Comparison of three methods to calibrate TDR for monitoring solute movement in undisturbed soil

Time domain reflectometry (TDR) is rapidly becoming a popular method for measuring solute concentrations in the laboratory as well as in the field. Success or failure of TDR to represent solute resident concentrations depends on the accuracy of the invoked calibration. In this study, we compared three commonly used calibration methods that relate the impedance, Z(0) as measured with TDR, to the solute concentration such as the inlet concentration, C-0. The comparison was carried out using solute transport data obtained from 1-m-long, 0.3-m-diam. undisturbed saturated soil columns. The first method comprised the application of a long enough solute pulse such that the concentration in a soil column became equal to the input concentration. The second method involved numerical integration of the observed response to a tracer pulse input function from which Z(0) could be obtained. The third method determined Z(0) using an independently measured relationship between the impedance and the solute concentration. The three calibration methods gave approximately the same results for the first observation depth at x = 0.05 m. However, the presence of heterogeneous transport processes involving solute diffusion from mobile to immobile water regions predicated the use of excessively long solute pulses in order to equilibrate the entire soil column to the input concentration. The first method hence was useful only for the shallower depths. The second method could be applied throughout the soil profile, provided impedance measurements were made for a reasonable time period, especially in the case of nonequilibrium transport. The procedure using an independently measured Z-C relationship underpredicted Z(0) in about 50% of the cases, presumably because of the use of repacked soil in the calibration.