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T <br />S E C 0 R <br />r <br />E 4.4 Estimated Mass of Petroleum Hydrocarbons in Groundwater <br />Concentration data from July 24, 2000 was used to develop area initial isoconcentration <br />contours while the most recent GWE system data from April 13, 2005 was used as an estimate <br />of mass removed to date (Table 7). The total mass of TPHg, benzene, and MtBE remaining in <br />groundwater are based on the change in mass estimates using area isoconce nt ration contours <br />and GWE system mass removal estimates. Mass balance calculations for soil and groundwater <br />are presented in Appendix D. Assumptions used for this mass estimate are listed below: <br />♦ The groundwater plume thickness is estimated at 52 ft. Groundwater is typically <br />encountered at approximately 55 to 58 ft bgs and the deepest recorded detection of <br />MtBE in groundwater was at MW -19C, which is screened from 105 to 110 ft bgs. The <br />assumption that the thickness of the dissolved plume is uniformly 52 ft is considered <br />conservative; <br />♦ Concentrations of constituents in the dissolved phase are assumed to vary linearly with <br />respect to distance between isocontour intervals. This assumption was made to <br />facilitate calculation by removing the logarithmic diffusion rate. Because concentrations <br />typically decrease exponentially with respect to distance, a linear relationship is likely <br />conservative; <br />♦ Porosity of the various soil types beneath the site range from 30% for sands to 29.9 to <br />40.7% for clays. An overall porosity of 35% has been selected to conservatively <br />estimate the amount of water available in saturated soils; and <br />♦ The highest representative dissolved hydrocarbon concentrations in the source area are <br />51,000 gglL for TPHg, 39,000 gglL for benzene, and 720,000 µglL for MtBE. These <br />values are based on monitoring well data. <br />Based upon these assumptions and known densities of gasoline, benzene, and MtBE, the total <br />mass remaining for each constituent has been calculated in the dissolved plume as follows: <br />Gasoline 55 10.8 43.2 19 <br />Hydrocarbons <br />Benzene 42 0.3 41.7 0.7 <br />MtBE 137 85.1 51.9 62 <br />95775 NFAR REPORT Final.doc 10 SECOR Intemational lncotporated <br />