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ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS Chevron 9-9840, 4344 Waterloo Road, Stockton, CA In March 1998, three 10,000-gallon gasoline underground storage tanks (USTs), one 1,000- gallon waste oil UST, one 500-gallon waste oil UST, product dispensers, associated product lines, two hoists, and a sump were removed as part of station upgrade activities The highest initial hydrocarbon concentrations were found at the west end of the former UST pit, and below the former product lines These areas were subsequently over- excavated (approximately 725 cubic yards) On September 22 and September 25, 1998, three onsite groundwater monitoring wells (MW-1, MW-2, and MW-3) were installed Soil analytical data from MW-3 showed no detectable hydrocarbon concentrations at depth Methyl-tertiary-butyl-ether (MtBE) was not detected in any of the soil samples collected during the advancement of the borings for the wells MW-1, MW-2, and MW-3 Soil analytical results from MW-1 and MW-2 reported total petroleum hydrocarbons quantified as gasoline (TPHg) at concentrations of 8580 mg/kg (MW-1@30') and 3 23 mg/kg (MW-2@60') Benzene was also reported in soil samples from MW-1 and MW-2 at concentrations of 12 1 mg/kg (MW-1@30'), and 0 995 mg/kg (MW-2@60') Second quarter 2001 groundwater analytical results from well MW-3 reported all analytes as non-detect MW-1 reported 92 ppb TPHg, 2 0 ppb benzene, 0 64 ppb toluene, 0 68 ppb ethylbenzene, and 180 ppb MtBE MW-2 reported 7 1 ppb benzene, 2 1 ppb toluene, and 23 ppb MtBE Groundwater flow was reported to the northeast at 0 002 foot/foot (ft/ft) i I