From: Jeff Edwards
Sent: Tuesday, March 1, 2022 7:24 AM
To: City Council
Cc: Whitney McDonald ; Matthew Bronson ;
Jeremy Ghent ; Will Clemens ; Carey Casciola
Subject: Regular City Council-3/1/22-Item 12.C.-Central Coast Blue (CCB)
External Email
Mayor and City Council,
Please find attached (three) recent letters from the Central Coast Water Board in connection with CCB. The substance of the agenda item relates to the application for a CWSRF loan from the State of California. Unfortunately, your staff report for 12.C. makes reference to the Water Board only by stating "The City received feedback from the Water Board on the draft resolution,...". It appears staff has failed to include the letters because they are not complimentary of the project. In fact, the letters highlight looming questions about the efficacy of the CCB injection regime.
I will be further expanding on the content of the letters at your City Council meeting tonight. Please let me know if you have any questions in the meantime.
Thanks,
Jeff
Julie Tacker
Administrative Assistant
J.H. Edwards Company
P.O. Box 6070
Los Osos, CA 93412
805.235.0873 - Jeff
805.235.8262 - Julie
805.528.3569 - Office
Central Coast Regional Water Quality Control Board
October 28, 2021
Sent Via Electronic Mail
Ben Fine
Director of Public Works
City of Pismo Beach
760 Mattie Road
Pismo Beach, CA 93449
Email: [email protected]
Dear Mr. Fine:
CENTRAL COAST WATER BOARD COMMENTS ON THE ANTIDEGRADATION TECHNICAL MEMORANDUM SUBMITTED TO THE PROPOSITION 1 PLANNING
GRANT TECHNICAL ADVISORY COMMITTEE FOR CENTRAL COAST BLUE
Central Coast Regional Water Quality Control Board (Central Coast Water Board) has staff reviewed the antidegradation technical memorandum (TM) submitted to the Proposition 1 Planning Grant Technical Advisory Committee (TAC) on September 17, 2021, and submits this letter to provide TAC member comments. The reports were submitted by Water Systems Consulting on behalf of the City of Pismo Beach to fulfill requirements of the Proposition 1 Planning Grant awarded by the State Water Resources Control Board to the City of Pismo Beach. The antidegradation TM provides information on water quality impacts expected from the Central Coast Blue project in support of the State Water Resources Control Board Resolution No. 68-16 “Statement of Policy with Respect to Maintaining High Quality of Waters in California,” also known as the antidegradation policy, which requires that the Central Coast Water Board consider whether water quality degradation caused by the project is permissible when balanced against the benefit to the public.
The antidegradation TM submitted by the City of Pismo Beach evaluates ambient groundwater quality and projected future water quality under Central Coast Blue project operations. The TM finds that for many chemical constituents, injection of advanced purified recycled water associated with Central Coast Blue will result in improved water quality, with the exception of boron and nitrate. Both boron and nitrate are expected to increase ambient groundwater concentrations only marginally and the TM concludes that beneficial uses will still be protected even under project operations. The Central Coast Water Board thanks the City of Pismo Beach for preparing and submitting this
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antidegradation assessment and finds that the approach and conclusions are reasonable. However, the Central Coast Water Board requests that additional information be provided in support of the antidegradation TM. Additional requested information is as follows:
• The antidegradation policy requires that the baseline quality of receiving water determine the level of water quality protection. Baseline quality is defined as the best quality receiving water that has existed since 1968. Section 3 of the antidegradation TM states that baseline water quality will be compared with water quality objectives. However, it doesn’t appear that baseline water quality was ever evaluated as part of the TM and rather the TM relies on a comparison of ambient groundwater quality, compiled from data collected between 2009-2020, to water quality objectives. We recommend that the TM be revised such that baseline water quality, as defined above, also be evaluated and compared to water quality objectives and to simulated concentrations under project conditions.
• If a discharge will degrade high quality water, the antidegradation policy requires demonstration that the change in water quality will be consistent with the maximum benefit to the people of this state. The antidegradation TM states that the discharge is consistent with the maximum benefit to the people of the state but doesn’t provide any justification to support this assertion. We recommend providing this justification.
