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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 January 1998, a Corrective Action Plan(CAP)was submitted to San Joaquin County Public Health Environmental Health Division (EHD). The purpose of the CAP was to provide an assessment of the impacts and, through an evaluation of potential alternatives, select a proposed action that would adequately protect human health, safety, and the environment, and that would restore or protect current or potential beneficial uses of water. Intrinsic remediation with a monitoring alternative was determined to be the most cost effective and least disruptive alternative available for the site. Human health receptors were not identified, and therefore, site-specific risk-based cleanup levels were not developed. In 2001, a Human Health Risk Assessment was conducted to evaluate potential exposure risk from benzene in groundwater to workers inside the HSA building. A risk-based analysis was conducted for the indoor air pathway using site-specific information and the Johnson and Ettinger Model for subsurface vapor intrusion into buildings. Using the 90th percentile concentration for benzene in groundwater (12,900 micrograms per liter [µg/L]), the incremental cancer risk was calculated at 8.5 x 10-7 (i.e., below acceptable risk levels). In 2006, the CAP was revised and resubmitted to EHD. The revised CAP recommended that remaining data gaps be addressed prior to considering the Site for a low-risk closure. The identified data gaps included an uncertainty in the groundwater flow direction and incomplete vertical and lateral extent of contamination. In 2007, the groundwater wells were re-surveyed, and groundwater flow direction was calculated to flow toward the east to northeast. This was consistent with historically observed groundwater flow directions. In June 2008, an investigation was conducted to assess the vertical extent of soil and groundwater contamination at the site in response to the findings of the revised CAP. Soil boring was BH-14 was drilled near well MW-2A and boring BH-13. Six soil samples were collected from boring BH-14 from 50, 55, 70, 85, 105, and 110 feet bgs—below the depth of those collected previously from BH-13. Two samples contained TPH-gasoline (TPH-g) concentrations of 35 mg/kg and 130 mg/kg from sampling depths of 50 feet and 55 feet, respectively. Benzene was detected in only one soil sample collected from 85 feet bgs at a concentration of 5.7 micrograms per kilogram (µg/kg). A groundwater "grab" sample was 2-3 February 2021