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' Well Installation Report 6 7-Eleven Inc., Store#14117, 2725 Country Club Boulevard,Stockton, CA August 12, D 4.0 RESULTS 4.1 Geology ' The site is located in the northern part of the San Joaquin Valley at an elevation of approximately sea level. The shallow stratigraphy in the area consists of Quarternary alluvial/deltaic sediments. The sand ' units are generally fresh-water-bearing east and southeast of Stockton, but brakish or saline water occurs in these sediments in the delta area west of Stockton (California Department of Water Resources, 1967, Bulletin No. 146). The saline water/fresh-water boundary trends north to south ' across the western part of Stockton. The brakish or saline groundwater west of this boundary is often overlain by a layer of fresh groundwater to a depth of approximately 50 feet BGS; therefore, shallow ' monitoring wells may encounter fresh groundwater on both sides of the boundary. The materials encountered during recent drilling at the site generally consisted of sandy/silty clay to clayey silt to approximately 15 feet BGS, followed by medium sand to clayey sand to 25 feet BGS, clayey silt to 35 ' feet BGS, and line to medium sand to total depths explored (51 feet BGS)(Appendix A). During drilling, groundwater was encountered at approximately 18 feet in the soil borings. Static water level in the wells following installation was approximately 7.5 feet below top of casing (TOC). 4.2 Results of Soil Sample Analyses Table 1 summarizes the laboratory analytical results for soil samples collected on May 13 and 14, 1999. The highest benzene and TPH-G concentrations were detected in sample MW-3-15' at 2.89 mg/kg and 429 mg/kg, respectively. The highest MTBE concentration by EPA method 8260 was detected in soil ' sample MW-4-20` at 0.119 mg/kg. Table la summarizes analytical results for soil samples analyzed for volatile organics by EPA method 8260. The highest methylene chloride, naphthalene, 1,1,2 ' trichloroethane concentrations were detected in sample MW-3-15' at 4.3 mg/kg, 10.7 mg/kg, and 10 mg/kg, respectively. Acetone was detected in sample MW-2-25' at 0.07 mglkg. Copies of the analytical results and chain-of-custody documents are included in Appendix C . ' Based on the samples collected during the p g installation of the four groundwater monitoring wells and the seventeen geoprobe soil borings the vertical and lateral extent of benzene, MTBE, and TPH-G in soil ' beneath the site has been delineated. Benzene, MTBE, and TPH-G concentrations in soil appear restricted to the immediate area of the former USTs and fuel dispenser island. West of the former LIST ' area maximum benzene and MTBE concentrations were detected in MW-3 at 15 feet BGS at 14117we11 ipl ",.ter