015 contour map; MO:
historic high based on spring 1998 contour
map
‐‐
Prop 68 MW‐7 ‐‐ not applicable Monitoring Well Eastern x 37.743410 ‐120.704350 242.6 242.3 300 275‐295 75 110 MT: based on minimum of available
measured data at this well. There is a lack of
water level data in this area of the Subbasin.
MO: based on historic high at CASGEM well
Paulsell‐1 (~2 miles south).
40 Prop 68 MW‐8 ‐‐ not applicable Monitoring Well Eastern x 37.732370 ‐120.632880 292.9 292.3 290 265‐285 75 110 MT: based on minimum of available
measured data at this well. Similar value to
nearby well on fall 2015 contour map. MO:
based on historic high at CASGEM well
Paulsell‐1
49 Prop 68 MW‐9 ‐‐ not applicable Monitoring Well Eastern x 37.649510 ‐120.535140 244.5 247.6 365 340‐360 150 180 MT: based on minimum of available
measured data at this well. There is a lack of
water level data in this area of the Subbasin.
MO: Based on similar operational range as
other eastern Subbasin wells (~30 ft)
138
Prop 68 MW‐10 ‐‐ not applicable Monitoring Well Eastern x 37.739630 ‐120.756490 265.1 264.7 265 240‐260 72 101 MT: based on historic low at a nearby DWR
WDL well ‐ Dec 2013 (data from 1990 to
2014); MO: based on historic high at nearby
DWR WDL well ‐ Nov 1997
63 Prop 68 MW‐11 ‐‐ not applicable Monitoring Well Eastern x 37.643970 ‐120.900997 116.3 116.1 175 150‐170 35 48 MT: based on historic low at nearby MOD‐
247; based on historic high at nearby MOD‐
247
‐‐
Notes: IMs were developed for wells in the Non‐District East Management Area and the OID Management Area, where water levels may continue to decline while projects are being brought online.
IMs were not assigned to wells in the Non‐District West Management Area and the MID Management Area, where water levels are relatively stable and consistent with established MTs and MOs.
IMs provided on this table represent 5‐year IMs (2027), as described in Section 7.1. The 10‐year IMs (2032) are the MTs and the 15‐year IMs (2037) are the midpoint between the MT and the MO (see Section 7.1).
Modesto Subbasin GSPSTRGBA GSA/Tuolumne GSA
7‐5
Revised July 2024
TODD GROUNDWATER
#25..
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Table 7‐2: Summary of Monitoring Network, Interconnected Surface Water Program Well ID State Well Number
CASGEM
Identification
Number
Well Use / Status Principal Aquifer MID
OID
Non‐District East
Non‐District West Latitude (NAD 83) Longitude (NAD 83) Ground Surface Elevation (feet msl) Reference Point Elevation (feet msl) Total Well Depth (feet bgs) Screen Interval Depths (feet bgs) Minimum Threshold (MT) (feet msl) Measurable Objective (MO) (feet msl) MT/MO Note Interim Milestone (2027) (feet msl) CASGEM Canfield 90 04S08E06L001M 26633 Active Irrigation Western Upper x 37.613113 ‐121.130799 52.0 52.3 151 40‐75 33 37 based on measured data at the well ‐‐ CASGEM Katen 69 03S07E25P001M 3147 Active Irrigation Western Upper x 37.637929 ‐121.149890 45.1 45.1 160 13‐148 27 33 based on measured data at the well ‐‐ CASGEM Allen OID‐01 02S10E16M001M 4430 Active Irrigation Eastern x 37.759897 ‐120.885401 145.6 145.7 415 0‐120 75 83 based on measured data at the well 61 CASGEM American 208 02S08E25P001M 3723 Active Irrigation Eastern x 37.728064 ‐121.041430 99.9 99.9 320 79‐272 48 55 based on measured data at the well ‐‐ CASGEM Birnbaum OID‐03 02S10E15N001M 4429 Active Irrigation Eastern