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t <br /> CLEAER <br /> G R O U P <br /> Environmental Services <br />' <br /> (except for denitrification which gelds the most free energy but will not occur if DO <br /> { p Y gY <br />' concentrations are >0 5 milligrams per liter (mg/L)) This results in electron acceptors being <br /> used up in the following preferential order DO, nitrate, ferric-iron oxides, sulfate, and carbon <br />' dioxide (methanogenesis) Since DO and nitrate are toxic to sulfate-reducing organisms, sulfate <br /> cannot be used as an electron acceptor until DO and nitrate have been sufficiently depleted4 <br />' Metabolism through iron reduction uses ferric-iron oxides and produces ferrous iron (dissolved) <br /> as a byproduct <br /> 1 <br /> Reduction-oxidation potential (Eh) is a measure of the electron activity in a solution As electron <br />' acceptors are consumed within the plume during biodegradation, Eh will drop within the plume <br /> Each biochemical pathway has an associated range of Eh values, depending on the influx of <br /> electrons to the system by groundwater recharge Eh values can thus be used to confirm the <br /> active biochemical pathway(s) determined on the basis of electron acceptor depletion <br /> 10 Alternatively, when electron depletion data is inconclusive due to high groundwater recharge, <br />' biodegradation can be confirmed and the active biochemical pathway can be assessed by <br /> evaluating Eh values Approximate Eh ranges for each biochemical pathway are <br /> Method of Metabolism Eh Range in milliyolts5 <br /> Aerobic Degradation +150 to +500 <br />' Denitrification -100 to +750 < <br /> Sulfate Reduction -770 to -220 <br /> Iron reduction -470 to +100 <br /> Alkalinity in the groundwater reacts with organic acid byproducts of biodegradation and thus acts <br />' as a buffering agent to maintain pH levels suitable for microbes This reaction causes alkalinity <br /> to decrease in the presence of biodegradation With sufficient alkalinity present to buffer the <br /> 1 <br /> 4Wiedemeier, T H , Wilson, J T , Kampbell, D H, Miller, R N and Hansen, J H (1995) Technical Protocol for <br />' implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination <br /> Dissolved in Groundwater Vol 1 AFCE,Technology Transfer Division,Brooks AFB,San Antonio,TX <br /> 5Cookson, J T , Jr, 1995 Bioremediation Engineering Design and Application (Ed Nalven, G) McGraw - Hill, <br />' Inc ,New York,NY <br /> ZB178C RAP 10 May 18,2004 <br />