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Report on Additional Site Assessment and First Quarter 2000 Monitoring 4 7-Eleven Inc Store#21756,853 E Yosemite Avenue Manteca CA February 21 2000 40 RESULTS ' 41 Geology and Hydrogeology rThe 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 interbedded below about 13 feet below grade with clayey silt Depth to groundwater as first encountered was about 16-19 feet below grade Equilibrated groundwater ' levels were about 17- 18 feet below grade in January 2000 Groundwater flow, as mapped from first quarter 2000 data, is to the northwest at a hydraulic gradient of about 0 002 foot per foot(Figure 2) ' 42 Results of Sol] 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 Sod MTBE was found in only one location (GP-4)at a concentration of 0 167 mg/kg 43 First Quarter 2000 Groundwater Monitoring Analytical Results Groundwater analytical data (Figure 2, Table 2) indicate dissolved benzene concentrations of up to ' 0 32 lag/L, dissolved TPH-G levels of up to 55 pg/L, and dissolved MTBE levels of up to 230 pg/L Dissolved tertiary butyl alcohol (TBA)was also detected at concentrations of 3,710 pg/L (well MW-1) (Table 2) The MTBE plume is defined downgradient of the hydrocarbon source area (the former ' underground storage tank and pump island locations) Copies of the analytical reports and chain-of-custody forms are Included in Appendix D SDissolved oxygen data ranged between 0 7 and 1 milligram per liter(mg/L)(Table 2), which is indicative of oxygen depletion to generally anearobic conditions r