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' 's E ' s September 26, 1996 �! Page 2 surface water bodies are the Mormon Slough located approximately 1/4-mile south of the site and the Port of Stockton located approximately one mile west of the site. 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 capacity and one 1,000-gallon capacity double-walled fiberglass USTs. The former USTs consisted of two 10,000- gallon gasoline, one 5,000-gallon unleaded gasoline, and one 1,000-gallon used oil tank. With oversight from Ms. Casey Folley of the San Joaquin County Environmental Health Department, GTI collected soil samples from the UST pits and product line trenches. GTI reported concentrations of hydrocarbons over 50 parts per million(ppm)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 ppm in samples collected from the bases of the over-excavated areas(GTI, April 23, 1990). In February 1991, GTI drilled four borings at the site and installed Wells MW-1 through MW-4 in the borings (GTI, April 5, 1991). Soils encountered consisted of clayey silt, silty to sandy clay, and lenses of gravelly sand to the depth explored of 60 feet 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 below ground surface (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 and 250 parts per billion(ppb), and 290 and 140 ppb, respectively. Nondetectable 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 (GTI, January 25, 1993). Three soil samples collected from the boring were reported to contain nondetectable concentrations of hydrocarbons. Groundwater samples from Well MW-5 were reported to contain nondetectable concentrations of hydrocarbons. In September 1993, GTI installed Well MW-6 offsite to further define the extent of gasoline hydrocarbons in groundwater west of the former USTs at the site(GTI, January 28, 1994). Seven soil samples collected from the boring were reported to contain nondetectable concentrations of TPHg and benzene; very low(0.012 to 0.022 ppm) concentrations of toluene and xylenes were reported in 2 of the 7 samples. Groundwater samples from Well MW-6 were reported to contain 980 ppb TPHg and 9 ppb benzene. In May 1994, GTI drilled five borings (SB-1 through SB-5) onsite to further define the extent of hydrocarbons in soil in the areas of the eastern pump island, former USTs, and 920-078.1 A/WPLAN