Soil-Plant Interactions Explaining Long-Term Plutonium Transport Experiments at the Savannah River Site

Improved understanding of flow and radionuclide transport in vegetated vadose zone sediments is fundamental to future planning involving radioactive materials. To that end, long-term experiments were conducted at the Savannah River Site, where a series of lysimeters containing sources of reduced plutonium (Pu) were placed in the shallow subsurface (21 cm depth) and exposed to the environment for approximately 11 years. After the experiments, Pu activity concentrations were measured along vertical cores from the lysimeters. Results showed anomalous activity distributions below the source, with migration of Pu above the source all the way to the soil surface. Based on previous laboratory experiments, a fully-transient flow and reactive transport model with surface-mediated redox reactions was developed to simulate water flow due to rainfall/transpiration and the resulting Pu transport due to advection, dispersion and adsorption. This model was able to explain the downward migration of Pu, but not the upward migration. Through a process of elimination it was concluded that upward migration must occur through Pu absorption by grass roots and upward movement in the transpiration stream. Therefore, the reactive transport model was extended to include solution uptake by roots and Pu transport through the root/stem xylem. The extended model produced simulations that capture the general behavior of the upward migration with no effect on the below-source fit. These results, with the support of additional isotope ratio measurements showing that Pu on the surface of the lysimeter was from the source used in the experiments, not atmospheric fallout, indicated that Pu root uptake and xylem transport is a valid explanation for observed upward migration and may play an important role in near-surface Pu transport. Inferred xylem cross-sections and required head drops are reasonable. Further research is needed to identify the uptake mechanisms and detailed Pu behavior within plant systems.