x 37.755921 ‐120.863872 149.4 149.8 293 55‐293 74 87 based on measured data at the well 61 CASGEM Head Lateral 8 194 02S08E27N001M 38870 Active Irrigation Eastern x 37.727189 ‐121.087002 79.5 79.8 302 148‐211 40 47 based on measured data at the well ‐‐ CASGEM Langdon Merle 241 02S09E28H001M 3876 Active Irrigation Eastern x 37.734908 ‐120.977526 128.4 128.5 595 160‐300 50 62 based on measured data at the well ‐‐ CASGEM Marquis OID‐10 02S10E20C001M 29436 Active Irrigation Eastern x 37.753232 ‐120.896930 138.4 138.8 125 27‐125 86 92 based on measured data at the well 78CASGEM Riverbank OID‐13 02S09E27G001M 49463 Active Irrigation Eastern x 37.735134 ‐120.964821 132.3 134.2 560 200‐550 42 54 based on measured data at the well ‐‐ Prop 68 MW‐4S ‐‐ not applicable Monitoring Well Eastern x 37.728639 ‐120.941518 136.6 136.3 165 140‐160 56 67 MT: based on fall 2015 contour
map; MO: historic high based on
spring 1998 contour map
‐‐
CASGEM Jones WID 228 03S11E29J001M 38872 Active Irrigation Eastern x 37.641798 ‐120.776177 166.4 166.4 324 188‐280 55 75 based on measured data at the well ‐‐ CASGEM Lateral one 195 03S10E32G001M 3877 Active Irrigation Eastern x 37.632523 ‐120.889283 126.0 126.0 260 141‐210 42 52 based on measured data at the well ‐‐ CASGEM Paradise 235 04S08E02L001M 2151 Active Irrigation Western Upper x 37.614186 ‐121.057863 73.7 73.9 258 96‐132 34 41 based on measured data at the well ‐‐ CASGEM Philbrick 201 04S08E02H001M 26591 Active Irrigation Western Upper x 37.619159 ‐121.050003 73.1 73.5 88 58‐74 38 43 based on measured data at the well ‐‐CASGEM Quesenberry 223 03S12E19G001M 27424 Active Irrigation Eastern x 37.659773 ‐120.689681 197.0 197.0 380 168‐208 89 110 based on measured data at the well 72 CASGEM Schmidt 227 03S11E27G003M 3897 Active Irrigation Eastern x 37.648671 ‐120.736000 192.3 192.2 248 113‐153 59 78 based on measured data at the well ‐‐ Prop 68 MW‐2S ‐‐ not applicable Monitoring Well Western Upper x 37.613886 ‐121.023442 71.1 70.7 135 110‐130 38 43 MT/MO: based on nearby CASGEM well (Philbrick) ‐‐ Prop 68 MW‐3S ‐‐ not applicable Monitoring Well Eastern x 37.630743 ‐120.967621 95.8 95.6 161 136‐156 26 32 MT: based on fall 2015 level at
nearby MOD‐225; MO: historic
high based on spring 1998 contour
map
‐‐
Prop 68 MW‐6S ‐‐ not applicable Monitoring Well Eastern x 37.646100 ‐120.752540 171.3 170.9 179 154‐174 65 83 MT: based on fall 2015 contourmap; MO: historic high based onspring 1998 contour map ‐‐
Prop 68 MW‐9 ‐‐ not applicable Monitoring Well Eastern x 37.649510 ‐120.535140 244.5 247.6 365 340‐360 150 180 MT: based on minimum of
available measured data at this
well. There is a lack of water level
data in this area of the Subbasin.
MO: Based on similar operational
range as other eastern Subbasin
wells (~30 ft)
138
Notes: IMs were developed for wells in the Non‐District East Management Area and the OID Management Area, where water levels may continue to decline while projects are being brought online.
IMs were not assigned to wells in the Non‐District West Management Area and the MID Management Area, where water levels are relatively stable and consistent with established MTs and MOs.
IMs provided on this table represent 5‐year IMs (2027), as described in Section 7.1. The 10‐year IMs (2032) are the MTs and the 15‐year IMs (2037) are the midpoint between the MT and the MO (see Section 7.1).
