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 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. 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.
Highlights
Fractured crystalline basement aquifers are predominantly made of granites and gneisses are widespread [1] and are locally important for rural water supplies and as a geothermal resource (e.g., [2,3])
To mimic the fluids expected to saturate the fractured aquifer in similar conditions as encountered at the site where the sample was taken, i.e., in a coastal setting of NW Scotland, we used four aqueous solutions to measure the streaming potential coupling coefficient: (1) the artificial groundwater (AGW) comprising the main salts, consistent with the groundwater chemical composition as reported by BGS [36] for groundwater samples taken from similar Lewisian gneiss on the Isle of Harris (about 50 km West of the site, blue circle in Figure 1a; (2) the artificial seawater (ASW) formulated from the main ionic species; (3) low-salinity NaCl (LS); (4) high-salinity NaCl (HS) solutions
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
Summary
Fractured crystalline basement aquifers are predominantly made of granites and gneisses are widespread [1] and are locally important for rural water supplies and as a geothermal resource (e.g., [2,3]). They are predominantly made of granites and gneisses and mostly occur in Precambrian shield regions covering large parts of Europe (especially Scandinavia), North and South America, Africa, India, Australia, and in Precambrian orogens such as the Lewisian basement of NW Scotland [3]. 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]). 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
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