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A S S O C I A T E S I N C Here, only benzene was detected above laboratory detection limits at a concentration of 1.7 ug/L (Table 2). Criterion 3: No existing water supply wells, deep aquifers, surface waters or other receptors are threatened by pollutants remaining in the aquifer. Utilizing the geophysical analytical results of soil samples collected from the upper sand strata by Smith Technology, the estimated time for the hydrocarbon plume to reach the on-site domestic well (the nearest groundwater receptor) is 20 years by advection alone. However, potential petroleum hydrocarbons that leach from the vadose zone and become dissolved in groundwater beneath the site are intrinsically biodegraded to these regulatory levels within about 11 years. The on-site well is the closest 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 bw 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 r. migrating vertically from above the overlying sand strata. The deep aquifer is protected from the overlaying confining Corcoran Clay layer. Criterion 4: The total pollutant mass remaining in the groundwater is decreasing at predicted rates and neither creates, nor threatens to create, a risk to human •- _ health and safety or future beneficial uses of the aquifer. wA95122Veports\RequeA for NFA doc 16