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16 assumed that all data were collected in 1992, corresponding to the first deep soil borings <br /> drilled at the site <br /> I <br /> For the scenario with a 55 feet thick vadose zone, the model calculated a maximum mass of <br /> 2 054 g/yr TPH as diesel leaching from the vadose zone at the groundwater interface at MW- <br /> 4 This corresponds to a leachate concentration of 0 420 mg/L The model predicted this <br /> maximum concentration to be observed after 1 year. <br /> For the scenario with a 80 feet thick vadose zone, the model predicted a maximum mass of <br /> 0 112 E-05 g/yr TPH as diesel leaching from the vadose zone at the groundwater interface at <br /> B-102 This corresponds to a leachate concentration of 6 11 E-07 mg/L The model <br /> predicted the maximum concentration to be observed after 25 years <br /> Appendix B provides the model output files in tabular and graphical format <br /> 4.0 Area-weighted Average Groundwater Concentration <br /> The model indicates that the maximum impact to the groundwater occurs during shallow <br /> rather than deep groundwater conditions, i e when the vadose zone thickness was 55 feet, <br /> rather than 80 feet Therefore, to estimate a "worst-case" scenario, the shallow vadose zone <br /> results were used to calculate the average groundwater concentration of TPH as diesel for 25 <br /> years An area-weighted average calculation method was used. Figure 3 illustrates the area <br /> that was used for the area-weighted average calculation Table 3 summarizes the calculated <br /> results Figure 4 provides a graph of the TPH as diesel concentration versus time for the <br /> area <br /> Conservative use of the model indicates that the maximum average TPH as diesel <br /> concentration in groundwater from the leachate would be 44 pg/L This is consistent with <br /> actual site groundwater data that has been collected quarterly since 1993 <br /> 5.0 Recommendations <br /> The results derived from the VLEACHSM model are consistent with actual groundwater <br /> monitoring data that have been obtained from quarterly groundwater sampling activities On <br /> an area-weighted basis, these "worst-case" concentrations indicated by the model are <br /> Mti11-01-96/VDBF/96-0022 LF 4 <br />