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r <br /> ranging from 15,000 mg/Kg near the source (at a depth of 15 feet in SB-1) to less than 10 <br /> mg/Kg at 40 feet from the source (at all depths in SB-2) <br /> r <br /> Bioassessment tests results indicate that the site contains indigenous micro-organisms capable <br /> of breaking down the residual TPH as diesel (Table 4) (PT, 1995b) Geotechnical data <br /> collected at the site also indicates that the soils contain the appropriate substrate for the <br /> growth of micro-organisms (Table 3) Therefore it is believed that mu=ic bioremediation <br /> has contributed to the reduction of TPH as diesel concentrations in the vadose zone <br /> Fourteen quarters of groundwater sampling at the site suggest that the TPH as diesel in not <br /> migrating laterally from the source. This is believed to be due to the source removal actions <br /> in 1986 and 1995, and the asphalt and concrete paved surface at the site that has minimized <br /> infiltration of surface water to the subsurface <br /> 10.0 Mass Balance Calculation <br /> A mass balance calculation was performed to estimate the mass of TPH as diesel in soil <br /> remedrated and estimate the residual mass of TPH as diesel remaining in the soil at the site. <br /> Method of Calculation <br /> The treated mass of TPH as diesel in soil was initially calculated Secondly, the residual <br /> mass of TPH as diesel in soil was calculated. Based on the results, the percentage of TPH as <br /> diesel mass treated was calculated, as described below. <br /> r <br /> Assumptions <br /> To perforin the mass balance calculation the following conservative assumptions were made. <br /> • The volume of soil treated was 330 cubic yards, based on the source removal <br /> action conducted in May 1995 There was no consideration of biodegradation of <br /> TPH as diesel that would have occurred since May 1986, when the USTs were <br /> initially removed. <br /> • Based on the geology at the site, the soil density was assumed to be 1.5 g/cm3 or <br /> 93 6421b/f0 This density corresponds to a silty soil, and is consistent with the <br /> geotechnical data from the site (Table 3) <br /> • The TPH as diesel density was assumed to be 0 9 g/cm3 <br /> r <br /> MZ'06-13-97/VDBF/'97-0009 BK 6 <br />