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18 February 1997 AGE-NC Project No 95-0I04 Page 20 of 22 I samples would be required to monitor and verify natural attenuation Furthermore, as stated in Section 6 4 4, a long-term monitoring program, perhaps 10 years, would be initiated I Overall costs for natural attenuation are difficult to estimate because of the lengthy attenuation period and the uncertainties involved in the process Assuming some additional characterization of the site would be necessary, natural attenuation could ultimately cost as much as $75,000 More accurate Icosts could be determined once regulatory approval for a long-term monitoring plan is received IS 0 RECOMMENDATIONS 8 1 REMEDIATION OF HYDROCARBON-IMPACTED SOIL I Petroleum hydrocarbon impacted soil in the vadose zone appears to be limited to the area beneath Y P the former UST However, the impacted soil appears to extend outward with an approximate fifteen Ifoot radius and appears to extend downward to depths of 45 bsg The greatest concentrations of impacted soil is present at depths between 15 and 30 feet, while declining concentrations are present lie from 30 and 45 feet A "smear" zone near the fluctuating ground water table has not been well documented in soil samples collected up to the present time I Excavation of all petroleum hydrocarbon impacted scil 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 requi e the implementation of I 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 in-situ method of remediation will be most effective at the site Both in-situ methods (bioremediation and soil vapor extraction) described in this CAP could be effective to remediate the site On a cost basis, implementation of an in-situ bioremediation program I could be comparable in cost to soil vapor extraction, with minimal site disturbance and little necessary equipment However, monitoring requirements, duration and limited control of the technology could increase the cost significantly Soil remediation methods, estimated durations and associated costs are summarized on Table 5 - Soil and Gi ound Water Cori ective Action Alternatives AGE has the following recommendations for remediation of hydrocarbon-impacted soil at the IMarlowe site IDesign and installation of an SVE system The existing extraction well should be replaced . with a new well, screened only within the zone of remaining contaminated soil, approximately_ 15 to 45 feet bsg The new extraction well should be dual or triple-completed-with multiple Iwell casings, including one screened within the capillary fringe The design of this extraction J I