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Site History and Previous Investigations Former Chevron Service Station No. 94054 installed in the former gasoline UST pit. SB-7 was installed northeast and SB-8 was installed east- southeast of the former gasoline UST pit. Soil samples were collected from the monitoring wells and soil borings and groundwater samples were collected from the monitoring wells (Groundwater Technology, Inc., 1992). The greatest concentrations of petroleum hydrocarbon constituents detected in the soil samples collected during the associated monitoring well and soil boring installations were in the soil samples collected from SB-3 at 15 feet bgs and SB-4 at 15 feet bgs. TPH-GRO, benzene, ethylbenzene and total xylenes were detected at concentrations of 5,100, 48, 87 and 560 mg/kg, respectively, in the soil sample collected from SB-3 at 15 feet bgs. Toluene was detected at a concentration of 380 mg/kg in the soil sample collected from SB-4 at 15 feet bgs. The greatest concentrations of petroleum hydrocarbon constituents detected in groundwater samples collected during the associated monitoring well installations were in the groundwater samples collected from MW-4 and MW-6. TPH-GRO, toluene, ethylbenzene and total xylenes were detected at concentrations of 29,000, 1,000, 730 and 3,000 pg/L, respectively, in the groundwater sample collected from MW-6. Benzene was detected at a concentration of 2,100 pg/L in the groundwater sample collected from MW-4. In February 1995, Terra Vac Corporation (Terra Vac) performed a well survey of the area within a 2,600 foot radius of the site. The survey included reviewing the San Joaquin County Department of Environmental Health records. One additional well not recorded during the 1987 EA well survey was noted within 2,000 feet of the site; a monitoring well located at 2080 Country Club Boulevard (Terra Vac Corporation, 1995a). In March 1995, Terra Vac supervised the installation of two groundwater monitoring wells (MW-8 and MW-9). Monitoring well MW-8 was installed along the southern edge of the site and MW-9 was installed in the southbound lane of Franklin Avenue, north of the site (Terra Vac Corporation, 1995b). The greatest concentrations of petroleum hydrocarbon constituents detected in the soil samples collected during the associated monitoring well installations were in the soil sample collected from MW-8 at 19 feet bgs. TPH-GRO and BTEX were detected at concentrations of 21, 0.10, 0.14, 0.19 and 0.090 mg/kg, respectively, in the soil sample collected from MW-8 at 19 feet bgs. In March 1995, Terra Vac performed a dual vapor extraction (DVE) test at the site. Static water levels were recorded and then a vacuum induced flow was created at monitoring wells MW-1, MW-2 and MW-3. Drawdown, water production rates, vapor flow rates, vapor concentrations and applied vacuum were recorded. The data collected from MW-1 indicated that the site could be conducive to DVE, however, the data collected from MW-2 and MW-3 indicated the site would not be conducive to DVE. Based upon a review of the DVE test and lithology Terra Vac recommended a combined soil vapor extraction and air sparging remediation system be installed at the site (Terra Vac Corporation, 1995c). During the third quarter of 1995, Gettler-Ryan, Inc. performed a well survey to locate undocumented wells within an area extending 430 feet east of the southwestern corner of the site and 380 feet north of the southwestern corner of the site. The survey was conducted by going door to door to the different residencies located within the targeted area. Undocumented wells were not identified within the targeted area (Terra Vac Corporation, 1998). Stantec 3