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November 12, 1996 Page 12 - -Wells MW=12 and-MW=14-The data suggest that ttie leading edge of the plume boundary is controIIed 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 nearest down- gradient receptor is a well located approximately 1,800 feet northeast of the site Therefore, remaining petroleum hydrocarbons are not likely to adversely impact 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 effes1wdy remediated petroleum h drocarbons in soil such that there is little potential for future impact to groundwater • Based on ata 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 sSVE 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 ` from 78-to 88 percent was achieved during the time period between SVE system activation and the present Since SVE system operational data indicate that approximately 12,000 pounds of petroleum hydro- carbons were removed by the SVE system, the remaining 6,000 to 21,000 pounds were removed as a result of natural attenuation proc- esses The migration potential has therefore been reduced by the reduction of total mass beneath the site 3 Impact of petroleum hydrocarbons on groundwater has been effec- tively remediated by SVE as evidenced by minor residual concentra- tions of TPPH-g and benzene in the source area and plume definition by non-detectable concentrations of petroleum hydrocarbons 4 The closest domestic well is 1,700 feet upgradient from the site The case closure checklist, which summarizes site conditions is presented as,Attach- ment E The site poses no significant threat to groundwater quality, therefore PACIFIC recommends case closure � . I 3201337131CLOSURE2 i