COMPARISON OF 3 METHODS TO CALIBRATE TDR FOR MONITORING SOLUTE MOVEMENT IN UNDISTURBED SOIL

Time domain reflectometry (TDR) is rapidly becoming a popular method f or measuring solute concentrations in the laboratory as well as in the field. Success or failure of TDR to represent solute resident concent rations depends on the accuracy of the invoked calibration. In this st udy, we compared three commonly used calibration methods that relate t he impedance, Z(0) as measured with TDR, to the solute concentration s uch as the inlet concentration, C-0. The comparison was carried out us ing solute transport data obtained from 1-m-long, 0.3-m-diam. undistur bed saturated soil columns. The first method comprised the application of a long enough solute pulse such that the concentration in a soil c olumn became equal to the input concentration. The second method invol ved numerical integration of the observed response to a tracer pulse i nput function from which Z(0) could be obtained. The third method dete rmined Z(0) using an independently measured relationship between the i mpedance and the solute concentration. The three calibration methods g ave 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 pred icated the use of excessively long solute pulses in order to equilibra te the entire soil column to the input concentration. The first method hence was useful only for the shallower depths. The second method cou ld be applied throughout the soil profile, provided impedance measurem ents were made for a reasonable time period, especially in the case of nonequilibrium transport. The procedure using an independently measur ed Z-C relationship underpredicted Z(0) in about 50% of the cases, pre sumably because of the use of repacked soil in the calibration.