Due to its high toxicity and a long half-life, processes that may enhance Pu mobility in the environment and possible transport and exposure pathways need to be better understood and identified. The results of long-term Pu field lysimeter experiments at the Savannah River Site showed anomalous distributions below the source, with significant upward migration above the source. A previously developed reactive transport model with an initial application of a steady downward velocity successfully simulated the below-source distribution of the lysimeter data. Development and coupling of a transient flow model with root water uptake to the reactive transport model yielded a downward distribution fit almost identical to that from the steady-state flow application. The model predicted very little upward migration, however. Additional evaluations done by testing several soil hydraulic- and chemistry-related mechanisms that may enhance upward migration yielded no improvement. We developed an extension of the reactive transport model to include and test a new mechanism: root Pu uptake and xylem transport. 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 the additional finding that elevated Pu concentrations in the lysimeter surface sediment originated from the source used in the experiments, indicated that Pu root uptake and transport is a valid explanation for the observed upward migration and may play an important role in near-surface Pu transport. Further research is needed to identify the uptake mechanisms and Pu behavior within plant systems, with special attention directed to the effect of Pu complexation with different chelating agents in soil and plants (siderophores, phytosiderophores, and others).

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