Distribution, Origin, and Safe Water Supply Implications of Fluoride in Groundwater of the Yinchuan Plain

Medical studies indicate that both excessively high (>1.0 mg/L) and excessively low (<0.5 mg/L) fluoride (F⁻) content in groundwater can cause varying degrees of harm to the health of populations relying on it as a drinking water source. Therefore, studying the distribution and genesis of fluoride content in the groundwater of the Yinchuan Plain is of significant importance for ensuring safe water supply. Based on hydrogeological surveys, groundwater sample collection, and groundwater level monitoring, the distribution characteristics of fluoride content were identified. Comprehensive methods, including Piper diagrams, Gibbs diagrams, and ion ratio relationship analysis, were used to explore the sources and influencing factors of F⁻. The results show that the fluoride content in the shallow (unconfined) groundwater of the Yinchuan Plain ranges from 0.11 to 5.18 mg/L, while in the confined groundwater it ranges from 0.06 to 4.60 mg/L. The proportion of samples with F⁻ concentrations <0.5 mg/L, 0.5–1.0 mg/L, and >1.0 mg/L is 49.02%, 28.43%, and 22.55% for shallow groundwater, and 54.88%, 27.07%, and 18.05% for confined groundwater, respectively. The predominant hydrochemical type for both is Cl·SO₄-Na. The fluoride distribution in both shallow and confined groundwater shows a pattern where areas east of the Yellow River are mostly high-value zones, while areas west of the river exhibit higher values in the central part and lower values on either side. Fluoride content and its distribution are primarily controlled by rock weathering, evaporation concentration, and groundwater flow conditions. The weathering and dissolution of silicate and evaporite minerals are the main sources of fluoride ions in groundwater, followed by carbonate mineral weathering and dissolution. Cation exchange and Yellow River irrigation are also significant influencing factors. Based on fluoride content as an indicator, the groundwater is classified into three safety levels for water supply: Suitable Area (0.5–1.0 mg/L), Relatively Suitable Area (<0.5 mg/L), and Unsuitable Area (>1.0 mg/L). Targeted safe water supply measures are proposed. It is hoped this study will draw more attention and discussion to serve the goal of safeguarding public health.