Building conceptual models of field-scale uranium reactive transport in a dynamic vadose zone-aquifer-river system

Subsurface simulation is utilized to create, evaluate, and integrate conceptual process models that enhance understanding of the persistence of uranium concentrations exceeding federal drinking water standards within a groundwater plume at the U.S. Department of Energy’s Hanford Site. The study focuses on a 0.4 km by 1.0 km area where significant fluctuations in water levels, driven by the Columbia River, impact both the unconfined aquifer and the variably saturated lower vadose zone sediments. This research employs one- and two-dimensional fluid flow and reactive transport simulations, based on laboratory-derived uranium sorption models, to investigate uranium transport in the vadose zone-aquifer-river system. Findings reveal that fluctuations in river stage significantly influence the inland transport of uranium above the average water table, countering the general trend of groundwater flow toward the river. Furthermore, the inclusion of a rate-limited uranium mass transfer model is critical for accurately representing river stage-driven groundwater flow compared to vadose zone flow initiated by natural recharge. The dynamic interaction between river water and groundwater significantly alters uranium mobility as depicted by a multicomponent uranium surface complexation model.