Considerations for measuring pressure head and water content in inclined boreholes

Talby

Yechieli

et al. 2009

Water percolation and tracer migration through the vadose zone underneath an ephemeral channel were studied using a vadose zone monitoring system (VMS) and application of a multitracer test. The VMS included flexible time‐domain reflectometry (FTDR) probes for continuous tracking of water content profiles, and vadose zone sampling ports (VSPs) for frequent sampling of the deep vadose pore water at multiple depths. The VMS was installed directly under an infiltration pond with several infiltration rings containing a traceable solution. Water content measurements by the FTDR probes allowed detailed visualization of the vadose wetting process; VSP samples allowed the establishment of tracer breakthrough curves at various depths. Flow velocities and fluxes were calculated from both the wetting process and the tracer breakthrough curves. The multitracer experiment revealed an unsteady flow pattern strongly affected by the layered structure of the sediments. The tracer breakthrough curves indicated domination of a mobile–immobile flow mechanism controlling contaminant migration across the vadose zone. The experimental setup demonstrated the ability of the VMS to provide real‐time monitoring of water flow and contaminant transport in the vadose zone.

Section: Methodsmentioning

confidence: 91%

In Situ Monitoring of Water Percolation and Solute Transport Using a Vadose Zone Monitoring System

Talby

Yechieli

et al. 2009

“…A comprehensive description of the FTDR probes, including application in field studies, calibration procedure, and data interpretation, was presented by Rimon et al (2007) Recently, Hinnell and Ferré (2008) showed that under dynamic flow conditions, TDR probes installed on top of an impermeable casing in inclined boreholes can potentially overestimate the measured water‐content values. Because a similar installation procedure and TDR probes were used in this study, it is possible that the water‐content values measured in this study overestimated the actual water content during the dynamic phase of the infiltration process.…”

Section: Methodsmentioning

confidence: 99%

Physical and Chemical Evidence for Pore‐Scale Dual‐Domain Flow in the Vadose Zone

Rimon

Nativ

2011

Water percolation and solute transport through an unsaturated sandy formation were investigated using a vadose‐zone monitoring system that enables in situ, continuous, real‐time 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 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 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.

“…Because a similar installation procedure was used in this study, the measured values of water content and water pressure presented here could potentially be overestimates of the actual water content in the undisturbed sediments away from the impermeable barrier. According to Hinnell and Ferré (2008), however, and under the water content and pressure conditions measured in this study, this overestimation is not expected to exceed the actual water content values by more than 1%. During rare high‐flux wetting events, water content overestimation could reach up to 3%.…”

Section: Study Sitementioning

confidence: 46%

“…Hinnell and Ferré (2008) reported that the use of an impermeable casing in inclined boreholes can potentially create denser flow lines in the upper part of the instrument. As such, water‐content measurements above this upper part under dynamic flow conditions might produce overestimations.…”

Section: Study Sitementioning

confidence: 99%

Vadose Zone Water Pressure Variation during Infiltration Events

Rimon

Nativ

2011

Measurements of vadose zone water pressure using custom‐made tensiometers provided insight into the dynamics of rainfall‐induced infiltration events in a 22‐m‐thick sandy formation. The tensiometers, based on vadose zone sampling ports, were assembled in a vadose zone monitoring system that allowed in situ, real‐time measurements of the temporal variation in vadose zone water content and pore water pressure. Results revealed the critical relationship between temporal variations in vadose zone water content and water pressure, as well as the dynamic connectivity of the vadose zone gas phase to the atmosphere. As expected, variation in the sediment water contents, induced by infiltration events across the vadose zone, resulted in corresponding variations in pore water pressure; however, the measured responses of sediment water pressure to wetting events were delayed compared with the measured variations in water content. The delay in the 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 well connected to the atmosphere; however, this connectivity was limited during rain events and, therefore, compensation of the measured water pressure variation for the measured atmospheric pressure fluctuation is not straightforward. Connectivity of the vadose zone gas phase to the atmosphere was reestablished simultaneously across the entire vadose zone following redistribution of the percolating water in the upper part of the cross‐section.