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I 18 February 1997 AGE-NC Project No 95-0104 Page 19 of 22 7 4 NATURAL ATTENUATION Natural attenuation, or passive bioremediation, is a remedial alternative that relies on natural I processes to degrade hydrocarbons Natural attenuation is not an "inactive" remediation method and does require specific characterization and long-term monitoring 7 4 1 PRINCIPLES As with natural attenuation in soil, biodegradation is the most important natural attenuation mechanism No additional bacteria or nutrients are added to the ground water Essential nutrients required for biodegradation are usually naturally present in the saturated zone However, where high concentrations of hydrocarbons are present, nutrients may be consumed before hydrocarbon remediation is complete Oxygen depletion in ground water is another characteristic of passive biodegradation Natural attenuation is normally used where low concentrations of hydrocarbons are present or after completion of another method of remediation lie 7 4 2 DURATION The duration for natural attenuation can be many years Costs for natural attenuation are generally lower than most active remediation options However, costs can be highly variable depending, on the duration and type of monitoring required Fate and Transport modeling may be used to demonstrate the potential for hydrocarbons to impact nearby ground water receptors 7 4 3 FEASIBILITY No feasibility testing or studies concerning natural attenuation have been performed at the site Prior to initiation of natural attenuation, additional ground water characterization would likely be necessary Fate and Transport modeling may be used to demonstrate that hydrocarbons in ground water do not pose a threat to area receptors I Hydrocarbon concentrations are relativelylow at the Marlowe site and natural attenuation may be � Y a cost-effective method of remediation However, additional characterization of the dissolved ground water plume may be necessary to evaluate and monitor natural attenuation 7 4 4 ESTIMATED COSTS It is hkel that some additional assessment including additional laboratory analysis of round water Y � g rY Y g i