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i <br /> I10 June 1999 <br /> AGE-NC Project No 97-0290 <br /> IPage 16 of 26 <br />' 7 4 NATURAL ATTENUATION <br /> Natural attenuation, or passive bioremediation, is a remedial alternative that relies on natural <br /> processes to degrade hydrocarbons Natural attenuation is not an"mactive"remediation method and <br /> does require specific characterization and long-term monitoring <br /> I7 4 1 Principles <br /> IBiodegradation is the most important natural attenuation mechanism No additional bacteria or <br /> nutrients are added to the soil Essential nutrients required for biodegradation are usually naturally <br /> present in the subsurface However,where high concentrations of hydrocarbons are present,nutrients <br /> Imay be consumed before hydrocarbon remediation is complete Oxygen depletion in the subsurface <br /> is another characteristic of passive biodegradation The core of a contaminant plume typically exists <br /> under anaerobic conditions and only the margins are aerobic Therefore, even though anaerobic <br /> biodegradation is much slower than aerobic biodegradation, anaerobic processes may dominate the <br /> degradation of contaminants Natural attenuation is normally used where low concentrations of <br /> hydrocarbons are present or after completion of another method of remediation <br /> 7 4 2 Duration <br /> The natural attenuation process is time consuming, it may take many years for the hydrocarbon <br /> plume to reach levels comparable to those of active remediation methods Costs for natural <br /> attenuation can be significantly lower than the active remediation options However, additional <br /> characterization costs and periodic monitoring costs would accumulate over time and could <br /> potentially be significant depending on the duration and type of monitoring requirements <br /> 7 4 3 Feasibility <br /> I Although it is difficult to determine the feasibility of natural attenuation, fate and transport modeling <br /> may be used to determine the probabilities of hydrocarbon-impacted soil affecting ground water <br /> Where ground water has already been impacted, fate and transport modeling may also be used to <br /> Ievaluate the migration and dilution of hydrocarbons <br /> Due to the high concentrations of hydrocarbons present in both soil and ground water, additional <br /> Ispreading of the hydrocarbon plume may occur It is likely that future natural attenuation would <br /> . occur slowly at the site Future construction at the site would inhibit monitoring hydrocarbons in soil <br /> and ground water <br /> Advanced GeoEnwonmental,Inc <br /> I <br />