Colloid Facilitated Transport of Radionuclides through the Vadose Zone

Collaborators: J. B. Harsh (Washington State University), J. M. Zachara (Environmental Molecular Science Laboratory, Pacific Nothwest Laboratories), Y. Jin (University of Delaware), P. C. Lichtner (Los Alamos National Laboratory), J. F. McCarthy (University of Tennessee)


Radioactive and hazardous waste stored in the underground tanks at the Hanford site has leaked or is suspected to have leaked into the vadose zone. Radionuclides, which are normally considered to be strongly sorbed (e.g., Pu and Cs), have been detected at much deeper depth than predicted based on current theories of vadose zone contaminant transport. There is strong indication that contaminants can be transported via colloids. Such colloid-facilitated transport is likely to occur at the Hanford site, where contaminants leaking from the storage tanks are released directly to the vadose zone. At Hanford, the potential for in situ formation of colloids as leaking tank waste enters the vadose zone is high. Tank waste supernatants are known to consist of solutions with high pH, ionic strength, and aluminate concentrations. When such alkaline solutions contact soils and sediments around the tanks, it is likely that soluble silica-rich allophane, feldspathoids, and zeolite-like materials are formed. These negatively-charged colloids are capable of adsorbing or coprecipitating with contaminant cations and moving through soil and sediment matrices. This project seeks to improve the basic understanding of colloid and colloid-facilitated transport of contaminants in the vadose zone. The radionuclide 137-Cs---which occurs abundantly in Hanford waste tanks and which has entered the soil beneath the tanks in considerable amounts---is selected as the contaminant. The specific objectives that will be addressed are: Colloids will be separated and characterized in terms of size, structure, composition, and surface charge characteristics. The interactions of Cs with colloidal particles isolated from the previous step will be investigated with batch sorption experiments and spectroscopic techniques. Transport and co-transport of colloids and the radionuclide Cs will then be studied with a series of laboratory column experiments using repacked Hanford sand material. Experiments will be carried out under unsaturated, steady-state as well as transient water flow to study the effect of water content on colloid transport. Magnetic Resonance Imaging will be used to visualize colloidal movement inside the porous medium. Sorption studies and column outflow data will be analyzed with numerical models to elucidate the relevant mechanisms responsible for contaminant sorption as well as colloid and radionuclide transport. Sorption and reaction models will be combined with transport models to quantitatively describe the column experiments.

The results of the proposed research will lead to a better understanding of colloid-formation, colloid-contaminant-soil interactions, colloid migration, and colloid-facilitated transport in the vadose zone. The experiments proposed use conditions specific to the Hanford site, and the results are therefore directly applicable to clean-up strategies and procedures for Hanford contamination problems. We expect to provide conclusive evidence under what conditions, if any, colloid-facilitated transport can be expected at the Hanford site, and what the quantitative magnitude of this transport process will be.

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Markus Flury
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