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I Commingled Plume#35 Chevron 9-42751ARC0#2133 Site Conceptual Model September 1, 2006 Page 2 i 2.0 SITE HISTORY 2.1 Site Location The Chevron 9-4275 facility and the ARCO 2133 facility are located at the Intersection of Benjamin Holt Drive and Plymouth Road in Stockton, California The Site Is located in a light commercial/residential area of Stockton in a relatively flat lying area A site location map is 5 provided in Figure 1 Both sites are operating service stations A site plan showing both facilities is shown on Figure 2 Chevron 9-4275 station features include two underground storage tanks (USTs), a station building, product dispensers under a common canopy, and a car wash The ARCO 2133 facility features include four USTs, an AM/PM mini-mart, product dispensers, and associated product lines 22 Previous Investigations Previous Investigations for the Chevron 9-4275 facility and ARCO 2133 facility are described below 2 2.1 Chevron 9-4275 Facility The original service station was constructed in 1973 At that time, four single walled, fiberglass • USTs were Installed A gasoline release from a product line was detected in 1985 A site plan of the Chevron 9-4275 facility is included on Figure 3 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) total petroleum hydrocarbons as gasoline (TPHg) and 31 mg/kg of benzene Soil samples from below the southern product lines were non-detect for all analyzed analytes, however, samples from below the northern product lines reported 310 mg/kg TPHg and 0 63 mg/kg of benzene Soil samples from the waste oil UST were non- detect for TPHg and benzene, however, total petroleum hydrocarbons as diesel (TPHd) were reported at a concentration of 0 0011 mg/kg A soil sample from beneath the former waste oil UST was analyzed for total oil and grease, halogenated volatile organic compounds (HVOCs), poly chlorinated biphenyls (PCBs), and pesticides None of these constituents were detected A new tank 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 extraction/air sparge (SVE/AS) wells (GM-1 through GM-9 and VW-1) were installed onsite through 2005 (Figure 3) Soil samples collected and analyzed during the Installation of monitoring wells MW-3, MW-4, MW-6, MW-7, and MW-8 did not report any detections of TPHg or benzene above laboratory detection limits The station was renovated to include additional pump 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 TPHg and 0 29 mg/kg of benzene t 94275-ARCO 2133 SCM 9-2006 2