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Mr Nuel Henderson,SJCEHD April 7,2006 . Well installation Report ARCO Facility No 5469 Stockton,California Page 6 Fine to coarse-grained sand was encountered from approximately 40 to 55 feet bgs at boring MW-10 (located near the northern ARCO property line) This sandy stratum may extend westward across South Wilson Way, but does not appear to extend to the south in the vicinity of the ARCO USTs and product dispensers Sand strata were also logged at boring CPT-1 between approximately 68 and 73 feet bgs and 90 to 92 feet bgs, the lateral extent of these sand units is not currently known Depth to groundwater beneath the site has ranged from approximately 36 to 45 feet bgs since monitoring was initiated in 2000 Depth to groundwater in the immediate site vicinity has been measured as low as approximately 60 feet bgs (Top Filling Station, 1995) Groundwater was first encountered at approximately 45 feet bgs within the soil borings completed during this phase of investigation East, southeast, and northeast groundwater flow directions have predominately been observed, at relatively low groundwater gradients Soil Analytical Results Petroleum hydrocarbon and fuel additive concentrations were reported below laboratory detection limits for each soil sample collected from the unsaturated zone MTBE (0 023 microgams per liter [ltg/L]) was reported in the sample collected from the base of well boring MW-11 (54 feet bgs) TBA (0 022 µg/L) was detected in the soil sample collected from approximately 44 5 feet bgs at well boring MW-12 Groundwater Analytical Results MTBE was detected in the samples collected from wells MW-11 (41 µg/L) and MW-13 (0 56 µg/L) 1,2-DCA was reported for the MW-12 and MW-13 samples, at concentrations of 0 74 µg/L and 7 4 µg/L, respectively Concentrations of TPHG, BTEX, TBA, ethanol, DIPE, ETBE, EDB, and TAME were reported below laboratory detection limits for the well MW-11 through MW-13 groundwater samples SUMMARY The following summarizes work completed at the site and the findings of this investigation • Fine grained soils (clay and silt mixtures) were predominately observed within the MW-11 through MW-13 well borings • Based on the findings of this and previous investigations, sandy strata in the upper portion of the saturated zone near the northern ARCO property line (MW-4 and MW-10 borings) does not extend southward to the area surrounding the ARCO • USTs and fuel dispenser islands K\Bp-Arco for URS\5469\Reports\ARC05469-weltreport20o6 doe 5IMMS