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i colo ical-rechnics/tic. � g Page 6 � Site Conceptual Model Project No. 989.2 December 17, 2003 ' MtBE mass estimate calculations indicate there is 0.0_ 11 gallons of MtBE remaining in the groundwater at the VWDC location (Table 7). See Section 4.6 for details on contaminant ' mass estimate calculations. 3.2 Extent of Contamination 3.2.E Extent of Soil Contamination The following assumptions and observations were made in calculating the extent of the ' contamination of TPH-G and BTEX constituents: • BTEX constituents closely follow the model of the TPH-G plume and will be included as in the "TPH-G" model. No TPI 4-G contamination was detected above 7 feet bgs or below 17 feet bgs. The soil zone between 7 and 17 feet bgs is assumed impacted. • Concentration values used in producing the TPH-G Soil Contour Map (Figure 6) are the ' average contamination levels for soil samples collected in each borehole between 5 and 17 feet bgs. ' • Most likely natural attenuation will have degraded the concentration of TPH-G and BTEX constituents in samples collected over the past two to eight years. o Comparing MW-1 soil data collected in 1995, we see that the 11-foot bgs sample contained 120 mg/Kg of TPH-G. In 2003, MW-101 was installed approximately 3 feet from MW-1 and the 11-foot soil sample indicated the presence of 3.5 mg/Kg of TPH-G. This two orders of magnitude reduction is also seen in the BTEX ' constituents. o For estimating the mass of contamination remaining in the soil it is assumed that measured TPH-G concentrations are one order of magnitude less than reported ' laboratory results if the sample was collected prior to January 1999. o For estimating the mass of contamination remaining in the soil the TPH-G concentrations are reduced by 33% from the reported laboratory results if the ' sample was collected prior to January 2001. o The reported laboratory concentrations for TPH-G are used for samples collected after January 2001. ' • The volume of soil removed during excavation was subtracted from the total volume of soil impacted by TPH-G. • GTI feels that the mass estimate calculations are conservative. These calculations may be an order of magnitude higher than current site conditions. The summary of soil analytical data (Table 1) suggests TPH-G is defined to the north by MW-5 and MW-9, to the northwest by MW-7, southeast by MW-3, to the east by MW-2, and vertically by MW-101. Mass estimate calculations indicate there is approximately 75 allons ' of TPH-G remaining in the soil (Table 6). Figure 6 shows the approximate lateral extent of TPH-G soil contamination in map view. Figures 8 and 9 show the approximated extent of TPH-G soil contamination in cross section view. M