Comparing Approaches for Simulating the Reactive Transport of U(VI) in Ground Water |
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Authors: | Gary P Curtis Matthias Kohler James A Davis |
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Affiliation: | (1) US Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, USA;(2) Department of Civil and Environmental Engineering, University of Maine, Orono, ME 04469, USA |
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Abstract: | The reactive transport of U(VI) in a well-characterized shallow alluvial aquifer at a former U(VI) mill located near Naturita,
CO, was predicted for comparative purposes using a surface complexation model (SCM) and a constant K
d approach to simulate U(VI) adsorption. The ground water at the site had U(VI) concentrations that ranged from 0.01 to 20 μM,
alkalinities that ranged from 2.5 to 18 meq/L, and a nearly constant pH of 7.1. The SCM used to simulate U(VI) adsorption
was previously determined independently using laboratory batch adsorption experiments. Simulations obtained using the SCM
approach were compared with simulations that used a constant K
d approach to simulate adsorption using previously determined site-specific K
d values. In both cases, the ground water flow and transport models used a conceptual model that was previously calibrated
to a chloride plume present at the site. Simulations with the SCM approach demonstrated that the retardation factor varied
temporally and spatially because of the differential transport of alkalinity and dissolved U(VI) and the nonlinearity of the
U(VI) adsorption. The SCM model also simulated a prolonged slow decline in U(VI) concentration, which was not simulated using
a constant K
d model. Simulations using the SCM approach and the constant K
d approach were similar after 20 years of transport but diverged significantly after 60 years. The simulations demonstrate
the need for site-specific geochemical information on U(VI) adsorption to produce credible simulations of future transport. |
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Keywords: | Reactive transport Uranium Environmental geochemistry Geochemical modeling |
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