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19 June 2000 <br /> AGE-NC Project No 96-4232 <br /> Page 32 of 35 <br /> 9 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 a passive remediation method and does <br /> require a high degree of site characterization and long-tem monitoring <br /> 9 4 1 Principles <br /> As with natural attenuation in soil, biodegradation is the most Important natural attenuation <br /> mechanism for Impacted ground water No additional bacteria or nutrients are added to the ground <br /> water Essential nutrients required for biodegradation are usually naturally present in the saturated <br /> zone However,where high concentrations of hydrocarbons are present,nutrients may be consumed <br /> before hydrocarbon remediation is complete Oxygen depletion in ground water is another <br /> characteristic of passive biodegradation Natural attenuation is normally used where low <br /> concentrations of hydrocarbons are present or after completion of another method of remediation <br /> 9 4 2 Duration <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 can be highly variable depending on the <br /> duration and type of monitoring required Fate and transport modeling may be used to demonstrate <br /> the potential for hydrocarbons to impact nearby ground water receptors <br /> 9 4 3 Feasibility <br /> Naturally occurring degradation of MTBE remains a marginal remedial technology due to the <br /> contaminants low biodegradability Although natural degradation has not yet been widely tested for <br /> treatment of MTBE, it is known that ethers, in general, biodegrade poorly and the compound has a <br /> low adsorption potential for soil Based on the assumption that the MTBE plume has occurred in a <br /> relatively recent time frame, then it can be presumed that little or no degradation or adsorption has <br /> occurred naturally within the core of the soil plume Natural attenuation is unlikely to arrest the <br /> migration of MTBE-impacted ground water and will provide little protection for drinking water <br /> wells in the area - <br /> Advanced GcoEmironmCntal,Inc <br />