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CLEARWATER • R d P En rn vironrnfa( Se Srrurces which takes into account the observed mass loss (contaminant decay rate), however, volatilization has been demonstrated to account of only 5% of benzene mass loss at a site with relatively favorable conditions for this process3 Advection is responsible for the off-site migration of contaminants in concentrations below the detection limit To evaluate the extent of a plume attenuated through advection, the dissolved constituents are assumed to move at the same velocity as the groundwater (i a no retardation) Using the hydraulic conductivity of the water bearing zone, average hydraulic gradient, and average effective porosity, an approximate volume of hydrocarbons moving off site at 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 Biodegradation 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 3Chiang,C Y et al (1989) Aerobic Biodegradation of Benzene,Toluene,and Xylene in a Sandy Aquifer-Data Analysis and Computer Modeling Groundwater Vol 27,No 6, p 823-834 • 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 ZB178C/4Q03 Monitoring Rpt 9 January 29,2004