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• 'r S 29'April-2005` r AGE-NC Project No. 98-0440- Page 12 of 18 - 6.2.6. Feasibility ; No feasibility studies for in-situ bioremediation have been performed at the site. If bioremediation. is:- chosen as the methodof remediation; soil-samples:-will- be collected and- analyzed •for y r physicochemical properties necessary for effective bioremediation.. The results will be used to evaluate`the need for nutritional,enhancement, if necessary, to facilitate effective biodegradation. 6.2.7. . Estimated Costs For In-situ Bioremediation The cost for in-situ bioremediation will vary, depending u on'the volume of inoculum re uired and P g P q the'number,of remediation wells to be inoculated; in-addition, longer-chain hydrocarbons. (diesel) Will probably require-a longer remediation period. 'Generally, the initiation cost for -in-situ ' bibremediation will range from$35,000 to$75,000.Weekly and monthly monitoring generally costs between•$10,000 to $20,000 annually; soil borings. to monitor'bioremediation progress typically ' averages $5;000-to $10,000 annually. Total costs for bioremediation of soil at the subject site is • . estimated-to be between'$70,000 and $160,000. 6.3. MONITORED NATURAL ATTENUATION Natural attenuation, or'passive bioremediation, is a remedial alternative that relies on natural processes to degrade hydrocarbons. Biodegradation is the most important natural attenuation ; mechanism.No additional bacteria or nutrients are added to the soil.Essential:nutrients required for.. ' 'biodegradation are commonly pre5ent''naturally - iri. '.the subsurface; however, where , high concentrations'-.of hydrocarbons are present, nutrients may be 'consumed before"hydrocarbon., remediation is'colnplete. i Oxygen depletion.iii the subsurface is another characteristic of passive biodegradation. The,core'of "a contaminant-plume typically exists under anaerobic conditions and onl '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 hydrocarbons are present,'after comletion ofanother .. p method of remediation,'.or in an area where no domestic or irrigation wells - are threatened ; The natural attenuation process is usually time consuming;" it can take many, years for the hydrocarbon plume to reach levels comparable to-those of active remediation methods. Costs for " natural attenuation are definitely lower.than.the active,remediation options, however, the periodic �1 Advanced GeoEnvironmental,,Inc.