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' Additional Site Assessment Report and Request for Closure 3 7-Eleven Inc,Store#17647, 1048 W Yosemite Avenue,Manteca,CA August 13, 1999 40 RESULTS 41 Geology The site is located in the central part of the San Joaquin Valley The shallow stratigraphy in the area ' consists of Quaternary alluvium and lake deposits The materials encountered during recent drilling at the site generally consisted of silty fine sand grading downward to medium-grained sand to a depth of about 15 feet below grade Clay soils underlie the sandy strata at depths of 15 feet below grade(the total depth of exploration) (Appendix A) During drilling, groundwater was encountered at approximately 13 feet below grade 1 ' 42 Results of Soil Sample Analyses Table 1 summarizes the laboratory analytical results for sod samples collected on June 18, 1999 (Figure 2) All sod hydrocarbon concentrations were non-detectable, with the exception of 0 026 mglkg MTBE detected at 16 feet below grade in boring GP-3 (located at the northwest corner of the UST pit area) Copies of the analytical results and chain-of-custody documents are Included in Appendix D 4.3 Results of Groundwater Sample Analyses Groundwater analytical data for samples collected on June 18, 1999, are presented in Table 2 (Figure 2) Benzene was not detected Dissolved TPH-G was non-detectable in all locations except ' GP-3 (19,400 pg/L) and GP-4(,128 pg/L) Dissolved MTBE was only detected in location GP-3 (126 pg/L) Dissolved TBA, DIPE, ETBE, and TAME were not detected in any of the samples collected Dissolved EDB and 1,2-DCA were not detected in the sample from GP-5(Appendix B) Copies of the analytical reports and chain-of-custody forms are included in Appendix D I ' I , I i i rl i i r i i I ! I i t I I r it � E