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15 February 2000 _ AGE-NC Project No 98-0448 Page 22 of 33 ' 8 3 b Feasibility In-situ bioremediation of hydrocarbon-impacted soil at the site has a low feasibility The results of ' the analysis Indicated that selected hydrocarbon-degrading microorganisms were present at 3 CFUs per gram from EW1-25 and 1 CFU per gram from MW8-25 The laboratory indicated that the physicochernical conditions at the site were adequate to support microbial growth and bioremedial activity However, the analysis indicated that biodegradation was occurring at a slow rate due to the small percentage of biodegraders The laboratory suggested that alternative carbon sources (I e organic debris) may have become the dominant food source for native microorganisms 8 3 7 Estimated Cost For In-Situ Bloremediation _ Due to the low number of hydrocarbon-degrading organisms at the site, the cost for in-situ I bioremediation is uncertain, and will depend upon the volume of Inoculant required, the number of remediation wells to be inoculated and the duration of bioremedial activity Generally, the Implementation cost for in-situ bioremediation will range from $35,000 to $75,000 Furthermore, weekly and monthly monitoring generally costs between $10,000 and $20,000 annually, soil probings to monitor bioremediation progress typically averages $10,000 to $15,000 annually Therefore, the total cost can range from $55, 000 and $110,000 annually 8 4 NATURAL ATTENUATION I z Natural attenuation, or passive bioremediation, is a remedial alternative that relies on natural processes to degrade hydrocarbons Natural attenuation is a passive remediation method and does require a high level of characterization and long-term monitoring 8 4 1 Principles Biodegradation is the most Important natural attenuation mechanism No additional bacteria or nutrients are added to the soil Essential nutrients required for biodegradation are usually naturally present in the subsurface However,where high concentrations of hydrocarbons are present,nutrients may be consumed before hydrocarbon remediation is complete Oxygen depletion in the subsurface is another characteristic of passive biodegradation The care of a contaminant plume typically exists iunder anaerobic conditions and only the margins are aerobic Therefore, even though anaerobic biodegradation is much slower than aerobic biodegradation, anaerobic processes may dominate the degradation of contaminants Natural attenuation is normally used where low concentrations of v Advanced GeoEmiron mental,Inc i i