Estimation of soil water content and resident and effluent solute concentrations using time domain reflectometry

Monitoring solute concentrations within an undisturbed soil core during saturated and unsaturated flow can provide first-hand information for better understanding solute transport processes as well as data required for numerical simulation of solute transport. Time domain reflectometry (TDR) has recently been used to monitor solute concentration in both laboratory and field soils. Although TDR has been shown to measure resident solute concentrations, C r, it has not been fully developed to measure solute transport. The objectives of this study are to evaluate TDR\u27s abilities to accurately estimate Cr and to predict transport of the flux average concentration also termed effluent solute concentration, Ce;Relationships of TDR-measured apparent soil dielectric constant (K a) to water content (thetav), Ka to theta v and bulk soil electrical conductivity (sigmaa), and sigma a to thetav and Cr were developed using data obtained from incremental addition of solutions with different solute concentrations to four packed soil cores. The Cr breakthrough curves (BTC) were estimated using the sigmaa-thetav-Cr relationship and TDR-measured sigmaa and -estimated theta v for packed and undisturbed cores under steady flow conditions with various flow velocities. Effluent BTC was predicted from the Cr BTC either using solute transport models or by mass balance;Effects of sigmaa on Ka or thetav were site-specific and were not explained by TDR theory. A Ka to sigma a and thetav relationship developed from packed core data did not adequately describe the effects of sigmaa on TDR-measured Ka for an undisturbed soil core, the effect of sigmaa was only removed by individual calibrations. The sigmaa-theta v-Cr relationship developed in this study fitted packed soil core data with higher r2 and gave more accurate Cr estimation than three published models. An increase in flow velocity increased variation of TDR-measured sigmaa, and thus variation of TDR-estimated Cr for measurements on undisturbed soil cores over a range of pore water velocity (nu, 0.5--86.1 cm hr--1). The effluent breakthrough curve (BTC) predicted with solute transport models using solute transport parameters obtained by fitting the same models to TDR-estimated Cr BTC usually deviated from the measured effluent BTC; whereas, a simple mass balance method demonstrated the capability of accurately predicting effluent BTC