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Additional Site Assessment Report 3 7-Eleven, Inc,Store#21756,_853 Yosemite Avenue,Manteca,_CA August 13, 1999 ' 34 Groundwater Sampling A 9 ' On June 17, 1999, grab groundwater samples were collected from the base of soil borings GP-1, GP-2, GP-3, GP-4, and GP-5 The groundwater samples were collected using disposable bailers, then transferred to 40-milliliter glass vials filled to zero headspace and acidified to a pH of 2 The samples were sealed using Teflon®-septum caps, labeled, placed on ice in an insulated container, and ' transported under chain-of-custody to a state-certified laboratory The groundwater samples were analyzed for BTEX and TPH-G using EPA Methods 8020/modified 8015, and for MTBE, other oxygenates, ethylene dlbromide (EDB) and 1,2-dlchloroethane (1,2-DCA) by EPA Method 8260 1 40 RESULTS 41 Geology ' The shallow stratigraphy In the area consists of Quaternary alluvium As shown on the drilling logs (Appendix A), materials underlying the site consist largely of fine-to medium-grained sand underlain at about 15 feet below grade by silt, and silty clay ' Depth to groundwater as first encountered was about 13 feet below grade ' 42 Results of Soil Sample Analyses Laboratory analytical data is summarized in Table 1 and shown on Table 2 for samples collected ' June 17, 1999 Benzene and TPH-G were not detected Soil MTBE was found In only one location (GP-4) at a concentration of 0 167 mg/kg 43 Results of Groundwater Analyses Groundwater analytical data collected June 17, 1999, are presented in Table 2 and shown on Figure 2 Samples from locations GP-1, GP-2, GP-4, and GP-5 were non-detectable for all analytes, the sample from boring GP-3 contained benzene at concentrations of 607 pg/L, TPH-G at ' 32,300 pglL, MTBE at 46,300 pgIL, and tert-butyl alcohol (TBA) at 7,760 pglL Copies of the analytical results and chain-of-custody documents are included in Appendix B 1