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Weston Solutions, Inc. Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California PREVIOUS INVESTIGATIONS monitoring events. The increasing contamination trend was potentially due to the dissolution of residual contamination in the vadose zone from the rising groundwater table. Based on the 2020 groundwater monitoring results of MW-2, a decreasing trend of benzene concentrations are observed corresponding to stabilized groundwater table elevation. Similar groundwater conditions are found in wells MW-2A and MW-7, although a notable increase of benzene in MW-7 occurred in September 2020 shortly after the removal of an obstruction (plastic bags, bottles, food) and redevelopment of the well. The well repair activities for MW-7 are documented in the August 2020 Semiannual Groundwater Sampling Report (WESTON, 2020d). Groundwater samples collected from MW-6 have only contained detectable concentrations of TPH-d slightly above laboratory reporting limits; TPH-g,BTEX, and MTBE were not detected. At the request of the Central Valley Water Board, the July 2020 monitoring event included additional sampling of wells MW-3A and MW-9. MW-3A was also sampled as part of December 2020 sampling event. TPH-d, TPH-g, BTEX, and MTBE were not detected at concentrations above laboratory reporting limits in the samples collected from MW-3A and MW-9 during July and December 2020 monitoring events. TPH-g and benzene concentrations from the 4th Quarter 2020 sampling event are shown on Figure 2-2 (WESTON, 2021). In September 2020, a FS (WESTON, 2020a) was prepared and submitted to Central Valley Water Board to identify and evaluate remedial alternatives for residual petroleum hydrocarbon contamination in groundwater. This FS recommended Alternative 1 (Source Area In-Situ Bioremediation and Monitoring) for remediation of the residual petroleum hydrocarbons in groundwater. In a letter dated 2 December 2020, the Central Valley Board concurred with the selected remedial alternative and instructed the San Joaquin County Capital Projects Division to submit a Remedial Action Plan, 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. 2-8 February 2021