# Laboratory Measurements of Zeta Potential in Fractured
## 4 Conclusions  
We report measurements of the zeta potential carried out for the first time on a fractured Lewisian gneiss sample saturated with NaCl solutions, artificial groundwater (AGW), artificial seawater (ASW) and under 4 MPa and 7 MPa of confining pressure. The results show that:* Zeta potentials of gneiss are unique and dissimilar to sandstones, carbonates and even to individual gneiss constitutive minerals, i.e., mica and feldspar. * The negative zeta potential decreases with increasing salinity when the sample is saturated with AGW and ASW, but the rate of the decrease is smaller compared to any other mineral. * The negative zeta potential is independent of salinity when using NaCl; this feature is similar to what was observed with clayey sandstone [45] but the zeta potential of gneiss was found to be more positive compared with clayey sandstones saturated with NaCl. * Significant amounts of feldspar and mica present in our gneiss sample were found to be responsible for the high sensitivity of the sample to the presence of divalent ions (Ca\({}^{2+}\), Mg\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\)) compared with quartz. Therefore, these ions were identified as PDIs for gneiss, making the sample respond to compositional and concentration variations in a fashion similar to that of carbonate samples. * The reported values of gneiss \(C_{SP}\) remained independent of the confining pressure, thus, suggesting that the surface electrical conductivity in gneiss aquifers could be neglected. A simple fracture model was developed to evaluate depths, and resulting in situ confining pressures, until which fracture aperture would remain large enough to neglect the surface electrical conductivity; this depth was found to be 380 m, which is much deeper than most water resources applications. The reported dependence of the fractured gneiss zeta potential on confining pressure, composition and salinity of aqueous solutions can be used to identify newly formed fractures along naturally weathered pre-existing clades from the measured signal, thus paving a way to a novel passive non-intrusive method of quantitative characterization of fracture network in crystalline rocks. The above results suggest that interpretation of SP field data requires knowledge of the zeta potential values, which should be used to calculate \(C_{SP}\) using Equation (1). Subsequently, using the computed values of \(C_{SP}\) and voltages measured in the field, the expected pressure gradients can be quantified without any requirement of accounting for surface electrical conductivity. Finally, the obtained values of the pressure gradients will provide essential information on hydraulically active fractures (zero SP response results in zero pressure gradient, hence, implies no flow in that direction) and average fracture permeability (higher pressure gradients correspond to lower average fracture permeability). The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12010180/sl](https://www.mdpi.com/article/10.3390/app12010180/sl)]([https://www.mdpi.com/article/10.3390/app12010180/sl](https://www.mdpi.com/article/10.3390/app12010180/sl)): S1: Geological Map Lewisian gneiss; S2: Summary of All Experimental Data and Error Analyses. J.V. : Conceptualization, Methodology, Validation, Formal analysis, Writing--original draft, Writing--review and editing, Supervision; M.H. : Validation, Formal analysis, Investigation, Writing--review and editing, Visualization; Y.K. : Investigation; D.H.: Conceptualization, Writing--review and editing, Supervision; J.-C.C. : Conceptualization, Validation, Writing--original draft, Writing--review and editing, Supervision. All authors have read and agreed to the published version of the manuscript. This research received no external funding. [PERSON] was supported by the Aberdeen-Curtin PhD studentship. [PERSON] was supported by a scholarship from the Government of India.