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Soil Vapor Sampling Work Plan Page 2 LSP Area,Lodi,California October 13,2021 Groundwater monitoring data collected from the LSP monitoring wells have indicated the presence of PCE in several LSP monitoring wells with increasing PCE concentrations observed in LSP well OS-2, which is located in the parking lot of the Site (Figure 2). The Water Board requested that the City perform PSG sampling to evaluate whether a potential unknown source may be contributing to the increasing PCE concentrations in monitoring well OS-2. CHT, on behalf of the City and as per the request of the Water Board, conducted passive soil gas(PSG)sampling in July and December 2020 and confirmatory PSG sampling in February 2021 beneath the cul-de-sac to define the lateral extent of PCE/TCE in the soil beneath the cul-de-sac and to identify the location of a potential unknown source/s of PCE/TCE that may be contributing to the increasing PCE concentrations in LSP groundwater monitoring well OS-2. The results of the PSG sampling indicated the presence of PCE and TCE in soil vapor beneath the cul-de-sac at concentrations ranging from 12.6 to 41,900 µg/m3 (Figure 3) and <1.31 (reporting limit)to 2,540µg/m3 (Figure 4),respectively. Additionally,the highest concentration of PCE and TCE in soil vapor were observed in an area south of the City's industrial pump station. The soil vapor data also indicates that the portions of the cul-de-sac have PCE and TCE concentrations in soil vapor that are greater than the San Francisco Bay Region Environmental Screening Levels (ESLs) of 67 gg/m3 for PCE and 100 gg/m3 for TCE for commercial/industrial scenarios. The Water Board in their July 21, 2021 Review of Additional Passive Soil Gas Sampling for the LSP concluded that source of the high concentrations of PCE and TCE in soil vapor may be from spills or leaks at the City's industrial waste pump station and historical manufacturing operations in that area, and the soil contamination in deeper soil may be acting as a significant source to groundwater. The Water Board also stated that "the LSP site is unlikely to be eligible for closure if remediation of soil vapors that are significantly greater that the ESLs is not conducted", and requested the City to prepare a Work Plan for performing additional investigation and remediation of the PCE and TCE in the soil vapors that are above the ESLs in the vicinity of the existing industrial waste pump station in the cul-de-sac. APPROACH As per the Water Board's request, CHT, on behalf of the City, recommends the following approach for additional investigation and/or remediation of the PCE and TCE in the soil vapors that are above the ESLs in the vicinity of the industrial waste pump station in the cul-de-sac— • Soil Vapor Sampling—CHT will perform discrete depth soil vapor sampling to delineate the vertical extent of the PCE and TCE in soil vapor at the two locations south of the City industrial pump station where the highest PCE and TCE concentrations were observed during the LSP PSG sampling events; • Installation of the SVE wells for the Pilot Test — Following completion of soil vapor sampling, CHT will design and install SVE wells for performing a SVE pilot test. 175 N EL MONTE AVENUE,LOS ALTOS,CA 94022 TEL:415-424-3009