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+ as measured with TDR, to the solute concentration such as the inlet concentration, CO. The comparison was carried out using solute transport data obtained from l-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 ZO could be obtained. The third method determined & 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 w a s 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.O in about 50% of the cases, presumably because of the use of repacked soil in the calibration. SEVERAL STUDIES have recently demonstrated the potential of TDR for measuring solute transport in laboratory soil columns as well as field soil profiles (e.g., Kachanoski et al., 1992; Wraith et al., 1993; Vanclooster et al., 1993, 1995; Mallants et al., 1994; Ward et al., 1994). Potential advantages of TDR have been discussed D. Mallants, M. Vanclooster, J. Vanderborght, and J. Feyen, Inst. for Land and Water Management, KULeuven, Vital Decosterstraat 102, 3000 Leuven, Belgium; N. Toride, Dep. of Agricultural Sciences, Saga Univ., 840 Saga, Japan; and M.Th. van Genuchten, USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Road, Riverside, CA 92507 CA. Received 3 Mar. 1995. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 60:747-754 (1996). at length in the above literature, and are not repeated here. A possible disadvantage of TDR is its usefulness primarily for nonreactive tracers and low EC soils. Success or failure of TDR to accurately measure solute concentrations depends strongly on the appropriateness of the calibration procedure being used. The assumed calibration is also important for EC measurements using conventional four-electrode salinity probes (Rhoades et al., 1989). Since TDR measures the total resistance or impedance, Z, of a soil to the flow of electromagnetic energy, solute concentrations can be inferred from impedance readings if a particular value of the impedance, ZO, can be related to a known value of the concentration, such as the concentration of the inlet solution, CO. A linear relationship between Z and C has been observed for instance by Ward et al. (1994) for different values of water content, 8, although at higher values of 8 some nonlinearity exists for low concentrations. By means of such Z-C relationships, impedance distributions can subsequently be translated back to solute distributions. The three most commonly used calibration methods are: TDR measurement of a continuous solute application until some constant value ZO is obtained (referred to hereafter as Method l), TDR measurement of a pulsetype solute application using the concept of convolution (Method 2), and determination of an independent Z-C calibration using disturbed or undisturbed soil (Method 3). Any of these three methods works well for relatively homogeneous sandy soils or repacked soil columns (Kachanoski et al., 1992; Vanclooster et al., 1993; Ward et al., 1994). However, calibrations may become problematic for soils exhibiting small-scale heterogeneities due to the presence of macropores, immobile water regions, or low-permeability zones. This problem was demonstrated by Mallants et al. (1994), who used TDR to measure solute concentrations in a sandy loam soil containing a large number of macropores. They showed that the TDR sampling volume for their macroporous soil may have been too small to yield accurate values Abbreviations: TDR, time domain reflectometry; EC, electrical conductivity; PVC, polyvinyl chloride; BTC, breakthrough curve. 748 SOIL SCI. SOC. AM. J VOL. 60, MAY-JUNE 1996 of the average concentration across the soil column. Mallants et al. (1994) subsequently employed Method 1 by adding a very long solute pulse so that the conditions that C = CO (or Z = ZO) at the detection volume was satisfied. This method, however, also has problems if zones of low permeability and/or stagnant water are present within the sampling volume of the TDR probe; the solute may then require an inordinate amount of time in order to spread uniformly by diffusion across the entire cross-section of the column. In this study we compared these three calibration methods for relating TDR impedance readings to solute concentrations using data measured at six different depths in 1 -m-long, 0.3-m-diam. water-saturated undisturbed soil columns. First, we evaluated the accuracy of Method 1 to calibrate the Z-C relationship from a very long tracer pulse, i.e., until Z becomes constant and equal to ZO. Second, the results from a numerical convolution of the solute pulse response was investigated (Method 2). Next, we examined the applicability of an independently measured relationship between the bulk soil EC (EC,) and the EC of the soil liquid phase (EC,) as a function of the soil water content, 8 (Method 3). Values for the calibration constant, ZO, obtained with Methods 2 and 3 were compared with impedance data obtained using Method 1. MATERIALS AND METHODS Solute transport processes were examined on 30 undisturbed soil columns taken 1 m apart at the Bekkevoort experimental field, east of Leuven, Belgium. Details of the experimental design and the transport experiments can be found in Mallants et al. (1995, unpublished data). Soil water contents, 0, and bulk soil electrical conductivities, EC, were monitored by means of TDR probes installed horizontally at six different depths (0.05, 0.15, 0.30, 0.45, 0.60, and 0.80 m) under constant temperature conditions. Time-averaged water contents based on all 30 columns were 0.37, 0.34, 0.33, 0.33, 0.33, and 0.35 m3 mm3 for the six observation depths mentioned above. Standard errors of observed 8 are very small ranging from 0.002 to 0.004, which is within the range of TDR instrument variability. The mean water flux was 0.0078 m h-‘, with a standard deviation of 0.0064 m h-‘. After establishing steady-state saturated flow using solute-free water, longduration solute pulses containing 7 X 10m3 M CaCl* were applied in order to “saturate” the soil column with the applied tracer solution (i.e., until C = Co in the columns). Solute application times (to) ranged from 79 h for 14 columns to more than 660 h. Measurements were made with a Tektronix cable tester (Tektronix 1502B, Beaverton, OR) and the channels were multiplexed manually. Values of Z were obtained for 0.25-m-long two-rod TDR probes (0.005-m diam. and 0.02-m spacing). The travel time of the electromagnetic wave and the impedance Z were taken from the screen of the cable tester. Values of Z are calculated by taking the reflection coefficient, p, at long times (f-03) on the reflected wave form (see Fig. 1). The impedance where the wave form has reached a stable level, Z, is calculated from: z =z(l+Pm) 03 Yl Pm) [ 1 ] where Zc is the impedance of the coaxial cable (50 Sz), and pm is the reflection coefficient at long times (Fig. 1). -1.2 +-/ I I ,