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Weston Solutions, Inc. Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California NATURE AND EXTENT OF RESIDUAL CONTAMINATION regional background levels for arsenic (upper estimate at 11 mg/kg). Although there were a few additional metals, PAH, and VOC analytes reported above detection limits, none of the concentrations exceeded ESLs. In December 2019 and July 2020, soil gas sampling was conducted at the Site. Analytical results of the soil gas sampling event did not indicate significant environmental impact within the shallow soil gas sampled from soil gas wells SV-I and SV-2. There were no TPH or VOCs detected at concentrations exceeding ESLs for commercial/industrial use in soil gas. Soil gas sampling results are shown on Figure 2-3. 4.2 GROUNDWATER 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 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 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. A summary of the analytical data for the 4th quarter 2020 groundwater sampling events is provided in Table 4-1. Groundwater samples collected from wells MW-3A and MW-9 did not contain detectable concentrations of petroleum hydrocarbons. The estimated extent of groundwater contamination and recent sampling results are shown on Figure 2-2. Shallow groundwater beneath the Site was historically impacted with high concentrations of TPH-g and BTEX. Presently, only benzene exceeds the California Maximum Contaminant Level for drinking water of 1.0 gg/L. Benzene concentrations in groundwater samples 4-2 February 2021