Analysis of Hydrochemical Characteristics of Groundwater in Different Aquifers and Causes of High Fluoride in Northern Henan Plain

To reveal the hydrochemical evolution patterns of groundwater in the northern Henan Plain and the enrichment mechanism of high-fluoride groundwater, and to support regional drinking water safety and water resources management, a total of 84 monitoring sites were set up and sampled in the study area. Mathematical statistics, Piper trilinear diagrams, Gibbs diagrams and ion ratio coefficients were employed to analyze the hydrochemical characteristics of groundwater, spatial distribution and controlling factors of high-fluoride groundwater in different aquifers of the study area. The results indicate that the groundwater in the study area is weakly alkaline as a whole. Surface water and 64.6% of shallow groundwater are low-mineralized, while 35.4% of shallow groundwater and deep groundwater are medium- to high-mineralized. The hydrochemical types of groundwater in the study area exhibit vertical zonation: surface water and shallow groundwater are primarily HCO₃-Ca·Mg types, while deep groundwater evolves into HCO₃-Na and Cl·SO₄-Na types. The hydrochemical genesis of surface water and groundwater in the study area mainly includes atmospheric precipitation recharge, water-rock interaction and evaporation concentration. High-fluoride groundwater is widely distributed across the study area. The high-value zones (F⁻ mass concentration >3 mg/L) are primarily located around Xinxiang City in the west and Nanle County in the northeast. The sub-high-value zones with F⁻ concentration in the range of (1.5,3] mg/L are strip-like distributed in the northeastern and central-western regions. A weakly alkaline environment (pH 7-8) facilitates fluoride enrichment in the area. The formation of high-fluoride groundwater in the study area is primarily attributed to the weathering and dissolution of fluoride-bearing minerals, and is also influenced by leaching, cation exchange adsorption and evaporation-concentration. This study clarifies the main formation mechanisms and spatial distribution of groundwater hydrochemical characteristics and high-fluoride water in the northern Henan Plain, providing a scientific basis for regional groundwater quality assessment, precise treatment of high-fluoride water and sustainable exploitation and utilization of water resources.