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Weston Solutions, Inc. Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California PREVIOUS INVESTIGATIONS In June 2018, a Site Assessment Work Plan was prepared to address the Central Valley Water Board concerns detailed in their March 2018 letter. The Work Plan was revised in August 2018 to included additional analyses. In July 2019,two soil gas wells(SV-1 and SV-2)were installed at the Site and five soil borings were drilled. The soil gas wells were sampled in December 2019 and July 2020 and contained maximum concentrations of 2,800 (estimated concentration below the sample reporting limit) micrograms per cubic meter (µg/m3) TPH-g and 0.20 (estimated concentration below the sample reporting limit) µg/m3 total xylenes (WESTON, 2020b and WESTON, 2020c). The soil gas well locations are shown on Figure 1-2 and Figure 2-3. Soil samples from depths of 5 feet and 10 feet bgs were collected from borings CB-I located near the former USTs adjacent to well MW-3A; borings CB-2 and CB-3 near the former used oil UST and three USTs of unknown contents along Washington Street; and CB-4 and CB-5 at the four former USTs of unknown contents along Sutter Street. The results of the soil assessment indicated that there is no environmental impact detected within the shallow soils sampled. There were no detections of TPH, PAH, VOC, SVOC, or PCB analytes at concentrations exceeding either direct exposure human health levels in commercial/industrial shallow soils nor soil screening levels for direct contact or volatilization to outdoor air. Deeper samples(30 feet bgs)were collected from borings CB-2 and CB-3; only the deep samples from boring CB-2 contained detectable concentrations of petroleum hydrocarbons. The soil boring locations are shown on Figure 2-3. In October 2019, a Sensitive Receptor Survey was conducted to identify water wells and water bodies within 1,000 feet of the Site. Seven wells were identified inside the 1,000-foot radius surrounding the site; four were identified as water wells, two were monitoring wells, and one was an air conditioning well (Figure 2-4). A well located downgradient of the site is at a commercial business at 520 E Main Street. This business has drinking water provided by the municipal water service (California Water System). Semi-annual groundwater monitoring of wells MW-2, MW-2A, MW-6, MW-7, and MW-10 has been conducted from 2016 through 2020. Increasing benzene concentrations, coinciding with historically high groundwater elevations, were observed in MW-2 during 2018 and 2019 2-7 Febraary 2021