# Laboratory Measurements of Zeta Potential in Fractured
## 2 Materials and Methods  
The rock sample used in this study was cored from a weathered gneiss boulder collected from the Precambrian Lewisian Gneiss Complex in Northwest Scotland, near Clashnessie. The cylindrical rock sample (38 mm diameter, 54 mm length) was subjected to uniaxial loading under controlled stress and artificially fractured along pre-existing cracks. To preserve the cylindrical shape of the sample, it was wrapped in duct tape during the loading. The initial and final state of the sample is shown in Figure 1.  
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. The ionic strength of LS and AGW was 7.35 \(\times\) 10\({}^{-3}\) M (M = moL\(\cdot\)L\({}^{-1}\)), while the ionic strength of HS and ASW was 0.7 M, hence the ionic strengths of NaCl and the corresponding complex solution were identical between all low- and all high-salinity experiments. Therefore, the impact of ionic strength on the zeta potential measured with NaCl in comparison with complex solutions was eliminated to isolate the effect of the chemical composition. The ionic composition and equilibrium pH of all solutions used in this study are presented in Table 1.  
Figure 1: Rock sample used in this study was cut from the weathered outcrop boulder (**a**) collected from the Clashnessie area denoted by the red star on the inset map (refer to the geological map of the area, S1, in the supplementary material); the core sample was naturally fractured as demonstrated by the top (**b**), bottom (**d**) and side (**f**) views; the sample was loaded along the longitudinal axis resulting in at least one fracture that extended from the top face (**c**) to the bottom one (**e**); to preserve its cylindrical shape during uniaxial loading, the sample was wrapped in duct tape (**g**) and experienced fracturing that corresponded to bulk displacements of 1 mm, 1.4 mm and 1.7 mm, as demonstrated by the load [kN] vs. displacement [mm] graph (**h**). To characterize the chemistry and mineralogy of the gneiss sample used in our experiments, XRD analysis was carried out on powdered offcuts. Moreover, a chip from the sample was polished to produce a thin section, which was analyzed using scanning electron microscopy (SEM). The main constituent minerals from the SEM analysis were identified to be: quartz (c. 35 wt.%), Na-feldspars (plagioclase including albite, c. 60 wt.% in total) and micas (muscovite and biotite, c. 5 wt.% in total). These mineral proportions are characteristic of felsic Lewisian gneiss, as previously reported (e.g., [37; 38]). Prior to the streaming potential measurements, the rock sample was cleaned using deionized water and methanol following the procedure described by [PERSON] et al. [39]. Note that the cleaning procedure did not have any noticeable effect on the sample, as measurements of the zeta potential and permeability were repeated several times after cleaning and returned the same values within the experimental uncertainty. Subsequently, the sample was saturated with the solution of interest and placed in a vacuum oven for 24 h to establish 100% of water saturation. The saturated rock sample was mounted in a core holder made of PEEK, similar to that described in detail by [PERSON] et al. [31].