9304850 Hendrickx Unstable wetting fronts are the least understood physical phenomenon in soils. They start out as horizontal wetting fronts that under certain conditions break into "fingers" or "preferential flow paths" as the front moves downward, much like rain running off a sheet of glass breaking into streams. These fingers facilitate the transport of contaminants to the groundwater at velocities many times those calculated if a stable horizontal front is assumed. The investigators describe theoretical, experimental, and field evidence that confirm the occurrence of unstable wetting fronts in field soils. The research objectives are: (1) Verify the hydrodynamic stability by applying Richard's flow equation for a wide range of field soils with lysimeter and field experiments; (2) Apply theory to derive the relations between soil physical characteristics, initial water content, infiltration flux, and the occurrence of unstable wetting fronts; (3) expand the theory and experiments to include heterogeneous soils and boundary conditions. The main experimental difficulty of this study is the labor intensive nature of the lysimeter and field work. The researchers propose to fill large lysimeters (volume 1.2 m3, diameter 1.0 m) with different soil types to study fingers with large diameters that have not been studied before. They will take about 500 volumetric water content samples at each sites for a precise description of the wetting patterns in field soils. Better understanding of the mechanism of unstable wetting fronts will lead to better risk assessments for groundwater contamination, allow construction of more effective landfills and storage ponds, and help in the design of more efficient groundwater recharge systems. ***
