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11 February 2003 AGE-NC Project No. 98-0520 _ Page 13 of 21 ' Therefore, even though anaerobic biodegradation is much slower than aerobic biodegradation, anaerobic processes may dominate the degradation of contaminants. ' Naturai attenuation is normally used where low,concentrations of hydrocarbons are present, after completion of another 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 man ears for the Y. Y 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. ' • 7.0. GROUND WATER REMEDIATION ALTERNATIVES AGE believes that ground water extraction, In--situ air sparging, In-situ bioremediation/bioenhancement, and natural attenuation are appropriate remedial technologies or. ' strategy to consider to address hydrocarbon-impacted ground water on'the subject site. 7.1. GROUND WATER EXTRACTION There are several different methods for treatment of extractedound water, which are collectively � � y called "pump and treat". As the name indicates, all methods involve extraction and transfer of contaminated ground water from the aquifer to an above-ground treatment or disposal/recycling ' facility. The extraction is performed using an automatic pump system to maintain capture characteristics in the aquifer and to assure that process volume requirements are maiintained. After treatment, the water is usually discharged into a sanitary sewer. ' 7.1.1. Ground Water Treatment Methods The documented presence of dissolved hydrocarbons will require treatment for the recovered water. Two of the most common methods of water treatment are air stripping and carbon adsorption. Air stripping involves removal of volatile organic compounds from ground water by'promoting the transfer of contaminants from the dissolved phase to the vapor phase. The process usually works Advanced GeoEnviron mental,Inc.