Adsorption and Transport Behavior of Uranium in the Vadose Zone of Sandy Soils in Arid Regions: A Hydrus-1D Numerical Simulation Study

Abstract Uranium (U), a key nuclide in nuclear energy development and carbon neutrality strategies, exhibits environmental adsorption and transport behaviors that directly affect the safety of nuclear waste geological disposal and the long-term risk assessment of contaminated sites. The sandy vadose zone in arid regions of Gansu, China, is a representative medium for uranium contamination; however, its vertical migration and retention mechanisms remain poorly quantified. In this study, laboratory column experiments combined with Hydrus-1D simulations were conducted to investigate the coupled effects of water flow and experimentally determined linear adsorption on uranium transport in a homogeneous sandy vadose zone. Uranium migrated rapidly downward by approximately 8 cm within the first 20 days. Scenario simulations (20–50 years) suggest that, under evaporation-driven negative pressure gradients, migration depth stabilizes, with most uranium retained within the top 7.9 cm. Sensitivity analysis shows that the dispersion coefficient (DL) and residual water content (θr) are the most influential intrinsic parameters, while the precipitation-evaporation balance is the key external driver controlling long-term retention. Furthermore, rainfall-induced wetting-drying cycles may promote a shift of uranium toward relatively less mobile fractions under the Tessier extraction scheme. This study suggests that, under evaporation-dominated climatic conditions and the assumptions of the present conceptual model, uranium transport exhibits a trend of rapid initial migration followed by long-term shallow retention, providing quantitative support for scenario-based assessment of radionuclide transport in homogeneous sandy vadose zones.