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CVA%TC r... ASSOCIATES I N C . groundwater sample from boring CPT 1 at approximately 52 feet bgs contained no detectable concentrations of TPHg or BTEX. MTBE and fuel oxygenates were below stated method detection limits for the groundwater samples collected from borings GPI, GP2, GP3, and CPTI. An isoconcentration map for benzene in the shallow water bearing zone (Figure 3) is used to estimate the lateral distribution of petroleum hydrocarbons dissolved in shallow groundwater beneath the site. Analytical results of groundwater samples are summarized in Table 2. L 5.0 SENSITIVE RECEPTORS According to City of Tracy officials contacted by ATC, no municipal drinking water supply is currently available east of MacArthur Drive which is located approximately three-quarters of a mile west of the site. In addition, the City currently has no plans to extend municipal water `r services eastward to the project site area. Instead, the site and the surrounding parcels rely on site wells to satisfy their water needs. The City does not currently have an aquifer or wellhead vulnerability program in place for the project site area. The on-site well is the closest �r groundwater receptor to the plume (within approximately 100 feet crossgradient of the plume). The well's total completion depth is 139 feet bgs with perforated casing extending from 130 feet to 139 feet bgs. A 100-foot cement grout sanitary seal extends from the surface to 100 feet below grade. As previously mentioned, clay is present from 22 to 51 feet bgs; and from 70 to 131 feet bgs. The second nearest groundwater receptor is located approximately 330 feet southeast, generally crossgradient, of the site's former UST locations. This well's total completion depth is 95 feet bgs with perforated casing extending from 65 feet to 70 feet bgs. A 50-foot cement grout sanitary seal extends from the surface to 50 feet below grade. A log of the well's borehole indicates the lithology includes sand from 18 to 24 feet bgs; clay from 24 to 46 feet bgs; alternating gravel and sand units from 46 to 90 feet bgs; and clay from 90 to 150 feet bgs. DWR well construction logs of wells within 2,000 feet of the site indicate that none are screened in the predominantly sandy water-bearing unit present at approximately ± 10 feet to ± 20 feet bgs. This sandy unit appears to be laterally continuous across the well survey area. Clay underlying the sand appears to be a continuous, confining stratum extending across the study area with a thickness of ± 25 feet. This would be expected act as a physical barrier against contaminants migrating vertically from above the overlying sand strata. 6.0 DISCUSSION The objective of this investigation was to evaluate the vertical extent of impacted soil in the vicinity of the former underground storage tanks (USTs) and possible lateral migration of impacted groundwater between wells MW4 and MW5. Based on the field observations and analytical results obtained during this investigation, the vertical and lateral extent of soil and groundwater in the vicinity of the former USTs has been defined. Although soil does not appear �.. to be impacted between wells MW4 and MW5, the groundwater samples collected from the s:\1 7861\reports\sum2004.doc 6 V