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encountered in any of the samples collected from boreholes SB-1 or SB-2 Both soil borings <br /> (SB-1 and SB-2) were drilled through the former tank location, adjacent to former borehole <br /> BH-1 and vapor extraction well MW-6 Comparison of the 1988, 1991, and I993 analytical <br /> results indicates a decrease in petroleum hydrocarbon concentrations, with a lack of detectable <br /> concentrations in later samples from depth <br /> Analysis of current soil samples "wall V, "wall 2", "wall 3", and "soil floor" (collected in <br /> November 1994 from the overexcavated former tank location) revealed detectable diesel fuel <br /> concentrations in the samples "soil floor" and "wall 3" only No detectable diesel <br /> concentrations were encountered in the "wall 1" or "wall 2" samples Soil samples "wall 1" <br /> and "wall 2" were collected at 9 feet below grade from the north and southeast walls <br /> respectively Soil sample "wall 3" was collected at 13 feet below grade along the southwest <br /> wall of the excavation and was found to contain 900 ppm diesel, soil sample "soil floor" was <br /> collected from the excavation floor at 18-feet below grade and was found to contain 740 ppm <br /> diesel fuel No detectable volatile petroleum hydrocarbons were encountered in any of the <br /> soil samples collected following overexcavation A review of this recent analytical data <br /> reveals a continuing decrease in petroleum concentrations from 2,900 ppm diesel in 1993 to <br /> 740 and 900 ppm diesel in 1994 (see Figures 5 and 6) <br /> Soil sample analytical results are summarized in Table 1 Current Analysis Request and <br /> Chain of Custody Record forms and laboratory results are contained in Appendices A and B <br /> respectively <br /> 7.2 Residual Diesel Fuel Calculations <br /> Review of soil sample analytical results to date indicates a decrease in volatile petroleum <br /> hydrocarbon concentrations to below detection limits in site soils Diesel fuel concentrations <br /> have been noted to decrease significantly, over the period observed, from a maximum of <br /> 7,100 ppm in 1988 to a maximum of 900 ppm in 1994 The following calculations provide <br /> an order of magnitude volume estimate of the remaining residual diesel <br /> Area - Zone immediately beneath the southern half of the former tank location, extending <br /> north to soil sample "wail P, east to soil sample "wall 2", south to well MW-8, west <br /> to well MW-9, and to 20 feet below grade to soil sample SB2-20, (note analysis of <br /> boundary samples revealed no detectable diesel fuel concentrations - see Figure 7) <br /> MZ/01-27-95/UST/95-0011 nbr 9 <br />