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Stantm N"01111 Site History and Previous Investigations Chevron and ARCO Commingled Plume No. CP0034 October 26, 2011 Background The Chevron 9-4275 service station and ARCO Service Station No. 2133 (site) are located at the intersection of West Benjamin Holt Drive and Plymouth Road in Stockton, California. The site is located in a light commercial and residential area of Stockton in a relatively flat lying area. ARCO 2133 is currently comprised of an AM/PM mini-mart that retails gasoline fuel with four underground storage tanks (USTs), dispenser islands, and associated product lines. Chevron 9- 4275 features include two double-walled, fiberglass USTs, a station building, six product dispensers under a common canopy, and a car wash. The site is bound by retail shopping to the north, residential homes to the south, and Interstate Highway 5 to the west. The site is bound by Plymouth Road to the east, beyond which are a golf course and residential homes. According to a United States Geological Survey Map dated 1976, the elevation of the site is at approximately 3 feet (ft) above mean sea level. The nearest open body of water is the Fourteen Mile Slough, located approximately 2,000 ft south of the site. The site is bound by retail shopping to the north, residential homes to the south, and Interstate Highway 5 to the west. Chevron 9-4275 Facility The original service station was constructed in 1973. At that time, four single-walled, fiberglass underground storage tanks (USTs) were installed. A gasoline release from a product line was detected in 1985. In January 1989, four single-walled USTs (three gasoline and one waste oil) were removed. No holes were found in any of the USTs. Soil samples were collected from beneath the tanks and beneath the product lines. Soil samples collected from below the excavated tanks contained up to 3,600 milligrams per kilogram (mg/kg) gasoline range organics (GRO) and 31 mg/kg of benzene. Soil samples from below the southern product line were non-detect for all analyzed analyses; however, samples from below the northern product lines reported 310 mg/kg GRO and 0.63 mg/kg of benzene. Soil samples from the wa e oil UST were non-detect for GRO and benzene; however, diesel range organics were reported at a concentration of 0.0011 m /k . A soil sample from be neat a former waste oil UST was analyze or total oil and grease, halogenated volatile organic compounds, polychlorinated biphenyls, and pesticides. The constituents were not detected above the laboratory method reporting limits (MRL). A new UST pit was excavated and five double-walled, fiberglass USTs were installed (four gasoline and one methanol). Four soil borings (SB-1 through SB-4), five cone penetrometer test (CPT) borings (CPT-1 through CPT-5), nine groundwater monitoring wells (MW-1 through MW-9), and ten soil vapor l� extraction/air sparge (SVE/AS) wells (GM-1 through GM-9 and VW-1) were installed onsite and off site from 1989 through 2006. Soil samples collected and analyzed during the installation of monitoring wells MW-3, MW-4, and MW-6 through MW-8 did not report any detections of GRO or benzene above the laboratory MRL. The station was renovated to include additional dispenser islands and product lines in late 1996 to early 1997. Soil samples taken from beneath the product lines reported a maximum concentration of 8.1 mg/kg GRO and 0.29 mg/kg of benzene. In 2001, a CPT investigation consisting'of five borings (CPT-1 through CPT-5) along the eastern side of Plymouth Road was conducted to assess soil stratigraphy and collect discrete `%NWI groundwater samples. Groundwater samples were collected from the identified water-bearing 94275 Site History and Previous Investigations.doc 1