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' 05 May 1997 <br /> . AGE-NC Project No 95-0173 <br />' Page 19 of 29 <br />' 7 4 1 Principles <br /> Biodegradation 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 /> may 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 /> 1 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 /> 10 characterization costs and periodic monitoring costs would accumulate ox er time and could <br /> potentially be significant depending on the duration and type of monitoring requirements <br />' 7 4 <br /> 3 Feasibility <br /> 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 /> evaluate the migration and dilution of hydrocarbons <br />' The results of the microbial enumeration and physicochemical analysis indicated that hydrocarbon <br /> degrading organisms were inadequate to support microbial growth and bioremedlal activity The <br /> analysis indicated that a degree of biotoxicity to the microbes may be present at the site,possibly due <br /> to agents such as heavy metals in the soil or to excessive concentrations of gasoline constituents <br /> Additionally,the analysis indicated that levels of nitrate-nitrogen were inadequate to support passive <br /> bioremediation <br />' Based on the assumption that a release first occurred circa 1970 and that historically ground water <br /> depths have occurred between 60 and 70 feet bsg, laboratory analysis of soil and ground water <br />' samples collected at the site at depths greater than 60 feet bsg suggests that little apparent natural <br /> . attenuation has occurred in approximately 25 years <br />' Advanced GcoEnvironmental,Inc <br />