Modeling Long‐Term Plutonium Transport in the Savannah River Site Vadose Zone

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, k o averaged 2.4 × 10 −7 h −1 ; k r averaged 7.1 × 10 −4 h −1 (standard deviations are 1.6 × 10 −7 h −1 and 1.6 × 10 −4 h −1 respectively).