# Laboratory Measurements of Zeta Potential in Fractured
###### Abstract  
Despite the broad range of interest and possible applications, the controls on the electric surface charge and the zeta potential of gneiss at conditions relevant to naturally fractured systems remain unreported. There are no published zeta potential measurements conducted in such systems at equilibrium, hence, the effects of composition, concentration and pressure remain unknown. This study reports zeta potential values for the first time measured in a fractured Lewisian gneiss sample saturated with NaCl solutions of various concentrations, artificial seawater and artificial groundwater solutions under equilibrium conditions at confining pressures of 4 MPa and 7 MPa. The constituent minerals of the sample were identified using X-ray diffraction and linked to the concentration and composition dependence of the zeta potential. The results reported in this study demonstrate that the zeta potential remained negative for all tested solutions and concentrations. However, the values of the zeta potential of our Lewisian gneiss sample were found to be unique and dissimilar to pure minerals such as quartz, calcite, mica or feldspar. Moreover, the measured zeta potentials were smaller in magnitude in the experiments with artificial complex solutions compared with those measured with NaCl, thus suggesting that divalent ions (Ca2\({}^{+}\), Mg2\({}^{+}\) and SO4\({}^{2-}\)) acted as potential determining ions. The zeta potential was also found to be independent of salinity in the NaCl experiments, which is unusual for most reported data. We also investigated the impact of fracture aperture on the electrokinetic response and found that surface electrical conductivity remained negligibly small across the range of the tested confining pressures. Our novel results are an essential first step for interpreting field self-potential (SP) signals and facilitate a way forward for characterization of water flow through fractured basement aquifers. 20 December 202120 December 2  
erosion, groundwater flow is mostly restricted to connected fracture networks and, while fracture porosity is generally low, permeability can be relatively high (e.g., [5; 6]). Understanding the fluid flow and storage properties in fractured rocks is challenging due to the discrete nature, heterogeneity and scale effect of fracture systems (e.g., [7; 8; 9]). It is of major importance to understand fractured systems in terms of hydraulically active fractures (versus hydraulically inactive) controlled by fracture connectivity, and fracture geometry including aperture and direction, which overall results in strong anisotropy in groundwater flow. Direct characterization of groundwater flow patterns in fractured rock aquifers usually involves borehole studies including hydraulic testing or tracer testing methods, or a combination of both (e.g., [10; 11]). Due to low permeability and storativity of fractured aquifers, both methods usually provide information on aquifer and fracture network properties only in the close vicinity of the borehole [12]. It is challenging to extrapolate the information to site- or region-scale due to large, multiscale heterogeneities of fracture systems (e.g., [7; 13; 14]). Surface geophysical methods are increasingly used to support interpolation to larger scales by providing indirect quantitative information on the heterogeneities of fracture networks and associated equivalent hydrodynamic properties (e.g., [12; 15]). Most common geophysical methods used for this purpose are electrical resistivity tomography (ERT, e.g., [16]), ground-penetrating radar (GPR, e.g., [17]), magnetic resonance [18], or a combination of these techniques. However, these methods are usually applied to provide equivalent isotropic flow/storage properties of the bulk aquifer over a certain volume, or structural information, rather than the flow properties of the fracture networks themselves. They are also expensive to deploy in the field in terms of effort and equipment.