HR: 14:15h
AN: H23H-03 INVITED     [Abstracts]
TI: Visualization of Colloid Deposition and Mobilization During Unsteady and Steady Porewater Flow Through Unsaturated Porous Media.
AU: * Saiers, J E
EM: james.saiers@yale.edu
AF: Yale University, School of Environmental Studies, 205 Prospect Street, New Haven, CT 06511 United States
AU: Gao, B
EM: bin.gao@yale.edu
AF: Yale University, School of Environmental Studies, 205 Prospect Street, New Haven, CT 06511 United States
AU: Ryan, J N
EM: joseph.ryan@colorado.edu
AF: University of Colorado, Department of Civil, Environmental, and Architectural Engineering, Boulder, CO 80309 United States
AB: Mineral colloids that are mobilized from near-surface soils during infiltration events may carry sorptive contaminants through the vadose zone and into drinking-water aquifers. The vadose-zone flux of colloid-associated contaminants depends, in part, on the difference between colloid mobilization rates and deposition rates. Our research is aimed at improving current understanding of colloid effects on subsurface-contaminant transport by identifying the mechanisms that govern colloid mobilization and deposition in unsaturated porous media. We present pore-scale observations of the transport of fluorescent microspheres through transparent flow cells packed with a thin layer of partially saturated sand. These visualization experiments were conducted under steady-flow and transient-flow conditions. In experiments in which the air phase was discontinuous and occurred as insular air bubbles, the negatively charged microspheres accumulated at the air-bubble surface and moved freely about this air-water interface. A fraction of these colloids eventually migrated from the air-water interface to the air-water-solid interface, whereupon their motion stopped. Destruction of the air bubbles during imbibition led to the release of colloids retained previously by the air-water interface, but not to the release of colloids held at the air-water-solid interface. Colloids were also trapped upon entry into dead-end water conduits that split from the primary flow channels. The exchange of colloids and water between a dead zone and primary flow channel was slow under steady flow; however, the reconnection of dead-end zones as moisture content increased during imbibition resulted in the mobilization of large concentrations of colloids. Our findings show that multiple mechanisms govern the deposition and mobilization of colloids in unsaturated porous media and provide direction for refining mathematical models for colloid and colloid-facilitated contaminant transport within the vadose zone.
DE: 1806 Chemistry of fresh water
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting