Zeta Potential of Fractured Gneiss Saturated with NaCl and Natural Aqueous Solutions – Impact of Composition, Concentration and Fracture Aperture on SP Signal in Response to Water Flow in Fractured Crystalline Bedrock

Previous field self-potential (SP) surveys have suggested this passive, non-intrusive geophysical method to be a powerful tool for characterizing subsurface flow of water in shallow fractured systems. However, to accurately interpret the measured signal requires knowledge of electrochemical properties of these settings. Despite the interest, 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 measured in a fractured gneiss sample, obtained from the Lewisian complex in NW Scotland, and 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 of gneiss was unique and dissimilar to pure minerals such as quartz, calcite, mica or feldspar. Moreover, the measured zeta potentials suggest that divalent ions (Ca²⁺, Mg²⁺ and SO₄²⁻) 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. Moreover, the impact of fracture aperture on the electrokinetic response was investigated and likely implications for characterization of fractured systems using SP analyzed. Our novel results are an essential first step for interpreting field SP signals and facilitate a way forward for characterization of water flow through fractured basement aquifers.