A Time Domain Reflectometry Coaxial Cell for Manipulation and Monitoring of Water Content and Electrical Conductivity in Variably Saturated Porous Media

method for determination of porous media dielectric permittivity (i.e., for soil water content) and electrical conductivity (EC). Characteri- differences in the bulk density of the sample and more zation of these properties using controlled wetting or drying of the importantly, changes in the structural properties that porous media is of interest for studying and quantifying porous media are associated with electrical and dielectric properties physical and chemical properties. Our objectives were to develop a (JacobsenandSchjonning,1993;Robinsonetal.,2003a). TDR measurement cell providing sample water content adjustment Investigators using TDR to provide information on and solution exchange capability without repacking or disturbing the water retention bring the soil sample to an equilibrium sample. A coaxial cell provides a well-defined sample packing volume potentialbyapplyingaconstantpressuregradientacross in which radial distribution of the electromagnetic field averages the the porous semi-permeable membrane, driving water transmissionlinemeasurementsover theentiresample.Usingconcenmovement while preventing air from entering a pressurtric porous stainless steel tubes as electrical conductors, complete or ized chamber. This facilitates adjustment of sample water partial saturation of the porous medium (via matric potential control) was possible with the additional key advantage of soil solution ex- content by removal of solution as in the pressure plate change without changes in pore space or sample porosity. Solution apparatus described by (Dane and Hopmans, 2002). exchange and suction adjustment were controlled through the center (Wraith and Or, 2001) used a 15 bar pressure plate in tube with influent solution (ECw) entering the sample through the combination with internally installed TDR probes to meaouter wall and for unsaturated measurements an air inlet port facili- sure water retention in finer textured soils. (Wildenschild tates air exchange. The cell was temperature controlled using an et al., 2001) incorporated tensiometers into a 1 bar presexternal PVC jacket connected to a circulating water bath. Both sure (Tempe) cell which served to provide single- and saturated and unsaturated measurements of electrical conductivity multi-step outflow experiments from water potential resulted in excellent agreement between measurements and model measurementsatvaryingwatercontents.(Friedmanand predictions. Permittivity measurements were dependent on cell orienJones, 2001) used two orthogonally oriented TDR probes tation, likely due to differences in the water distribution within the cell.TheutilityofthisnewcoaxialTDRpressurecelldesignforporous to measure the anisotropy in permittivity and electrical media research is the ability to simultaneously measure dielectric and conductivity measurements in mica under drying condiECb in a fixed pore space where water content and solution electrical tions. Some limitations in these systems are that they conductivity are adjustable. can’t readily facilitate reintroducing water into the sample or cycling water for adjustment of the soil solution electrical conductivity for example.