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 (D_L) and residual water content (θ_r) 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.