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Additional Site Assessment and Second Quarter 1999 Monitoring and Sampling Report 3 7-Eleven Inc,Store#19976,1399 N Main St.Manteca,Cai forma -- July 15,1999 methods 8020/modified 8015, and for oxygenates MTBE,TBA, DIPS, ETRE, and TAME using EPA ' method 8260 Table 2 summarizes the laboratory analytical results of groundwater samples collected from three soil ' bonngs GP-1, GP-2,and GP-3 on April 29, 1998 Benzene and TPH-G ranged from 21 micrograms per liter(pg/L) and 1,600 pg/L., respectively in sample GP-2-W,to 5,800 pg/L and 290,000 pg/L, respectively in sample GP-3-W MTBE was detected in samples GP-1-W, GP-2-W, and GP-3-W at concentrations of 430 pg/L, 16 pg/L, and 4,200 pg/L, respectively MTBE was confirmed in sample GP- 1-W and GP-3-W at a concentration of 380 pg/L and 3,700 pg/L, respectively, using EPA method 8260 ' In June of 1998 SJCPHS required a work plan for further subsurface investigation of the vertical and ' lateral extent of contamination at this site On September 2 and September 3, 1998,to fulfill the requirements of the SJCPHS, Fluor Daniel GTI ' supervised the completion of three groundwater monitoring wells, (MW-2,MW-3 and dual-nested MW-i/MW 1A) at the locations shown on Figure 2 Using a drilling ng equipped with an 8-inch and 10- inch diameter hollow-stem augers and a split-spoon sampler lined with 2-inch-diameter by 6-4nch-long brass sample tubes discrete soil samples were collected from each of the soil bonngs Soil samples were collected at 5-foot intervals beginning at 5 feet below grade ' Twelve Soil samples were selected for analysis of BTEX,MTBE, and TPH-G by EPA methods ' 8020/8015 modified, and three of the twelve soil samples were also analyzed for the oxygenates MTBE, TBA, DIPS, ETBE, and TAME using EPA method 8260 Table 1 summarizes the laboratory analytical results for soil samples collected on September 2 and September 3, 1998 The highest TPH-G concentration was detected in sample MWA @45'at 17,000 mg/kg The highest benzene concentration was detected in sample MW-2@25'at 0 0708 mg/kg, and highest MTBE concentration was detected in sample MWA @20' at 0 0829 mglkg ' Additional assessment was conducted to further delineate the vertical and lateral extent of hydrocarbons in soil and groundwater at the site, as required by the SJCPHS r1997sg2 sar