San Joaquin River
Stanislaus RiverTuolumne River
Management Area
Modesto Subbasin GSPSTRGBA GSA/Tuolumne GSA
7‐6
Revised July 2024
TODD GROUNDWATER
#25..
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Modesto Subbasin GSP
STRGBA GSA/Tuolumne GSA 7-7
Revised July 2024
TODD GROUNDWATER
Hydrographs for each monitoring network well are provided in Appendix G. The hydrographs include well screen interval, ground surface elevation, the MT and MO for each well, and the 2027 IM, where applicable. Hydrograph presentation meets the data and reporting standards for hydrographs in Article 3 of the GSP regulations (§352.4(e)).
In addition to the representative wells in the monitoring networks, the GSAs will measure groundwater elevations in over 40 existing wells. These wells will be designated as SGMA monitoring wells and will not be used to monitor the sustainability indicators, and therefore do not have MTs and MOs. However, groundwater elevation data collected from the SGMA monitoring wells will be used for monitoring overall groundwater conditions and support analyses, such as the preparation of groundwater elevation contour maps. As part of the GSP five-year update, water level data from the SGMA monitoring wells will be compared to data from representative monitoring wells and these wells can be added to the monitoring network to reduce uncertainty or address data gaps, as needed. This task will be a part of the overall monitoring network assessment required by GSP regulations (§354.38(a)). The SGMA monitoring wells are summarized in Table 7-3.
A data gap analysis has been incorporated into the GSP Implementation Plan to address current data gaps and other improvements needed for the current GSP monitoring network (see Section 9.5.1).
The monitoring networks for each sustainability indicator are described in the following sections.
7.1.1. Chronic Lowering of Groundwater Levels
The monitoring network for chronic lowering of groundwater levels for each of the three principal aquifers is presented on Figures 7-1, 7-2 and 7-3. The wells in this monitoring network are summarized in Table 7-1.
Well density was an important consideration in identifying monitoring network wells for this sustainability indicator. DWR guidance (DWR, 2016b, see Table 1) generally recommends between one and ten monitoring wells per 100 square miles. This monitoring network is consistent with this guidance.
The following is a description of the monitoring network in each principal aquifer of the Subbasin.
7.1.1.1. Western Upper Principal Aquifer
The monitoring network for the Western Upper Principal Aquifer is illustrated on Figure 7-1.
The monitoring network is composed of 17 wells, including 12 CASGEM wells, 2 City of Modesto monitoring wells, 2 Proposition 68 monitoring wells, and 1 USGS well. The STRGBA GSA is working with the USGS to obtain ownership and access to the USGS monitoring well.
Well data are summarized in Table 7-1.
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Table 7‐3: Summary of SGMA Monitoring Wells
Program Well ID State Well Number
CASGEM
Identification
Number
Well Use / Status Principal Aquifer
MID
OID
Non‐District East
