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CLEARWATER G a o u r Enwronmenta! Scrorces (DO) In anaerobic degradation, compounds other than DO are used as electron acceptors The reactions that yield the most energy take precedence over those reactions that yield less energy (except for denitrification which yields the most free energy but will not occur of DO concentrations are >0 5 mmllngrams per liter (mg(L)) This results in electron acceptors being used up in the following preferential order DO, nitrate, ferric-iron oxides, sulfate, and carbon dioxide (methanogenesis) Since DO and nitrate are toxic to sulfate-reducing organisms, sulfate cannot be used as an electron acceptor until DO and nitrate have been sufficiently depleted4 Metabolism through iron reduction uses femc-iron oxides and produces ferrous iron (dissolved) as a byproduct Reduction-oxidation potential (Eh) is a measure of the electron activity in a solution As electron acceptors are consumed within the plume during biodegradation, Eh will drop within the plume Each biochemical pathway has an associated range of Eh values, depending on the influx of electrons to the system by groundwater recharge Eh values can thus be used to confirm the active biochemical pathway(s) determined on the basis of electron acceptor depletion Alternatively, when electron depletion data is inconclusive due to high groundwater recharge, biodegradation can be confirmed and the active bmochenucal pathway can be assessed by evaluating Eh values Approximate Eh ranges for each biochemical pathway are Method of Metabolism Eh Range in millivolts5 _ Aerobic Degradation +150 to+800 Denitrification -100 to+750_ y-.-........................... �-gyp�-�� .................. ...«.....»........_........� y Sulfate Reduction -770 to-220 Iron reduction r -470 to+100 Alkalinity in the groundwater reacts with organic acid byproducts of biodegradation and thus acts as a buffering agent to maintain pH levels suitable for microbes This reaction causes alkalinity to decrease in the presence of biodegradation With sufficient alkalinity present to buffer the products of biodegradation, pH values remain constant inside and outside of the plume If the groundwater does not contain sufficient alkalinity, the organic acids may build up, reducing the pH and eventually creating an environment inhospitable to the hydrocarbon- utilizing microbes Results of Monitored Natural Attenuation Evaluation The monitored natural attenuation (MNA) study focused on aerobic and anaerobic biodegradation processes Positive results for some or all natural attenuation pathways were anticipated because contaminant concentrations appear to be declining based on determination of first-order decay rates The results of this study suggest that both aerobic and anaerobic 4Wiedemeier,T H, Wilson,J T,Kampbell,D R,Miller,RN and Hansen,J H (1995) Technical Protocol for implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater Vol 1 AFCE,Technology Transfer Division,Brooks AFB, San Antonio,TX 5Cookson,J T,Jr, 1995 Bioremediation Engineering Design and Application (Ed. Nalven, G) McGraw-Hill, Inc,New York,NY ZB178C/3Q03 Momtonng Rpt 7 October 14,2003