LABORATORY MEASUREMENTS OF SOLUTE TRANSPORT USING TIME-DOMAIN REFLECTOMETRY

Measurement of solute transport at different depths in layered soil columns provides more information than traditional outflow measurements. The objective of this study was to extend the time domain reflectometry (TDR) method of measuring soil water content, theta, and bulk electrical conductiVitY, sigma(T), to the nondestructive measurement of resident concentration, C(R), of an added electrolytic tracer. Calibration curves of C(R) vs. theta and sigma(T) Were obtained directly, by mixing a sandy soil with different amounts of tracer and water and taking TDR readings of impedance. Similar soil was packed into a Plexiglas box fitted with curved TDR probes at different radial distances from one corner. A constant flux of water was applied at the corner of the box and a pulse of KCl tracer added. Measurements of theta and C(R) as a function of time and radial distance were made during the three-dimensional flux experiment. In a one-dimensional flux experiment, undisturbed soil columns (1.5 m long, 0.15-m diam.) were instrumented with straight TDR probes installed horizontally at 0.10-m increments. Indirect calibration of C(R) vs. theta and sigma(T) were obtained under steady-flow conditions by using a step increase in tracer, and by numerically convolving measurements from a pulse application of tracer. The final value of sigma(T) was equated to the input concentration of the tracer to obtain the calibration relationship. The method overcomes many of the limitations associated with soil solution samplers, allowing frequent measurements at close spacings and low cost.