| Experiments on microbial in-situ reduction of U(VI) are underway at the Field Research Center in Oak Ridge, TN. Theories and methodologies to calculate flow rates and residence times within in-situ reactors are developed to aid the design of in-situ reactors. A nested-cell design with four extraction-injection wells is proposed for the U(VI) bioreduction experiment: two downgradient extraction and two upgradient injection wells that create an inner cell nested within an outer cell. Mass-transfer limitation is found to control not only the time scale for aquifer preconditioning, but the U(VI) bioavailability during the experiment as well. To identify the heterogeneous mass transfer properties, we performed a novel forced-gradient tracer test, which involved the addition of bromide, the displacement of nitrate, and the rebound of nitrate after completion of pumping. According to simulations, the nitrate stored in the slowly interacting immobile domain will be reduced by an order of magnitude over a period of about a year. By contrast, the mobile domain rapidly responds to flushing, and a low average nitrate concentration can be maintained if the nitrate is removed as soon as it enters the mobile domain. We therefore conclude that the nitrate leaching out of the immobile pore space must continuously be removed by in-situ denitrification to maintain favorable conditions.; A travel-time based reactive-transport model is developed to simulate the in-situ experiment of U(VI) bioreduction. The model considers aquatic equilibrium chemistry of uranium and other groundwater constituents interacting with U in aqueous speciation, uranium sorption and precipitation, electron-accepting processes of microbial activities for the reduction of nitrate, sulfate and U(VI) Kinetic sorption/desorption of U(VI) was characterized by mass transfer between stagnant micro-pores and mobile flow zones. Effective U(VI) reduction rate and sorption site distributions in the mobile and immobile domains are determined by fitting the model simulation to an in-situ experiment. By assuming linear transport, the model can be conveniently characterized by the method of transfer function to estimate the degradation rate of injected electron donor. |
