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Modeling the Interaction of Sorption and Biodegradation on Transport in Ground Water In Situ Bioremediation Systems

Numerical experiments examining the effect of linear equilibrium adsorption of the electron donor and its subsequent effect on the biodegradation rate gave new insight on the behavior of homogeneous systems. Two regions of behavior, an initial rapid growth period (Region 1) and a long-term pseudo-steady-state (Region 2), were identified in the numerical experiments for the one-dimensional homogeneous system. The Region 2 long-term pseudo-steady-state cyclic phenomenon was examined in detail in order to determine the cause of this behavior. In the absence of significant biological growth, the injected electron-acceptor front travels faster than the retarded electron donor front. This overlap leads to a region of simultaneously high electron donor and acceptor, which leads to biomass growth. Biodegradation utilizes the electron donor and acceptor, which results in a speed up of the retarded electron donor front and a slow down of the electron acceptor front until biomass growth peaks. This separation of the fronts diminishes the region of simultaneously high electron donor and acceptor, resulting in biomass decay. The resultant cyclic phenomenon is thus explained based upon the results of numerical experiments and to date has not been reported in the literature. The lag time to the onset of Region 1 behavior increased as a result of increased sorption and decreased advection of the electron donor, which results in a decreased electron acceptor flux into the system due to increased biological growth. As the retardation factor of the electron donor increases in the experiments, the rate of biodegradation of the electron donor also increases. This is caused by the “reservoir” effect with increasing sorption of the electron. donor, which is augmented further by increasing overlap of the electron donor and electron acceptor fronts. For a retarded electron donor, decreasing flow velocity increases the biodegradation rate in Region 1, and this effect is due to increasing the overlap of the electron donor and acceptor within the domain.