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2 Background This section provides background information regarding Site description, environmental setting, history, and previous environmental investigations. 2.1 Site Description The Site encompasses 8.3 acres with approximately 86,160 square feet of single-story commercial/retail space. The Site is located at the southwest corner of the intersection of North Tracy Boulevard and Clover Road within a mixed commercial and residential area of San Joaquin County. The shopping center is currently occupied by retail commercial establishments including a supermarket, restaurants, nail and hair salons, insurance and financial outlets, an automobile parts store, and various professional services. Quality Cleaners, located at 3081 North Tracy Boulevard began operating at the shopping center in 1992 (Figure 2). 2.2 Site History Quality Cleaners reportedly used PCE in a closed loop system as part of the dry-cleaning process until 2007 when they switched to a hydrocarbon-based solvent. A subsurface investigation initiated in 2014 identified CVOCs in soil gas at concentrations exceeding applicable San Francisco Regional Water Quality Control Board Commercial Environmental Screening Levels (ESLs) thereby potentially indicating a release had occurred. Subsequent subsurface investigations, completed by Partner, confirmed impacts to soil, soil vapor and groundwater beneath and around the drycleaner. Since 2014, multiple investigations have been completed at the Site, key takeaways from these investigations are provided below. Historical data summary tables for soil gas, soil vapor extraction (SVE) wells, and groundwater monitoring are included in Tables 1 through 3. The chemicals of potential concern (COPCs) have been identified as chlorinated solvents, primarily PCE. Partner installed five groundwater monitoring wells at the Site (MW-1 through MW-5). Groundwater depth has been measured at between 8.51-feet below grade surface(bgs)and 9.42-feet bgs with groundwater flow direction calculated to be to the north/northwest. The screened interval for all five monitoring wells extended from 8 to 20 ft bgs. PCE concentrations exceeded the California Maximum Contaminant Level (MCL) level of 5.0 micrograms per litre (µg/L) only in monitoring Well MW-1 which is located behind (south) of the drycleaner. DTSC has requested additional groundwater samples be collected to the north and northwest of MW-1 to further evaluate the lateral and vertical delineation of the groundwater plume. The proposed scope of work to collect the additional groundwater samples is provided in Section 3. PCE in soil vapor has been detected at concentrations up to 72,000-micrograms per cubic meter (µg/m3) with the highest concentrations located behind (south) the drycleaner. Elevated PCE concentrations (up to 2,000 µg/m3) in soil vapor was also observed along the western boundary of the Site. Partner operated a SVE system at the Site to address the elevated PCE concentrations in soil vapor. It is Roux's understanding that the SVE system was shut down and that soil vapor has not been sampled following shutdown. Between November 14, 2018, and May 29, 2019, soil vapor testing of Wells SVE-1 through SVE-9 showed a reduction in VOC concentrations (Table 2). The results suggest that operation of the SVE system reduced concentrations of COPCs in soil vapor in the area of the former dry cleaner site towards the southern property boundary. 3989.0001S10VR Revised Data Gap Investigation Work Plan I ROUX 13