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i <br /> Chevron I <br /> t/ Chevron <br /> ATTACHMENT <br /> Estimate of Hydrocarbons Remaining in Soil <br /> The mass of hydrocarbons remaining in soil was estimated to be 1.5 pounds. The remaining mass <br /> is confined within a 3,048 square foot area measuring 5 feet thick which extends northeast beyond <br /> the overexcavation limits. The residual hydrocarbon mass estimate was based on the following <br /> assumptions: <br /> - All hydrocarbons remaining in soil onsite are located downgradient from the limits of the <br /> overexcavation(i.e., northeast beyond overexcavation soil samples TX-26 and TX-27). <br /> -The lateral extent of hydrocarbons remaining in soil is confined within the area defined by t/ <br /> overexcavation sample locations TX-26 and TX-27 and former wells MW-7 and MW-9. <br /> The vertical extent of hydrocarbons is confined between,31.0,and 36.0 feet below surface. These <br /> confining depths are based on the shallowest residual contamination measured at 31.0 feet in <br /> sample TX-27 and the vertical extent of contamination defined at 36.0 feet by overexcavation <br /> sample TB-8. <br /> - 6.0 ppm TPHG(measured at sample TX-26) was applied uniformly throughout the estimated <br /> soil volume as the representative residual hydrocarbon concentration. <br /> -A bulk density value of 1.35 glcc'was used to represent the confined soil volume. This is a <br /> published mean value described to be representative of all soil types. <br /> It should be pointed out that the above estimate is conservatively high. The soil concentration <br /> which was uniformly applied throughout the entire estimated volume is probably greater than <br /> actual. The residual hydrocarbon detected in soil at TX-26 is thoughP g t to represent the down radient V <br /> extent of a formerly migrating';groundwater plume. Therefore, any residual hydrocarbon <br /> concentrations -beneath and downgradient from this location would be lower. in.addition, the <br /> estimated volume of contaminated soil remaining is probably.greater than actual. The estimated <br /> extent of residual hydrocarbons beyond the overexcavation is defined by the non-detect soil <br /> samples obtained at former wells MW-7 and MW-9. The actual downgradient extent probably <br /> exists between the overexcavation-limits and MW-7 and MW-9. <br /> Based on the small estimated mass and composition-of hydrocarbons remaining in soil onsite, it is <br /> believed that no significant threat to.groundwater in this area exists. <br /> �� ' <br />