## 3 Results and Discussion
### Multivariate Analysis  
Principal component analysis (PCA) was conducted using all measured water quality parameters. The first two principal components accounted for approximately 43% of the total variance (Figure 2a). PC1 showed medium to strong correlations with ammonium nitrogen (NH4+-N, \(-\)0.61), nitrite nitrogen (NO2-N, \(-\)0.69), total phosphorus (TP, \(-\)0.75), electrical conductivity (EC, \(-\)0.77), and chloride ions (Cl-, \(-\)0.75), while PC2 was associated with water temperature (0.71), pH (\(-\)0.60), and dissolved oxygen (DO, \(-\)0.83) (Table 7). \begin{table}
\begin{tabular}{l c c} \hline \hline \multirow{2}{*}{**Indicator**} & \multicolumn{3}{c}{**Loading Values**} \\ \cline{2-3}  & **PC1** & **PC2** \\ \hline T & \(-\)0.16 & **0.71** \\ pH & \(-\)0.13 & \(-\)**0.60** \\ TSSs & \(-\)0.08 & 0.04 \\ DO & \(-\)0.05 & \(-\)**0.83** \\ BOD5 & 0.07 & \(-\)0.51 \\ NH4*-N & \(-\)**0.61** & 0.15 \\ NO2-N & \(-\)**0.69** & 0.05 \\ TP & \(-\)**0.75** & 0.27 \\ EC & \(-\)**0.77** & \(-\)0.09 \\ Alc. & \(-\)0.46 & \(-\)0.24 \\ TH & \(-\)0.34 & \(-\)0.52 \\ Cl– & \(-\)**0.75** & \(-\)0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Loadings of principal components (PC1 and PC2) for the analyzed water quality indicators. Figure 2: Results of PCA analysis using concentrations of all indicators. Position of eigenvalue vectors of the analyzed indicators (**a**) and position of the studied rivers (**b**) in relation to the first two principal components PC1 and PC2. The red arrow represents anthropogenic gradients. The green arrows represent natural gradients. These variables represent the key factors shaping the hydrochemistry of the studied rivers. The presence of both nitrogen forms, elevated phosphorus, chloride ions, and their proportional relationship with EC suggest an anthropogenic origin. Increased nutrient concentrations in surface waters are widely recognized as a consequence of human activity--ranging from agricultural practices (e.g., fertilizer application, manure management) to the discharge of municipal and industrial wastewater [40; 51; 52; 53; 54]. Pearson correlation coefficients further support the shared origin of these pollutants (Table 8). Significant positive correlations were found between NH4\({}^{+}\)-N and NO2\({}^{-}\)-N (r = 0.51), NH4\({}^{+}\)-N and TP (r = 0.43), and NO2\({}^{-}\)-N and TP (r = 0.40). These variables were also correlated with chloride and EC values (r ranging from 0.29 to 0.51), reinforcing their link to human-induced nutrient and ion enrichment. Elevated chloride levels in surface waters are commonly associated with anthropogenic pressures, as confirmed in other studies from the region [55; 56; 57]. [PERSON] et al. [57], for example, identified chloride, nitrogen, and phosphorus as dominant chemical contributors in a PCA of the Ina River basin. Water temperature was the only parameter clearly governed by natural conditions, reflecting seasonal atmospheric variation typical of temperate climates. In contrast, pH and DO were influenced by biogeochemical processes such as aerobic decomposition of organic matter and subsequent hydrogen ion release, which explains their moderate correlation (r = 0.41). Both parameters were also affected by atmospheric oxygen diffusion, which is inversely related to water temperature. This is reflected in the significant negative correlations between temperature and pH (r = \(-\)0.17), and temperature and DO (r = \(-\)0.58) (Table 8). The PCA ordination plot (Figure 2b) clearly separated the Sapolna River from the others, driven by its high concentrations of nitrogen, phosphorus, chloride, and EC (Table 6). These values reflect significant anthropogenic pressure, consistent with findings by [PERSON] et al. [15], who reported a negative impact from wastewater discharges originating from the Nowogard treatment plant. The Loznica River formed a distinct group characterized by the highest average alkalinity and water hardness.