Investigation of Colloid Transport under Unsaturated Steady Flow Conditions Using a Geocentrifuge

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Subsurface colloids can enhance the movement of strongly sorbing contaminants, a phenomenon called colloid-facilitated contaminant transport. In the presence of mobile subsurface colloids, contaminants may move faster and farther than in the absence of colloids, thereby bypassing the filter and buffer capacity of soils and sediments. Fate and transport models neglecting colloid-facilitated transport therefore often underpredict contaminant movement. Long-term predictions of contaminant fate and transport as well as risk assessment rely on an accurate representation of subsurface processes, and in the case of strongly sorbing contaminants, need to consider mobile colloids as potential contaminant carriers. The overall goal of this research project is to elucidate the relevant mechanisms controlling colloid fate and transport in unsaturated porous media, with emphasis on soils and sediments. The specific objectives are to (1) improve experimental measurement techniques to quantify colloid transport under unsaturated flow conditions; (2) delineate the importance of water content vs flow rate on colloid mobilization and transport in unsaturated porous media; and (3) modify an existing colloid transport code (Hydrus-1D) by incorporating mathematical formulations of the relevant colloid transport processes. The use of a geocentrifuge offers completely new experimental opportunities to study colloid transport in porous media. By changing the acceleration of the experimental setup the water content and the hydraulic flow rate of the porous media can be controlled independently. Therefore, the role and the interplay of these factors can be elucidated to obtain a mechanistic understanding of colloid transport and mobilization. Experiments can also be conducted with materials of low hydraulic conductivity, and the experimental time can be shortened. I will design a novel experimental system for use in a geocentrifuge, and will conduct a series of colloid transport experiments under unsaturated flow. Additionally, I will validate and generalize the experimental measurements and results by using a new modelling approach. An unsaturated flow and transport code (Hydrus-1D) will be modified to include variable accelerations (higher than gravity) to accommodate the geocentrifuge system. The modified Hydrus code will be used to extrapolate and scale measurements back to normal gravity.