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• Non-detect values reported for TPH as diesel were assumed to have a value of 0 5 mg/kg, corresponding to one-half of the typical detection limit of TPH as diesel in soil (1 mg/kg) Treated Mass of TPH as Diesel in Soil The mass of TPH as diesel treated was calculated by multiplying the volume of soil excavated by the average TPH as diesel concentration measured in the excavated soil Approximately 330 cubic yards (8,910 ft) of TPH as diesel impacted soil was excavated and disposed of during the interum corrective action implemented in May 1995 The average TPH as diesel concentration in the excavated soil was 2,975 mg/kg This corresponded to a treated mass of 2,482 lbs of TPH as diesel Table 6 provides the detailed calculations performed Residual Mass of TPH as Diesel in Soil To estimate the residual mass of TPH as diesel in soil, a statistical method was used since an extreme variation in the TPH as diesel concentration was measured at the site (5 orders of magnitude) The following method was followed to ensure a realistic, yet conservative estimate of the residual mass of TPH as diesel in soil. • A histogram was plotted to determine the distribution of TPH as diesel in the soil (Figure 11). All the available data from soil borings drilled at the site between 1987 and 1995, totaling seventy (70) data points. The resulting histogram exhibited a log-normal distribution with an extreme right skew spanning five orders of magnitude (<1 mg/kg to 130,000 mg/kg). A log-normal distribution of TPH as diesel in the soil is consistent with contaminant distributions typically observed in soil and groundwater (USEPA 1989, 1992, 1994) More than 80 percent of the data yielded concentrations of TPH as diesel less than 100 mg/kg, and 50 percent of the data yielded non-detect concentrations • Data analysis used procedures recommended by Ott (Ott, 1995) to determine the residual mass of TPH as diesel in the soil. A log-normal distribution of TPH as diesel in the soil was assumed. The residual mass of TPH as diesel in the soil at the site was calculated to be 1203 lbs, corresponding to 160 gallons Table 7 provides the sequential method that was employed when calculating the residual mass. Mass Balance Calculation The treated mass of TPH as diesel was calculated to be 2,482 lbs (198 gallons) in 8,910 ft' (330 cubic yards) of impacted soil This was treated by excavation of impacted soil in the source area The residual mass of TPH as diesel in the soil was calculated to be 1,203 lbs MWO&I 3-97/VDBF/97-0009 BK 7