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1 ' 18 February 1997 • AGE-NC Project No 95-0104 ' Page 13 of 22 ' 6 4 1 PRINCIPLES Biodegradation is the most important natural attenuation mechanism No additional bacteria or ' nutrients are added to the soil Essential nutrients required for biodegradation are usually naturally present in the subsurface However, where high concentrations of hydrocarbons are present, nutrients may be consumed before hydrocarbon remediation is complete Oxygen depletion in the subsurface ' is another characteristic of passive biodegradation The core of a contaminant plume typically exists under anaerobic conditions and only the margins are aerobic Therefore even though anaerobic biodegradation is much slower than aerobic biodegradation, anaerobic processes may dominate the ' degradation of contaminants Natural attenuation is normally used where low concentrations of hydrocarbons are present or after completion of another inethod of remediation ' 6 4 2 DURATION ' The natural attenuation process is time consuming, it may take many years for the hydrocarbon plume to reach levels comparable to those of active remediation methods Costs for natural attenuation can '. be significantly lower than the active remediation options Howevei, additional characterization costs and periodic monitoring costs would accumulate over time and could potentially be significant depending on the duration and type of monitoring requirements 6 4 3 FEASIBILITY IAlthough it is difficult to determine the feasibility of natural attenuation, fate and transport modeling may be used to determine the probabilities of hydrocarbon-impacted soil affecting ground water ' Where ground water has already been impacted, fate and transport modeling may also be used to evaluate the migration and dilution of hydrocarbons I6 4 4 ESTIMATED COSTS It is likely that some additional assessment, including additional laboratory analysis of soil samples ' and/or confirmation soil sampling would be verify to monitor and required vera natural attenuation q Furthermore, a long-term monitoring program, perhaps 10 years, would be initiated Overall costs for natural attenuation in soil cannot be estimated at this time Much of the cost associated with natural attenuation would be for long-term ground water monitoring, discussed in Section 7 4 4 Other costs for natural attenuation in soil would include periodic soil borings or probings to monitor the natural attenuation rate I