Improved understanding of flow and radionuclide transport in vadose zone sediments is fundamental to future planning involving radioactive materials. Long-term experiments were conducted at the Savannah River Site (SRS), where a series of lysimeters containing sources of different Pu oxidation states were placed in the shallow subsurface and exposed to the environment for 2 to 11 years. After the experiments, Pu activity concentrations were measured along vertical cores. The distributions were anomalous in nature, with irregular downward migration and unexpected upward migration. Previous studies suggested that surface-mediated, oxidation/reduction (redox) reactions may be responsible for the anomalies. This hypothesis is tested by performing transport simulations that include retardation and first-order redox reactions on mineral surfaces within a steady-state, net downward flow field. Profiles below the source are captured well by this steady-state flow model, but there was no mechanism to capture the observed upward migration.
The natural climate created a dynamic hydrologic system for the lysimeter experiments. Thus, for the purpose of evaluating the effects of flow dynamics on Pu migration, a variably saturated flow model with root water uptake (transpiration) was developed. These simulations did not show any significant differences in the resulting Pu distributions compared to those from the steady-state flow simulations. Therefore, transient flow simulations also could not explain the observed upward migration of Pu.
Very small amounts of Pu root uptake and translocation within the transpiration stream has been reported in the literature. Therefore, a model was developed in this study for Pu root uptake and translocation within the plant as a cause for the observed upward Pu movement. Simulations done with the reactive transport model with variably saturated flow and Pu root uptake/translocation yielded a realistic fraction of Pu migrating upward in the plant domain regardless of the oxidation state of the source.
The results of this research indicate that the hypothesis of surface-mediated redox reactions is consistent with the observed transport behavior of Pu from the lysimeter experiments. Upward migration of Pu is likely due to an additional mechanism such as root Pu uptake and translocation within the plant. This hypothesis deserves further study.