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Geosyntec consultants 3. INVESTIGATION OBJECTIVES AND APPROACH The objectives of this investigation are as follows: • Investigate soil vapor concentrations at select on-Site locations; • Evaluate the correlation of select VOCs (1,2,3-TCP, 1,2-DCP, 1,2-dibromo-3- chloropropane [DBCP], and chloroform) concentrations in soil vapor and shallow groundwater; and • Confirm that the 2005 Threshold Concentrations are protective of human health or, if warranted, establish new Threshold Concentrations based on Site-specific data and updated residential soil vapor environmental screening levels (ESLs). Soil vapor samples will be collected from two on-Site locations(Figure 3). On-Site locations were selected for three primary reasons: 1. 1,2-DCP and 1,2,3-TCP Groundwater Concentrations:the locations were chosen based on the 1,2-DCP and 1,2,3-TCP concentrations observed in the most recent groundwater monitoring event on February 18, 2021, and were selected to include locations where groundwater monitoring data indicate the highest concentrations (at monitoring well MW-513)and near-highest concentrations(at monitoring well MW-713)of 1,2-DCP and 1,2,3-TCP (Geosyntec, 2021). Therefore, soil gas probes will be installed approximately 10 to 15 feet to the east of wells MW-513 and MW-713, respectively. By evaluating soil vapor concentrations at on-Site locations with highest or near-highest groundwater concentrations of 1,2-DCP and 1,2,3-TCP, a "worst-case" scenario will be assessed for soil vapor concentrations; 2. Vadose Zone Lithology: based on review and comparison of boring logs from on-Site and off-Site locations, the vadose zone at both areas appears to be consistent in that fine-grained material predominates(Geomatrix,2008;APEX,2018;Geosyntec,2019); and 3. Accessibility: no access constraints are present at the two proposed on-Site locations. and there will be no need for access agreements. At each proposed location, soil vapor probes will be installed in adjacent borings at approximately 5 and 10 ft bgs using direct push technology (DPT). The actual locations and depths may be modified depending on field conditions and access constraints. Soil cores will be collected during drilling and will be visually logged and screened for the presence of VOCs using a hand-held photoionization detector(PID). After the borehole is advanced at each location, soil vapor probes will be constructed in the borehole and,following an equilibration time of two to 48 hours,sampled in accordance with the California Department of Toxic Substances Control's (DTSC) Active Soil Gas Advisory (Advisory; DTSC, 2015). Soil vapor samples will be submitted to a State-of- California certified analytical laboratory under chain-of-custody procedures for analyses of 1,2,3- TCP, 1,2-DCP,DBCP,and chloroform. During soil vapor probe installation,physical soil samples will be collected using a slide hammer. In addition, groundwater samples will be collected from monitoring wells MW-513 and MW-713. Work Plan for Vapor Intrusion Investigation 4 July 30,2021