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.', February 13, 1998 <br /> Page 2 <br /> approximately one mile west of the site. The Highway 4 overpass is located south of <br /> the site across Washington Street. <br /> As previously reported by Groundwater Technology, Inc. (GTI), the site has been <br /> operated as a retail gasoline station since 1969. In April 1990,the existing single- <br /> walled steel underground storage tanks (USTs) and associated piping were removed <br /> from the site and replaced with three 10,000-gallon capacity and one 1,000-gallon <br /> capacity double-walled fiberglass USTs. The former USTs consisted of two 10,000- <br /> gallon gasoline, one 5,000-gallon unleaded gasoline, and one 1,000-gallon used oil tank. <br /> With oversight from Ms. Casey Folley of the San Joaquin County Environmental Health <br /> Department, GTI collected soil samples from the UST pits and product line trenches. <br /> GTI reported concentrations of hydrocarbons over 50 parts per million(ppm)present in <br /> samples collected from the UST pit, used oil tank pit, and southern end of the eastern <br /> product island. GTI oversaw subsequent over-excavation in these areas and reported <br /> hydrocarbon concentrations less than 50 ppm in samples collected from the bases of the <br /> over-excavated areas (GTI,April 23, 1990). <br /> In February 1991, GTI drilled four borings at the site and installed Wells MW-1 through <br /> MW-4 in the borings (GTI,April 5, 1991). Soils encountered consisted of clayey silt, <br /> silty to sandy clay, and lenses of gravely sand to the depth explored of 60 feet bgs. No <br /> detectable concentrations of hydrocarbons were reported in the soil samples collected <br /> from these borings. Groundwater was reported at a depth of approximately 45 feet <br /> below ground surface (bgs) and the direction of groundwater flow was reported to the <br /> southeast. Total petroleum hydrocarbons as gasoline (TPH-g) and benzene were <br /> reported in groundwater samples collected from Wells MW-1 and MW-2 at <br /> concentrations of 1,400 and 250 parts per billion(ppb), and 290 and 140 ppb, <br /> respectively. Nondetectable concentrations of TPH-g and benzene were reported in <br /> Wells MW-3 and MW-4. <br /> In November 1992, GTI installed Well MW-5 offsite to further define the extent of <br /> gasoline hydrocarbons in groundwater southeast(downgradient) of the site (GTI, <br /> January 25, 1993). Three soil samples collected from the boring were reported to <br /> contain nondetectable concentrations of hydrocarbons. Groundwater samples from <br /> Well MW-5 were reported to contain nondetectable concentrations of hydrocarbons. <br /> In September 1993, GTI installed Well MW-6 offsite to further define the extent of <br /> gasoline hydrocarbons in groundwater west of the former USTs at the site (GTI, <br /> January 28, 1994). Seven soil samples collected from the boring were reported to <br /> contain nondetectable concentrations of TPH-g and benzene; very low(0.012 to <br /> 0.022 ppm) concentrations of toluene and xylenes were reported in 2 of the 7 samples. <br /> Groundwater samples from Well MW-6 were reported to contain 980 ppb TPH-g and <br /> 9 ppb benzene. <br /> In May 1994, GTI drilled five borings (SB-1 through SB-5) onsite to further define the <br /> extent of hydrocarbons in soil in the areas of the eastern pump island, former USTs, and <br /> 7G007-013-01/workplan <br />