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' 15 June 1999 • AGE-NC Project No 9$-0450 ' Page 15 of 27 Furthermore,weekly and monthly monitoring of remediation wells generally costs between $10,000 to $20,000 annually In addition, soil borings to monitor and confirm bioremediation progress typically averages $5,000 to $10,000 annually The total cost of bioremediation for the duration (24 ' to 36 months) is estimated range from $$0,000 to $165,000 ' 4 4 NATURAL ATTENUATION Natural attenuation, or passive bioremediation, is a remedial Iternative t at relies on natural ' processes to degrade hydrocarbons Natural attenuation is not "inactive"re ediation method and does require extensive characterization of site and contamin t plum and ong-term monitoring 4 4 1 Principles ' The processes involved in natural attenuation of petroleum hydrocarbons include aerobic and anaerobic biodegradation, dispersion, volatization, and adsorption Biodegradation is the predominant mechanism of natural attenuation Petroleum hydrocarbons are generally biodegradable as long as petroleum-degrading indigenous microorganisms are present and have an adequate supply of nutrients and biological activity are not inhibited by toxic substances The essential nutrients for biodegradation are usually naturally present in the subsurface Aerobic biodegradation consumes oxygen which, if not replenished, can limit the effectiveness of further aerobic activity Soil consisting of finer-grained material commonly have relatively low permeability which reduces the rate of oxygen replenishment In addition, less permeable materials typically are finer grained and contain higher percentages of organic carbon ' which favors adsorption and retardation of contaminant movement resulting in contaminants remaining relatively undegraded but in close proximity to the original source Anaerobic biodegradation is also a significant attenuation process Oxygen depletion in the subsurface is a characteristic of biodegradation of hydrocarbons and is a result of the rate of ' metabolic oxygen utilization exceeding the natural capacity for oxygen replenishment Commonly, the core of a contaminant plume is typically under anaerobic conditions and only the margins of the plume have aerobic conditions Anaerobic biodegradation is typically 10 to several hundred times slower than aerobic biodegradation ' 4 4 2 Monitoring Activities • Installation of soil vapor monitoring wells would be required to monitor the effects of natural Advanced GeoEntiLronmental,Inc