Noninvasive 3‐D Transport Characterization in a Sandy Soil Using ERT: 2. Transport Process Inference

Abstract

In this study, we examined four tracer displacement experiments in a large undisturbed soil column under unsaturated hydraulic steady‐state conditions. Using electrical resistivity tomography (ERT), we were able to investigate the corresponding solute transport processes in unprecedented three‐dimensional (3‐D) spatiotemporally resolved detail. In addition, time‐domain reflectometry (TDR) probes and tensiometers provided hydrologic state variables relevant to solute transport. One important result from the study is that despite considerable differences between soil hydrologic state variables of the low‐ and high‐flux experiments, the 3‐D–resolved transport patterns remained approximately stable and were aligned to a structural feature in the topsoil. Furthermore, the ERT image data indicated that the solute front evolutions were influenced by spatiotemporal heterogeneities of the irrigation system. Another result is that the column‐scale mixing regime was found to be convective–dispersive, although the transport process exhibited considerable heterogeneities at smaller scales. We introduced an upscaling approach that allowed us to predict the column‐scale apparent dispersivity on the basis of the convection–dispersion equation parameters at the scale of the ERT voxels. The approach proved to be useful to evaluate at which scale the apparent dispersivity was generated. We consider the investigation of the relationship between apparent dispersivity and lateral scale as a promising tool for characterizing solute transport in future studies.

Meta-analysis of the effects of soil properties, site factors and experimental conditions on solute transport

“…However, it is also plausible that features at the interfaces between topsoil and subsoil in these columns, e.g. plow pans, enhance the spreading of a solute plume, such as observed for example byÖhrström et al (2002) and Koestel et al (2009b). As samples taken from only the topsoil are always restricted to lengths between 20 and 40 cm and because longer samples taken from only the subsoil have seldom been investigated, it is not possible to appraise to what degree interfaces between topsoil and subsoil add to the scaling effect of the apparent dispersivity, λ app , with travel distance.…”

Links between soil properties and steady‐state solute transport through cultivated topsoil at the field scale

“…The sampling points in the grid were placed either at the grid intersects or 1 m away from the grid intersects in random directions, avoiding wheel tracks. This was also done because the sampling grid was aligned with the plow direction and plowing is expected to cause periodical bias [ Koestel et al ., ; Petersen et al ., ] in soil properties that we sought to avoid. The columns were pushed into the topsoil with a hydraulic press from a tractor until the edge of the column was level with the soil surface.…”

Three‐Dimensional Electrical Resistivity Tomography to Monitor Root Zone Water Dynamics

“…Waxman and Smits (1968) (W‐S) developed such a pedoelectrical model based on Archie's law (Archie, 1942) for the use in geological applications. Recently, it has been successfully applied by several authors for quantifying transport processes in the unsaturated zone using ERT (e.g., Koestel et al, 2009a,b; Garré et al, 2010). For this study, we used a simplified empirical equation closely related to the W‐S model: where a (S m −1 ), b (S m −1 ), and n are fitting parameters.…”