Using a Geocentrifuge to Study Colloid Transport In Unsaturated Porous Media.

Colloids are ubiquitous in the subsurface and play an important role in contaminant fate and transport. Flow and transport experiments in porous media can take a long time, particularly in unsaturated porous media where the hydraulic conductivity decreases exponentially with decreasing water saturation. Advantages of using centrifuges to conduct flow and transport experiments are that the driving force can be varied without affecting the moisture content, experiments can be conducted with materials of low hydraulic conductivity, and the experimental time can be greatly reduced. Geocentrifuges have been found to be useful and promising tools to study colloid transport in saturated porous media, but no experiments have been conducted with unsaturated porous media so far. The overall goal of this research project is to identify and quantify the relevant mechanisms controlling colloid fate and transport in unsaturated porous media, with emphasis on soils and sediments. The specific objectives for the geocentrifuge experiments are to improve experimental measurement techniques to quantify colloid transport under unsaturated flow conditions and to delineate the importance of water content vs flow rate on colloid mobilization and transport in unsaturated porous media. We built a new test platform for unsaturated flow and transport experiments and have designed a series of tests using sieved silica sand and two different polystyrene colloids (26 nm and 200 nm) in solutions of different ionic strength. Centrifugal acceleration will be varied to determine the impact of flow rates and water content on the mobilization and transport of colloids. The data will be analyzed using filtration theory, and compared with a modified version of the Hydrus code adapted to variable body forces. Future test series include kaolinite and hematite colloids as well as in situ colloids of undisturbed Hanford sediments.