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Title: FLUID FLOW AND SOLUTE MIGRATION WITHIN THE CAPILLARY FRINGE

Author
item SILLIMAN, STEPHEN - UNIV OF NOTRE DAME
item BERKOWITZ, BRIAN - WEIZMANN INSTITUTE,ISRAE
item SIMUNEK, JIRI - UC RIVERSIDE, CA
item Van Genuchten, Martinus

Submitted to: Ground Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2001
Publication Date: 1/15/2002
Citation: Silliman, S.E., Berkowitz, B., Simunek, J., Van Genuchten, M.T. 2002. Fluid flow and solute migration within the capillary fringe. Ground Water. Vol 40:76-84

Interpretive Summary: A key question in the characterization, exploitation and remediation of groundwater aquifers concerns identification of pathways traveled by water, and the impact of these pathways on the movement of chemicals and the viability of microbial populations. In this study, we are particularly interested in observations of fluid flow and chemical transport within the capillary fringe, with emphasis on the impact of physical heterogeneity and exchange of water between the capillary fringe and the region below the water table. We define the capillary fringe as the zone immediately above the water table where the water content remains at or very close to saturation. Laboratory experiments involving both homogeneous and heterogeneous porous media to demonstrate that fluid flow and chemical transport will occur regularly in the capillary fringe including both vertical (upwards as well as downwards) and horizontal flow. Horizontal flow above the water table appears to be limited primarily to the region of high water saturation, an observation supported by numerical modeling and consistent with the literature. Our experiments suggest that the capillary fringe may affect, far more significantly than is usually assumed, the natural geochemical and microbial conditions present in the region of transition from unsaturated to saturated groundwater flow. This conclusion is important for proper designs of various remediation strategies.

Technical Abstract: Laboratory experiments involving both homogeneous and heterogeneous porous media are used to demonstrate that fluid flow and chemical transport will occur regularly in the capillary fringe (CF) including both vertical (upwards as well as downwards) and horizontal flow velocities. Horizontal flow above the water table appears to be limited primarily to the region of high water saturation (i.e., the CF), an observation supported by numerical modeling and consistent with the literature. Beyond observations presented in prior literature, it was observed that exchange of water within the CF with water below the water table is quite active, with flux both from the CF downward across the water table and from the region below the water table, upward in the CF. This flux is enhanced by the presence of physical heterogeneity. These findings strongly contrast the common conceptualization of predominantly downward vertical fluid flow through the unsaturated zone, with transition to fully three-dimensional flow only below the water table. Based on these observations, it is suggested that the CF may affect, far more significantly than is usually assumed, the natural geochemical and microbial conditions present in the region of transition from unsaturated to saturated groundwater flow.