Solute transport under transient flow conditions estimated using time domain reflectometry

Improved methods having high resolution in space and time are needed to monitor transport of soluble chemicals in soils. We used time domain reflectometry (TDR) to measure breakthrough curves (BTCs) for Br- during miscible displacement in repacked soil columns under nonsteady water flow conditions. A simple linear model was used to estimate soil solution electrical conductivity (sigma(w)) based on TDR-measured bulk soil electrical conductivity (sigma(a)) and volumetric water content (theta). An efficient calibration method was developed whereby required model parameters were estimated in the same soil volume and under the same Bow conditions as were used in subsequent transport experiments. Breakthrough curves measured using TDR were similar to those based on electrical conductivity or Br- concentration in effluent fractions, although peak sigma(w) were substantially lower for TDR than for effluent from two of the four soils. Effluent sigma(w) BTCs were adequately predicted based on TDR sigma(a) and theta for all four soils when parameters of the simple linear model or two alternative models were optimized to the effluent data. Time moments and convection-dispersion equation (CDE) transport parameters based on time moments of measured BTCs were similar for TDR and effluent analyses. Our results indicate that TDR has potential to monitor the status of representative concentrations of ionic solutes under laboratory and field conditions, without the need to impose steady water flow.