# Detection of Preferential Water Flow by Electrical Resistivity Tomography and Self-Potential Method
## 2 Materials and Methods
Before inversion data were processed with the ZondRes Quality control module, unrealistic data points were removed. Ten inversion iterations were applied to maximally reduce the difference between observed and calculated apparent resistivity. Streaming potential is one of the sources causing self-potential signals, which are triggered by the movement of electrolytic groundwater in porous media [34]. The magnitudeof the streaming potential can range up to 100 mV [37]. Moreover, in unconfined aquifers along the preferential flow path, a negative anomaly can be observed in the upstream, and a positive anomaly can be observed in the downstream part of the flow path [18; 34]. When conducting the self-potential measurements, the base electrode is commonly kept in a fixed location as a reference, while the removable electrode is replaced along a specified raster [34]. Initially, self-potential mapping with a coarse raster of 2 \(\times\) 5 m was conducted for the identification of locations with self-potential anomalies in the study area. Further in the location of anomalies, measurements were repeated with a refined raster of 2 \(\times\) 2 m. A regular measurement raster composed of several separated plots was marked in the field using measuring tape, wires, and wooden sticks. Furthermore, the corner points of the each plot were georeferenced using a GNSS receiver. Two Ag/AgCl non-polarizing electrodes and a high impedance voltmeter with a sensitivity of 0.1 mV were used. The repeatability of the self-potential measurements in the locations of the anomalies was validated over a period of one year, and regular measurements showed that the anomalies remain stable. A 3D geological model was developed to provide a geological context for the interpretation of geophysical measurements. GeoModeller software was employed for the building of a 3D geological model applying 19 boreholes. Layer interfaces from the 3D geological model were further used for the building of a hydrogeological model. The 3D hydrogeological model with a focus on preferential flow was developed (Figure 3). The aim of the model was to find a physically-based explanation for observed groundwater heads by applying different subsurface flow mechanisms. When only flow in porous media was simulated, we were not able to explain the observed groundwater flow in the strata of less conductive marl; therefore, preferential flow was considered in the model's simulations. A discrete fracture modeling approach applying the HydroGeopSphere software was used. The HydroGeopSphere (HGS) is a 3D fully-integrated surface and subsurface flow simulator [45; 46], and with this, surface flow and subsurface flow in porous, fractured, and dual media for both saturated and unsaturated conditions can be modeled [45]. This allows for the application of the HGS code for advanced simulations in the vadose zone [47; 48; 49]. A detailed explanation of the governing equations used in the HGS code are available in the HGS user manual [46] and in a publication by [PERSON] and [PERSON] [45]. In the hydrogeological simulation, the recharging of groundwater in the model's domain occurs through the first-type boundary condition of the rainfall rate, which is applied to the model's surface domain (Figure 3a). Initially, the average precipitation amounts were applied for a period of several years, which allowed for the saturation of the model's domain. Furthermore, daily precipitation amounts observed by the meteorological observation station of Nittel from 7 March 2018 to 1 April 2019 were used in the model [50]. The first-type (Dirichlet) boundary condition of the prescribed hydraulic head was applied in the eastern part of the model's domain to consider the infiltration of groundwater from the Mosel River, and also in the western part, which was equal with the groundwater table in borehole No 6 to consider the efflux of groundwater from the carstic network in the eastern part.