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' 01 June 1999 AGE-NC Project No 95-0137 Page 15 of 26 Longer-chain hydrocarbons(diesel)in the vicinity of former the USTs will probably require a longer ' remediation period If bioremediation is chosen as the method of remediation,additional soil samples should be collected and analyzed for physicochemical properties necessary for effective ' bioremediation The results will be used to evaluate the need for nutritional enhancement, if necessary, to facilitate effective biodegradation 5 3 7 Estimated Costs For In-situ Bioremediation ' The cost for in-situ bioremediation will vary, depending upon the volume of inoculum required and the number of remediation wells to be inoculated Generally, the initiation cost for in-situ bioremediation will range from$35,000 to$75,000 Weekly and monthly momtonng generally costs between $10,000 to $20,000 annually, soil bonngs 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 $60,000 and$110,000 t 14 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 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 core of a contaminant plume typically exists under 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 vNhere low concentrations of hydrocarbons are present, after completion ' of another method of remediation or in area which no threatened wells are located 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 costs could accumulate over time and potentially be considerable depending on the duration and type ' of monitoring requirements ' Fate and Transport modeling may be used to demonstrate that hydrocarbons in soil do not pose a threat to ground water Where ground water has been impacted modeling may be used to evaluate the possibility of hydrocarbons to impact nearby ground water receptors Advanced GeoEn►ironmental,Inc