Non‐District West Latitude (NAD 83) Longitude (NAD 83) Ground Surface Elevation (feet msl) Reference Point Elevation (feet msl) Total Well Depth (feet bgs) Screen Interval Depths (feet bgs) CASGEM Basso 2 03S08E18C001M 3865 Active Irrigation Western Upper x 37.677888 ‐121.136328 49.0 49.0 200 1‐119 CASGEM Gove 18 03S08E19Q001M 3868 Active Irrigation Western Upper x 37.653607 ‐121.128597 54.7 54.7 136 36‐96 City of Modesto MOD‐MWA‐1 ‐‐ not applicable Monitoring Well Eastern x 37.643037 ‐120.931769 ‐‐ 103.9 109 84‐104 City of Modesto MOD‐MWA‐3 ‐‐ not applicable Monitoring Well Eastern x 37.642945 ‐120.931770 ‐‐ 103.7 285 260‐280 City of Modesto MOD‐MWA‐4 ‐‐ not applicable Monitoring Well Eastern x 37.642905 ‐120.931769 ‐‐ 103.6 356 331‐351 City of Modesto MOD‐MWB‐3 ‐‐ not applicable Monitoring Well Western Lower x 37.690560 ‐121.044196 ‐‐ 78.7 299 274‐294City of Modesto MOD‐MWB‐4 ‐‐ not applicable Monitoring Well Western Lower x 37.690561 ‐121.044144 ‐‐ 78.7 385 360‐380City of Modesto MOD‐MWC‐1 ‐‐ not applicable Monitoring Well Eastern x 37.672249 ‐120.940957 ‐‐ 105.5 135 110‐130 City of Modesto MOD‐MWC‐2 ‐‐ not applicable Monitoring Well Eastern x 37.672250 ‐120.941012 ‐‐ 105.3 191 166‐186 City of Modesto MOD‐MWC‐4 ‐‐ not applicable Monitoring Well Eastern x 37.672250 ‐120.941058 ‐‐ 105.3 445 420‐440 City of Modesto MOD‐MWD‐2 ‐‐ not applicable Monitoring Well Western Upper x 37.649920 ‐121.048682 ‐‐ 73.3 179 154‐174 City of Modesto MOD‐MWD‐4 ‐‐ not applicable Monitoring Well Western Lower x 37.649919 ‐121.048652 ‐‐ 73.0 325 300‐320 City of Modesto MOD‐MWE‐2 ‐‐ not applicable Monitoring Well Eastern x 37.635224 ‐121.010426 ‐‐ 83.9 200 175‐195City of Modesto MOD‐MWE‐3 ‐‐ not applicable Monitoring Well Eastern x 37.635184 ‐121.010427 ‐‐ 83.8 265 240‐260City of Modesto MOD‐MWE‐4 ‐‐ not applicable Monitoring Well Eastern x 37.635272 ‐121.010426 ‐‐ 83.8 430 405‐425 USGS FPA‐1 003S009E08K005M not applicable Monitoring Well Eastern x 37.686194 ‐121.000917 ‐‐ 91.0 37 30‐35 USGS FPA‐3 003S009E08K003M not applicable Monitoring Well Eastern x 37.686194 ‐121.000917 ‐‐ 91.0 222 215‐220 USGS FPA‐4 003S009E08K002M not applicable Monitoring Well Eastern x 37.686194 ‐121.000917 ‐‐ 91.0 350 343‐348 USGS FPB‐1 003S009E08H003M not applicable Monitoring Well Eastern x 37.692611 ‐120.997333 ‐‐ 95.0 39 30‐35 USGS FPB‐2 003S009E08H002M not applicable Monitoring Well Eastern x 37.692611 ‐120.997333 ‐‐ 95.0 194 187‐192 USGS FPB‐3 003S009E08H001M not applicable Monitoring Well Eastern x 37.692611 ‐120.997333 ‐‐ 95.0 335 328‐333 USGS FPD‐1 003S009E04G003M not applicable Monitoring Well Eastern x 37.705972 ‐120.983250 ‐‐ 104.0 35 28‐33USGS FPD‐2 003S009E04G002M not applicable Monitoring Well Eastern x 37.705972 ‐120.983250 ‐‐ 104.0 174 167‐172USGS FPD‐3 003S009E04G001M not applicable Monitoring Well Eastern x 37.705972 ‐120.983250 ‐‐ 104.0 359 334‐339 USGS FPE‐1 003S009E09L003M not applicable Monitoring Well Eastern x 37.687722 ‐120.988056 ‐‐ 96.0 39 30‐35 USGS FPE‐2 003S009E09L002M not applicable Monitoring Well Eastern x 37.687722 ‐120.988056 ‐‐ 96.0 106 98‐103 USGS FPE‐3 003S009E09L001M not applicable Monitoring Well Eastern x 37.687722 ‐120.988056 ‐‐ 96.0 211 203‐208 USGS OFPA‐1 003S009E16C003M not applicable Monitoring Well Eastern x 37.680000 ‐120.986000 ‐‐ 94.0 38 30‐35USGS OFPA‐2 003S009E16C002M not applicable Monitoring Well Eastern x 37.680000 ‐120.986000 ‐‐ 94.0 105 95‐100USGS OFPA‐3 003S009E16C001M not applicable Monitoring Well Eastern x 37.680000 ‐120.986000 ‐‐ 94.0 200 188‐193 USGS OFPB‐1 003S009E11F003M not applicable Monitoring Well Eastern x 37.690194 ‐120.951417 ‐‐ 104.0 36 28‐33 USGS SA 003S009E09F001M not applicable Monitoring Well Eastern x 37.692361 ‐120.987333 ‐‐ 99.0 39 30‐35 Management Area
