## 5 Conclusions  
The present research focuses on the hydrochemical characteristics, quality, spatial variations, and controlling processes of groundwater in urbanized valley basins on the Tibetan Plateau. A typical urbanized valley basin named the Beichuan River basin was investigated and analyzed with integrated approaches of entropy water quality index,  
Figure 11: Relationship Diagram of (NO3\({}^{-}\)/Na\({}^{+}\)) versus (Cl\({}^{-}\)/Na\({}^{+}\)) ratios of groundwater within the basin. Figure 10: Spearman correlation analysis of groundwater chemical parameters. human health risk assessment, irrigation quality indices, and hydrochemical analysis. The main findings are as below:  
1. Groundwater resources are slightly alkaline, with the pH in the range of 7.13 to 8.31. Groundwater presents a gradual increase of salinity from the upstream to the downstream and a relatively great variation, with the TDS ranging from 206 mg/L to 3066 mg/L. Groundwater hydrochemical facies are dominated by the HCO\({}_{3}\)-Ca type (54.17%), followed by the Cl-Na type (33.33%), and the mixed Cl-Mg-Ca type (12.5%). Groundwater toxic substances of NO\({}_{3}\)\({}^{-}\), NO\({}_{2}\)\({}^{-}\), and F\({}^{-}\) exceed the desirable limits for drinking purposes with the maximum concentration of 69.33 mg/L, 0.04 mg/L, and 3.12 mg/L, respectively. 2. Groundwater within the basin has an EWQI value between 26.7 and 143.4. Approximately 50.00% of groundwater samples have the EWQI value less than 50 and belong to the overall water quality of Rank 1. About 33.30% and 16.70% of groundwaters have the EWQI value in the range of 50-100 and 100-150, respectively, and are categorized as the overall water quality of Rank 2 and Rank 3. Groundwater across the basin can be used for domestic usage, but that located in the lower basin should be not directly used for drinking based on the EWQI assessment. 3. The exceedingly toxic substances would pose potential health hazards to various populations with the HI (overall non-carcinogenic health risk) value in the range of 0.592-6.255 for infants, 0.367-3.876 for children, 0.263-2.763 for adult females, and 0.339-3.411 for adult males, respectively. Only infants (29.2%), rather than the other three populations, face the high risk posed by the toxic substances across the basin. All populations are with the potential medium risk posed by the toxic substances and account for 62.5% of sampled sites for infants, 66.7% of sampled sites for children, 58.3% of sampled sites for adult females, and 66.7% of sampled sites for adult males, respectively. The health risks of groundwater for all populations are located in the downstream area. 4. Groundwater quality for irrigation purposes has been assessed using the single indicators of EC, SAR, %Na, RSC, KR, PI, and PS. Predominant groundwater samples are suitable for long-term agricultural irrigation according to these single-indicator assessment results. Groundwater with relatively poor irrigation is dominantly located in the middle downstream area. The integrated irrigation quality assessment of the USSL, Wilcox, and Doneen diagram also demonstrated groundwater in dominated sites is suitable for long-term irrigation. 5. Groundwater chemistry in the basin is naturally governed by water-rock interaction and evaporation. Groundwater chemical solutes are dominantly from silicate dissolution and also sporadically from evaporated dissolution. The influence of water-rock interaction on groundwater chemistry is predominantly observed in the upper reaches of the stream, while the impact of evaporation is more pronounced in the middle to lower reaches. Human activities have regulated the hydrochemical composition of groundwater in the basin. Both agricultural activities and municipal sewage are responsible for the elevated NO\({}_{3}\)\({}^{-}\) content in groundwater. While only municipal sewage is responsible for elevated salinity and F\({}^{-}\) content in groundwater.