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Site History and Previous Investigations Former Chevron Service Station No. 94054 Former Chevron Service Station 94054 is located in a mixed commercial and residential area at the intersection of Country Club Boulevard and Franklin Avenue in Stockton, California. The site address is 2103 Country Club Boulevard. The former service station was in operation from 1962 to 1986 and was razed in late 1988 (Terra Vac Corporation, 1998). The site is currently an unpaved vacant lot owned by Mr. Mark Springs. The region consists of Holocene age alluvium and soils from the Victor Formation, primarily unconsolidated Quaternary age deposits, to roughly 150 feet below ground surface (bgs). The regional lithology is heterogeneous and consists of laterally discontinuous lenses of sand and gravel, with varying amounts of sand and silt interstratified with clay and silt. Site specific boring logs indicate that site lithology is consistent with regional lithology, comprised primarily of clay and silt with discontinuous lenses of poorly to well graded sand and gravel (SECOR International Incorporated, 2003). The nearest surface waters are the San Joaquin River, located approximately 1.75 miles west of the site, the Calaveras River, located approximately 1 mile north of the site, and the Smith Canal, located approximately 1,320 feet south of the site (SECOR International Incorporated, 2003). The site is currently believed to have four separate water bearing zones: the A zone (screened from approximately 5 to 35 feet bgs), the B zone (screened from approximately 40 to 51.5 feet bgs), the C zone (screened from approximately 65 to 81.5 feet bgs) and the D zone (screened from approximately 95 to 101 feet bgs). Depth-to-water measurements at the site have ranged from approximately 3 to 23 feet below the top of well casing. Groundwater elevations within the A zone were at their recorded lowest in November 1992, at which point they rose steadily until their peak in February 1998; subsequently groundwater elevations have declined slightly and remained fairly stable. Current depth-to-water measurements at A, B, C and D zone wells are approximately 10, 11, 12 and 12 feet below top of well casing, respectively. In August 1983, Pronanic Professional Tank Investigation Corporation performed a tank system test which indicated that the manifold piping system at the site was leaking (Terra Vac Corporation, 1998). In November 1986, Groundwater Technology Inc. (GTI) supervised the installation of three groundwater monitoring wells (MW-1, MW-2 and MW-3). Monitoring well MW-1 was installed east of the underground storage tanks (UST), MW-2 was installed north of the USTs and MW-3 was installed east of the former pump island area, south of the USTs. Hydrocarbon odors and free product were recorded during drilling activities. Measurable free product was also observed following the completion of monitoring well MW-1, however further monitoring did not yield the presence of free product (Groundwater Technology, Inc., 1986). The greatest concentrations of petroleum hydrocarbon constituents detected in the soil samples collected during the monitoring well installation were in the soil samples collected from MW-1 at 9 feet bgs and MW-2 and 10.5 feet bgs. Total petroleum hydrocarbons and xylenes were detected at concentrations of 4.1 and 1.3 milligrams per kilogram (mg/kg), respectively, in the soil sample collected from MW-1 at 9 feet bgs. Benzene and toluene were detected at concentrations of 0.36 and 0.48 mg/kg, respectively, in the soil sample collected from MW-2 at 10.5 feet bgs. In 1987, EA Engineering, Science, and Technology, Inc. (EA) conducted a risk assessment for the site. It was concluded that the risk to environmental , and human health attributed to contamination from the site was insignificant, due to a predicted concentration of benzene in the nearest municipal well far below the most conservative drinking water standard and action Sta ntec 1