Modelling field-scale uranium mass transfer at the Hanford IFRC site

The variation of geochemical conditions caused by Columbia River water intrusion combined with kinetic uranium adsorption/desorption behaviours observed in the laboratory suggest that a multi-rate surface complexation model (SCM) is needed to account for uranium transport at the 300 Area of the Hanford site in southeast Washington State, USA. This study incorporates the laboratory research results into a field-scale multi-rate SCM and investigates its application to a cross-section near the 300A site as part of the Integrated Field-Scale Subsurface Research Challenge (IFRC) programme supported by the US Department of Energy. The reactive transport simulation results indicate that the U(VI) plume is strongly adsorbed by aquifer sediments. Generally, river water intrusion enhances uranium adsorption by lowering the carbonate and increasing pH in groundwater. Employing the multi-rate SCM, U(VI) migration is much more dynamic and synchronic with the groundwater flow field. Even though the general extent of the U(VI) plume does not change significantly after a certain time period, the plume's high concentration zone remains quite dynamic. In contrast, in the simulations with an equilibrium-based SCM, the plume shows very limited seasonal movement. Overall, the results from the multi-rate SCM appear to be more consistent with the field observations of a dynamic uranium plume at the 300A site.