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C A M B R I A November <br /> Lori Duncan <br /> 18,2005 <br /> constituents. Therefore, for this calculation, the impacted groundwater thickness was assumed to <br /> extend from depth to water(37 fbg)to 70 fbg,a total of 33 feet. Because the deeper groundwater <br /> (represented by MW-13) is not impacted by TPHg or benzene, it is assumed that it does not <br /> contain significant TPHg or benzene mass. <br /> Isoconcentration contour maps of shallow TPHg and benzene concentrations in groundwater are <br /> presented as Figures 14 and 15, respectively. The porosity was assumed to be 40 percent, based <br /> on typical porosity values for soil types published in Groundwater, by Freeze and Cherry, 1979. <br /> Since the site soil types contain sand, silt, and clay, the midpoint of where the ranges of porosity <br /> for these soil types overlap was used. Porosity values for various soil types are presented in the <br /> © notes section of Table 7. The average contaminant concentration per contoured interval was <br /> determined by the same method as for soil concentrations. The average contaminant <br /> concentration was assumed to be half the value of the higher contour interval. For the center <br /> contour interval, we assumed that the average contaminant concentration was half the highest <br /> detected value. <br /> TPHg in Groundwater: The TPHg isoconcentration contour map (Figure 14) and historical <br /> laboratory analytical data show that wells south of the subject site (cross-gradient) contain <br /> tetrachloroethene (PCE), which is reported by the laboratory as TPHg. Because this constituent <br /> did not originate on site, data from these wells was not used to calculate the TPHg mass in <br /> groundwater. Table 7 presents the estimates and calculations for TPHg mass remaining in <br /> groundwater. Based on the approach and assumptions presented above,the estimated TPHg mass <br /> in groundwater is approximately 83 pounds. <br /> Benzene in Groundwater: Table 7 presents the estimates and calculations for benzene mass <br /> remaining in groundwater. Based on the approach and assumptions presented above, the <br /> estimated benzene mass in groundwater is approximately 3 pounds. <br /> These contaminant mass remaining estimates are based on available site data and an assumed <br /> static environment versus the true dynamic environment. The error associated with these <br /> estimates should be considered with any decision-making. <br /> 15 <br />