Comparison of time domain reflectometry, fiber optic mini-probes, and solution samplers for real time measurement of solute transport in soil

Characterizing water and solute transport in soil requires sampling at a scale and temporal resolution appropriate for the question being studied. Sampling scale is also an important factor to consider when using devices to obtain transport parameters for modeling. In fact, probes that monitor solute transport at different sample volumes may actually produce dissimilar results; consequently, it is necessary to understand how devices produce different breakthrough curves (BTCs). Three in situ nondestructive methods to monitor solute transport were tested and compared in a single repacked clay loam soil column. The methods compared included time domain reflectometry (TDR), fiber optic mini-probes (FOMP), and continuous pore water small tube samplers (STS). All three probes-TDR, FOMP, and STS-performed well in measuring solute transport in the soil column. We found that a single point linear calibration is sufficient to correlate concentration to electrical conductivity for horizontally inserted TDR probes. In addition, the convective dispersive equation (CDE) described adequately the ETC measured with TDR, FOMP, and STS. The average effective pore water velocity, estimated by fitting the data to the CDE, were similar for all of the probes. Finally, we point out that effective dispersion values estimated using data from each type of probe were systematically different. This difference may be an artifact of a lower mass balance resulting from the calibration procedures, a scale issue resulting from the probe sampling volumes, or both. Nevertheless, the fit from each probe produced a different transport parameter for effective dispersion, and this must be considered when comparing results obtained with different monitoring devices.