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' 36 mg/kg Laboratory analysis of groundwater samples detected 5,500 µg/1 TPH and 830 µg/1 benzene in MW-7, and 130,000 µg/1 TPH and 7,200 µg/1 benzene in MW-8 On July 11, 1995, the three (3) vadose wells (VWA, VWB, and VWC) and three groundwater monitoring wells(MW-1, MW-2, and MW-8) were properly abandoned prior to station remodeling ' activities On July 25, 1995, five single-walled, steel USTs were removed from the Site and disposed of by ' Erickson Richmond of California Approximately 1,500 cubic yards of soil was removed from the UST excavation and transported to Forward Incorporated, of Stockton, California for disposal Nine (9)soil samples were collected beneath the five (5) USTs after excavation and seven (7) soil samples were collected at a depth of 2 to 4 feet beneath the product lines TPH and benzene was detected in soil samples collected from beneath the USTs at concentrations up to 12,000 mg/kg TPH and 33 mg/kg benzene On September 14 1995 Thrifty Oil Company contracted Alton Geoscience (Alton) to perform a vapor extraction test at the Site to assess the possibility of using this technology for soil remediation ' Results of the test indicated an effective radius of influence of approximately 33 feet with a flow rate pP Y ' of approximately 105 8 SUM and a pressure drawdown of approximately 0 10 inches of water (inches H2O), therefore, vapor extraction was identified as a feasible remediation option for the mitigation of hydrocarbon affected soil in the vadose zone beneath this site On May 28 through 30, 1996 wells VW-2, VW-4, VW-5, VW-6, MW-10/VW-1 and MW-11/VW-3 were installed onsite to depths ranging from 16 5 to 58 5 feet bgs Monitoring wells MW-10/VW-1, and MW-11/VW-3 were installed as nested wells Well VW-2 was completed as a dual-completion vadose zone monitoring well Wells VW-4, VW-5, and VW-6 were completed as triple-completion vadose zone monitoring wells Wells MW-10/VW-1 and MW-11/VW-3 were completed as dual- completion groundwater/vadose zone monitoring wells Groundwater monrtonng and sampling has occurred at the Site since June 6, 1991 Summaries of historical soil and groundwater laboratory analytical results are included as Tables 2 and 3 respectively ' 2.3 Physical Description ' 2 3 1 To o ra h The Site is located in the city of Stockton, California at an elevation of approximately 11 feet above mean sea level (MSL) The topography in the vicinity of the Site is relatively flat with a slope to the south/southeast at approximately 30 feet per mile A.11528J1 ASA 4