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' 05 May 1997 • AGE-NC Project No 95-0173 ' Page 27 of 29 I8 4 4 Estimated Costs It is likely that some additional assessment, including additional laboratory analysis of ground water ' samples would be required to monitor and verify natural attenuation Furthermore, as stated in Section 6 4 4, a long-term monitoring program, perhaps 25 years, would be initiated Generally, the cost for natural attenuation would range from $6,000 to $20,000 annually Quarterly monitoring and sampling generally costs between $5,000 to $15,000 annually, report preparation would generally cost between $1,000 to $5,000 annually ' 9.0. RECOMMENDATIONS I91 REMEDIATION OF HYDROCARBON-IMPAC FED SOIL Petroleum hydrocarbon-impacted soil in the vadose zone appears to be limited to the area containing ' wells MW-1 through MW-3, VW-1, VW-2, AW-1, AW-2, soil borings B1 through B3 and B6 Impacted soil at the site has an estimated north-south diameter of 130 feet, a east-west diameter of 120 feet and appears to extend downward to depths in excess of 75 bsg The greatest concentrations of impacted soil is present at depths between 15 and 40 feet, while declining concentrations are present from 40 and 75 feet A "smear" zone near the fluctuating ground water table has not been ' well documented in soil samples collected up to the present time Excavation of all petroleum hydrocarbon-impacted soil within the vadose zone would require the ' removal of a large volume of overburden soil and is not logistically feasible at the site However, excavation of a portion of impacted soil is feasible, but would require the implementation of additional remedial methods at greater depths Furthermore, excavation of shallow impacted soil ' (less than 25 feet bsg) would not facilitate remediation of hydrocarbon-impacted ground water Therefore, an to-situ method of remediation will be most effective at the site The in-situ method soil vapor extraction described in this CAP could be effective to remediate the site Soil remediation methods, estimated durations and associated costs are summarized on Table 7 AGE recommends the followingactions for remediation of hydrocarbon-impacted soil at the site ' - Preparation of a Remedial Action Work Plan (RAWP)'detailing the proposed SVE system - Design and Install a SVE systefn - Assessment of the thermal oxidation unit should be performed every 6 months of operation to evaluate progress If necessary, an activated carbon SVE system should be installed to I Advanced GcoEn►Iron mental,Inc 4 Z)