## 5 Discussion
### Hydrochemical Characteristics and Evolution of Spring Water
#### 5.1.1 Hydrochemical Facies and Mixing Indicators  
Piper diagram analysis indicates that cold springs in the study area are predominantly enriched in Ca\({}^{2+}\) and Mg\({}^{2+}\), with Ca\({}^{2+}\) being the dominant cation. Only the thermal springs S13 and S14, along with cold springs S6, S7, and S16, fall within the Na\({}^{+}\) and K\({}^{+}\) enriched zones (Figure 2), indicating the presence cation exchange processes. These cold springs are mainly of the Ca-HCO\({}_{3}\) and Ca-Mg-Cl types, typically reflecting interactions with carbonate lithologies or contributions from young, recently recharged groundwater. Elevated Ca\({}^{2+}\) concentrations likely result from carbonate mineral dissolution, releasing Ca\({}^{2+}\) and HCO\({}_{3}\)\({}^{-}\) and forming Ca-HCO\({}_{3}\) type waters. Mg\({}^{2+}\) enrichment may arise from geochemical reactions with dolomite [CaMg(CO\({}_{3}\))\({}_{2}\)] or other Mg-rich carbonates.  
Thermal spring S13 belongs to the Na-HCO\({}_{3}\) type, whereas S14 and cold spring samples S6, S7, and S16 exhibit Na-Cl characteristics. These discrepancies likely result from hydrothermal activity, evaporation, or extended water-rock interaction. The enrichment of Na\({}^{+}\) and K\({}^{+}\) indicates active ion exchange, particularly in environments with feldspathic or clay-rich host rocks (e.g., illite, montmorillonite), which modulate Na\({}^{+}\) and Ca\({}^{2+}\) concentrations and affect the carbonate equilibrium HCO\({}_{3}\)\({}^{-}\) and CO\({}_{3}\)\({}^{2-}\). In Na-Cl waters, elevated Na\({}^{+}\) is associated with ion exchange with sodium-rich minerals (e.g., albite) in surrounding rocks [30; 66; 67].  
The Na-HCO\({}_{3}\) and Na-Cl types in S13 and S14 suggest hydrothermal influences or evaporative concentration. Evaporation processes can enhance total dissolved solids, particularly in semi-confined aquifers or zones with active surface-groundwater interaction, raising conductivity and altering ion composition. Hydrothermal processes play a critical role in modifying the fluid chemistry. Rising thermal fluids, when mixing with cold spring water, can markedly alter ionic ratios. These fluids transport dissolved gases (e.g., CO\({}_{2}\), H\({}_{2}\)S) and minerals (e.g., sulfates, chlorides) [68; 69], altering cation (Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\)) and anion (Cl\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), HCO\({}_{3}\)\({}^{-}\)) balances. The following chemical equations illustrate these reactions:  
\[\text{CaMg(CO${}_{3}$)}_{2(5)}+2\text{CO${}_{2(aq)}$}+2\text{H${}_{2}$O${}_{( l)}$}{\rightarrow}\text{Ca${}^{2+}$}_{(aq)}+\text{Mg${}^{2+}$}_{(aq)}+4\text{HCO${}_{3}$}^{-} _{(aq)}\]  
\[\text{NaAlSi${}_{3}$O${}_{8(8)}$}+4\text{CO${}_{2(aq)}$}+4\text{H${}_{2}$O${}_{( l)}$}{\rightarrow}\text{Na${}^{+}$}_{(aq)}+\text{Al(OH)}{_{4}$}^{-} _{(aq)}+3\text{SiO${}_{2(aq)}$}+2\text{HCO${}_{3}$}^{-}_{(aq)}\]  
\[\text{CaSO${}_{4(8)}$}+\text{CO${}_{2(aq)}$}+\text{H${}_{2}$O${}_{(l)}$}{ \rightarrow}\text{Ca${}^{2+}$}_{(aq)}+\text{SO${}_{4}$}^{2-}_{(aq)}\]  
Figure 2: Piper trilinear diagram of hot spring water samples.  
Variability in groundwater flow paths and water-rock interactions likely drive distinct hydrochemical signatures. Na\({}^{+}\) and K\({}^{+}\) enriched samples (S13, S14, S6, S7, S16) reflect progressive geochemical evolution under complex hydrogeological settings, where multi-directional fluid migration occurs along varied lithologies. In the MDF, extended water-rock interactions and complex flow paths may enhance ion concentration diversity. Tectonic activity (e.g., fault activation) may promote mixing between thermal and cold springs, forming hybrid waters with distinct chemical signatures. These mixed compositions may display anomalous signatures, indicating diverse hydrogeological conditions and flow trajectories.