LABORATORY MEASUREMENTS OF SOLUTE TRANSPORT USING TIME-DOMAIN REFLECTOMETRY

Measurement of solute transport at different depths in layered soil co lumns provides more information than traditional outflow measurements. The objective of this study was to extend the time domain reflectomet ry (TDR) method of measuring soil water content, theta, and bulk elect rical conductiVitY, sigma(T), to the nondestructive measurement of res ident concentration, C(R), of an added electrolytic tracer. Calibratio n curves of C(R) vs. theta and sigma(T) Were obtained directly, by mix ing a sandy soil with different amounts of tracer and water and taking TDR readings of impedance. Similar soil was packed into a Plexiglas b ox fitted with curved TDR probes at different radial distances from on e corner. A constant flux of water was applied at the corner of the bo x 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-dimen sional flux experiment. In a one-dimensional flux experiment, undistur bed soil columns (1.5 m long, 0.15-m diam.) were instrumented with str aight 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 numericall y convolving measurements from a pulse application of tracer. The fina l value of sigma(T) was equated to the input concentration of the trac er to obtain the calibration relationship. The method overcomes many o f the limitations associated with soil solution samplers, allowing fre quent measurements at close spacings and low cost.