Wednesday, November 7, 2007 - 2:20 PM
284-5

Simulating Long-Term Plutonium Transport Experiments in the Vadose Zone at the Savannah River Site.

Inci Demirkanli1, Fred J. Molz1, Daniel Kaplan2, Robert Fjeld1, and John Coates1. (1) Clemson University, Environmental Engineering & Earth Science, 342, Computer Court, Anderson, SC 29625-6510, (2) Savannah River National Laboratory, WSRC, P.O. Box 616, Building 773-42A, Aiken, SC 29801

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, 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 11years.  After the experiments, Pu activity concentrations were measured along vertical cores from the lysimeters.  Results showed anomalous activity distributions below the source, with significant migration of Pu above the source. Based on previous laboratory experiments, a fully-transient flow and reactive transport model with surface-mediated redox reactions was developed to simulate the lysimeter results. Several mechanisms were tested that might explain the observed upward migration, including air-content dependent oxidation, hysteresis, different root distributions, and Pu uptake by vegetation rooted in the lysimeter soil. This later mechanism required the development of a root water uptake and translocation model as part of the overall transport 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.  Calibrated reaction rate constants were consistent throughout all simulations, and the redox hypothesis reproduced the observed Pu profiles below the source.  However, the soil flow system itself could not create a significant upward migration of Pu.  Upward  movement required dissolved Pu uptake by roots and transport in the plant xylem.

Back to Symposium--Evaluating Water Flux in the Vadose Zone: Computational Methods and Geostatistical Approaches--Modeling Approaches to Problem Solving: In Honor of Art Warrick/Div. S01 Business Meeting
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Back to The ASA-CSSA-SSSA International Annual Meetings (November 4-8, 2007)