• Staff was confused by the explanation provided in the TM to describe which data sets were used for which parts of the TM and the use of different data sets for different parts of the analysis appears to be inconsistent. As an example, it appears that the Northern Cities Management Area (NCMA) wells were used to determine ambient conditions for TDS, chloride, sodium, and sulfate. However, determination of ambient conditions for boron and nitrate utilized NCMA wells plus a subset of wells included in State Water Resources Control Board Groundwater Ambient Monitoring and Assessment Program (GAMA) (no agricultural wells). However, inputs to the groundwater model used only GAMA data (including agricultural wells) but no NCMA (or Nipomo Mesa Management Area [NMMA]) well data.
Please provide better clarity regarding which data sets were used for which parts of the antidegradation analysis and better justification for using different data sets for different parts of the analysis. If possible, use the data sets consistently across all parts of the analysis. We recommend including an additional table or tables that explains the data sets and/or wells that were used in the different parts of the antidegradation analysis.
• Inputs to the groundwater flow model included in Tables 9 and 10 utilized boron and nitrate data from GAMA but not from NCMA or NMMA. It is unclear why the NCMA or NMMA data was not utilized since it is available in areas that correspond to the groundwater model input locations. In addition, as explained in
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the previous comment, it seems inconsistent to utilize NCMA data for the evaluation of ambient conditions but not in simulations of groundwater quality flows within the numerical model.
• Only data from the NCMA was used to evaluate ambient groundwater quality for TDS, chloride, sodium, and sulfate. The TM did not use data from the GAMA groundwater database because the NCMA “have the most consistent timeframe and Project partners have confidence in the reliability of the data.” However, much of the data included in GAMA comes from municipal water purveyors and data associated with these purveyors and included in GAMA is collected at a regular frequency and is likely reliable and of high quality. In addition, data submitted to the Central Coast Water Board’s Irrigated Lands Program and included in GAMA was analyzed by laboratories certified by the State Water Board’s Environmental Laboratory Accreditation Program (ELAP) and should also be reliable.
We recommend that the TM be revised such that the ambient concentration determinations include TDS, chloride, sodium, and sulfate data from the GAMA database or that the TM include an expanded justification for not using the GAMA data.
• It is unclear if agricultural wells outside the urbanized area and included in the map in Figure 5 were included in the expanded groundwater quality assessment used to evaluate boron and nitrate ambient groundwater quality.
Ambient groundwater quality for boron and nitrate utilized data from the NCMA and the State Water Board’s GAMA online groundwater database, though it appears that only wells within the urbanized areas of Arroyo Grande, Oceano, and Grover Beach were used. However, a map included in the TM of nitrate and boron concentrations shows wells within the NCMA but outside the urbanized area that were not included in the evaluation of ambient groundwater quality.
These additional wells are within the same zone of the groundwater model (Zone 2) as the project wells and within the NCMA boundary and should be included in the determination of ambient groundwater quality. Furthermore, these wells appear to have higher nitrate concentrations relative to the wells in the urbanized area and it is important that this degraded water quality be included in the assessment of ambient groundwater quality for the area.
• Page 12 of the TM, under the section Expanded Groundwater Quality Assessment, second paragraph, fourth sentence, says that if Grover Beach and Arroyo Grande use the wells with higher nitrate concentration, nitrate reduction methods are implemented. It is unclear if this means that the concentrations used in the antidegradation TM are the concentrations measured after nitrogen reduction. If so, please explicitly state this in the TM. Also, if the concentrations used in the TM are after nitrogen reduction has been implemented, the authors should attempt to determine what the native groundwater concentrations (i.e.,
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before nitrogen reduction) are. The concentrations reported after nitrogen reduction don’t represent ambient groundwater quality.
• Results of the antidegradation TM rely on denitrification being implemented at the Pismo Beach Wastewater Treatment Plant to reduce nitrate concentrations from 23 milligrams per liter (mg/L) nitrate as N to 4.6 mg/L. However, the antidegradation TM doesn’t include any information regarding the feasibility of denitrification or the likelihood that this change in treatment process could actually occur. The report only says that the City of Pismo Beach “intends to test denitrification to lower total nitrogen/nitrate.” We recommend including some additional details that help the reader understand the likelihood that denitrification can be implemented.
