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SITE CONCEPTUAL MODEL AND REMEDIAL ACTION PLAN 1 7-Eleven Store No 14117 2725 Country Club Blvd Stockton, CA June 20 2002 ' 10 SUMMARY OF SITE ASSESSMENT DATA ' 1 1 Soil Data Summary The underground storage tanks (USTs) at the site were replaced in July 1998 (Fluor Daniel GTI, 1998) ' Soil samples collected from beneath the tanks at depths of up to 14 5 feet below grade (bg) contained up to 63 milligrams per kilogram (mg/kg) benzene, 7,200 mg/kg total petroleum hydrocarbons as gasoline ' (TPH-G), and up to 40 mg/kg methyl tertiary butyl ether(MTBE) The new tanks were installed in a tank pit located to the east and slightly north of the original tank location (Figure 2) 1 Four groundwater monitoring wells were installed in May 1999 (IT Corporation, 1999) To obtain vertical soil hydrocarbon/oxygenate plume delineation data, the wells were overdrilled to depths of up to 51 feet lag and backfilled to 30 feet bg Soil samples collected during installation of the wells contained up to 2 89 ' mg/kg benzene, 429 mg/kg TPH-G, and 0 119 mg/kg MTBE, with highest concentrations at 15 feet bg Selected halocarbons (analyzed at the request of the San Joaquin County Public Health Services [SJCPHS]) indicating possible dry-cleaner plume contributions were also detected in samples from well MW-3 The base of the MTBE plume in soil was identified at less than 25 feet bg Additional site assessment consisting of the installation of 17 soil borings and collection of soil and groundwater samples was completed between September 29 and October 2, 1998 (Fluor Daniel GTI, 1998) The borings were installed both at the 7-Eleven site and on adjacent residential property located to the north Soil concentrations were non-detectable for benzene, toluene, ethylbenzene, and total xylenes (BTEX) as measured using EPA Method 8020 Soil concentrations were also non-detectable for TPH-G as measured using EPA Method 8015, and for MTBE, tert-amyl methyl ether(TAME), diisopropyl ether ' (DIPS), tert-butanol (TBA), and ethyl-tert-butyl ether(ETBE) as measured by EPA Method 8260 On October 30, 2001, IT Corporation installed groundwater monitoring wells MW-5, MW-6, and MW-7 (Figure 2) Soil samples collected from the capillary fringe (at about 10 feet bg) and from total depth (about 20 feet bg) were found to contain non-detectable levels of BTEX, MTBE, and other oxygenates (except for 0 006 mg/kg total xylenes in well MW-7) These data indicate lateral delineation of the soil hydrocarbonloxygenate plume to the west, northwest, and south of the UST/pump island source areas, and vertical plume delineation ata th of less than 25 feet bgS, - ✓ 1 14117Scm2 doc