A typical area, Gaomi City in China, was chosen to discuss the enrichment process of groundwater fluorine in seawater intrusion area. The groundwater had fluorine levels of 0.09–10.99 mg/L, with an average concentration of 1.38 mg/L. The high-fluorine groundwater was mainly distributed in the unconsolidated Quaternary sediments, where concentrations in 83.6% of the samples exceeded the national limit of 1.0 mg/L. The groundwater in the Quaternary sediments also had higher levels of Cl⁻, TDS, Mg²⁺, pH and lower levels of Ca²⁺, Co, Ni, Cu than that in the bedrock. The groundwater fluorine levels in the Quaternary sediments are positively correlated with Cl⁻, TDS, Mg²⁺, pH, and negatively correlated with Ca²⁺, γCa²⁺/γMg²⁺, Co, Ni, Cu. Geochemical indexes of Cl⁻ and TDS indicate seawater intrusion in the Quaternary high-fluorine groundwater area (F⁻ > 1.0 mg/L), while they do not in the bedrock area. The chemical weathering of minerals was intensified with the intrusion of seawater. Cation exchange was confirmed to occur in the Quaternary sediments, and was promoted by seawater intrusion. Cation exchange consumes part of groundwater Ca²⁺ and permits more F⁻ dissolving. Consequently, in the Quaternary sediments, the groundwater was super-saturated with CaF₂ minerals and under-saturated with MgF₂ minerals when F⁻ > 1.0 mg/L, while CaF₂ and MgF₂ minerals both are under-saturated when F⁻ < 1.0 mg/L. Thus, the chemical weathering of minerals and cation exchange caused by seawater intrusion are the crucial processes controlling the groundwater fluorine levels, which should be considered when the groundwater fluorine enrichment mechanism is discussed along coastal zones.

中文翻译：

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高密市海水入侵区地下水富氟过程

选取我国高密市一个典型地区，探讨海水入侵区地下水氟的富集过程。地下水氟含量为0.09~10.99 mg/L，平均浓度为1.38 mg/L。高氟地下水主要分布在松散的第四纪沉积物中，83.6%的样品浓度超过国家1.0 mg/L的限值。与基岩相比，第四纪沉积物中地下水的Cl ^-、TDS、Mg ²⁺、pH值较高，而Ca ²⁺、Co、Ni、Cu的含量较低。第四纪沉积物中地下水氟含量与Cl ^-、TDS、Mg ²⁺、pH呈正相关，与Ca呈负相关²⁺ , γCa ²⁺ /γMg ²⁺ , Co, Ni, Cu。^{Cl -}和TDS的地球化学指标表明第四纪高氟地下水区（F ^-  > 1.0 mg/L）有海水侵入，而基岩区则没有。随着海水的侵入，矿物的化学风化作用加剧。证实第四纪沉积物中发生了阳离子交换，并受到海水入侵的促进。阳离子交换消耗部分地下水Ca ²⁺并允许更多的F ^-溶解。因此，在第四纪沉积物中_，当F⁻  > 1.0 mg/L，而CaF ₂和MgF _2矿物在F ⁻  < 1.0 mg/L时均处于欠饱和状态。因此，海水侵入引起的矿物化学风化和阳离子交换是控制地下水氟含量的关键过程，在讨论沿海地区地下水富氟机制时​​应考虑这一点。