# Laboratory Measurements of Zeta Potential in Fractured
## 3 Results and Discussion
### Effect of the Ionic Strength and Chemical Composition  
It is generally agreed that zeta potential decreases in magnitude with increasing ionic strength as a result of a collapse of the electrical double layer (e.g., [63]) and our results with ASW and AGW, as shown in Figure 3a, were consistent with this behavior. The measured zeta potentials of these systems remained negative but became smaller in magnitude with increasing ionic strength. In contrast, the zeta potentials measured with NaCl solutions remained constant within the experimental uncertainty with increasing salinity, as demonstrated by the horizontal dash-dotted line in Figure 3b. Our results are denoted by the red and blue symbols in Figure 3.  
As shown in Figure 3a, the rate at which the zeta potential decreases in magnitude with increasing salinity in gneiss samples (blue and red symbols) is significantly lower than that reported for clean sandstones (green dashed line) and carbonates (black dashed line) saturated with NaCl solutions. However, a recent study [45] reported an anomalous salinity dependence of the zeta potential in clayey Berea samples. In this study the zeta potential obtained with NaCl solutions varied from \(-18.3\) mV at 0.005 M to \(-21.4\) mV at 0.5 M (green circles in Figure 3), thus, indicating much weaker and reversed concentration dependence compared with that of clean sandstones. At the same time, the magnitude of zeta potential in Berea NaCl experiments remained significantly larger compared with our guess results. In contrast, the zeta potentials of approximately \(-7\) \(\pm\) 0.3 mV measured in Berea-CaCl\({}_{2}\) systems (black circles in Figure 3) remained unchanged between ionic strength of 0.008 M and 0.8 M. In this sense, the independant of salinity zeta potential in the clayey sandstones saturated with CaCl\({}_{2}\) was qualitatively similar to the observed trend in the genesis sample saturated with NaCl, but the magnitude of the latter was consistently larger.  
The zeta potentials in our genesis sample saturated with aqueous solutions of complex composition became more negative with decreasing salinity and was consistently more positive compared with the results obtained with NaCl. Moreover, the zeta potential of \(-7.2\) \(\pm\) 0.2 mV obtained with ASW (0.7 M) were comparable with those measured in Berea samples saturated with CaCl\({}_{2}\) (\(-7.0\)\(\pm\) 0.3 mV; [45]) and Portland limestone saturated with equilibrated NaCl (\(-8.0\)\(\pm\) 0.5 mV; [34]).  
Figure 3 clearly indicates that zeta potentials of Lewisian gneiss saturated with aqueous solutions are unique in their response to varying ionic strength and brine composition. These observations suggest that gneiss mineralogy affects the zeta potential that strongly responds to presence of divalent ions, hence Ca\({}^{2+}\), Mg\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\) are PDIs for this rock type, similar to the generally agreed feature of carbonates ([PERSON] et al. [34]), feldspars ([PERSON] et al. [61]) and micas ([PERSON] et al. [59]). Therefore, accurate laboratory measurements of the zeta potential in gneiss should be carried out prior to attempting interpretation of filed SP data.