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Weston Solutions, Inc. Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California 4 NATURE AND EXTENT OF RESIDUAL CONTAMINATION This section briefly summarizes the nature and extent of soil, soil gas, and groundwater contamination from the recently published FS (WESTON, 2020a) and the Semiannual Groundwater Sampling Report for 4th Quarter 2020 (WESTON, 2021). Additional details, including complete historical analytical results and concentration trends of VOCs in groundwater are presented in the FS (WESTON, 2020a). 4.1 SOIL AND SOIL GAS Previous soil sample locations identifying the lateral extent of soil contamination and excavation limits are shown on Figure 2-1. Recent (2019) soil sampling results are shown on Figure 2-3. Based on data collected during the investigations,low concentrations of petroleum hydrocarbons were detected in the Site soil including BTEX and PAHs including naphthalene. The historical detections of analytes in soil are generally below the depth of 10 feet (depth threshold used in Low-Threat Underground Storage Tank Case Closure Policy [LTCP]) for evaluating risk to impacted soil through direct contact and outdoor air exposure pathways) (Figure 2-1), and most of these detections are currently below the water table. Soil samples were collected from boring BH-14 to a depth of 110 feet bgs and contained detectable concentrations of TPH-g to a maximum depth of 55 feet bgs. A comparison of petroleum hydrocarbon concentrations in deeper soil at BH-14 (drilled in 2008) and BH-13 (drilled in 1996), indicated a significant reduction of gasoline hydrocarbons over the 12-year period between investigations. TPH-g and benzene at concentrations of 60 mg/kg and 900 gg/kg, respectively, were previously found in samples from approximately 50 feet in boring BH-13. Soil samples from the same interval in boring BH-14 contained TPH-g and benzene concentrations of 35 mg/kg and less than 50 gg/kg, respectively. These results indicate that natural attenuation of the petroleum hydrocarbons is occurring at the Site. Most samples collected during the 2019 soil investigations were from depths of less than 10 feet bgs.Figure 2-3 shows detected compounds from the 2019 investigations.The only analyte reported with concentrations above soil Environmental Screening Levels (ESLs) was arsenic, with concentrations between 2.5 mg/kg and 11 mg/kg. These concentrations were within 4-1 February 2021