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z � Irl 1 t , 16 August 2001 AGE-NC Project No 97-0312 ' Page 12 of 20 1 1 7 3 2 Duration 1 Natural attenuation, or passive bioremediation, is_a remedial alternative that relies on natural ' processes to degrade hydrocarbons The natural attenuation process is time consuming, it will take a number of years for the hydrocarbon plume to reach levels comparable to those of active remediation methods Costs for natural attenuation are definitely lower than the active remediation ' options However, the periodic costs could accumulate over time and potentially be considerable depending on the duration and type of monitoring requirements h ' Fate and transport modeling may indicate that hydrocarbons in soil do not pose a threat to ground water Where ground water has been impacted or has a high potential to become impacted,modeling maybe used to evaluate the potential for the hydrocarbons to impact nearby ground water receptors ' 7 3 3 Feasibility I Although locally high concentrations of petroleum hydrocarbon-impacted soil occur at the site,the major portion of impacted soil contains only minor concentrations Additionally, most soil contamination is composed of relatively immobile long-chain hydrocarbons Considering the depth to ground water and that a source for further contamination does not exist at the site, it is unlikely ' that significant additional ground water contamination will occur Aittioub`ri the feasimliiy for natural attenuation as a iemediatieti alternative at rhe sire -s uiicensin, ' it may be the more appropriate alternative thus far in view of the large volume of impacted soil,the relatively immobile characteristics of the contaminants and'the significant cost that would be required to remediate soil at the site 7'3 4 Estimated Costs tAdditional assessment,including additional laboratory analysis of soil samples and/or confirmation soil sampling,would be required to monitor and verify the effectiveness of passive remediation A ' long-term monitoring program,perhaps greater than 10 years,would be initiated Generally,the cost for quarterly ground water monitoring and sampling generally costs between $5,000 to $10,000 annually, semiannual soil borings to monitor the progress of passive remediation typically would ' average $5,000 to $10,000 annually, report preparation would generally cost between $1,000 to $2,000 annually Therefore, annual cost for passive remediation should be on the order of$22,000 ' Based on a duration of 10 years of monitoring, the total cost for passive remediation is estimated to be approximately$220,000 1 1 Advanced GeoEnvironmental,Inc