Comparison of parameter sensitivities between a laboratory and field‐scale model of uranium transport in a dual domain, distributed rate reactive system

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 geochemical complexity to identify the importance of individual processes in controlling the fate of U(VI), as well as to elucidate the characteristic differences between well‐defined laboratory and the more complex field‐scale conditions. Therefore, a rigorous sensitivity analysis was carried out for the various simulation scenarios. The underlying conceptual and numerical model, originally developed from column experiment data, includes distributed rate surface complexation kinetics of U(VI), aqueous speciation, and physical nonequilibrium transport processes. The field scenarios accounted additionally for highly transient groundwater flow and variable geochemical conditions driven by frequent water level changes of the nearby Columbia River. The results of the sensitivity analysis showed not only similarities but also important differences in parameter sensitivities between the laboratory and field‐scale models. It was found that the actual degree of sorption disequilibrium, actual concentration of sorbed U(VI), and the sorption extent (i.e., theoretical concentration of sorbed U(VI) at equilibrium) are the major controls for the magnitude of the calculated parameter sensitivities. These internal model variables depended mainly on (1) the groundwater flow conditions, i.e., the relatively long phases of limited groundwater movement in the field scale (intercepted by short peak flow events) and the long sustained flow phases in the column experiment (intercepted by relatively short stop flow events), and (2) the sampling location in the field‐scale model, i.e., plume fringe versus plume center.