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-��P CLEAkiWATER R0Uf' Envrranmrnfaf Servrces concentrations at or below the detection limit can be estimated using Darcy's Law However, this process typically accounts for a relatively small percentage of apparent contaminant mass loss at most sites Biode radation Processes and Indicators During biodegradation, microbes utilize electron acceptors to convert hydrocarbons to carbon dioxide, water, and biomass In aerobic degradation, the electron acceptor is dissolved oxygen (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 if DO concentrations are >0 5 milligrams 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 ferric-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 biochemical pathway can be assessed by evaluating Eh values Approximate Eh ranges for each biochemical pathway are Method of Metabolism Eh Range in m1111volts5 Aerobic Desradation +150 to +500 Denitrification -100 to +750 Sulfate Reduction -770 to -220 Iron reduction -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 4Wiedemeier,T H,Wilson,J T , Kampbell,D H ,Miller,R N 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 I 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/IQ03 Monitoring Rpt 7 April 23,2003