## 5 Discussion
### Geochemical Processes Affecting Groundwater  
The results above show that most of the major ions (Na\({}^{+}\), Mg\({}^{2+}\), Ca\({}^{2+}\), SO\({}_{4}\)\({}^{2-}\), and Cl\({}^{-}\)) directly correlated with TDS (Figure 5a-f), indicating that increases in their concentrations were consistent with the flow path from upstream (northeast) to downstream (southwest) (Figure 1). Both geochemical and physical processes (dissolution and evaporation) as well as water-rock interaction were controlling factors for salinity variations in the study area (Figure 8; [54]). Figure 8 represents the ratios of ((Na\({}^{+}\) + K\({}^{+}\))/Na\({}^{+}\) + K\({}^{+}\) + Ca\({}^{+2}\)) and major anions (Cl\({}^{-}\)/Cl\({}^{-}\) + HCO\({}_{3}\)\({}^{-}\)) separately, as a function of TDS. The plot indicates that groundwater samples of the studied aquifers were primarily distributed in the evaporation dominance field. This suggests that the groundwater chemistry in the area was primarily controlled by the evaporation process, as well as the water-rock interaction factor, because the annual rainfall and groundwater recharge were insignificant.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Aquifer**} & \multirow{2}{*}{**Cl**} & \multirow{2}{*}{**\(\mathbf{\xi^{18}\text{O}}\)**} & \multirow{2}{*}{**\(\mathbf{\xi^{2}\text{H}}\)**} & \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Aquifer**} & \multicolumn{2}{c}{**Cl**} & \multirow{2}{*}{**\(\mathbf{\xi^{18}\text{O}}\)**} & \multirow{2}{*}{**\(\mathbf{\xi^{2}\text{H}}\)**} & \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Aquifer**} & \multicolumn{2}{c}{**Cl**} & \multirow{2}{*}{**\(\mathbf{\xi^{18}\text{O}}\)**} & \multirow{2}{*}{**\(\mathbf{\xi^{2}\text{H}}\)**} \\ \cline{3-3} \cline{8-11}  & & ppm & & (\%) & & & ppm & (\%) & & & ppm & & (\%) \\ \hline  
The influence of hydrochemical processes that affect water quality such as ion exchange, mixing, and leaching can be detected using chemical ion ratios [55; 56; 57]. In the current study, the relations between different ionic concentrations were used to understand the relationships between the ions and factors affecting groundwater chemistry (Figure 9). A higher concentration of sodium in the groundwater of the NSA and a lower concentration in the groundwater of the PNA indicate silicate weathering [58]. In contrast, the groundwater in the Quaternary aquifer and in most of the PNA samples showed a slightly lower concentration of Na\({}^{+}\), which may be attributed to the Ca/Na exchange process (Figure 9a) due to the presence of clay interbeds in the aquifers. The relations of Ca\({}^{+2}\)/Na\({}^{+}\) versus HCO\({}_{3}\)\({}^{-}\)/Na (Figure 9b) show that most of the groundwater samples of the three aquifers had more Na\({}^{+}\) than Ca\({}^{+2}\), Mg\({}^{+2}\), and HCO\({}_{3}\)\({}^{-}\), which could be explained by silicate weathering. The relation of (Cl\({}^{-}\) - Na\({}^{+}\))/Cl\({}^{-}\) vs. TDS (Figure 9c) shows that 51.61% of Quaternary samples and 83.33% of Post-Nubian samples in the study area had a positive value; this indicates a direct cation exchange process between Na\({}^{+}\) and K\({}^{+}\) dissolved in groundwater with Ca\({}^{2+}\) and Mg\({}^{2+}\) embedded in the aquifer matrix. In contrast, the rest of the Quaternary samples, the Post-Nubian samples and all Nubian groundwater samples had negative values that indicate a reverse cation exchange process.  
Figure 8: [PERSON]’s diagram with all water samples represents ratios of Na/(Na + Ca) and Cl/(Cl + HCO\({}_{3}\)) as a function of TDS. The chemistry of the rainwater sample is from [48].