[Hydrochemical Characteristics and Mechanism Analysis of Shallow Groundwater in Beijing Plain Area]

Groundwater is an important resource for alleviating global water scarcity. Therefore, understanding its hydrochemical characteristics and mechanisms is of great significance for groundwater management. A total of 210 groundwater samples from shallow monitoring wells in the plain area of Beijing were collected in this study. Statistical analysis, Piper diagram, spatial interpolation, Pearson correlation analysis, and the ion ratio method were used to study the hydrochemical characteristics, spatial distribution, and formation mechanism of groundwater in the plain area of Beijing. The results showed that ① The groundwater of the six groundwater systems in the plain area was mainly HCO₃-Ca type. The water chemistry along the downstream of the alluvial fan of the Yongding River was HCO₃-Na·Ca type. The spatial variation coefficients of chemical components in groundwater were relatively large, indicating certain spatial differences. ② A strong and significant positive correlation exists between Fe and Mn in groundwater, and their spatial distribution is similar. High concentrations were distributed in the Tongzhou area downstream of the Chaobai River, mainly due to the hydrogeological structure of thin clay layers and alternating sand. Fe and Mn in groundwater mainly came from the dissolution of iron and manganese oxides. High concentrations of F^- were mainly distributed in the Wenyu River alluvial fan and Tongzhou area, which was related to the convergence of groundwater flow and the hydrogeological structure of multiple layers of clay and sand in the quaternary strata in Tongzhou. SO₄^2- and NO₃^- had similar spatial layouts, with high concentrations distributed upstream of the Yongding River alluvial fan, which may have been related to the historical drainage of infiltration wells and pits. ③ The hydrochemical types of the six groundwater systems were mainly controlled by the dissolution of carbonate and sulfate rocks, and certain sampling points downstream of the Yongding River alluvial fan were affected by the dissolution of silicate rock minerals to a certain extent. The ion ratio method showed that the hydrochemical components of groundwater in the Chaobai River alluvial fan mainly came from the dissolution of calcite. The groundwater in the alluvial fans of the Wenyu River and Yongding River mainly came from the dissolution of dolomite. The hydrochemical components of groundwater in other systems were influenced by the dissolution of both the two carbonate minerals. In addition, the milligram equivalent concentration of SO₄^2-+Cl^- in the study increased with that of NO₃^-, confirming the influence of human activities on the hydrochemical composition of groundwater. The research results will provide data support for the evaluation and management of groundwater in the plain areas of Beijing.