Noninvasive 3-D Transport Characterization in a Sandy Soil Using ERT: 1. Investigating the Validity of ERT-derived Transport Parameters

We used time-lapse electrical resistivity tomography (ERT) and time-domain reflectometry (TDR) probes to noninvasively capture three-dimensional solute transport during four tracer experiments under different steady-state irrigation rates in a large unsaturated undisturbed soil column (140 cm length and 116 cm inner diameter). The transport was characterized by means of apparent convection-dispersion parameters that were derived from breakthrough curves (BTCs) at different lateral scales: the ERT voxel scale, the sampling volume of TDR, and the cross-section of the column. We validated the ERT-derived data by means of mass balance, TDR probes, and the effluent BTC. We observed an excellent mass recovery by ERT. The ERT-derived transport velocities exhibited minimal bias and high precision at the scale of the TDR measurements. On average the ERT-derived column-scale transport velocities were also not biased; however, the spatial variability of the voxel-scale velocities within the column's cross-sections underestimated the true velocity variability. In contrast to the transport velocities, the ERT-derived dispersivities exhibited a large bias and low precision and were sensitive to temporal smoothing. Unlike previous studies, we did not find evidence that the ERT-derived voxel-scale dispersivities increase with decreasing ERT sensitivity. Although ERT provided unprecedented information about transport processes, resolution and uncertainty analyses remain important issues requiring further investigation.

Publication year:2009