Additionally, it may be worthwhile to evaluate scenarios in which denitrification is not able to be implemented at the Pismo Beach WWTP. Lastly, the antidegradation policy requires that the best practicable treatment and control (BPTC) be implemented for any activity that discharges to high-quality waters.
We recommend that the antidegradation TM consider other BPTC that could be implemented to reduce nitrate concentrations in the event that denitrification is not able to be implemented at the Pismo Beach WWTP.
• During construction of test injection well IW-4, it was discovered that the upper target injection zone was not a viable target for injection due to poorer than expected transmissivity. It is possible that transmissivities may be poor in the upper zone for the remaining four proposed injection wells. We recommend that the antidegradation TM consider the possibility that only the lower zone is used as an injection target.
The Central Coast Water Board appreciates the opportunity to participate on the Central Coast Blue Technical Advisory Committee and to provide comments on various technical documents. Comments on the antidegradation TM are provided by Central Coast Water Board staff engineering geologist James Bishop. If you have questions or would like to discuss these comments in greater detail, please contact James Bishop at (805) 542-4628 or by email at [email protected] or Jennifer Epp at [email protected].
Sincerely,
for Matthew T. Keeling
Executive Officer
City of Pismo Beach – Central Coast Blue October 28, 2021 - 5 -
Cc:
Rob Morrow, Water Systems Consulting, [email protected] Dan Heimel, Water Systems Consulting, [email protected] Michael Cruikshank, Water Systems Consulting, [email protected] Kira Smith, Division of Financial Assistance, [email protected] Alex Huang, Division of Financial Assistance, [email protected] Saeedreza Hafeznezami, Division of Drinking Water,
[email protected]
Jeff Densmore, Division of Drinking Water, [email protected] Leah Lemoine, [email protected]
Monique Gaido, [email protected]
Jennifer Epp, [email protected]
Harvey Packard, [email protected]
WDR Program, [email protected]
Central Coast Regional Water Quality Control Board
October 28, 2021
Sent Via Electronic Mail
Ben Fine
Project Director
City of Pismo Beach
760 Mattie Road
Pismo Beach, CA 93449
Email: [email protected]
Dear Mr. Fine:
CENTRAL COAST WATER BOARD COMMENTS ON THE AERIAL GEOPHYSICS
AND GROUNDWATER MODELING UPDATE TECHNICAL MEMORANDUMS
SUBMITTED TO THE PROPOSITION 1 PLANNING GRANT TECHNICAL ADVISORY
COMMITTEE FOR CENTRAL COAST BLUE
Central Coast Regional Water Quality Control Board (Central Coast Water Board) staff have reviewed the aerial geophysics and groundwater model update technical memorandums (TM) submitted to the Proposition 1 Planning Grant Technical Advisory Committee (TAC) on July 27, 2021, and submit this letter to provide TAC member comments. The reports were submitted by Water Systems Consulting on behalf of the City of Pismo Beach to fulfill requirements of the Proposition 1 Planning Grant awarded by the State Water Resources Control Board to the City of Pismo Beach.
The aerial geophysics TM describes the results of aerial electromagnetic surveying conducted in the northern part of the Santa Maria groundwater basin. Interpretations of seawater intrusion and the seawater-freshwater interface acquired from the aerial geophysical surveys were incorporated into the updated groundwater model and results of this updated modeling were described in the updated groundwater modeling TM. The updated groundwater modeling TM evaluates simulated groundwater elevations and seawater intrusion that would occur with and without the Central Coast Blue indirect potable reuse project. Specific comments on these reports are included below.
