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r ' 5 i 15 February 2000 AGE-NC Project No 98-0448 ' Page 23 of 33 ' hydrocarbons are present or after completion of another method of remediation ' 8 4 2 Duration The natural attenuation process is time consuming, it may take many years for the hydrocarbon ' concentration to reach levels comparable to those of active remediation methods Costs for natural attenuation can be significantly lower than the active remediation options However, additional characterization costs and periodic' monitoring costs would accumulate over time and could ' potentially be significant depending on the duration and type of monitoring requirements $ 4 3 Feasibility Natural attenuation at the site has a low feasibility, the results of the microbial enumeration and physicochemical analysis indicated that hydrocarbon degrading organisms were generally inadequate to support microbial growth and bioremedial activity The analysis indicated that biodegradation was occurring at a slow rate due to the small percentage of biodegraders of petroleum hydrocarbons The laboratory suggested that an alternative carbon sources (i a organic debris) may be the dominant food source for native microorganisms Based on the assumption that the gasoline release occurred ' a minimum of 20 years ago, laboratory analysis of soil and ground water samples collected during the ongoing site investigation suggests that little apparent natural attenuation has occurred during the 20 year period ' 8 4 4 Estimated Costs It is likely that some additional assessment, including additional laboratory analysis of soil samples and/or confirmation soil sampling would be required to monitor and verify natural attenuation Furthermore, a long-term monitoring program, perhaps greater than 25 years, would be initiated Generally,the cost for natural attenuation would range from $11,000 to-$30,000 annually Quarterly ' ground water monitoring and sampling generally costs between $5,000 to $15,000 annually, semiannual soil probings to monitor passive bioremediation progress typically would average$5,000 to $10,000 annually, report preparation would generally cost between $1,000 to $5,000 annually 9.0. GROUND WATER REMEDIATION ALTERNATIVES ' Four methods of remediation of hydrocarbon-impacted ground water on the site have been evaluated The methods include in-situ IAS, ground water extraction (pump and treat), bioremediation and Advanced GeoEnvironmentst,Inc. 1