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' 18 July 2002 AGE-NC Project No 96-0254 ' Page 13 of 17 613 Duration ' Experience has shown that all systems involving ground water extraction require much more time ' than would be anticipated from aquifer tests and calculations of ground water discharge rates, and the method seldom achieves regulatory goals for ground water clean-up Systems typically operate for several years During this time,pumps and other equipment may need replacement several times Currently, the primary utility of pump-and-treat systems is in control migration of contaminated ground water t6 14 Cost Ground water extraction is usually a very costly method of ground water treatment Costs for installation of a"pump and treat' system typically ranges from $20,000 to $35,000, including the installation of large-diameter extraction wells Once extracted, the contaminated water must be I treated or disposed Treatment is highly dependant upon the concentrations of dissolved hydrocarbons in the ground water, the cost per gallon could be as great as $2 based on the extraction/treatment of approximately 100,000 gallons of contaminated ground water Costs for the '• entire treatment project would likely be between $100,000 and $300,000 ' 62 IN-SITUBIOREMEDIATION/BIOENHANCEMENT In-situ bioremediation involves the introduction of hydrocarbon-degrading microbes and/or supplemental nutrients to enhance naturally-occurring or injected bacteria, through wells into the groundwater The microbes degrade hydrocarbons and spread laterally(and vertically)by migration Iand reproduction There is considerable variation in the types of bacteria used by different remediation consultants for ' bioremediation The bacteria are cultured and selected for their ability to degrade petroleum hydrocarbons They are microaerophilic and facultatively anaerobic,and therefore abundant oxygen is not necessary for the process to occur Exotic forms of oxygen (such as hydrogen peroxide) are ' usually not required, but free oxygen does increase the metabolic rates in the bacteria Therefore a compressed or pressurized source of delivering atmospheric air to the subsurface at a rate of at least 4 to 6 scfm per infection well can be used to enhance air flow and oxygen concentrations during the ' process For in-situ bioremediation of the ground water,hydrocarbon-degrading bacteria are propagated in Ia liquid medium or solid,which is then injected directly into gtound water and inoculation wells • screened into ground water Once in the contaminant plume, the bacteria metabolize and therefore Advanced CeoEn"mnmentai,Inc I