Modeling of depleted uranium transport in subsurface systems

Groundwater and soil contamination with depleted uranium (DU) is an important public concern because of its long-term toxicity. In this study, the DU risk in groundwater was assessed through modeling of its sorption equilibrium and kinetics, as well as modeling of its transport in natural subsurface systems. Whenever possible, simulation results were compared with published experimental and field data. Equilibrium modeling studies showed that DU sorption increased sharply from 0 to 100% in the pH range of 3.5 to 5.0 and maximum immobilization was established at pH > 5. Kinetic simulations indicated that the sorption of DU in subsurface systems is a rapid process. Simulations of DU mobility due to groundwater flow and due to infiltration, as well as modeling of DU fate, were carried out by using a metal ion transport model, which included aqueous speciation, redox, precipitation, and sorption reactions. The results showed that the DU mobilization is a relatively slow process. Precipitation, redox, and sorption reactions resulted in the immobilization of DU. Among these reactions, sorption played a major role, and the pH of soils was critical in the immobilization; higher pH in soils resulted in greater immobilization of DU. The impact of DU transport from infiltration was estimated based on four extreme cases of climate and existing conditions of uranium penetrator fragments. The simulations demonstrated that the transport of DU in groundwater is slow due to the low infiltration rate and low DU concentration resulted from the penetrators. Finally, modeling of DU fate showed that the natural cleanup of the DU-contaminated sites is a slow process.