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' 18 July 2002 AGE-NC Project No 96-0254 ' Page 16 of 17 The duration for natural attenuation can be many years Costs for natural attenuation are generally ' lower than most active remediation options,however,costs can be highly variable depending on the duration and type of monitoring required iFate and Transport modeling may be used to demonstrate the potential for hydrocarbons to impact nearby ground water receptors Quarterly ground water monitoring may also be used to demonstrate Ia stable and declining ground water plume 7.0. RECOMMENDATIONS FOR REMEDIATION OF HYDROCARBON- IMPACTED SOIL Vadose zone soil impacted by TPH-d, TPH-g,BTEX and MTBE extends vertically to the saturated zone under the former UST and surrounding areas to an approximate depth of 10 feet bsg(Figure 3) Excavation would require the removal of a minor volume of overburden soil in the former UST and surrounding areas Figure 7 illustrates the approximate area of proposed excavation Because there appears to be a low feasibility for bioremediation and monitored natural attenuation,AGE believes that excavation would be most effective for remediating impacted soil at the site.Soil remediation Imethods, estimated durations and associated costs are summarized on Table 5 A Remedial Action Work Plan detailing the proposed excavation should be prepared 8.0. RECOMMENDATIONS FOR REMEDIATION OF HYDROCARBON- IMPACTED GROUND WATER AGE believes that an in-situ method would be more effective in both cost and performance over a "pump and treat" method Both in-situ methods described in this CAP could be effective in remediating the site However,due to limited regulatory acceptance,in-situ bioremediation of ground water may not be the most cost-effective remedial alternative for the site, and should not be relied upon as the primary method for remediation of impacted ground water Because there appears to be a low feasibility for bioremediation and pump and treat,AGE believes that coupled with the excavation of the hydrocarbon source (impacted soil), monitored natural attenuation would be the most feasible method for remediating impacted ground water at the site I Ground water remediation alternatives, estimated durations and associated costs, as applied to the whole site, are summarized on Table 5 IAdvanced GeoEnvironmental,Inc