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Groundwater Model Update Technical Memorandum
Alternative seawater intrusion prevention scenarios
In general, results of the updated groundwater modeling show that seawater intrusion is simulated to impact water quality in one water supply well located near the community of Oceano (PB Well 23). None of the other wells in the model domain are simulated to be impacted by seawater intrusion under the “no project” scenario. According to the groundwater model simulations, the Central Coast Blue seawater intrusion prevention project will be successful in mitigating seawater intrusion at PB Well 23 in both model layers 6 and 8. However, another option for minimizing or mitigating seawater intrusion at the subject well might be to shift pumping that occurs at this well elsewhere, either by increasing pumping at existing inland wells or developing one or more new extraction wells further inland. Shifting pumping away from the coast may be a more cost-effective and less resource-intensive strategy for mitigating seawater intrusion at the single supply well that is simulated to experience seawater intrusion. We recommend that future investigations evaluate seawater intrusion mitigation by moving pumping away from the coast.
In follow-up discussions with Dan Heimel and Michael Cruikshank from Water Systems Consulting, Mr. Heimel and Mr. Cruickshank explained that previous groundwater modeling reports developed for Central Coast Blue, but not submitted to the TAC, demonstrated that the injection of 900 acre-feet per year of advanced purified recycled water created a hydraulic dam which allowed for approximately 1500 acre-feet per year of extraction to safely occur. In essence, the project resulted in a water supply volume benefit in excess of the volume of advanced purified recycled water produced and injected into the aquifer. We recommend that this additional information be explicitly included in the groundwater model update submitted to the TAC. For the purposes of comparing various scenarios, it would be useful if report authors compared the volume of safely extractable water under no-project scenarios versus project scenarios.
Revised simulations for model layers 6 and 8 based on test well 4 construction The existing groundwater model update includes simulations for injection into model layers 6 and 8. However, the development of test injection well 4 has revealed that the hydrogeologic horizon that corresponds to model layer 6 is not a viable target injection zone, at least at test well 4. This finding has significant impacts to the results of simulations included in the groundwater model update TM because the TM demonstrates that the seawater intrusion prevention project will be adequate in preventing seawater intrusion to PB Well 23 for both model layers 6 and 8. However, if model layer 6 isn’t a viable injection target, then seawater may only effectively be mitigated for the portion of PB Well 23’s well screen that corresponds to model layer 8; seawater intrusion may still occur in the portion of the PB Well 23 well screen that corresponds to model layer 6, even if the Central Coast Blue seawater intrusion injection barrier is implemented. We recommend that the updated groundwater model simulate scenarios in which injection only occurs into model layer 8 to reflect actual
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hydrogeologic conditions identified in test well 4.In addition, it is possible that model layer 6 for won’t be a viable target injection zone for any of the five proposed injection wells. As such, we also recommend that model scenarios be evaluated in which only model layer 8 is utilized as a target injection zone for all five proposed injection wells.
Simulations with variable salt-fresh water interface locations The aerial geophysics TM identifies 6 ohm-m as the resistivity value that corresponds to the salt-fresh water interface. The location of this resistivity value is utilized in the groundwater model as the starting location for seawater intrusion simulations. The geophysical TM states that determination of 6 ohm-m as the salt-fresh water interface was based on visual inspection of resistivity cross-sections. In areas where the interpreted salt-fresh water interface is sharp, slight variations in interpretation of the resistivity that corresponds to the saltwater interface may not make much of a difference with respect to the location of the saltwater interface. However, in areas where the interface may be diffuse, small variations in the interpretation of the resistivity value that corresponds to the saltwater interface can have a dramatic impact on the location of the interface relative to the coast. The determination of the location of this interface can in turn have a substantial impact on simulations of seawater intrusion conducted using the updated numerical groundwater model. We recommend that the interpretation of the location of the salt-fresh water interface be reviewed and if necessary, additional numerical groundwater simulations of seawater intrusion be conducted that utilize alternative plausible starting locations for the salt-fresh water interface.
