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kfFf{, 27 June 1996 AGE-NC Project No. 95-0118 Page 21 of 23 _l Typical costs for monthly monitoring of an in-situ bioremediation system, in addition to standard quarterly sampling, would be approximately $1,000.00 per month. Based on a two-year treatment period, the total cost for in-situ bioremediation at the Elkhorn site could be between $75,000 and r= $110,000. !,,_d 9.0. RECOMMENDATIONS FOR REMEDIATION OF HYDROCARBON-IMPACTED ' SOIL =� Based upon analytical results of soil samples collected during the investigation of the former UST area, it appears that hydrocarbon-impacted soil extends approximately 15 feet northwest and 10 feet east of the former UST excavation, having a radius of approximately 25 feet. The impacted soil appears to be distributed between 10 and 40 feet bsg. Approximately 1,000 to 1,200 cubic yards of hydrocaron-impacted soil is residual on the site. A lithologic cross section of the impacted area is depicted in Figure 6 - Lithologic Cross Section. Since impacted soil extends beneath the maintenance building, excavation is probably not feasible. Therefore,an in-situ method should be considered.Both in-situ methods described in this CAP could be equally effective in remediating the site. From a cost basis, implementation of an in-situ bior�TOation program could potentially be less expensive than soil vapor extraction. However, r monitoring requirements, duration and limited controls of the technology could increase the cost significantly. Therefore, we recommend installation of an SVE system at the site. .= The use of an SVE system at the Elkhorn site could remove up to 90%to 95% of the "pollutants" from the soil. Thus,the anticipated remaining concentrations of petroleum hydrocarbons would be - - small: Soil remediation methods, estimated durations and associated costs are summarized on Table ` 7. r=1 Because of the relatively high flow rates measured during the SVE pilot test, a unit capable of at GJ_r' least 100 scfm will be required for remediation at the site;a thermal oxidation or internal combustion SVE unit would be the most cost-effective choice. A 6 to 18 month soil remediation period can be _ expected. Installation of an SVE system should include installation of two additional soil vapor extraction wells within the contaminated soil plume; this will allow the rapid removal of hydrocarbon contaminants- One soil vapor extraction well should be installed north of MW-2; a second soil vapor extraction well should be installed south of MW-3. The SVE well network should include all extraction wells and monitoring wells MW-2 and MW-3. A Remedial Action Work Plan detailing the proposed installation and operation of the SVE system ;. :.1 should be prepared, •1