A simple approach to determine reactive solute transport using time domainreflectometry

Time domain reflectometry (TDR) possesses potential for determining solute- transport parameters, such as dispersion coefficients and retardation facto rs for reactive solutes. We developed a simple method based on peak-to-peak measurements of water and solute velocities through the soil using TDR. Th e method was tested by carrying out unsaturated leaching experiments in the laboratory on two soil columns packed with a South Pacific soil from Mare, which is a ferrasol with variable surface charge. One column was left bare and the other was planted with mustard. Pulses of CaBr2 and Ca(NO3)(2) wer e applied to the surface of either wet or dry soil and then leached by wate r from a rainfall simulator applied at a steady rate of between 30 and 45 m m h(-1). Water and solute transport were monitored by collecting the efflue nt. Contemporaneous in situ measurements of the water content and electrica l conductivity were made using TDR, Transport parameters for the convection -dispersion equation, with a linear adsorption isotherm, were obtained from the flux concentration and the solute resident concentrations measured by TDR, Anion retardations between 1.2 and 1.7, and dispersivities between 1 a nd 9 mm, were found. Retardations also were calculated using our simple app roach based on TDR-measured water and solute front velocities. These used T DR measurements of soil water content and bulk soil electrical conductivity with time, and were similar to those obtained from the effluent. The agree ment suggests TDR could be a valuable in situ technique for obtaining the p arameters relating to reactive solute transport through soil.