Solute channeling in unsaturated heterogeneous porous media

Numerical simulations have been performed to study flow and solute transport phenomena in strongly heterogeneous, variably saturated porous media. Different saturation scenarios were applied varying from fully saturated to highly unsaturated conditions, corresponding to different infiltration rates into the soil. It was found that the solute travels along preferred flow paths, which may be called channels. The degree of channeling, the location of channels, and the hydraulic properties along channels are a function of the mean saturation in the flow domain. Strong channeling effects were obtained in both fully saturated and in low-saturation cases. At intermediate saturation values, channeling effects are less significant, and the system exhibits a more homogeneous flow pattern. The dispersion of solutes as shown in the calculated tracer breakthrough curves essentially reflects the degree of channeling and thus is saturation dependent; in cases when flow channeling is less evident we observe much smaller dispersion than in cases with strong channeling. The hydraulic properties of the channels appeared to be an invariant of the actual location and geometry, indicating that they may be an intrinsic characteristic of the soil heterogeneity and the water saturation.