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1.0 INTRODUCTION AND SITE DESCRIP'T'ION <br /> 1.1 Introduction <br /> Thrifty Oil Co (Thrifty), has performed additional site assessment activities at station No <br /> 172 located at 7647 Pacific Avenue in Stockton, California fture 1) in response to the San <br /> Joaquin County Environmental Health Division's (SJCEHD) request to assess the lateral and <br /> vertical extent of the hydrocarbon plume in soil This work was approved by the SJCEHD <br /> 1.2 Site Description <br /> Thrifty Oil Station No 172 is an operating TOSCO retail gasoline station/food store located <br /> at the southwest corner of the intersection of Pacific Avenue and Rivara Road in Stockton, <br /> California (Figure 1). Four underground storage tanks (USTs) exist onsite along with three <br /> dispenser islands (Figure 2) <br /> 2.0 PREVIOUS SITE ASSESSMENTS <br /> In July, 1991, SSB Environmental Consultants, Inc performed an initial investigation at the <br /> site This investigation consisted of the installation of four 2-inch diameter groundwater <br /> monitoring wells (MW 1 through MW4) to a depth of approximately 75 feet below ground <br /> surface (bgs) Total petroleum hydrocarbon (TPH) was detected in soil samples at <br /> concentrations ranging up to 560 mg/kg in soil and benzene was detected in groundwater <br /> samples at concentrations ranging up to 5,720 ug/l (Thrifty, 1995) <br /> On April 22, 1994, Thrifty installed three vadose wells (VWA, VWB and VWC) and two <br /> groundwater monitoring wells (MW7 and MW8) TPH was detected at concentrations <br /> ranging up to 21,000 mg/kg in soil and benzene was detected at concentrations ranging up <br /> to 7,100 ug/l in groundwater (Thrifty, 1995). <br /> On June 26, 1995, Alton Geoscience performed soil vapor extraction tests on soil vapor <br /> wells VWA, VWB and VWC An average radius of influence was estimated to be <br /> approximately 33 feet for a flow rate of 16 to 17 cfm in the test wells. Laboratory analysis <br /> of air samples collected from the test wells indicated that significant concentrations of <br /> hydrocarbons (purgeable hydrocarbons at concentrations ranging from 1,700 ppmv to 9,500 <br /> ppmv during the soil vapor extraction tests) can be removed by soil vapor extraction from <br /> these wells At the flow rates testes, the estimated gasoline hydrocarbon removal rates were <br /> approximately 31 pounds per day (lb/day) for well VWA, 15 lb/day for well VWB and 61 <br /> lb/day for well VWC (Alton Geoscience, 1995) <br /> 1 <br />