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CLEAiKVE-R <br /> G R d U P <br /> Envcronmenlal Services <br /> Reduction-oxidation potential (Eli) 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 /> 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 millivolts5 <br /> Aerobic Degradation +150 to +800 <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 <br /> • acts as a buffering agent to maintain pH levels suitable for microbes This reaction causes <br /> alkalinity to decrease in the presence of biodegradation With sufficient alkalinity present to <br /> buffer the products of biodegradation, pH values remain constant inside and outside of the <br /> plume If the groundwater does not contain sufficient alkalinity, the organic acids may build up, <br /> reducing the pH and eventually creating an environment inhospitable to the hydrocarbon- <br /> utilizing microbes <br /> Results of Monitored Natural Attenuation Evaluation <br /> The monitored natural attenuation (MNA) study focused on aerobic and anaerobic <br /> biodegradation processes Positive results for some or all natural attenuation pathways were <br /> anticipated because contaminant concentrations appear to be declining based on determination of <br /> first-order decay rates The results of this study suggest that both aerobic and anaerobic <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/4Q03 Monitoring Rpt 10 January 29,2004 <br />