Shallow low-resistivity area near Oceano Airport
The aerial geophysics TM indicates that there is an area of likely shallow subsurface seawater intrusion near the Oceano Airport. This low-resistivity area shown in the aerial geophysics TM extends as far east as Highway 1 and is located approximately 0.25-0.5 miles from PB Well 23. If this area of shallow seawater intrusion continues to migrate landward, it could potentially migrate vertically downward and impact water quality in PB Well 23. Because the proposed seawater intrusion prevention injection barrier will not inject into this shallow zone, the project may be ineffective at preventing this shallow seawater intrusion. There could be a situation in which PB Well 23 experiences shallow seawater intrusion, even with the Central Coast Blue project. We recommend that evaluations be conducted to determine the likelihood that this low-resistivity area will continue to migrate landward and vertically downward to impact PB Well 23.
Aerial Geophysics Technical Memorandum
Salt-fresh water interface determination
As described above, the aerial geophysics TM interprets the salt-fresh interface as corresponding to the 6 ohm-m resistivity. The report bases this interpretation on visual inspection and provides the reader with one resistivity cross-section to demonstrate the interpretation. We recommend updating the technical memorandum to provide additional justification for the use of 6 ohm-m as the resistivity value that corresponds to the salt-fresh water interface. If necessary, we also recommend that the report include
City of Pismo Beach – Central Coast Blue October 28, 2021 - 4 -
other plausible resistivity values that could corresponds to the salt-fresh interface and maps that show alternative locations for the salt-fresh water interface.
Shallow low-resistivity area near Oceano Airport
The aerial geophysics TM shows an area of low resistivity near the Oceano Airport that is interpreted to be an area of shallow seawater intrusion. The report does not make any interpretations regarding the mechanism for this seawater intrusion. We recommend that the report be updated to include an interpretation as to the cause of this shallow seawater intrusion based on the available geophysical and hydrogeologic data.
Other minor comments
• Section 6.3 – low resistivities are identified in the northern portion of the survey area, near the City of Pismo Beach, and are interpreted to indicate extensive seawater intrusion. There is only one borehole located within this area. If there is water quality data from this borehole that can corroborate the geophysical interpretation, we recommend including this information in the report. If no water quality data is available from this borehole, we recommend stating this in the report.
• Figure 16 and Figure 18 – the borehole IDs included in the cross-section in Figure 18 don’t appear to match the borehole IDs used in the plan view map for section LL’ in Figure 16. Specifically, Figure 16 refers to well 30N but well 30N isn’t identified in Figure 18. We recommend utilizing consistent well naming in these two figures.
• The agreement between the State Water Resources Control Board and the City of Pismo Beach for the Proposition 1 Planning Grant states that the geophysical investigation shall recommend optimal injection well locations. We didn’t see this recommendation included in the report and suggest that the report be revised to include this.
• We recommend that the lithology of the groundwater model be updated based on the newly collected geophysical data.
• All of the figures say “Groves Beach,” this should be changed to Grover Beach.
• The in-text references to figure numbers don’t appear to correspond to the correct figure starting with Figure 23 and above. We recommend reviewing these in-text references and associated figures and update accordingly.
The Central Coast Water Board appreciates the opportunity to participate on the Central Coast Blue Technical Advisory Committee and to provide comments on various technical documents. Comments on the aerial geophysics and updated groundwater model TM are provided by Central Coast Water Board staff engineering geologist James Bishop. If you have questions or would like to discuss these comments in greater
City of Pismo Beach – Central Coast Blue October 28, 2021 - 5 -
detail, please contact James Bishop at (805) 542-4628 or by email at [email protected] or Jennifer Epp at
[email protected].
