Water percolation and solute transport through an unsaturated sandy formation were investigated using a vadose-zone monitoring system (VMS) that enables in-situ, real-time, monitoring of the percolating water. The VMS includes flexible time-domain reflectometry (FTDR) probes which allow continuous monitoring of the temporal variations of the vadose zone water contents, and vadose-zone sampling ports (VSPs) which are designed to allow frequent sampling of the sediment pore-water as well as measurements of the pore-water pressure. Several years of continuous operation of the VMS provided insight into the dynamics of rainfall-induced infiltration events in a 22-m thick sandy formation. Measurements of the temporal variations in vadose-zone water contents as well as continuous monitoring of the vadose-zone pore water, allowed detailed tracking of the wetting fronts' propagation velocities and determination of flow patterns governing solute transport. It has been shown that the chemical composition of mobile flowing water along the vadose zone is not in equilibrium with the total soluble solute potential of the sediment. This phenomenon is usually attributed to preferential flow. However, wetting-front propagation patterns, as monitored continuously over four rainy seasons through the entire vadose zone, as well as a tracer experiment, showed relatively uniform wetting-front propagation with no direct evidence for significant preferential flow. These results were confirmed HYDRUS simulation. The contradictory observations on matrix and preferential flow as governing mechanisms led to conceptualization of the percolation process as pore-scale dual domain flow. Measurements of vadose zone water pressure through a separate set of VSPs, revealed the critical relationship between temporal variations in vadose zone water contents and water pressure, as well as the dynamic connectivity of the vadose zone gas phase to the atmosphere. As expected, variation in the sediments' water contents, induced by infiltration events, resulted in corresponding variations in pore water pressure. However, the measured responses of water pressure to wetting events were delayed compared to the measured variations in water contents. The delayed pressure response to a wetting process varied with location, as well as between wetting events. Most of the time, the vadose zone gas phase was found to be well connected to the atmosphere. However, this connectivity was limited during rain events. Therefore, compensation of the measured water pressure against the measured atmospheric pressure is not straightforward. Connectivity of the vadose zone gas phase to the atmosphere was reestablished simultaneously across the entire vadose zone upon redistribution of the percolating water in the upper part of the cross section.
|Geophysical Research Abstracts
|Published - 1 Dec 2010
- 1838 HYDROLOGY / Infiltration
- 1875 HYDROLOGY / Vadose zone
- 1895 HYDROLOGY / Instruments and techniques: monitoring