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Report on Additional Site Assessment 2 7-Eleven Inc. Store#19976 1399 N.Main St.,Manteca,California March 5,2001 A workplan for initial site assessment was requested in February 1998 by the SJCPHS. Geoprobe soil borings GP-1 through GP-3 were subsequently installed on April 29, 1998 near the hydrocarbon source area (the underground storage tank(UST) pad and pump island) (Figure 2). Nineteen soil samples were collected from depths ranging between 5 and 45 feet below grade for analysis for BTEX, MTBE, and TPH-G (EPA Methods 8020/modified 8015). One sample from each soil boring was also analyzed for MTBE,TBA, DIPE, ETBE,and TAME using EPA method 8260. Hydrocarbons were reported in detectable levels in seven of the nineteen tested samples; most of the samples containing detectable concentrations were obtained from depths of 15-25 feet below grade (the upper part of the shallow water-bearing zone). Soil MTBE identified using EPA Method 8020 in some of the samples was not detected when the same samples were analyzed by EPA Method 8260 (Table 2). Anomalously high levels found at 35 feet below grade in boring GP-3 were bracketed by non-detectable concentrations at 30 and 40 feet below grade (Table 2);these anomalous data have since been unverifiable by later sampling, and are now thought to be indicative of borehole wall caving from shallower soil depths during drilling of GP-3. Grab groundwater samples collected from the base of each boring were also analyzed for the same cofistituents and contained up to 5,800 pg/L benzene and 290,000 pg/L TPH-G. Dissolved MTBE levels ranged up to 3,700 pg/L (EPA Method 8260) (Table 3). Additional assessment was requested by the SJCPHS in June 1998. Groundwater monitoring wells MW- 1, VIW-2, and MW-3 were installed in September 1998 (Figure 2). Well MW-1A is a dual-nested well completed to depths of 30 and 66 feet below grade. Twelve soil samples were analyzed for BTEX, MI-BE, and TPH-G by EPA methods 8020/8015 modified, and two of the samples were also analyzed for oxygenates MTBE,TBA, DIPE, ETBE, and TAME using EPA method 8260 (Table 2).The highest benzene concentration was detected in capillary fringe soils (0.0708 mg/kg in MW-2@25');the highest confirmed MTBE concentrations were detected at 50 feet below grade in soil from well MWA (0.0187 mg/kg). The highest TPH-G concentration was detected in sample MW-1 @45' at 17,000 mg/kg. 19976ASA