## 4 Results and Discussion  
### Statistical Analysis of Groundwater Hydrochemistry  
The hydrochemical parameters of groundwater sampled from the Beichuan River basin are statistically presented in Table 2. The table also includes the drinking water standards recommended by the World Health Organization [51] and Chinese guidelines [52] for comparative analysis. The groundwater samples exhibit a slightly alkaline character, with pH values ranging from 7.13 to 8.31 and a mean pH of 7.59. Groundwater in the Beichuan River basin displays a broad spectrum of Total Dissolved Solids (TDS) and Total Hardness (TH) concentrations, with TDS ranging from 206.00 to 3066.00 mg/L and TH from 32.60 to 866.00 mg/L. High TDS levels in drinking water can cause kidney damage and soil salinization, in addition to restricting its use for both consumption and industrial purposes. Thus, attention should be paid to the high TDS groundwater. However, the majority of the groundwater samples exhibit low Total Hardness (TH) levels, with only 4.17% surpassing the threshold stipulated by the Chinese guidelines [52]. While 29.17% of the groundwater samples with Total Dissolved Solids (TDS) exceeding the 1000 mg/L threshold are categorized as brackish or saline water. The spatial distribution of Total Dissolved Solids (TDS) reveals that the majority of brackish or saline groundwaters are concentrated in the downstream region of the basin (Figure 2). Furthermore, a progressive increase in salinity is observed in groundwater as one moves from the upstream to the downstream areas. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Index** & **Unit** & **Min** & **Max** & **Mean** & **SD*** & **Guideline** &
\begin{tabular}{c} **\% of the Sample** \\ **Exceeding the Guideline** \\ \end{tabular} \\ \hline pH & / & 7.13 & 8.31 & 7.59 & 0.29 & 6.5–8.5 ** & / \\ TDS & mg/L & 206.00 & 3066.00 & 824.50 & 770.05 & 1000 ** & 29.17\% \\ TH & mg/L & 32.60 & 866.00 & 171.75 & 176.03 & 450 ** & 4.17\% \\ K\({}^{+}\) & mg/L & 1.41 & 13.04 & 4.13 & 2.66 & / & / \\ Na\({}^{+}\) & mg/L & 10.82 & 1132.00 & 176.90 & 274.03 & 200 ** & 25.00\% \\ Ca2\({}^{+}\) & mg/L & 45.11 & 154.60 & 86.51 & 30.20 & 75 *** & 58.33\% \\ Mg2\({}^{+}\) & mg/L & 7.45 & 88.00 & 23.96 & 18.80 & 50 *** & 8.33\% \\ Cl\({}^{-}\) & mg/L & 6.40 & 1283.22 & 160.90 & 289.18 & 250 ** & 16.67\% \\ SO4\({}^{2-}\) & mg/L & 42.19 & 648.38 & 201.12 & 195.78 & 250 ** & 25.00\% \\ HCO3\({}^{-}\) & mg/L & 136.61 & 558.00 & 270.68 & 113.75 & / & / \\ F & mg/L & 0.12 & 3.32 & 0.78 & 0.75 & 1.0 ** & 33.33\% \\ NO3\({}^{-}\) & mg/L & 0.06 & 69.33 & 26.94 & 21.75 & 50.0 *** & 20.83\% \\ NO2\({}^{-}\) & mg/L & 0.00 & 0.04 & 0.00 & 0 & 0.02 ** & 12.50\% \\ \hline \hline \end{tabular}  
* Standard Deviation; ** Chinese Guideline [52]; *** WHO Guideline [51].  
\end{table} 
Table 2: Statistical summary of groundwater hydrochemical composition within the basin. Figure 2: Spatial distribution of **(a)** TDS, **(b)** NO\({}_{3}\)\({}^{-}\), and **(c)** F\({}^{-}\) content in groundwater within the basin. In the Beichuan River watershed, the solute chemistry of groundwater is predominantly influenced by the cation Ca2+ (Figure 3). The concentration of Ca2+ spans from 45.11 mg/L to 154.60 mg/L, with an average concentration of 86.51 mg/L. Na+ and Mg2+, as the second and third most abundant cations, exhibit concentration ranges of 10.82 to 1132.00 mg/L and 7.45 to 88.00 mg/L, respectively. Among the cations, K+ was detected at the lowest concentrations, with values ranging from 1.41 mg/L to 13.04 mg/L and an average of 4.13 mg/L (Table 2). Among the major anions of groundwater in this region, a clear dominance is observed in the order of HCO3- > SO42- > Cl- (Figure 3). The concentrations of these predominant anions exhibit a broad range (Table 2). The concentration of HCO3- varies within the range of 136.61 to 558.00 mg/L. SO42- and Cl- concentrations span from 42.19 mg/L to 648.38 mg/L and 6.40 mg/L to 1283.22 mg/L, respectively.