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1 <br /> 1 18 February 1997 <br /> • AGE-NC Project No 95-0104 <br />' Page 17 of 22 <br />' control to prevent spreading of the contaminant plume <br />' 7 2 4 DURATION <br /> Experience has shown that all systems involving ground water extraction require much more time <br />' than would be anticipated from aquifer tests and calculations of ground water discharge rates, and <br /> seldom achieve regulatory goals for ground water clean-up Systems typically operate for several <br /> years During this time, pumps and other equipment may need replacement several times A treatment <br />' period of 12 to 24 months is estimated for the site <br />' 7 2 5 COST <br />' Ground water extraction is usually a very costly method of ground water treatment Costs for <br /> installation of a "pump and treat" system typically ranges from $25,000 to $40,000, including the <br /> installation of one or more large-diameter extraction wells Once extracted, the contaminated water <br /> must be treated and/or will require disposal Treatment is highly dependant upon the concentrations <br /> of dissolved hydrocarbons in the ground water The cost per gallon could be as great as $2 00, based <br /> upon the extraction/treatment of approximately 100,000 gallons of contaminated ground water Costs <br /> Ifor the entire treatment project would likely be between $200,000 and $250,000 <br /> I7 3 IN--SITU BIOREhIEDIATION/BIOENHANCEMENT <br /> 7 3 1 BASIC PRINCIPLES <br /> Bioremediation involves the introduction of hydrocarbon-degrading microbes or supplemental <br /> nutrients to enhance naturally-occurring bacteria, through wells into the ground water The microbes <br /> Idegrade hydrocarbons and spread laterally (and vertically) by migration and reproduction <br /> Hydrocarbon-degrading microbes are able to metabolize hydrocarbons and assimilate the resulting <br /> Iorganic chenucals into biomass for reproduction and growth Typically, the light-end hydrocarbons <br /> are degraded first, followed by heavier compounds <br /> 73 <br /> 2 NUTRIENT REQUIREMENTS AND TOXICITY <br /> The microbes require certain essential nutrients that are usually available in ground water If <br /> • supplemental nutrients are required, such as nitrogen and phosphorus as ammonium phosphate in <br /> liquid phase, the quantity is usually very low However, a Waste Discharge Permit is required by the <br />