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February 14, 1997 Page 14 5 5 . bons in groundwater samples collected in plume source areas have decreased This trend indicates that SVE was effective in rernediating hydrocarbon-affected soils in the soul ce ai ea • As shown on the geologic cross-sections, soil analytical data collected from borings and wells indicate that soils beneath the site are defined to non-detectable levels for TPPH-g and benzene laterally and verti- cally • The dissolved petroleum hydrocarbons plume is essentially defined to non-detectable or very low levels, is stable and decreasing in size • Water quality data for on- and,off-site wells infer that significant hydrocarbon degradation occurs across the site fi om the former primary source area near Well MW-9 toward downgradient off-site Wells MW-12 and MW\14 The data suggest that the leading edge of the plume boundary is controlled by intrinsic remediation Residual petroleum hydrocarbons in soil and groundwater have been defined to non- detectable concentrations and have been shown to be attenuating The installation of additional monitoring wells is not necessary The soil vapor survey performed prior to source removal did not detect petroleum hydrocarbons vapors emanating to near surface soils The nearest downgradient receptor is a well located approximately 1,800 feet northeast of the site Therefore, the potential for any human health or environmental risk is negligible and the remaining petroleum hydrocarbons are not likely to have any adverse impact on water quality SUMMARY AND CONCLUSIONS PACIFIC recommends that the site be closed based on the following 1 All USTs and associated piping have been removed from the site 2 SVE operation has effectively remediated petroleum hydrocarbons in soil such that there is little potential for future impact to groundwater • Based on data collected from borings drilled before and after SVE system operation, hydrocarbon concentrations in soil and groundwater have significantly been reduced' Soil chemistry data from borings indicate that TPPH-g mass in the vicinity of the former USTs was estimated to range between 18,000 and 33,000 pounds mass prior to SVE system operation, after SVE system operation TPPH-g mass in the vicinity of the former USTs is estimated to be 4,000 pounds mass Based on these figures, a total reduction of TPPH-g mass ranging 3201 337B\cLosLTRE3 , 4