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<ovember 24, 1997 zgre 2 SITE BACKGROUND AND PREVIOUS INVESTIGATIONS The site is the location of an operating Chevron Service Station located on the northwest corner of Washington Street and South Center Street in Stockton, California, and is within an area of primarily commercial use. Topography in the area of the site is relatively flat and the site is approximately 12 feet above mean sea Ievel- The nearest surface water bodies are the Morrison Slough located approximately 114-mile south of the site, the Yacht Harbor located approximately 1/4-mile north 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, (11`I drilled four borings at the site and installed Wells MW-1 through i MW-4 in the borings (GTI, April 5, 1991). Soils encountered consisted of cla)-ey silt, silty to sandy clay, and lenses of gravely 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 (TPH-g) 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 TPH-g 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 frons 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 USI's at the site (GTI, January 28, 1994). Seven soil samples collected from the boring were reported to 920-078.1 A/S01,G W RPT