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1 <br /> 27 December 1996 <br /> AGE-NC Project No 95-0143 <br />' Page 18 of 19 <br /> Natural attenuation is normally used where low concentrations of hydrocarbons are present or <br />' following completion of another form of remediation Concentrations of hydrocarbon have shown <br /> a decrease in monitoring well MW-9 during each of the last five monitoring events Hydrocarbon <br />' concentrations have also decreased in MW-3 Hydrocarbon concentrations have varied in monitoring <br /> wells MW-6 and MW-7, but have remained relatively stable Continued ground water monitoring <br /> would be required to show a stable plume and decreasing hydrocarbon concentrations <br />' The duration for natural attenuation can be many years Costs for natural attenuation are generally <br /> lower than most active remediation options However, costs could be highly variable depending on <br />' the duration and type of monitoring required Fate and Transport modeling may be used to <br /> demonstrate the potential for hydrocarbons to impact nearby ground water receptors <br /> 7 0. RECOMMENDATIONS FOR REMEDIATION OF HYDROCARBON- <br /> IMPACTED SOIL <br />' <br /> Impacted soil in the vadose zone extends tends under the building on-site Additional impacted soil is <br /> present laterally in the saturated "smear" zone Excavation would require the removal of a large <br /> volume of overburden soil Therefore, an in-situ method of remediation should be considered <br />' Both in-situ methods described in this CAP could be effective for remediating the site From a cost <br /> basis, implementation of an in-situ bioremediation program could be comparable in cost to soil vapor <br /> extraction,but without the site disturbance However, monitoring requirements,duration and limited <br />' control of the technology could increase the cost significantly Therefore, we recommend installation <br /> of an SVE system at the site Vadose zone vapor extraction may also reduce floating product and <br /> dissolved hydrocarbon concentrations Soil remediation methods estimated durations and associated <br />' costs are summarized on Table 5 <br /> An extraction unit capable of approximately 70 scfm will be required for soil remediation at the site <br /> Due to the high concentration of contaminants at the site, an IC unit would be the most cost-effective <br /> choice An electric blower and carbon filtration system may be used during the final stages of <br /> remediation A 12 to 18 month soil remediation period can be expected Installation of an SVE <br /> system would include installation of soil vapor extraction wells within the contaminated soil plume <br /> 8 0. RECOMMENDATIONS FOR REMEDIATION OF HYDROCARBON- <br /> IMPACTED GROUND WATER <br /> Ground water monitoring at the site indicates that MW-17 is the down-gradient well Free product <br />. has been detected in MW-3 during monitoring High concentrations of dissolved hydrocarbons have <br />