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19 June 2000 AGE-NC Project No 96-0232 Page 33 of 35 9 4 4 Estimated Costs It is likely that some additional laboratory analysis of ground water samples would be required to monitor and verify natural attenuation A long-term monitoring program, perhaps 25 years, would be initiated Quarterly monitoring and sampling could costs between $5,000 to $15,000 annually, and report preparation could cost between $1,000 to $5,000 annually Therefore, the total cost for natural attenuation could range from $6,000 to $20,000 annually 10.0. RECOMMENDATIONS AGE has evaluated four methods to remediate hydrocarbon- and MTBE-impacted soil and ground water on the subject site In view of site specific conditions,the principles of the remedial methods, the advantages and disadvantages of eachremedial method and AGE,s experience with each method, AGE recommends that impacted soil and ground water at the site be addressed as described below ; 101 REMEDIATION OF HYDROCARBON-IMPACTED SOIL Petroleum hydrocarbon-and MTBE-impacted soil in the vadose zone appears to be limited to the area containing soil borings Pl,P6,P12,P14,P19,P22,P23, P24,P31 and P32 The plume of impacted soil at the site has an estimated north-south diameter of approximately 40 feet,an east-west diameter of approximately 200 feet and appears to extend downward more than 30 feet bsg (See Figures $, 9 and 41) The calculated volume of hydrocarbon-and MTBE-impacted soil is approximately 3,149 cubic yards (Appendix F) The greatest contaminant concentrations in impacted soil are present at depths between 10 and 20 feet, while lesser concentrations are present from 20 to 35 feet bsg The feasibility for soil vapor extraction, excavation, bioremediation and natural attenuation to effectively address the impacted soil is low Since MTBE is highly soluble and does not adhere to soil well, a passive remediation appears to be the best remedial alternative The major portion of the soil plume is submerged beneath the ground water table, MTBE should dissolve and migrate into the ground water medium, where it can be addressed by the ground water remediation effort Intuitively, the rate that adsorbed MTBE dissolves into ground water should be related to the soil types,less permeable fine-grained soil retarding diffusion and physical migration with ground water movement AGE does not recommend aggressive remediation of hydrocarbon-and MTBE-impacted soil at the site Rather, the soil plume should be monitored periodically with confirmation soil borings until the contaminants have been adequately dissipated Soil remediation methods,estimated durations and associated costs are summarized on Table 1 I Advanced CcoEnWonmental,Inc