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

Abstract

Subsurface simulation is used to build, test, and couple conceptual process models to better understand the persistence of uranium concentrations above federal drinking water standards in a 0.4 km by 1.0 km groundwater plume beneath the 300 Area of the U.S. Department of Energy's Hanford Site in eastern Washington State. At this location, the unconfined aquifer and the variably saturated lower vadose zone sediments are subject to significant variations in water levels driven by diurnal, weekly, and seasonal fluctuations in the Columbia River stage. In the near‐river aquifer, uranium‐contaminated sediments in the highly transmissive Hanford formation are subject to high groundwater velocities, daily flow reversals, and exposure to river water. One‐ and two‐dimensional simulations of variably saturated flow and reactive transport based on laboratory‐derived models of uranium sorption are used to assess the representation of uranium transport processes in the vadose zone‐aquifer‐river system. The simulations show that the various frequencies of river stage fluctuation are capable of driving significant inland transport above the average water table, which is in contrast to the net groundwater flow to the river. Inclusion of a rate‐limited uranium mass transfer process model is notably more important to the timescales of the river stage‐driven groundwater flow than for vadose zone flow driven by natural recharge. Spatially and temporally variable solution chemistry from the dynamic exchange of river water and groundwater in the unconfined aquifer is shown to significantly alter uranium mobility as represented by a multicomponent uranium surface complexation model.