## 5 Discussion
### Water-Rock Interaction and Mixing Model  
Chemical data from the groundwater samples and isotopic data were used in the NETPATH Model to estimate geochemical reaction and mixing with other sources [15,50]. This model estimates net geochemical reactions and observed variations in groundwater chemistry between initial and final groundwater wells along the groundwater subsurface flow path. However, this approach is limited by the input data related to the subsurface groundwater aquifer [59]. In this study, the NETPATH geochemical model is constrained by major dissolved ions in the groundwater including carbon (carbonate and bicarbonate ions) sulfur (to represent sulfate anions), calcium, magnesium, sodium, chloride, and silica (Table 3). The essential minerals embedded in the aquifer sediment were used as input phases to represent the interaction and hydrolysis between the groundwater and aquifer matrix. Calcite, dolomite, and halite minerals dominate terrestrial alluvial deposits (Quaternary aquifer) and the Post-Nubian carbonate aquifer. Gypsum, montmorillonite, and illite are dominant in the clay sheet intercalations of the Quaternary aquifer and the impervious Quseir variegated shale overlying the NSA [1]. Anorthite, alunite, and chlorite are used to simulate the basement aquifer and the main watershed area of the Qena basin. The Qena watershed is in Egypt's arid zone; therefore, the evaporation parameter was selected to simulate the impact of aridity and scarce rainfall on groundwater salinization. A mixing parameter was used to simulate the possibility of recharge from the River Nile and mutual leakage from the multiple aquifer hydrologic system (confined, unconfined, and semi-confined). The model results show two main factors controlling the groundwater geochemistry: water-rock interaction and mixing models (Table 4). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Constraints** & **Phases** & **Processes** \\ \hline \hline  & Albié, Alunite, Calcite, & \\ Calcium, Carbon, Magnesium, & Chlorite, (\(\pm\)) Dolomite, & \\ Potassium, Sodium, Sulfur, & (\(-\)) Ca-Montmorillonite, & Reaction and/or Evaporation \\ Chloride, Silica & K-Mica, Illite, Gypsum, Sio\({}_{2}\), & and Mixing \\  & (+) Halite NaCl, (\(-\)) Anorthite, & \\  & (\(\pm\)) Exchange & \\ \hline \hline \end{tabular} Note: (\(\pm\)) Dissolution and precipitation, (\(+\)) dissolution only, and (\(-\)) precipitation only.  
\end{table} 
Table 3: Constraints, phases, and processes used in NETPATH models. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Model**} & \multirow{2}{*}{**Aquire**} & \multirow{2}{*}{**Initial**} & \multirow{2}{*}{**Initial**} & \multirow{2}{*}{**Final**} & \multicolumn{4}{c}{**Mixing Percent**} & \multicolumn{4}{c}{**Phases Precipitated or Dissolved**} \\  & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & \\  & & & & & & & & & & & & & & & & & & & & & & & & & & & & \\  & & & & & & & & & & & & & & & & & & & & & & & & & & & \\  & & & & & & & & & & & & & & & & & & & & & & & & & & & \\ \hline \multirow{9}{*}{Reaction Models} & \multirow{9}{*}{Quaternary aquifer (Upstream)} & 8 & Rain & 9 & 87.2 & 12.8 & 1.36 & – & 3.66 & 16.85 & – & – & 0.39 & – & 1.84 & 3.39 &  
The water-rock interaction models well describe the evolution and salinization processes of the Quaternary groundwater located at the upstream watershed of Wadi Qena. Geochemical water-rock interaction modeling results suggest the dissolution of calcite, gypsum, halite, silica, illite, and chlorite as groundwater flows downward; dolomite, albite, anorthite, and alunite are precipitated, and some cation exchange occurs (e.g., from initial water-1 represented by sample 54 to final water at sites 55, 57, 58; from site 57 to 59; site 59 to 66, and site 66 to 71; see Table 4 and Figure 10). The evaporation factor for groundwater in this area ranges from 1.04 to 1.2.