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Humar, Services .Agency December 2020 145 S. Sutter St. Stockton, San Joaquin County In the Study, Weston concludes that Alternative 1 is the most cost-effective approach, stating that it "is projected to have longer remediation timeframe, but would cause the least disruption to traffic flow and the community", as Alternatives 2 and 3 would require trenching across S. Sutter Street. Additionally, in the Study, Weston identifies four water supply wells within 1,000 feet of the Site. Three of these wells are located northwest or southwest (cross -gradient) of the Site. One of the wells is located approximately 750 feet northeast (downgradient) of the Site, but the associated building is supplied with drinking water by California Water Service Stockton District. Central Valley Water Boards staff have the following comments: 1. Central Valley Water Board staff concur with Weston's conclusion that the risk posed to the nearby water supply wells appears low. 2. Based on data provided by Weston, it appears that the majority of remaining hydrocarbon impact to groundwater beneath the Site is near well MW -2. Weston indicates that the proposed injection depth for Alternative 1 is between 15-45 feet bgs and within a 20 -foot by 20 -foot square around well MW -2. Historical Site soil data indicates that the majority of hydrocarbon impact to soil beneath the Site is within 30 feet of the surface. 3. Central Valley Water Board staff concurs with the recommended approach. By 5 February 2021, please submit a Remedial Action Plan detailing your proposal for implementing remedial Alternative 1, including the following: • Proposed locations of remedial injection points; • Proposed depth of injections; • Proposed number and frequency of injection events; • Proposed total injectant volume and volume per injection point for each injection event, and total cumulative volume of injectant for all events; • Per requirements of Order R5-2015-0012, identification of proposed discrete remedial treatment, transition, and compliance zones, and identification of wells to monitor these zones; o If insufficient wells exist to properly monitor.these zones, then new monitoring wells are to be proposed • Proposed remedial monitoring schedule, and analytical suite to include at least hydrocarbons, arsenic, iron, manganese, chromium, hexavalent chromium, bromide, and bromate; o If BOS-200 is used, the injection of included electron acceptors such as nitrate and sulfate may require additional sampling in accordance with the Central Valley Salinity Alternatives for Long-term Sustainability (CV - SALTS) program • Proposed groundwater sampling to determine baseline concentrations for determination of remedial Action Levels for Order compliance; and