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I ' 20 January 2005 AGE-NC Project No 97-0343 ' Page 13 of 21 ' 9 2 3 Estimated Costs For Excavation Once excavated, impacted soil could be treated on-site or transferred to an off-site disposal or ' recycling facility It is unlikely that on-site treatment would be amenable to facility operations Replacement backfill material must be imported if off-site disposal is chosen The cost for excavation, treatment/disposal and backfilling would likely be between $110 00 and $125 00 per ' cubic yard, depending upon actual hydrocarbon concentrations encountered during excavation Therefore, based on the estimated volume of 1,600 cubic yards,the estimated cost to remediate the site utilizing excavation would be between $175,000 and $200,000 or more as special excavation ' techniques or shoring would likely be required to remove all contaminated soil 9 3 NATURAL ATTENUATION/PASSIVE REMEDIATION Passive remediation is a process that relies on natural processes to degrade petroleum hydrocarbons Natural attenuation,or passive bioremediation, is a passive remediation method and requires a high level of characterization and long-term monitoring 93i Principles IBiodegradation 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 I present in the subsurface However,where high concentrations of hydrocarbons are present,nutrients maybe 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 where low concentrations of hydrocarbons are present or after completion of another method of remediation I9 3 2 Duration Natural attenuation, or passive bioremediation, is a remedial alternative that relies on natural processes to degrade hydrocarbons The natural attenuation process is time consuming, it will take a number of 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 Ioptions However, the periodic costs could accumulate over time and potentially be considerable depending on the duration and type of monitoring requirements Advanced GeoEnvironmenYal,Inc I