PNNL

Abstract

A laboratory-derived conceptual and numerical model for U(VI) transport at the Hanford 300A site, Washington, USA, was applied to a range of field-scale scenarios of different complexity to systematically evaluate model parameter sensitivities. The model, originally developed from column experiment data, included distributed-rate surface complexation kinetics of U(VI), aqueous speciation, and physical non-equilibrium transport processes. A rigorous parameter sensitivity analysis was carried out with respect to different state variables: concentrations, mass fluxes, total mass and spatial moments of dissolved U(VI) for laboratory systems, and various simulation scenarios that represented the field-scale characteristics at the Hanford 300A site. The field-scenarios accounted for transient groundwater flow and variable geochemical conditions driven by frequent water level changes of the nearby Columbia River. Simulations indicated that the transient conditions significantly affected U(VI) plume migration at the site. The parameter sensitivities were largely similar between the laboratory and field scale systems. Where differences existed, they were shown to result from differing degrees of U(VI) adsorption disequilibrium caused by hydraulic or hydrogeochemical conditions. Adorption disequilibrium was found to differ (i) between short duration peak flow events at the field scale and much longer flow events in the laboratory, (ii) for changing groundwater chemical compositions due to river water intrusion, and (iii) for different sampling locations at the field scale. Parameter sensitivities were also found to vary with respect to the different investigated state variables. An approach is demonstrated that elucidates the most important parameters of a laboratory-scale model that must constrained in both the laboratory and field for meaningful field application.