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• Remedial Action Plan and Further Site Assessment 3 7-Eleven Store No 19976 1399 N Main St Manteca CA May 30 2000 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 8015/8020 and for MTBE and the other oxygenates by EPA Method 8260 The base of the benzene,TPH-G, and MTBE plume in the area of boring SB-1 was identified at between 40 and 55 feet below grade, as evidenced by non-detectable i, 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) (IT Corporation, 2000) ' Upon receipt of these data, the SJCPHS requested further delineation Well MW-6 was Installed in December, 1999, to a depth of 30 feet below grade (Figure 3) Three soil samples were analyzed for ' BTEX, MTBE, and TPH-G by EPA Method 801518020, and for MTBE and other oxygenates by EPA Method 8260(IT Corporation, 2000) Soil benzene was not detected Soil TPH-G concentrations ranged up to 15 4 mg/kg, with highest levels detected at 20 feet below grade Soil MTBE levels were non- detectable Groundwater concentrations (in the monitoring wells) have varied over time between non-detectable (wells MW-3, MW-4, MW-4A, MW-5A), and up to 6,990 ug/L benzene, 57,000 ug/L TPH-G (well MW-2), and ' 1,240 ug/L MTBE (well MW-1) Deeper well MW-1A has generally low concentrations, Indicating vertical attenuation of the dissolved hydrocarbon plume with depth (IT Corporation, 2000) Well MW-6 had detectable benzene and TPH-G, but no detectable MTBE as of the first quarter 2000 The SJCPHS has requested installation of at least one additional monitoring well preferentially screened from 30 -45 feet below grade near well MW-1 Two additional wells are proposed to be installed to monitor conditions in this depth interval (Figure 3) The wells will be constructed of 2-inch diameter PVC, screened from 30-45 feet below grade Sand filter packing will occupy the annular space from total ' depth to 29 feet below grade Three feet of bentonite seal saturated in place overlain by concrete to grade will complete the wells, which will be capped with water-tight locking caps and boxed with traffic-rated ' street boxes at grade The wells will be surveyed relative to existing wells for piezometric surface data calculations, and will be sampled during the next quarterly monitoring event following installation Two soil ' samples per well will be collected for analysis by EPA Method 8015/8020 for BTEX and TPN-G, and by EPA Method 8260 for MTBE and other oxygenates