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1 Report on Additional Site Assessment and First Quarter 2000 Monitoring and Sampling Report 2 • 7-Eleven Inc Store#19976, 1399 N Main St Manteca California February 4 2000 1 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, ETRE, and TAME using EPA method 8260 Concentrations ranged up to 15 mg/kg benzene, 27 mg/kg TPH-G, and no detectable MTBE (EPA Method 8260) (Table 1) Highest concentrations were at a depth of 35 feet below grade Grab groundwater samples collected from the base of each boring were also analyzed for the same constituents 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 2) Additional assessment was requested by the SJCPHS in June 1998 Groundwater monitoring wells MW- 1, MW-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, MTBE, and TPH-G by EPA methods 8020/8015 modified, and three of the samples were also analyzed I• for oxygenates MTBE, TSA, DIPE, ETBE, and TAME using EPA method 8260 (Table 1) The highest benzene and MTBE concentrations were detected in capillary fringe soils (0 0708 mg/kg In MW-2@25' and 0 0829 mg/kg in MW-1 @20', respectively) The highest TPH-G concentration was detected in Isample MW-1@45' at 17,000 mg/kg Soil boring SB-1 and cluster wells MW-414A and MW-5/5A were Installed in May 1999, after receipt of a letter from the SJCPHS requesting additional lateral and vertical plume delineation In April 1999 (Figure 2) Soil boring SB-1 was installed to about 65 feet below grade near the hydrocarbon source area to evaluate the vertical extent of the hydrocarbon plume The cluster wells were Installed to access the saturated zone at depths of 20 - 30 feet below grade (MW-4 and MW-5), and 45 - 50 feet below grade (MW-4A and MW-5A) to provide further Information on the lateral and vertical extent of the dissolved hydrocarbon plume Fourteen soil samples were analyzed for BTEX, MTBE, and TPH-G by EPA methods 8020/8015 modified and for oxygenates MTBE, TBA, DIPE, ETBE, and TAME using EPA method 8260 (Table 1) The base of the benzene, TPH-G, and MTBE/oxygenates plume In the area of boring SBA was identified at between 40 and 55 feet below grade, as evidenced by non-detectable concentrations of all analytes Soil samples from wells MW-4 and MW-4A were non-detectable for all constituents, and samples from well MW-5 contained low levels of hydrocarbons from 30 - 35 feet below grade (below which concentrations were non-detectable for all constituents) i 19976 3SA 1