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I05 May 1997 <br /> AGE-NC Project No 95-0173 <br /> Page 26 of 29 <br /> 8 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 /> I <br /> 8 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 /> iduration 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 /> 8 4 3 Feasibility <br /> No feasibility testing or studies concerning natural attenuation have been performed at the site Prior <br /> to initiation of natural attenuation, additional ground water characterization would probably be <br /> necessary as it is necessary to be able to demonstrate stability of the ground water plume Fate and <br /> Transport modeling may be used to determine whether or not hydrocarbons in ground water pose a <br /> threat to area receptors <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 ground water samples <br />' collected at the site at depths greater than 60 feet bsg suggests that little apparent natural attenuation <br /> has occurred in approximately 25 years <br /> Hydrocarbon concentration-, are relatively high at the s,te� anti therefnre, natural attpni,at,nn 14Z <br /> probably not a cost-effective method of remediation Additional assessment of the dissolved ground <br /> water plume is necessary northwest, east and south of the site to evaluate and monitor natural <br /> attenuation <br /> Advanced CeoEm ironmental,Inc <br />