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r MMC M PROBLEM ASSFSSMENT REPORTANM M REMEDIAL ACTION PLAN -9- Former Beacon Station No. 494, 2185 East Fremont Street Stockton, CA—AMV Project No. 19005.07 ' The results from the limited soil gas survey generally correlate with the soil sample analytical data. The highest soil vapor concentrations observed were from vapor extraction wells VW-1, VW-1b, VW-2a, VW-2b, and VW-4. The soil sample analytical data and the soil gas survey results indicate the extent of petroleum ' constituents in soil is limited to the vicinity of the former underground storage tank basin. The highest TPHg concentrations (9 10 to 5,400 mg/kg) appear to occur in sand units underlying the vicinity of the south wall of the former tank basin at a depth of approximately 30 feet below grade, as indicated by analytical results from the borings for monitoring well MW-3 and vapor extraction well VW-1. Analytical results of soil samples collected deeper in the same borings ' (between 45 and 50 feet below grade) indicate decreasing TPHg concentrations (730 to 790 mg/kg) in sand and silty sand units. In the vicinity of the western wall of the former tank basin, the analytical results for soil samples from the boring for extraction well VW-2 at depths of 30 and 40 feet below grade contained TPHg concentrations of 200 and 410 mg/kg, respectively. The boring log for vapor extraction well VW-2 indicates a sand unit between 30 and 36 feet below grade. In six other samples analyzed from the boring for extraction well VW-2, the highest TPHg result was 4.7 mg/kg. The soil boring logs for monitoring/extraction wells MW-3, VW-1, and VW-2 all indicate the ' presence of silty clay units beginning at about 35 feet below grade, which underlay the sand unit(s) containing the highest TPHg concentrations. Soil sample analytical results indicate that the (apparently) reduced permeability of the silty clay units at approximately 35 feet below grade resulted in reduced TPHg concentrations at depth. The silty clay units observed at depths of 35 to 50 feet below grade would presumably act to further impede downward migration. ' 5.2 Ground Water Conditions and Distribution of Petroleum Constituents in Ground Water Current (July 1993) depth to ground water is approximately 73 feet below surface grade. This depth is about 40 feet below the sand units containing the highest TPHg concentrations. The highest concentrations of dissolved benzene in ground water have been detected-in samples ' collected from monitoring well MW-3, located near the southwest corner of the former underground storage tank basin (Figure 6). The July 1993 depth to ground measurements indicate a general direction of ground water flow toward the northeast at a gradient of approximately 0.008 foot per foot. Previous depth to ground water measurements have indicated widely variable ground water flow directions. ' Resurvey of monitoring well reference elevations in July 1993 indicate the July 1993 results are probably the most accurate obtained. At least two additional rounds of ground water monitoring ' Moos--