The Enrichment Process of Groundwater Fluorine in Sea Water Intrusion Area of Gaomi City, China

A typical area, Gaomi City in China, was chosen to discuss the enrichment process of groundwater fluorine in sea water 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+, and pH and lower levels of Ca2+, Co, Ni, and 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+, gamma Ca2+/gamma Mg2+, Co, Ni, Cu. Geochemical indices of Cl- and TDS indicate sea water intrusion in the Quaternary high-fluorine groundwater area (F- > 1.0 mg/L), while they do not indicate any intrusion in the bedrock area. The chemical weathering of minerals was intensified with the intrusion of sea water. Cation exchange was confirmed to occur in the Quaternary sediments and was promoted by sea water intrusion. Cation exchange consumes part of groundwater Ca2+ and permits more F- dissolving. Consequently, in the Quaternary sediments, the groundwater was supersaturated with CaF2 minerals and undersaturated with MgF2 minerals when F- > 1.0 mg/L, while CaF2 and MgF2 minerals both are undersaturated when F- < 1.0 mg/L. Thus, the chemical weathering of minerals and cation exchange caused by sea water 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.