## 4 Results and Discussion
### Processes Regulating Groundwater Hydrochemistry Variation  
Groundwater hydrochemistry is generally regulated by both natural and anthropogenic processes [61]. Gibbs diagram is considered an effective method for differentiating the dominant processes governing water chemistry in nature [62]. As shown in Figure 8, groundwater sampled from the upper-stream areas is distributed in the rock dominance of the Gibbs diagram. This indicates that the interactions between water and rock are the main natural regulating process of groundwater chemistry. Groundwater samples from the middle and lower reaches are found distributed in the evaporation dominance of the Gibbs diagram (Figure 8), suggesting evaporation plays a dominant role in the natural regulating processes of these groundwaters. Spatially, as one progresses downstream along the Beichuan River, the predominant natural mechanism governing groundwater chemistry in the basin transitions progressively from water-rock interaction to evaporation (Figure 8). Concomitant with the shift in groundwater regulation mechanisms, there is a corresponding alteration in the Total Dissolved Solids (TDS) levels. Groundwater that is predominantly influenced by evaporation exhibits elevated Total Dissolved Solids (TDS) concentrations. These groundwaters are found located in the middle and lower stream areas where, with more densely human activities, as a result, are also more vulnerable to anthropogenic impacts. To further refine the natural mechanisms regulating groundwater chemistry in the basin, the end-member diagram is introduced for description [63]. Previous research has demonstrated the effectiveness of this method. In the basin, groundwater samples are primarily distributed between the evaporites and silicate end-members on the diagram (Figure 9). Groundwater with lower TDS levels in the upper reaches of the Beichuan River is primarily distributed near the silicate end-member (Figure 9). This observation indicates that in the upstream regions, where water-rock interaction predominantly governs the groundwater dynamics, the interaction primarily involves silicate minerals. Additionally (Figure 9), two groundwater samples with high TDS values are found located in the evaporite dominance, implying the contribution of evaporite dissolution to groundwater chemistry. As illustrated in Figure 2a, these high TDS groundwaters are located in the middle lower stream area. Evaporation is also evidenced as the dominant regulation of  
Figure 8: The Gibbs diagram of (**a**) TDS versus Na\({}^{+}\)/(Na\({}^{+}\)+Ca\({}^{2+}\)) and (**b**) TDS versus Cl\({}^{-}\)/(Cl\({}^{+}\)+HCO\({}_{3}\)\({}^{-}\)) of groundwater within the basin. groundwater chemistry in these areas (Figure 8). Evaporites could be formed in these sites during the fluctuation of groundwater level and then dissolve into groundwater again under proper circumstances and provide the sources of groundwater chemicals. Overall, groundwater chemical solutes are naturally dominated by the dissolution of silicates, followed by evaporite dissolution at some sporadic sites. In recent years, the urbanization process has gradually accelerated, and agricultural activities are becoming increasingly intense. All of these human activities would disturb the natural hydrochemistry of groundwater in the basin. NO3- in an important indicator of anthropogenic inputs for groundwater chemicals. Naturally, the content of NO3- in natural surface and subsurface water is within 10 mg/L. Any water with NO3- content exceeding this geological limit is regarded as influenced by human activities. As statistically listed in Table 2, a lot of groundwater samples (79.17%) have NO3- content beyond 10 mg/L, indicating the disturbance of anthropogenic inputs. It can be seen from Figure 10 that most major ions presented a relatively high Spearman correlation coefficient with NO3- in groundwater, implying that the anthropogenic inputs of nitrogen contaminants also bring other chemicals into groundwater and result in the elevated salinity of groundwater.