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139 South Center Street, Stockton, California June 28, 1994 <br /> Chevron USA Products Company Page 2 <br /> 2.0 BACKGROUND <br /> The site has been in operation as a retail gasoline service station since 1969. In April 1990, the original, <br /> single-wall steel tanks were removed and replaced with three 10,000-gallon and one 1,000-gallon <br /> double-wall fiberglass tanks. The original tanks consisted of two 10,000-gallon unleaded gasoline, and <br /> one 5,000-gallon used-oil tank. At the time of removal, soil samples were collected and analyzed for <br /> gasoline constituents. The analytical results revealed elevated concentrations of petroleum <br /> hydrocarbons in soil samples collected from the gasoline tank pits and the pump island areas. <br /> On February 14 and 15, 1991, as a result of these preliminary findings, Groundwater Technology drilled <br /> and sampled four on-site soil borings, and installed 2-inch-diameter groundwater monitoring wells within <br /> each boring ( MW-1 through MW-4). Analyzed soil samples from the four borings did not contain <br /> detectable concentrations of petroleum hydrocarbons. Detailed discussions of the well Installation were <br /> documented in Groundwater Technology's Site Assessment Repo , dated April 1991. One additional j <br /> groundwater monitoring well (MW-5) was installed in September 1992 to further define the boundaries of <br /> hydrocarbon-impacted groundwater. <br /> On September 14, 1993, Groundwater Technology was contracted by Chevron to conduct an additional <br /> investigation which involved the installation of one groundwater monitoring well (MW-6) to a depth of 60 <br /> feet below ground surface (BGS). Benzene and total petroleum hydrocarbons-as-gasoline (TPH-G) were <br /> detected in groundwater from MW-6 at 9 parts per billion (ppb) and 980 ppb, respectively. Results of <br /> this phase of investigation are outlined in Groundwater Technology's Additional Assessment Report, <br /> dated January 21, 1994. <br /> The most recent phase of assessment was conducted on May 12 and 13, 1994 and consisted of drilling <br /> and sampling five soil borings. The results of this investigation are summarized in this report. <br /> C+ti. GROUNDWATER <br /> 5039SU M.RPT TECHNOLOGY <br />