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 sc ale and temporal resolution appropriate for the question being studied. Sam pling scale is also an important factor to consider when using devices to o btain transport parameters for modeling. In fact, probes that monitor solut e transport at different sample volumes may actually produce dissimilar res ults; consequently, it is necessary to understand how devices produce diffe rent breakthrough curves (BTCs). Three in situ nondestructive methods to mo nitor solute transport were tested and compared in a single repacked clay l oam soil column. The methods compared included time domain reflectometry (T DR), fiber optic mini-probes (FOMP), and continuous pore water small tube s amplers (STS). All three probes-TDR, FOMP, and STS-performed well in measur ing solute transport in the soil column. We found that a single point linea r calibration is sufficient to correlate concentration to electrical conduc tivity for horizontally inserted TDR probes. In addition, the convective di spersive equation (CDE) described adequately the ETC measured with TDR, FOM P, 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 o f probe were systematically different. This difference may be an artifact o f a lower mass balance resulting from the calibration procedures, a scale i ssue 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 wit h different monitoring devices.