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's abilities to accurately estimate C_r and to predict transport of the flux average concentration also termed effluent solute concentration, C_e.; Relationships of TDR-measured apparent soil dielectric constant (K _a) to water content (&thetas;_v), K_a to &thetas; _v and bulk soil electrical conductivity (σ_a), and σ _a to &thetas;_v and C_r were developed using data obtained from incremental addition of solutions with different solute concentrations to four packed soil cores. The C_r breakthrough curves (BTC) were estimated using the σ_a-&thetas;_v-C_r relationship and TDR-measured σ_a and -estimated &thetas; _v for packed and undisturbed cores under steady flow conditions with various flow velocities. Effluent BTC was predicted from the C_r BTC either using solute transport models or by mass balance.; Effects of σ_a on K_a or &thetas;_v were site-specific and were not explained by TDR theory. A Ka to σ _a and &thetas;_v relationship developed from packed core data did not adequately describe the effects of σ_a on TDR-measured K_a for an undisturbed soil core, the effect of σ_a was only removed by individual calibrations. The σ_a-&thetas; _v-C_r relationship developed in this study fitted packed soil core data with higher r2 and gave more accurate C_r estimation than three published models. An increase in flow velocity increased variation of TDR-measured σ_a, and thus variation of TDR-estimated C_r for measurements on undisturbed soil cores over a range of pore water velocity (ν, 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.