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1 <br />' containing detectable concentrations of MTBE were further analyzed <br /> using EPA Method 8260 <br /> • Monitoring well MW-7 was surged and re-developed as per <br /> PHS-EHD requirements detailed in thew letter dated <br />' November 19, 1998 The well was purged until the parameters of pH <br /> and conductivity had stabilized and effluent was clear Groundwater <br /> from the well was extracted and pumped into an on-site Baker tank <br /> • The findings of this investigation were incorporated into this <br /> Additional Assessment and Remedial System Design Report <br /> 2.0 BACKGROUND <br />' 21 Facility Description <br />' Thnfty Service Station#172 is an operating retail gasoline station/food store operated by TOSCO <br /> and located at the southwest corner of the intersection of Pacific Avenue and Rivara Road in <br /> Stockton, California(Figure 1) Site improvements include one (1)building, three (3) underground <br /> storage tanks (USTs) and three (3) dispenser islands The USTs were upgraded in July 1995 to a <br /> dual-containment system The Site will continue to be operated by TOSCO as a retail gasoline <br /> station/food store A Site plot plan is presented as Figure 2 The Site is concrete surfaced with a <br /> station building located along the southwestern portion of the Site <br />' 2.2 Summary of Previous Assessments <br /> From May 23 through June 6, 1991, SSB Environmental Consultants, Incorporated (SSB) installed <br /> four(4) groundwater monitoring wells (MW-1, MW-2, MW-3, and MW-4) at locations indicated on <br />' Figure 2 Two (2) monitoring wells were located adjacent to the USTs and two (2) monitoring wells <br /> were installed near the pump island The monitoring wells were installed to depths of 76 to 80 feet, <br /> and groundwater was encountered at 58 feet below ground surface (bgs) Laboratory analysis of soil <br /> samples collected from the well MW-2 at a depth of 20 feet indicated the highest concentration of <br /> TPH and benzene at 380 milligrams per kilogram (mg/kg) and 33 mg/kg, respectively Laboratory <br /> analysis of groundwater samples collected ranged from 36 4 micrograms per liter (µg/1) to 5,270 µg/1 <br /> benzene in wells MW-4 and MW-2, respectively <br />' On April 22, 1994, Thrifty Oil Company installed three (3) vadose zone wells (VWA, VWB, and <br /> VWC) to depths of 35 feet, 48 feet, and 45 feet bgs Thrifty also installed two (2) groundwater <br /> monitoring wells MW-7 and MW-8 to 56 feet and 58 feet, respectively(Figure 2) Groundwater was <br />' encountered at approximately 50 feet bgs Laboratory analysis of soil samples detected TPH <br /> concentrations from ND to 21,000 mg/kg, and detected benzene concentrations from ND to <br /> 1 <br /> A.11528Ji ASA 3 <br />