Improved understanding of flow and radionuclide transport in vadose zone sediments is fundamental to future planning involving radioactive materials. To that end, 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 yr. After the experiments, Pu activity concentrations were measured along vertical cores from the lysimeters. Plutonium distributions were anomalous in nature—transport from oxidized Pu sources was less than expected, and a small fraction of Pu from reduced sources moved more. Studies conducted with these lysimeter sediments indicated that surface-mediated, oxidation–reduction (redox) reactions may be responsible for the anomalies. This hypothesis is tested by performing transient Pu transport simulations that include retardation and first-order redox reactions on mineral surfaces within a steady-state flow field. These simulations affirm the consistency of the surface-mediated, redox hypothesis with observed Pu activity profiles below the source. Such profiles are captured well by a steady-state, net downward flow model. The redox model explains how Pu(V/VI) sources release activity that moves downward more slowly than expected, and how Pu(III/IV) sources result in a small fraction of activity that moves downward farther than expected. The calibrated parameter values were robust and well defined throughout all simulations. Approximate retardation factors for Pu(V/VI) were 15, and for Pu(III/IV) were 10,000. For these values, ko averaged 2.4 × 10−7 h−1; kr averaged 7.1 × 10−4 h−1 (standard deviations are 1.6 × 10−7 h−1 and 1.6 × 10−4 h−1 respectively).

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