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+ r ATTACHMENT B SUMMARY OF PREVIOUS INVESTIGATIONS As previously reported by Groundwater Technology, Inc. (GTI), the site has been operated as a retail gasoline station since 1969. In April 1990, the existing single-walled steel underground storage tanks (USTs) and associated piping were removed from the site and replaced with three 10,000-gallon and one 1,000-gallon capacity double-walled fiberglass USTs. The former USTs consisted of two 10,000-gallon and one 5,000-gallon gasoline and one 1,000-gallon used oil tank. GTI collected soil samples from the UST pits and product line trenches. GTI reported concentrations of hydrocarbons over 50 mg/kg present in samples collected from the UST pit, used oil tank pit, and southern end of the eastern product island. GTI oversaw subsequent over-excavation in these areas and reported hydrocarbon concentrations less than 50 mg/kg in samples collected from the bases of the over-excavated areas. In February 1991, GTI drilled four borings at the site and installed wells MW-1 through MW-4 in the borings. Soils encountered consisted of clayey silt, silty to sandy clay, and lenses of gravelly sand to the depth explored of 60 feet below ground surface (bgs). No detectable concentrations of hydrocarbons were reported in the soil samples collected from these borings. Groundwater was reported at a depth of approximately 45 feet bgs, and the direction of groundwater flow was reported to the southeast. Total petroleum hydrocarbons as gasoline (TPHg) and benzene were reported in groundwater samples collected from wells MW-1 and MW-2 at concentrations of 1,400 ug/l and 250 ug/l, and 290 ug/l and 140 ug/l, respectively. Non-detectable concentrations of TPHg and benzene were reported in wells MW-3 and MW-4. In November 1992, GTI installed well MW-5 offsite to further define the extent of gasoline hydrocarbons in groundwater southeast (downgradient) of the site. Three soil samples collected from the boring were reported to contain non-detectable concentrations of hydrocarbons. Groundwater samples from well MW-5 were reported to contain non-detectable concentrations of hydrocarbons. In September 1993, GTI installed well MW-6 off-site to further define the extent of gasoline hydrocarbons in groundwater west of the former USTs. Seven soil samples collected from the boring were reported to contain non-detectable concentrations of