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.

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