[Analysis on Hydrochemical Evolution of Shallow Groundwater East of Yongding River in Fengtai District, Beijing].

In order to enhance the support for groundwater development and utilization, as well as pollution control and prevention in Fengtai District, Beijing, a comprehensive study was conducted based on long-term monitoring data of shallow groundwater in the eastern area of Yongding River during the dry season. The mathematical statistics, Piper diagram, Gibbs diagram, and ion ratio analysis and other methods were employed to explore the pattern of groundwater hydrochemical evolution, the formation mechanism, and sources of pollution in Fengtai District. The findings were as follows:① Overall, the current groundwater quality in the study area was poor. The average concentration of each index in groundwater increased and then decreased from 1976 to the present. The pollution range of Cl-, SO42-, and TH generally expanded, whereas the pollution range of TDS and NO3- expanded before 2005 and then decreased with 2005 as the turning point. ② The hydrochemical types of groundwater samples displayed a complex regional variation each year, as well as along the groundwater direction. The dominant anion in groundwater was HCO3-, and the dominant cation was Ca2+ each year. The number of groundwater hydrochemical types in 1976 was 8, in which the predominant type was HCO3·SO4-Ca·Mg·Na, accounting for 40%. However, the number of groundwater hydrochemical types in 2021 was 17, in which the predominant type was HCO3·Cl·SO4-Ca·Na·Mg, accounting for 23.88%. The groundwater hydrochemical type showed a complex trend within the region and upstream along the flow direction each year, whereas the migration characteristics of groundwater samples, as depicted on the Piper diagram, indicated that the hydrochemical components of groundwater were significantly affected by human activities during its evolution. ③ The groundwater chemistry in the study area was influenced by both rock weathering and evaporative crystallization processes, with evaporation playing a major role. The alternation of groundwater cations was relatively weak, and the dissolution of carbonate minerals served as the primary source of Ca2+ and Mg2+. ④ The ion ratio analysis suggested that exogenous sources, mainly agricultural activities and urban sewage, contributed to the input of NO3- and Cl-. The pollution impact from agricultural activities was significant before 2005, which aligned with the historical presence of numerous seepage pits, seepage wells, and direct discharge of industrial and domestic sewage for irrigation purposes in the study area. These activities were closely associated with the high levels of pollution. However, pollution input from agricultural activities notably decreased in 2021, likely due to the effective implementation of water environmental protection programs and action plans in recent years.