# Detection of Preferential Water Flow by Electrical Resistivity Tomography and Self-Potential Method
## 1 Introduction  
The aim of this study is the identification and characterization of preferential flow in a study site by using hydrogeological and geophysical investigation techniques. Field investigation delivers boundary conditions for groundwater simulation, which allows for better characterization of preferential flow in the subsurface. Preferential groundwater flow has been extensively studied in recent decades because it has a growing importance in several research fields such as in contaminant transport [1], the construction of nuclear waste repositories [2, 3], the prevention of dam leakages [4, 5], and in slope failures [6, 7, 8, 9]. Various types of discontinuities in rock or soil such as surface or subsurface layer topography, tree root decay, animal caves, or soil/rock cracks can result in preferential flow [10, 11]. Preferential flow routes can form complex networks [12], and moreover, they can evolve over time [13]. Recent advances in numerical modeling allow for the consideration of preferential flow in hydrogeological simulations [14, 15]. [PERSON] and [PERSON] [16] identified the following preferential flow modeling approaches: single continuum, dual continuum, dual permeability, and dual porosity. In the case of single continuum, dual continuum, and dual permeability modeling approaches, the morphology of separated fractures is not represented in the model. Instead, modified hydraulic conductivity functions for single continuum models or regularly distributed permeable inclusions are applied to themodel [16]. Such an approach allows for the consideration of the impact of preferential flowpaths in a generalized way if the impact of individual conduits can be neglected. The dual-porosity approach considers the morphology of individual flowpaths, and the water exchange between the conduit and the encompassing matrix is simulated [16]. However, this approach is numerically challenging and demands advanced investigation techniques for the identification and parametrization of the preferential flow conduits in the subsurface. Consequently, depending on the demanded model's resolution, an appropriate approach of preferential flow conceptualization in the model should be selected. Data characterizing preferential flow in the field site can be attained by selecting appropriate field investigation techniques, and they are a prerequisite for the building of a representative groundwater model. Various geophysical and hydrogeological approaches have been used recently for the identification of preferential flow pathways [17; 18; 19; 20; 21; 22]. The main hydrogeological approaches are single-borehole tests, hydraulic tests, cross-borehole hydraulic tests, and tracer tests [23]. However, these methods have limited applicability if the groundwater-bearing fracture is not in the proximity of the borehole [17]. Non-invasive geophysical techniques such as electrical resistivity tomography (ERT) have been widely applied for the investigation of lithological settings [18; 19] and groundwater bodies [20; 21; 24; 25]. ERT determines the resistivity distribution in the subsurface, which allows for the identification of saturated zones of soil and rock. Both the flow of groundwater and of electrical current in saturated rock or soil rely on similar physical principles, which can be conceptualized through the flow in porous and fractured media [24]. The conduits within the rock matrix can radically increase conductivity both in terms of water and electrical current [6; 24; 26]. By using the discrete-dual-porosity modeling approach, the impact of anisotropy on the electrical resistivity of rock massif has been illustrated [24]. It should be added that anisotropy of electrical conductivity in subsurface is affected by other factors such as solute concentration variability. This effect is particularly emphasized under partially saturated conditions [27]. Preferential flowpaths can be caused by different types of heterogeneities in the subsurface.