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3 <br /> Report on Additional Site Assessment March 5 2001 <br /> 7-Eleven Inc,Store#19976 1399 N Main St,Manteca,California <br /> Soil boring SB-1 and cluster wells MW-414A and MW-515A were installed in May 1999, after receipt of a <br /> letter from the SJCPHS requesting additional lateral and vertical plume delineation in April 1999 <br /> (Figure 2) Soil boring SB-1 was installed to about 65 feet below grade near the hydrocarbon source <br /> area to evaluate the vertical extent of the hydrocarbon plume The cluster wells were installed to access <br /> the saturated zone at depths of 20-30 feet below grade (MW-4 and MW-5), and 45-50 feet below <br /> grade (MW-4A and MW-5A)to provide further information on the lateral and vertical extent of the <br /> dissolved hydrocarbon plume Fourteen soil samples were analyzed for BTEX, MTBE, and TPH-G by <br /> EPA methods 8020/8015 modified and for oxygenates MTBE,TBA, DIPS, ETBE, and TAME using EPA <br /> ' method 8260 (fable 2) The base of the benzene,TPH-G, and MTBE/oxygenates plume in the area of <br /> boring SB-1 was identified at between 40 and 55 feet below grade, as evidenced by non-detectable <br /> concentrations of all analytes Soil samples from wells MW-4 and MW-4A were non-detectable for all <br /> constituents, and samples from well MW-5 contained low levels of hydrocarbons from 30-35 feet below <br /> grade (below which concentrations were non-detectable for all constituents) <br /> Additional wells were requested by the SJCPHS as outlined in their August 16, 1999 correspondence,in <br /> response a workplan was submitted on October 18, 1999 Groundwater monitoring well MW-6 was <br /> installed on December 22, 1999 (IT Corporation,2000) Soil samples collected during well installation <br /> contained no detectable levels of benzene or MTBE, and up to 15 4 mglkg TPH-G detected at 20 feet <br /> ' below grade <br /> Groundwater analytical data trends over time indicate highest dissolved hydrocarbon levels are <br /> consistently present in well MW-2 (Table 3) Benzene,TPH-G, and MTBE concentrations in this well <br /> have ranged up to 6,990 micrograms per liter (pg/L),68,000 pg1L, and 1,400 pg/L, respectively Low to <br /> non-detectable concentrations in downgradient wells MW-5, MW-5A, and well MW-6 indicate adequate <br /> lateral and vertical plume delineation (Figures 3 and 3A) Well MW-7 data will be included in the first <br /> quarter 2001 monitoring and sampling report <br /> 19976 4SR <br />