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Geosyntec �UI IJUIt31its 2. BACKGROUND The Site is a 30-acre retail shopping center in Stockton, California where several dry-cleaners conducted business and disposed wastewater containing tetrachloroethylene (PCE) into sanitary sewer lines beginning in the late 1950's(Figure 2).These historical waste disposal practices caused PCE and its degradation products to impact soil and groundwater beneath and downgradient of the Site, including the northern parking lot which is located in the vicinity of one of the former source areas. The Site soil vapor clean-up goal of 603 4g/m3 (commercial CHHSL for PCE) was achieved through historic SVE activities in the northern parking lot including several rounds of SVE optimization and confirmation testing (Geosyntec, 2013). The SVE system was operated in this area for over 10 years. The CVRWQCB approved closure of the on-Site soil source areas and decommissioning of the SVE system in 2013 (Cal/EPA RWQCB,2014). Between 1998 and 2016, historical groundwater remediation activities have largely remediated groundwater impacts under the northern parking lot with only low-level residual concentrations remaining as suggested by recent concentrations at MW-007A and MW-009A (Geosyntec, 2021a). As described in the Vapor Intrusion Assessment Report (Geosyntec, 2021 a), elevated PCE concentrations of up to 22,000 4g/m3 were reported in shallow soil vapor at SG-100, a permanent soil vapor sampling probe located within the previously remediated northern parking lot. Additional investigation conducted in early 2021 utilizing temporary soil vapor probes showed that the lateral and vertical extent of the PCE impacts was generally localized to the vicinity of SG-100(Geosyntec,2021 b;Figure 2).PCE concentrations were generally found in two main depth intervals, from 5 to 10 feet below ground surface (bgs) localized around SG-100, and from 15 to 20 feet bgs immediately to the north of SG-100, suggesting residual PCE mass sorbed onto low permeability soils volatilizing into previously remediated higher permeability intervals. Notwithstanding the elevated PCE soil vapor concentrations near SG-100, PCE was not reported in indoor air samples in nearby buildings demonstrating that Site-specific attenuation factors of approximately 0.001 to 0.0007, developed from data collection, are significantly lower than the default factor of 0.03 being suggested in pending guidance (Geosyntec, 2021b; CVRWQCB, 2021). Vapor intrusion as a result of the northern parking lot impacts is not a concern. The residual groundwater impacts at MW-007A and MW-009A as described previously,are likely a result of residual PCE mass sorbed onto the historic capillary fringe immediately overlying the groundwater table (Geosyntec, 2021 b). Northern Parking Lot Remedial Plan 2 August 6,2021