Investigating Preferential Flow Processes in a Forest Soil Using Time Domain Reflectometry and Electrical Resistivity Tomography

We compared the well-established time domain reflectometry (TDR) method and electrical resistivity tomography (ERT) to monitor bulk electrical conductivity, sigma(b), during a saline tracer experiment. The experiment was conducted at a forest site on the premises of the Forschungszentrum Julich. To parameterize solute transport processes, the convection-dispersion equation (CDE) and the mobile-immobile (MIM) model were fitted to the data. Although sigma(b) derived from ERT was lower than TDR measurements at almost all depths, the estimated pore water velocities of the CDE model were very similar. Early peak arrival times at lower depths and long tailings of the breakthrough curves clearly indicated preferential flow phenomena that could not be described with an appropriate parameterization using classical transport approaches such as the CDE. Adoption of the MIM model did not lead to more reasonable solute transport parameters. Additionally, preferential flow was reflected in high peak velocities in the lower depths, which exceeded piston flow velocities. The strong decline in peak sigma(b) with depth showed that the volume through which transport took place decreased with depth. Typical features of preferential transport could be detected and the spatial variability of the preferential transport process could be imaged by ERT.