Temporal and spatial evolution characteristics of main constant index concentrations of groundwater and hydrochemistry in the upper and middle reaches of Yongding River alluvial fan

In order to improve the accuracy of groundwater pollution prevention and control in the middle and upper reaches of Yongding River alluvial fan, a comprehensive study based on the long-term monitoring data of pore groundwater in the middle and upper reaches of Yongding River alluvial fan during the normal season was carried out. Spatial analysis, piper diagram Gibbs diagram and ion proportion analysis are used to explore the temporal and spatial evolution characteristics and formation mechanism of main ions and hydrochemistry of groundwater in this area. The results are as follows: ① Nitrate nitrogen, total hardness and total dissolved solids in the study area all showed the trend of increasing first and then decreasing from single aquifer zone-transition zone-stable confined zone, in which the change range of total hardness and total dissolved solids is relatively smaller, and the change range of nitrate nitrogen is larger. ② The hydrochemical types of groundwater in the single aquifer area are more complicated, and the groundwater in the recharge area has been influenced by human since the 1990s. The hydrochemical types of groundwater in the single aquifer area are relatively complex. The groundwater in the upstream recharge area of a single aquifer has been influenced by humans since the 1990s. The hydrochemical types of stable confined water areas are relatively stable, while the concentration changes of cations and anions in the transition zone are between them. ③ The groundwater in single aquifer area is mainly affected by evaporation and crystallization, while the groundwater in transition zone and confined area is controlled by both rock weathering and evaporation and crystallization. In the water-rock interaction, the groundwater in the study area is mainly influenced by the dissolution of silicate minerals and carbonate minerals, but it is gradually strengthened with the direction of alluvial and diluvial deposits. ④ The NO3− and Cl− imported from outside sources in the groundwater of study area mainly come from agricultural activities and urban sewage. The underground water is more affected by urban sewage when urbanization and industrial development earlier, where the influence of industrial activities is also stronger. The underground water with later urbanization development is more affected by agricultural activities.