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Table 7‐3: Summary of SGMA Monitoring Wells
Program Well ID State Well Number
CASGEM
Identification
Number
Well Use / Status Principal Aquifer
MID
OID
Non‐District East
Non‐District West Latitude (NAD 83) Longitude (NAD 83) Ground Surface Elevation (feet msl) Reference Point Elevation (feet msl) Total Well Depth (feet bgs) Screen Interval Depths (feet bgs) Management Area
USGS SB 003S009E10D001M not applicable Monitoring Well Eastern x 37.692944 ‐120.973389 ‐‐ 104.0 36 30‐35 USGS SC 003S009E10L001M not applicable Monitoring Well Eastern x 37.685722 ‐120.971500 ‐‐ 99.0 41 30‐35 USGS MRWA‐1 003S008E33R003M not applicable Monitoring Well Western Upper x 37.624121 ‐121.086103 ‐‐ 64.0 35 25‐30 USGS MREA‐1 003S010E17K004M not applicable Monitoring Well Eastern x 37.674092 ‐120.891361 ‐‐ 132.0 46 40‐45 USGS MREA‐2 003S010E17K003M not applicable Monitoring Well Eastern x 37.674092 ‐120.891361 ‐‐ 132.0 56 51‐56USGS MREA‐3 003S010E17K002M not applicable Monitoring Well Eastern x 37.674092 ‐120.891361 ‐‐ 132.0 266 100‐260 Prop 68 MW‐4D ‐‐ not applicable Monitoring Well Eastern x 37.728568 ‐120.941473 136.7 136.4 255 230‐250 Prop 68 MW‐5D ‐‐ not applicable Monitoring Well Eastern x 37.763080 ‐120.825350 191.8 191.5 285 260‐280 Prop 68 MW‐6D ‐‐ not applicable Monitoring Well Eastern x 37.646090 ‐120.752510 171.3 170.8 261 236‐256 Modesto Subbasin GSPSTRGBA GSA/Tuolumne GSA
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The wells in this monitoring network were chosen based on the following scientific rationale:
• Known locations and construction, with screen intervals or total depth above the Corcoran Clay (in the Western Upper Principal Aquifer). • Spatial distribution and density of wells throughout the Western Upper Principal Aquifer.
• Length, completeness, and reliability of historical groundwater level record. • Accessibility for future water level measurement.
Hydrographs for the wells in this monitoring network are presented in Appendix G. The CASGEM wells have historical water level records, many with water level data since the start of the GSP study period (water year (WY) 1991). As described in Chapter 6, the MT for the chronic lowering of groundwater level sustainability indicator is the historical low groundwater elevation observed from WY 1991 to WY 2020 and the MO is the midpoint between the historical high groundwater elevation during this time period and the MT. The MTs and MOs for the CASGEM wells were based on direct measurements in each well.
The City of Modesto monitoring wells, USGS wells and Proposition 68 monitoring wells have limited water level data. The MTs and MOs at these wells are based on the groundwater elevation contour maps in fall 2015 and spring 1998 (see Figures 3-26 and 3-27) or nearby wells with historical data.
The USGS well (MRWA-2) and one of the City of Modesto monitoring wells (MOD-MWD-1) are part of well clusters. At each of these locations, there are two wells screened in the Western Upper Principal Aquifer (and wells screened in the Western Lower Principal Aquifer). One representative well was chosen for the monitoring network from each location based on a review of the water level data, lithologic logs, and geophysical logs. The wells chosen for the monitoring network are screened in conductive sand or gravel units and have similar water levels to the other well in the cluster. The remaining well at each location are SGMA monitoring wells and are summarized in Table 7-3.
