SITE INFORMATION AND CORRESPONDENCE

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conservative estimate made no attempt to average TPHG concentrations throughout the bulk soil volume. If this were done, the average TPHG concentration would include detections at 26 feet and 36 feet in former well MW-1 and the single detection at MW-4. The resulting hydrocarbon mass estimate would be 237 pounds if the average TPHG concentration was used. An additional aspect of the conservative mass estimate was the use of TPHG detections at former well MW-1 to establish the lateral and vertical extent of hydrocarbons remaining in place. The rationale for this was that these detections represented the furthest extent of a small contaminant plume that may have been present at one time beneath the site. It would then be expected, that samples obtained closer to the source within the former plume would detect higher concentrations of TPHG. This probably would have been demonstrated with samples at MW-4 and SB-1 locations if they were drilled when MW-1 was. However, MW-4 and SB-1 were drilled approximately four to six years after MW-I and as a result, the hydrocarbons remaining have been biodegraded to near non-detect levels. It is reasonable to believe then, that hydrocarbons at MW-1 (further from the source) are completely biodegraded now and the actual lateral extent of hydrocarbons remaining is probably between MW-4 and the former location of MW-1. The vertical extent of residual hydrocarbons should also be reduced due to the fact that detected concentrations of soil contamination below 15 feet at well MW-4 are lower than the maximum LUFr recommended action levels. The effective vertical extent of hydrocarbons remaining would then be between 15 feet and 20 feet. Considering these reductions in the lateral and vertical extent of hydrocarbons and an applied TPHG concentration of 130 ppm, the actual mass of hydrocarbons remaining is probably closer to 108 pounds. The estimated mass of benzene remaining in soil beneath the site is zero pounds. This estimate was based on the fact that benzene has never been detected in soil boring samples. The only benzene detected in soil beneath the site was from samples collected beneath the former UST complex. However, these hydrocarbon detections have been discounted for reasons explained earlier. Considering the conservative mass estimate, the composition of the remaining hydrocarbons mass (i.e., no benzene), the fact that the bulk of the mass is at shallow depths (c 20 ft.), and that groundwater has remained below 50 feet for more than five years, Chevron believes no significant threat to groundwater remains at this site.