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F ------------------------------------ <br /> C'LEARWATEIK <br /> ,,;... <br /> G R 0 v 11 <br /> Enviranmenlaf Services <br /> average effective porosity, an approximate volume of hydrocarbons moving off site at <br /> concentrations at or below the detection limit can be estimated using Darcy's Law. However, <br /> , <br /> this process typically accounts for a relatively small percentage of apparent contaminant mass <br /> `___1 <br /> loss at most sites. <br /> =:-_, <br /> 2.3.3 Biodegradation Processes and Indicators <br /> During biodegradation, microbes utilize electron acceptors to convert hydrocarbons to carbon <br /> E j dioxide, water, and biomass. In aerobic degradation, the electron acceptor is dissolved oxygen <br /> (DO). In anaerobic degradation, compounds other than DO are used as electron acceptors. The <br /> r. .� reactions that yield the most energy take precedence over those reactions that yield less energy <br /> ::a Y gY P Y� gY <br /> (except for denitrification which yields the most free energy but will not occur if DO <br /> j concentrations are >4.5 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 /> C j <br /> cannot be used as an electron acceptor until DO and nitrate have been sufficiently depieted3. <br /> 17, <br /> Metabolism through iron reduction uses ferric-iron oxides and produces ferrous iron (dissolved) <br /> as a byproduct. <br /> Reduction-oxidation potential (ORP) is a measure of the electron activity in a solution. As <br /> electron acceptors are consumed within the plume during biodegradation, ORP will drop within <br /> the plume. Each biochemical pathway has an associated range of ORP values, depending on the <br /> iY <br /> influx of electrons to the system by groundwater recharge. ORP values can thus be used to <br /> confirm the active biochemical pathway(s) determined on the basis of electron acceptor <br /> depletion. Alternatively, when electron depletion data is inconclusive due to high groundwater <br /> recharge, biodegradation can be confirmed and the active biochemical pathway can be assessed <br /> by evaluating ORP values. Approximate ORP ranges for each biochemical pathway are: <br /> 3Wiedemeier, T.H., Wilson, J.T., Kampbeil, D.H., Miller, R.N. and Hansen, J.H. (1.995). 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 /> j{ <br /> ZB178E iSOC System Installation/Start-up 6 Nov.24,2004 <br />