Abstract: Groundwater is a critical water resource supporting industrial and agricultural activities in the plain area of the Yongding River Basin. Understanding its hydrochemical characteristics, formation mechanisms, and sources of typical excessive indicators is of great significance for ensuring the water supply security of the Beijing-Tianjin-Hebei urban agglomeration and guiding the rational development and utilization of groundwater resources. Based on 89 groundwater samples collected from May to August 2021, this study systematically analyzed the hydrochemical characteristics, component over-standard status, and spatial distribution patterns. By comprehensively adopting the Piper diagram, Gibbs diagram, ion ratio method, principal component analysis, and Spearman correlation analysis, the hydrochemical formation mechanism of groundwater was elucidated, with an emphasis on revealing the enrichment mechanism and sources of nitrate (\mathrmNO_3^- -N) and fluoride (F⁻). The results show that the groundwater in the study area is weakly alkaline, and most sampling points belong to fresh water. The dominant ions are \mathrmHCO_3^- and Na⁺, and the hydrochemical type is dominated by the HCO₃-Ca type. The mass concentrations of TH, TDS, Ca²⁺, Mg²⁺, \mathrmSO_4^2- , Cl⁻, \mathrmHCO_3^- , and \mathrmNO_3^- -N in phreatic water are significantly higher than those in confined water, showing a decreasing trend with the increase of burial depth. The over-standard rate of F⁻ in confined water is higher than that in phreatic water. The hydrochemical formation is mainly controlled by the weathering and dissolution of carbonate rocks, followed by evaporation and concentration. Stronger cation exchange processes in confined water promoted Na⁺ enrichment and facilitated fluoride release. The over-standard rate of \mathrmNO_3^- -N in phreatic water is relatively high, mainly derived from agricultural activities and domestic sewage discharge. Local high concentrations of \mathrmNO_3^- -N in confined water are caused by interlayer cross-contamination induced by mixed groundwater exploitation and leakage recharge. F⁻ enrichment is predominantly controlled by geological factors; the leaching of fluorine-bearing minerals, the reduced activity of Ca²⁺ under weakly alkaline conditions, and cation exchange and adsorption jointly promote the enrichment of F⁻. Based on the understanding of hydrochemical formation, targeted risk prevention and control strategies are proposed. It is recommended to restrict phreatic water from being used as a drinking water source, and prioritize confined water for urban water supply with focused monitoring of fluoride pollution risks. Sewage collection and treatment facilities should be promoted in high nitrate risk areas, and water quality early warning thresholds should be established in fluoride over-standard areas. The research results reveal the hydrochemical characteristics and formation mechanisms of groundwater in typical urban agglomerations, providing a scientific basis for groundwater resource protection and water security guarantee in the river basin.