Long-Term Colloid Mobilization and Colloid-Facilitated Transport of Radionuclides in a Semi-Arid Vadose Zone
Collaborators: J. B. Harsh (Washington State University), Glendon W. Gee and Fred Zhang (Pacific Northwest Laboratory), P. C. Lichtner (Los Alamos National Laboratory), Earl D. Mattson (Idaho National Laboratory)
Background: Colloid-facilitated transport of radionuclides over large distances can occur in water-saturated porous media (i.e., in groundwater). However, in unsaturated porous media (e.g., the ``vadose zone''), the effectiveness and importance of colloid-facilitated transport is largely unknown. We do know that colloid transport, hence colloid-facilitated contaminant transport, decreases as the water content of the medium decreases. In semi-arid regions (e.g., the Hanford Site) water contents are low, but episodic precipitation and snow melt events likely cause colloid mobilization and transport. Current theory and experimental evidence do not allow us to develop a defensible conceptual and predictive model for colloid transport at the Hanford vadose zone. Such a model is critical at Hanford and elsewhere around the world-wherever contaminants are entering the subsurface and there are concerns about their transport and fate.
Goal and Objectives:
Our general goal is to improve the fundamental mechanistic understanding and quantification of long-term colloid mobilization and colloid-facilitated transport of radionuclides in the vadose zone, with special emphasis on the semi-arid Hanford site. Our specific goal is to develop a defensible conceptual model for long-term colloid-facilitated transport at Hanford. We will (1) determine the mechanisms of colloid mobilization and colloid-facilitated radionuclide transport in undisturbed Hanford sediments under unsaturated flow, (2) quantify in situ colloid mobilization and colloid-facilitated radionuclide transport from Hanford sediments under \textbf{field conditions}, and (3) develop a field-scale conceptual and numerical model for colloid mobilization and transport at the Hanford vadose zone, and use that model to predict long-term colloid and colloid-facilitated radionuclide transport.
Methods: (1) We will conduct in situ colloid mobilization experiments with meso-scale, undisturbed sediment columns using natural and forced flow rates and water contents representative of the Hanford site. We will further use a geocentrifuge (a) to separate effects of flow rates and water contents on colloid mobilization and (b) to speed up the experiments. At the micro-scale, we will use tensiometry to quantify forces between colloids and the the air-water interface. (2) We will study colloid mobilization and transport at the field-scale at Hanford using natural and forced flow rates. Colloids in Hanford pore water will be collected with innovative vadose zone flux meters. (3) We will develop a numerical field-scale model to describe colloid and colloid-facilitated contaminant transport through the Hanford vadose zone. The modeling will allow us to extend the experimental results beyond the experimental conditions and to make long-term predictions.
Benefits: This work will lead to a better understanding of long-term colloid mobilization, colloid migration, and colloid-facilitated transport in the vadose zone. The experiments proposed here use conditions specific to the semi-arid Hanford site and the results are therefore directly applicable to clean-up strategies for Hanford contamination problems. We will delineate the conditions under which colloid-facilitated transport can be expected at Hanford and predict its magnitude. Moreover, these conditions are not unique to Hanford-they occur at other sites, hence are important far beyond Hanford's borders. As a result, the model should be extendable to less arid conditions, thus broadening its use.
Publications
Markus Flury 
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