# Detection of Preferential Water Flow by Electrical Resistivity Tomography and Self-Potential Method
## 4 Discussion  
This study was conducted in a study site in the valley of Upper Mosel, Luxembourg, where mass movements are frequently present. The aim of the study was to explore the hydrogeological conditions of a slope affected by a recent landslide. Observations of the groundwater heads in piezometer wells indicated very diverse responses to changing precipitation in the study site. The almost constant hydraulic heads with high flow rates in two boreholes could be explained through preferential flow in the marl strata. The dissolution of gypsum could have caused thin, lateral conduits in the marl deposits. In boreholes No 3 and 5, constant groundwater heads are sustained because water discharges into these conduits. In the inclined borehole No 6, which was installed in the dolomite cliff (Figure 2b), alternation between the two governing groundwater heads can be observed (Figure 4). This phenomenon could be explained through presence of two fracture planes dissecting the piezometer's screen. Possibly if the conductivity of the lowest conduit is relatively low and high inflow of groundwater occurs, the level of the next lateral conduit is reached where it remains stable until the water inflow decreases. Hydrogeological field testing and observations in boreholes at the field site also revealed that preferential flowpaths have high flow capacity. However, the piezometer observations delivered no information about the morphology and spatial extension of preferential flowpaths. The newest 3D geophysical surveys are promising techniques for the investigation of the morphology of subsurface flowpaths. The 3D ERT indicated possible locations of saturated groundwater bodies associated with preferential flow heading toward the Mosel River; however, the resistivity method was not able to detect increased flowrate in the subsurface, which is characteristic of preferential flow. The self-potential method was employed for the measurement of streaming potentials associated with preferential flow into the subsurface. When integrating both geophysical methods--ERT and self-potential--conducted in the study area, it can be observed that the main groundwater pathways toward the Mosel River start from spring discharges (Figures 6 and 7). Both geophysical methods reveal self-potential anomalies and highly conductive zones in the same locations. Two main groundwater pathways starting from spring discharge could be recognized both in the ERT and self-potential surveys. The pathway in the northern part disappears from geophysical surveys; perhaps it diverges or moves toward deeper layers in the marls. The pathway in the southern part continues in both surveys through the study area toward the Mosel River. The aforementioned field research allowed the measurement of signals associated with preferential flow such as steaming potentials and the increase of electrical conductivity along the flowpath; however, possible signals of flowing groundwater are diffusely spread over a relatively large area. Despite intensive testing, the morphology of the flowpath remained unclear. A detailed analysis of the drillcores indicated that a possible cause of intense preferential flow may be the dissolution of gypsum interlayers in the marls of Middle Muschelkalk. Thin, laterally extended conduits, caused by the dissolution of the gypsum, are proposed as the most likely morphological shapes of preferential flow. To validate this hypothesis, a 3D hydrogeological model considering flow both in porous and fractured media was developed. This model with a lateral preferential flow path embedded in a porous matrix (Figure 3c) provides an excellent correspondence with observations in piezometers where preferential flow is present. Nevertheless, many unknowns still remain such as lateral extension, aperture, or the discharge location of the flowpath. Furthermore, the impact of the hydrogeological settings on the stability of the slope is unclear and will be investigated in a separate article.