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Appendix C <br /> Mass Calculations <br />' This section describes theP rocedures and rationale ERM used to estimate the mass <br /> balance for West Weber Sites #1 and #2 <br /> ERM compared the remedial goals for TPH and BTEX to the confirmation and post- <br /> excavation sampling results to determine where and at what depths contaminated soil <br /> may still be present In order to estimate the remaining mass of TPH/BTEX in site soils, it <br /> was necessary to mathematically extrapolate the concentrations that are likely to be <br />' present below and in between the sample depth intervals The confirmation samples <br /> collected during excavation were taken at only one depth interval, S and 5 feet bgs for <br /> Sites #1 and #2, respectively, and subsequently collected soil boring samples were taken <br /> at three depth intervals, i e 9, 10 5 and 15 feet bgs The mathematical extrapolation using <br /> a polynomial curve-fit was performed for chemical concentration data from each of the <br />' soil borings, and resulting polynomial equation was applied to other sample locations <br /> This was done by proportionally scaling coefficients in the equation to the known <br /> chemical concentration values for confirmation samples from adjacent locations Once <br /> extrapolation was complete, all the concentrations of the constituent were averaged for <br /> the site This estimate of the average concentration of TPH/BTEX at each site was used in <br /> calculating the mass of each constituent remaining on the sites (see Tables C-1 through <br /> C-4) <br /> The thickness used for calculating the post-excavation volume of soil was based on the <br /> results of the concentration extrapolations The thickness chosen was the depth below the <br /> sample points where all the extrapolation values were at or below the remedial goals (see <br /> Tables C-1 through C-4) The pre-excavation thickness was set at the depth of the lowest <br /> soil boring sample interval This thickness is 11 feet below the first sample depth and a <br /> total of 16 feet below ground surface <br /> EThe surface areas used for calculating the pre- and post-excavation volumes are shown in <br /> Figures C-1 and C-2 The boundaries of the post-excavation surface areas were set at the <br /> Iestimated distance to the remedial goal concentration This estimation was obtained by <br /> linearly interpolating between samples with detected and non-detected chemical <br /> I concentrations Pre-excavation surface areas were based on the soil excavation boundary <br /> for Site #1 and the soil excavation boundary plus the post-excavation surface area for Site <br /> #2 (see Figures C-1 and G2) <br /> The pre- and post-excavation volumes were calculated by multiplying the surface areas by <br /> the thickness Using these volumes, an assumed soil density, and the average <br /> concentrations, the pre- and post-excavation TPH/BTEX mass estimates were calculated <br /> C-1 <br />