Abstract

Evolution of a hexavalent uranium [U(VI)] plume at the Hanford 300 Area bordering the Columbia River was investigated to evaluate the roles of labile and nonlabile forms of U(VI) on the longevity of the plume. A high fidelity, three-dimensional, field-scale, reactive flow and transport model was used to represent the system. Richards’ equation coupled to multicomponent reactive transport equations were solved for times up to 100 yr, taking into account rapid fluctuations in the Columbia River stage resulting in pulse releases of U(VI) into the river. The petascale computer code PFLOTRAN developed under a Department of Energy Scientific Discovery through Advanced Computing (SciDAC-2) project was used in the simulations and executed on Oak Ridge National Laboratory’s Jaguar XT5 Cray supercomputer. Labile U(VI) was represented in the model through surface complexation reactions and its nonlabile form through dissolution of metatorbernite used as a surrogate mineral. Initial conditions were constructed corresponding to the U(VI) plume already in place to avoid uncertainties associated with the lack of historical data for the waste stream. The cumulative U(VI) flux into the river was compared for cases of equilibrium and multirate sorption models and for no sorption, and its sensitivity on the initial plume configuration was investigated. The presence of nonlabile U(VI) was found to be essential in explaining the longevity of the U(VI) plume and the prolonged high U(VI) concentrations at the site exceeding the USEPA maximum contaminant level for U(VI).

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