Physical and chemical evidence for pore-scale dual-domain flow in the vadose zone

Water percolation and solute transport through an unsaturated sandy formation were investigated using a vadose-zone monitoring system that enables in situ, continuous, realtime monitoring of the percolating water. Measurements of the temporal variations in vadose-zone water content as well as continuous monitoring of the vadose-zone pore water allowed detailed tracking of the propagation velocities of the wetting front and determination of the flow patterns governing solute transport. It has been shown that the chemical composition of mobile fl owing water along the vadose zone is not in equilibrium with the total soluble solute potential of the sediment. This phenomenon is usually attributed to a flow mechanism controlled by preferential flow. Wetting-front propagation patterns, as monitored continuously during four rainy seasons throughout the entire vadose zone, as well as a tracer experiment, showed relatively uniform wetting-front propagation with no direct evidence for significant preferential flow. Contradictory observations of matrix and preferential flow as governing mechanisms led to conceptualization of the percolation process as pore-scale dual-domain flow.