Sincerely,
for Matthew T. Keeling
Executive Officer
Cc:
Rob Morrow, Water Systems Consulting, [email protected] Dan Heimel, Water Systems Consulting, [email protected] Michael Cruikshank, Water Systems Consulting, [email protected] Kira Smith, Division of Financial Assistance, [email protected] Alex Huang, Division of Financial Assistance, [email protected] Saeedreza Hafeznezami, Division of Drinking Water,
[email protected]
Jeff Densmore, Division of Drinking Water, [email protected] Leah Lemoine, [email protected]
Monique Gaido, [email protected]
Jennifer Epp, [email protected]
Harvey Packard, [email protected]
WDR Program, [email protected]
Central Coast Regional Water Quality Control Board
October 28, 2021
Sent Via Electronic Mail
Ben Fine
Director of Public Works
City of Pismo Beach
760 Mattie Road
Pismo Beach, CA 93449
Email: [email protected]
Dear Mr. Fine:
CENTRAL COAST WATER BOARD COMMENTS ON THE AQUIFER LEACHING STUDY TECHNICAL MEMORANDUM SUBMITTED TO THE PROPOSITION 1
PLANNING GRANT TECHNICAL ADVISORY COMMITTEE FOR CENTRAL COAST BLUE
Central Coast Regional Water Quality Control Board (Central Coast Water Board) staff has reviewed the aquifer leaching study technical memorandum (TM) submitted to the Proposition 1 Planning Grant Technical Advisory Committee (TAC) on July 27, 2021, and submit this letter to provide TAC member comments. The reports were submitted by Water Systems Consulting on behalf of the City of Pismo Beach to fulfill requirements of the Proposition 1 Planning Grant awarded by the State Water Resources Control Board to the City of Pismo Beach.
The aquifer leaching study TM describes the results of literature reviews and laboratory experiments that evaluate the potential for geochemical reactions that may mobilize problematic constituents as a result of the injection of advanced purified recycled water into Careaga sand as part of the Central Coast Blue groundwater recharge project. The literature review and experimental results described in the TM provide valuable information relevant to evaluating the potential for problematic geochemical reactions.
However, there are still a number of unanswered questions related to the potential for problematic constituent mobilization as a result of the injection of advanced purified recycled water. Central Coast Water Board staff comments and recommended additional evaluations or report modifications are described below:
• The leaching study conducted laboratory column experiments on aquifer materials that correspond to the lower target injection zone, but no leaching
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studies have been conducted on materials collected from the upper target injection zone. For the leaching study, aquifer materials were collected at depths of 250-280 feet below ground surface from the test injection well, IW-4. This depth interval corresponds to the lower of two potential target injection zones identified by project proponents. At IW-4, the upper target injection zone was deemed to be not viable due to poor transmissivity. However, the project intends to inject into the upper target zone at the other four or more injection wells that are planned to be constructed.
Because the leaching study only collected samples from the lower target injection zone, no information is available on the potential for problematic geochemical interactions in the upper injection zone, which may be targeted with the other planned injection wells. This upper target zone likely corresponds to the Paso Robles formation, which is a non-marine gravelly alluvial deposit and may be substantially different, at least from a geochemical interaction perspective, compared to the lower target zone made up of the marine Careaga sand.
It is unclear if leaching study results conducted on materials collected from the lower marine Careaga sand can reasonably be extrapolated to anticipate reactions that may occur in the upper non-marine alluvial Paso Robles gravels.
We recommend that if the other injection wells that are planned for the project will be injecting into the upper target zone that geochemical interaction analyses be conducted using aquifer materials collected from the upper zone.
• The leaching study doesn’t provide any information on the oxidation-reduction (redox) potential of native groundwater. Differences in redox potential between the injected water and the in-situ environment likely has the greatest effect on the potential for problematic geochemical interactions. Because the target injection zones are confined aquifers, it is reasonable to suspect that native groundwater is anaerobic or anoxic. Injected product water will be aerobic, and injection of this aerobic product water into an anaerobic or anoxic environment creates the potential for problematic geochemical interactions.
Information on in-situ redox conditions is relevant to the leaching study because the leaching study occurred under aerobic conditions for all three test columns.
An understanding of native groundwater redox conditions allows for better qualitative understanding of how well the laboratory experiments were able to simulate in-situ conditions. We strongly recommend that sampling occur to assess the oxidation-reduction potential of native groundwater.