Static groundwater elevations will be measured twice a year in these monitoring wells to represent seasonal high and seasonal low groundwater conditions. The wells in this monitoring network will be monitored by one of the STRGBA GSA member agencies.
The SGMA monitoring wells in the Western Upper Principal Aquifer will also be monitored twice a year. These wells can be added to the monitoring network if problems arise with current monitoring network wells.
7.1.1.2. Western Lower Principal Aquifer
The monitoring network for the Western Lower Principal Aquifer contains five wells, as illustrated on Figure 7-2 and summarized in Table 7-1. The monitoring network includes two City of Modesto monitoring wells, two Proposition 68 monitoring wells, and one USGS monitoring well.
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The wells in this monitoring network were chosen because they have known locations and construction, with discrete screen intervals in the Western Lower Principal Aquifer (below the Corcoran Clay), and because they will be accessible for water level measurement in the future. As described in Section 3.1.4, The Corcoran Clay is the primary aquitard in the Subbasin and separates the alluvial aquifers above and below the clay, creating confined conditions in the Western Lower Principal Aquifer. The STRGBA GSA is working with the USGS to obtain ownership and access to the USGS monitoring well.
The two City of Modesto wells in this monitoring network (MOD-MWB-2 and MOD-MWD-3) are part of well clusters with two or three wells screened in the Western Lower Principal Aquifer at each location. One representative well was chosen for the monitoring network from each location based on a review of the water level data, lithologic logs, and geophysical logs. The wells chosen for the monitoring network are screened in conductive sand or gravel units and have similar water levels to the other well at the same location.
The remaining well(s) at each location are SGMA monitoring wells and are summarized in Table 7-3.
As shown on Figure 7-2, most of the wells in the monitoring network are in the eastern region of the Western Lower Principal Aquifer, with one City of Modesto monitoring well in the southwestern Western Lower Principal Aquifer. There is a lack of well coverage in the central and western regions of the aquifer. This data gap of groundwater elevations in the Western Lower Principal Aquifer is identified in Section 3.2.9. Further improvements to the monitoring network are described in the data gap analysis included in the GSP Implementation Plan in Chapter 9 (Section 9.5.1).
Hydrographs for wells in this monitoring network are presented in Appendix G. There are no measured data from Fall 2015 at any of these monitoring network wells. Historic data from other wells in the western aquifers suggest the historic low water level occurred during the recent drought in 2015 and have recovered to some degree since then. As noted in Table 7-1, the MTs selected for the Western Lower Principal Aquifer wells are based on estimates from the Fall 2015 groundwater elevation contour map (see Figure 3-27) or Fall 2015 model groundwater elevation contours. The MOs are based on the Spring 1998 contour map (see Figure 3-26) or available measured data at the well.
Static groundwater elevations will be measured in these monitoring wells twice a year, once in the spring and once in the fall, to represent seasonal high and seasonal low groundwater conditions. The wells will be monitored by one of the STRGBA GSA member agencies.
7.1.1.3. Eastern Principal Aquifer
The monitoring network for the Eastern Principal Aquifer consists of 39 wells, as shown on Figure 7-3. The monitoring network includes CASGEM wells, City of Modesto monitoring wells, Proposition 68 monitoring wells and USGS monitoring wells. Well data are summarized in Table 7-1.
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distribution throughout the Eastern Principal Aquifer. The STRGBA GSA is working with the USGS to obtain ownership and access to the USGS monitoring wells.
The monitoring network wells are distributed throughout most of the Eastern Principal Aquifer but are sparse in the eastern Subbasin. This data gap of groundwater elevations in the Eastern Principal Aquifer is identified in Section 3.2.9. The four Proposition 68 monitoring wells constructed in the eastern Subbasin in 2021 (MW-7, MW-8, MW-9, and MW-10) help to fill this data gap. However, additional monitoring wells are necessary to fully characterize groundwater levels and flow in the eastern Subbasin. Further improvements to the monitoring network are described in the data gap analysis incorporated into the GSP implementation Plan in Chapter 9 (Section 9.5.1).
