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ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS Chevron 9-4275, 2905 Benjamin Holt Drive, Stockton, CA • The service station was constructed in 1973 At that time, four single walled, fiberglass underground storage tanks (USTs) were installed A gasoline release from a product line was detected in 1985 In January 1989, the four single walled USTs (three gasoline and one waste oil) were removed No holes were found in any of the USTs Soil samples were collected from beneath the tanks and beneath the product lines Soil samples collected from below the excavated tanks contained up to 3,600 milligrams per kilogram (mg/kg) total petroleum hydrocarbons quantified as gasoline (TPHg) and 31 mg/kg of benzene Soil samples from below the southern product line were non- detect for all analyzed analytes, but samples from below the northern product lines reported 310 mg/kg TPHg and 0 63 mg/kg of benzene Soil samples from the waste oil UST were non-detect for TPHg and benzene, however, total petroleum hydrocarbons quantified as diesel (TPHd) were reported at a concentration of 0 0011 mg/kg A soil sample from beneath the former waste oil UST was analyzed for total oil and grease, halogenated volatile organic compounds (HVOCs), poly chlorinated biphenyls (PCBs), and pesticides None of these constituents were detected A new tank pit was excavated and five double-walled, fiberglass USTs were installed (4 gasoline and 1 methanol) Nine groundwater monitoring wells (MW-1 through MW-9), and ten soil vapor extraction/air sparge (SVE/AS) wells (GM-1 through GM-9 and VW-1) have been installed onsite In addition, four soil bonngs (SB-1 through SB-4) have also been drilled Soil samples collected and analyzed from the installation of monitoring wells MW-3,4, 6, 7, and 8 did not contain report any detections of TPHg and benzene The station was renovated to include additional pump islands and product lines in the winter of late 1996 to early 1997 Soil samples taken from beneath the product lines reported a maximum concentration of 8 1 mg/kg TPHg and 0 29 mg/kg of benzene On March 29, 2001, five soil samples designated A, B, B2, B3, and C were collected from the excavations underlying the methanol product line and methanol dispenser Samples were collected in a trench approximately 3-feet deep directly beneath the area of the former piping The granular backfill of the piping was removed to reveal the native clayey sand soils This depth interval was sampled at each location No visual evidence or odors were present that suggested a release emanated from the product line or dispenser The laboratory did not detect concentrations of TPHg, BTEX, fuel oxygenates, or methanol at or above laboratory method detection limits Corrective action is currently underway which includes remediation by SVE/AS Only the AS system is currently operating, the SVE well screens are submerged An ARCO service station located south (upgradient) of the site, across Benjamin Holt Drive, had 40 a spill of approximately 2,000 gallons of gasoline in May of 1989 Another release of gasoline