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139 South Center Street, Stockton, California January 21, 1994 �- Chevron U.S.A. Products Company Page 2 �- 2.0 BACKGROUND The site has been In operation as a retail gasoline service station since 1969. In April 1990, the original tanks were removed and replaced with three 10,000-gallon, and one 1,000-gallon double-wall fiberglass tanks. The original tanks that were removed consisted of two 10,000-gallon unleaded gasoline, one 5,000-gallon leaded gasoline, and one 1,000-gallon used-oil tank. All four tanks were constructed of single-wall steel. At the time of removal, soil samples were collected and analyzed for gasoline constituents. The analytical results revealed elevated concentrations of petroleum hydrocarbons in the gasoline and used-oil tank pits, and the pump island areas. On February 14 and 15, 1991, as a result of these preliminary findings, Groundwater Technology drilled and sampled four on-site soil borings, and installed 2-inch-diameter groundwater monitoring wells within each boring (MW-1 through MW-4). Soil samples analyzed from the four borings did not contain detectable concentrations of petroleum hydrocarbons. Detailed discussions of the well installation were documented in Groundwater Technology's report dated April 1991, She Assessment Report, Chevron Service Station No. 9-0557 139 South Center Stockton California. One additional groundwater monitoring well (MW-5) was installed in September 1992 to further define the boundaries of hydrocarbon-impacted groundwater. A quarterly groundwater monitoring and sampling program has been maintained at the site since March 1991. Dissolved petroleum hydrocarbons have been consistently detected in MW-1 and MW-2, and lower concentrations have occurred sporadically in the other wells. Groundwater monitoring and sampling reports are prepared quarterly to document groundwater conditions beneath the site. GROUNDWATER -- -TECHNOLOGY