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C A M B R I A <br /> of hydrocarbon mass detected was in soil less than 20 ft deep (Tables 1, 2 and 3, Attachment <br /> B) Hand-annotated hydrocarbon distribution figures are presented in Attachment C <br /> Mass Balance Calculations <br /> To estimate the hydrocarbon mass remaining in soil, we first estimated the mass of <br /> hydrocarbon-bearing soil We then estimated the average hydrocarbon concentrations in the <br /> impacted soil volume Once we estimated the volume of soil and hydrocarbon <br /> concentrations, we could estimate the hydrocarbon mass remaining to soil These steps are <br /> discussed below Tables 1, 2 and 3 present the estimated hydrocarbon mass remaining in soil <br /> Volume of Hydrocarbon-Bearing Soil. We determined the volume of hydrocarbon-bearing <br /> soil by contouring the horizontal distribution over discrete depth intervals Hand-annotated <br /> hydrocarbon distribution figures for the various depth intervals are presented in Attachment <br /> C To be conservative, we multiplied the length by the width of each contour and assumed a <br /> rectangular contour shape In reality, the contour shape is ovoid so the estimated soil <br /> volume is likely only 70% to 80% of the volume calculated assuming a rectangular shape <br /> Estimated Hydrocarbon Concentrations The data set available was too small for significant <br /> statistical analysis Therefore, we assumed that the average concentrations for each volume <br /> of soil were half of the maximum concentration detected in the impacted volume Since <br /> concentrations typically decreases exponentially from hydrocarbon source areas, this <br /> approach is very conservative and will over-estimate the hydrocarbon mass <br /> Hydrocarbon Mass in Soil Based on the approach and assumptions presented above, the <br /> estimated PHg, enzen and lel-mass remaining in soil are`_34�1, 0L011 and L2L61D gal tons, <br /> respectively (Tables 1, 2 and 3) As indicated above, these estimates are conservative <br /> because of the assumptions used for soil volume and concentrations In addition, the <br /> concentration data used for these calculations is three to four years old It is likely that <br /> current concentrations are well below initially detected concentrations due to hydrocarbon <br /> biodegradation and attenuation Therefore, the actual hydrocarbon mass is likely much less <br /> than the conservative hydrocarbon mass estimates presented here <br /> 1 <br /> 2 <br />