SITE INFORMATION AND CORRESPONDENCE

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It 1 ATTACHMENT 1 Revised Estimate of Hydrocarbons Remaining in Soil The revised mass of hydrocarbons remaining in soil was estimated to be 11.5 pounds. The remaining mass is confined within a 4,550 square foot area measuring 5 feet thick which extends northeast beyond the overexcavation limits. The revised residual hydrocarbon mass estimate was based on the following assumptions: - All hydrocarbons remaining in soil are located downgradient from the northeast limits of the overexcavation. -The areal extent of hydrocarbons remaining in soil is confined between the downgradient limits of the overexcavation and the downgradient extent of the former dissolved hydrocarbon plume (Attachment 2). -The vertical extent of hydrocarbons is confined between 31.0 and 36.0 feet below surface. These confining depths are based on the shallowest residual contamination measured at 31.0 feet in sample TX-27 and the vertical extent of contamination defined at 36.0 feet by overexcavation sample TB-8. - 6.0 ppm TPHG (measured at sample TX-26) was applied uniformly throughout the estimated soil volume as the representative residual hydrocarbon concentration. -A bulk density value of 1.35 glcc was used to represent the confined soil volume. This is a published mean value described to be representative of all soil types. -The actual calculations used to derive this revised mass estimate are provided in Attachment 3. It should be pointed out that the revised mass estimate is conservatively high. The TPHG concentration which was uniformly applied throughout the revised bulk soil volume is probably greater than actual. The residual hydrocarbon detected in soil at TX-26 is thought to represent the downgradient extent of a formerly migrating groundwater plume. Therefore, any residual hydrocarbon concentrations beneath and downgradient from this location would be lower due to natural attenuating factors. Support for this was demonstrated by non-detect levels of TPHG measured during the destruction of well MW-6. Additionally, the revised contaminated soil volume is probably greater than actual. The revised areal extent was increased to account for the detection of trace levels of xylenes at well MW-8 and to account for the possibility that TPHG remains at concentrations lower than 10 ppm (former detection limit) in soil within the area of the former dissolved hydrocarbon plume. The percent of benzene estimated to be remaining within the revised contaminated soil volume is zero. This estimate was based on the fact that benzene has never been detected in soil samples downgradient from the overexcavation limits. This is easily explained by the fact that benzene biodegrades more rapidly than other BTEX constituents and by the fact that it volatilizes more readily. Considering the revised mass estimate and the composition of the remaining hydrocarbons, Chevron still maintains there is no significant threat to groundwater remaining at this site.