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CLEARWATER <br />' G R O U P <br /> Environmental Services <br />' 2 3 3 Ph sical Processes So tion Volatilization and Advection and Indicators <br /> _ rp � <br /> Sorption typically affects only a very small fraction of the plume mass It generally does not play <br /> a role in the ongoing attenuation of the plume, except possibly at the plume edges if the plume <br />' continues to migrate Sorption generally occurs early in the spill history, as contaminated <br /> groundwater contacts clean soils <br /> Volatilization of dissolved contaminants may play a significant role in the natural attenuation of <br /> volatile organic constituents such as benzene Volatilization can be estimated using a formula <br /> which takes into account the observed mass loss (contaminant decay rate), however, <br /> fvolatilization has been demonstrated to account of only 5% of benzene mass loss at a site with <br /> relatively favorable conditions for this process3 <br /> Advection is responsible for the off-site migration of contaminants in concentrations below the <br /> detection limit To evaluate the extent of a plume attenuated through advection, the dissolved <br /> constituents are assumed to move at the same velocity as the groundwater (i e no retardation) <br /> Using the hydraulic conductivity of the water bearing zone, average hydraulic gradient, and <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, this <br /> process typically accounts for a relatively small percentage of apparent contaminant mass loss at <br /> most sites <br /> 2 3 4 Biodegradation Processes and Indicators <br /> During biodegradation, microbes utilize electron acceptors to convert hydrocarbons to carbon <br /> dioxide, water, and biomass In aerobic degradation, the electron acceptor is dissolved oxygen <br /> I (DO) In anaerobic degradation, compounds other than DO are used as electron acceptors The <br /> reactions that yield the most energy take precedence over those reactions that yield less energy <br /> 3Chiang, C Y et al (1989) Aerobic Biodegradation of Benzene, Toluene, and Xylene in a Sandy Aquifer-Data <br /> Analysis and Computer Modeling Groundwater Vol 27,No 6,p 823-834 <br /> ZB 178C RAP 9 May 18,2004 <br />