SOLUTE TRANSPORT UNDER TRANSIENT FLOW CONDITIONS ESTIMATED USING TIME-DOMAIN REFLECTOMETRY

Improved methods having high resolution in space and time are needed t o monitor transport of soluble chemicals in soils. We used time domain reflectometry (TDR) to measure breakthrough curves (BTCs) for Br- dur ing miscible displacement in repacked soil columns under nonsteady wat er flow conditions. A simple linear model was used to estimate soil so lution electrical conductivity (sigma(w)) based on TDR-measured bulk s oil electrical conductivity (sigma(a)) and volumetric water content (t heta). 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, al though peak sigma(w) were substantially lower for TDR than for effluen t from two of the four soils. Effluent sigma(w) BTCs were adequately p redicted based on TDR sigma(a) and theta for all four soils when param eters of the simple linear model or two alternative models were optimi zed to the effluent data. Time moments and convection-dispersion equat ion (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 concentratio ns of ionic solutes under laboratory and field conditions, without the need to impose steady water flow.