# Detection of Preferential Water Flow by Electrical Resistivity Tomography and Self-Potential Method
## 3 Results  
From a hydrogeological viewpoint, the two most conductive geological units in the study site are the cystic dolomites and the overlaying conductive landslide mass. However, field investigation revealed that in the strata of less conductive marks highly permeable conduits exist. Figure 4 illustrates that two systems of groundwater flow are present, namely one above the less conductive marks and another in the marl sediments, which itself is represented by boreholes 3 and 5.  
of these cavities is present. Drawings of the cores indicate that the cavities are located in the proximity of gypysiferous interlayers. Therefore, it can be speculated that the preferential flowpaths in the new boreholes No 3 and 5 are also caused by dissolution of the gypysiferous interlayers. After a detailed analysis of the new drillcore records in borehole No 5, a thin gypsum interlayer was discovered at the same depth where preferential groundwater flow occurs (Figure 5). For detailed insight into the geological settings, a geological section through borehole No 5 was drawn (Figure 2b). Both ERT and self-potential mapping were used to obtain detailed information about the preferential flowpaths in the study area. The ERT was employed for the identification of groundwater bodies caused by groundwater discharging from the cystic network and flowing downslope through the landslide mass to the Mosel R  
Figure 4: Temporal changes of the groundwater heads in piezometer wells Nos 1–7 from 7 March 2018 to 1 April 2019. Figure 5: Gypsum interlayer in the core records of borehole No 5.  
mapping was used for the identification of streaming potential caused by preferential groundwater flow. The results of the self-potential mapping are explicitly illustrated in Figure 6. Relatively homogenous self-potential values can be observed in the northern part of the study area; however, downslope from the discharge area of groundwater springs, emphasized self-potential anomalies are present. Several couples of positive and negative self-potential anomalies can be observed in the study site (Figure 6). The maximum magnitude of the self-potential signal exceeds approximately \(-103/+98\) mV. The polarity, magnitude, and orientation of the self-potential anomalies indicate that they might be caused by the streaming potential. In Figure 6, the main self-potential anomalies are marked by blue arrows. They are orientated in the direction of the expected groundwater flow with a positive polarity downslope. An exception is the anomaly in the southern part, which is probably associated with a groundwater flowpath adjoining the main pathway. A closer look at the subsurface morphology of the less conductive marl indicates a local depression that corresponds with the direction of the supposed streaming potentials. It can be speculated that the surface of the marl constrains groundwater flow, which corresponds with the direction of the measured streaming potentials. Figure 6: Location self-potential mapping in the study area. The intense preferential flow within the marls, as revealed in boreholes No 3 and 5, may not cause the streaming potential anomalies. Possibly self-potential signals occur due to groundwater flow in the conductive landslide mass directed by the topography of the underlain marl surface. When considering the ERT model, highly conductive zones can be observed in deeper sections of the ERT survey while low conductivities can be observed in the uppermost part (Figure 7). Moreover, the less conductive zones are more fragmented, as they may perhaps represent landslide mass in the unsaturated zone with large spaces between the dolomite blocks resulting in increased electrical resistivity. Highly conductive zones are better connected as they may be related through groundwater pathways (Figure 7a,c). Figure 7: _Cont._  
A 3D hydrogeological model of the study site was compiled for the time period from 7 March 2018 to 1 April 2019 when the results of groundwater monitoring were available.