Hydrographs for wells in this monitoring network are presented in Appendix G. Several methods were used to develop MTs and MOs, based on available data. Most of the wells in the monitoring network are CASGEM wells with sufficient historical water level records and therefore, MTs and MOs are based on measured data at the wells. The City of Modesto, Proposition 68 and USGS monitoring wells, however, do not have sufficient historical measured water levels so their MTs and MOs were developed with a variety of methods.
For these wells, MTs were either based on the Fall 2015 groundwater elevation contour map (see Figure 3-27), groundwater elevations at nearby wells, or the limited measured water level data at the well. MOs were based on either measured historic high groundwater levels or estimates from the Spring 1998 contour map (see Figure 3-26). A summary of the MT/MO development method for each well in the monitoring network is provided in Table 7-1.
The City of Modesto wells (MOD-MWA-2 and MOD-MWC-3) and the USGS wells (FPA-2 and OFPB-2) are part of well clusters with two or four wells at each location. One representative well was chosen for the monitoring network from each location based on a review of the water level data, lithologic logs, and geophysical logs. The wells chosen for the monitoring network are screened in conductive sand or gravel units and have similar water levels to the other well at the same location. Similarly, the three Proposition 68 monitoring wells (MW4S, MW-5S and MW-6S) have two wells at each location and the shallower of the two wells at each location were chosen for the monitoring network. The remaining well(s) at each location are SGMA monitoring wells and are summarized in Table 7-3.
Static depth to water will be measured twice a year in these monitoring network wells to represent seasonal high and seasonal low groundwater conditions. The wells will be monitored by one of the STRGBA GSA member agencies.
As summarized on Table 7-3, there are SGMA monitoring wells in the Eastern Principal Aquifer that will be monitored on a semi-annual basis. Future water level data from these wells will be evaluated, and some of these wells may be added to the monitoring network during the GSP five-year update.
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7.1.2. Reduction of Groundwater in Storage
As described in Section 6.4, the sustainable management criteria for chronic lowering of groundwater levels will be used as a proxy for the reduction of groundwater in storage indicator. Accordingly, the monitoring network for the reduction of groundwater in storage is the same as the monitoring network for the chronic lowering of groundwater levels. This monitoring network is described above in Section 7.1.1, summarized in Table 7-1, and illustrated on Figures 7-1, 7-2, and 7-3.
Static groundwater elevations will be measured twice a year in these monitoring network wells to represent seasonal high and low groundwater conditions.
In addition to the required reporting of groundwater levels over time, regulations also require that the GSP annual reports provide an annual estimation of the change in groundwater in storage (§354.34(c)(2)). As described in Chapters 5 and 6, the historical reduction of groundwater in storage is estimated at about 43,000 AFY. As discussed in Section 6.4, both the change in groundwater in storage and corresponding water levels in the Subbasin will be documented annually in the GSP annual reports. Collectively, these data will allow the connection between the reduction of groundwater in storage to groundwater elevations to be documented on an annual basis, providing further justification for the use of a groundwater elevation proxy for this indicator.
7.1.3. Seawater Intrusion
As described in Section 6.5, the STRGBA GSA found that seawater intrusion is not an applicable sustainability indicator for the Modesto Subbasin. Specifically, the STRGBA GSA determined that seawater intrusion is not present in the Modesto Subbasin and is not likely to occur in the future. Therefore, neither sustainable management criteria nor a monitoring network has been established for this sustainability indicator (§354.34(j)).
7.1.4. Degraded Water Quality
As summarized in Section 6.6.1.3, undesirable results for degraded water quality are defined as significant and unreasonable adverse impacts to groundwater quality caused by GSA projects, management actions, or other management of groundwater such that beneficial uses are affected and well owners experience an increase in operational costs.