• The leaching study does not evaluate the potential for geochemical reactions that could result in plugging of aquifer pore space. This plugging is commonly observed in projects that use injection wells to recharge groundwater, and the degree to which this plugging is problematic can vary from little problems to project-inhibiting. Irreversible transmissivity losses have occurred in fine-grained aquifers where the salinity of injected water was substantially less than that of the
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receiving water. The aquifer material characterization report prepared by Mineralogy Inc. and included as an appendix in the leaching study TM concludes that injection of advanced purified water may cause in-situ corrosion of feldspar grains, and subsequent migration of these corroded grains/clays may result in “brush-piling, potentially contributing to fluctuations in aquifer transmissivity (especially during periods of peak injection).” It doesn’t appear that the leaching study incorporated the conclusions from this aquifer material characterization report, and we recommend that the leaching study report be revised such that it considers the potential problems identified by Mineralogy Inc. In addition to the potential problems identified in the aquifer material characterization report, other commonly identified issues are manganese precipitation, clay swelling, and corrosion of conveyance pipelines resulting in elevated total suspended solids concentrations, all of which can result in reduced transmissivity and injectivity.
We recommend that other potential aquifer pore space plugging mechanisms also be evaluated. The results of aquifer pore space plugging should be used to develop an operational strategy for well backflushing and rehabilitation frequency and/or product water stabilization that may reduce the likelihood for problematic geochemical interactions that inhibit injectivity.
• The report explains that there was no pyrite detected using x-ray diffraction (XRD). Pyrite is a common host mineral for arsenic and its presence is a clue that can indicate a potential for problematic arsenic mobilization. However, XRD has fairly coarse resolution and pyrite concentrations that are non-detectable using XRD can host quantities of arsenic that, if mobilized, can result in exceedances of the maximum contaminant level (MCL) for drinking water. There are clues in the results of the leaching study that indicate that pyrite was present in the experimental soil columns and that the pyrite was oxidized, releasing dissolved arsenic. For example, sulfate concentrations in the outlet waters for both AWPF columns show multifactor increases relative to inlet concentrations.
This may indicate the oxidation of iron sulfides (e.g. pyrite) to sulfate after saturation with AWPF water. These concentration patterns appear to be consistent with those exhibited by arsenic and iron, which further supports the hypothesis that pyrite is present and is being oxidized by AWPF water.
We recommend that the report be revised to explain the coarse resolution of XRD as it relates to pyrite and evaluate the various clues that may be indicators of pyrite oxidation and arsenic mobilization.
• Results of bulk soils chemical analysis indicate that arsenic was present at a concentration of 2.4 mg/kg of dry solids. The report states this concentration was “low abundance.” We recommend that the report compare the arsenic concentrations measured in these soils to concentrations measured in soils at other recharge locations that were described in the literature review, Section 2.3 of the report.
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For comparison, the Orange County Water District (OCWD) observed arsenic mobilization associated with advanced purified recycled water recharge via infiltration basins, and the maximum arsenic soil concentration was 2.9 mg/kg, similar to the concentrations collected as part of the Central Coast Blue leaching study. OCWD had to augment the chemical composition of its product water to mitigate arsenic mobilization. Although the arsenic concentration in soils measured as part of the Central Coast Blue leaching study is low compared to global averages, the concentrations are high enough to result in problematic arsenic mobilization, as has been demonstrated at the OCWD facility.
The Central Coast Water Board appreciates the opportunity to participate on the Central Coast Blue Technical Advisory Committee and to provide comments on various technical documents. Comments on the leaching study TM are provided by Central Coast Water Board staff engineering geologist James Bishop. If you have questions or would like to discuss these comments in greater detail, please contact James Bishop at (805) 542-4628 or by email at [email protected] or Jennifer Epp at [email protected].
Sincerely,
for Matthew T. Keeling
Executive Officer
Cc:
Rob Morrow, Water Systems Consulting, [email protected] Dan Heimel, Water Systems Consulting, [email protected] Michael Cruikshank, Water Systems Consulting, [email protected] Kira Smith, Division of Financial Assistance, [email protected] Alex Huang, Division of Financial Assistance, [email protected] Saeedreza Hafeznezami, Division of Drinking Water,
[email protected]
Jeff Densmore, Division of Drinking Water, [email protected] Leah Lemoine, [email protected]
Monique Gaido, [email protected]
Jennifer Epp, [email protected]
Harvey Packard, [email protected]
WDR Program, [email protected]