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[� Stantec Consulting Corporation tom/ 3017 Kilgore Road Suite 100 Rancho Cordova CA 95670 Tel:(916)861-0400 Fax: (916)861-0430 Stantec RECEIVED December 8, 2010 DEC 0 8 2010 &MRONMENTAL HEALTH Ms. Victoria McCartney PERMIT/SERVICES San Joaquin County Environmental Health Department 600 East Main Street Stockton, CA 95202 • RE: Vapor Risk Assessment Report and Work Plan for Additional Soil Gas Assessment 7-Eleven Store #19976 1399 North Main Street �2, Manteca, California too Stantec Project No.: 211501028.210.0537 (S L G r Dear Ms. McCartney: This report was prepared by Stantec Consulting Corporation (Stantec) on behalf of 7-Eleven Inc. (7-Eleven) in order to evaluate the potential for vapor intrusion into structures at 7-Eleven Store Number 19976 located at 1399 North Main Street in Manteca, California (Figures 1 and 2). This report was prepared in response to the San Joaquin County Environmental Health Department (SJCEHD) letter dated September 22, 2009 (Attachment A). i- SITE BACKGROUND Underground storage tank (UST) replacement and associated soil sampling activities were conducted in September 1996. Five soil samples (T1-N, T1-S, T2-S, T3-N, T3-S) (Figure 2) were collected at depths of 15 feet below ground surface (bgs) from beneath the USTs. Benzene, total petroleum hydrocarbons as gasoline (TPHg), and methyl-tert-butyl ether (MtBE) were detected at concentrations of 0.93 milligrams per kilogram (mg/kg), 220 mg/kg, and 0.35 mg/kg, respectively. After overexcavation, analytical results of soil samples E1-W, E1-C, E1-E, E2-W, E2-C, and E2-E (Figure 2), collected at the edge of the excavation, contained TPHg, benzene, toluene, ethylbenzene, and total xylenes (BTEX), and MtBE at low concentrations. Prior to backfilling the excavation, two horizontal vent lines were installed in the northern part of the tank pit floor at a depth of 11 to 11.5 feet bgs (Figure 2). The vent lines were constructed of 4-inch diameter Schedule 40 poly-vinyl-chloride (PVC) piping, each line with 20 feet of screened interval. The lines were stubbed up inside a fiberglass box located in a planter east of the tank pit. The new USTs were installed close to (partly overlapping) the location of the original USTs (Figure 2). Geoprobe soil borings GP-1 through GP-3 were advanced on April 29, 1998, near the hydrocarbon source area (the UST pad and pump island) (Figure 2). Nineteen soil samples were collected from depths ranging between 5 and 45 feet bgs. Concentrations ranged up to 15 r