Characteristics and mechanisms of fluorine enrichment in the geothermal water of south central Shandong Province

Objective Yishu fault zone geothermal field and central Shandong uplift geothermal field are typical geothermal fields in south central Shandong Province. In order to investigated the distribution characteristics and mechanisms of fluorine enrichment in the geothermal water of the this area, Methods hydrochemical diagrams, geochemical simulations and principal component analysis were employed. Results The results show that the geothermal water in the study area is mainly Na-Ca-Cl-, Na-Ca-SO4-Cl- and Na-Cl-SO4-type water, dominated by weakly alkaline water. The predominant cation is the sodium ion, with fluorine concentration between 0.38 and 4.5 mg/L. Sodium-rich and weakly alkaline environments are faciliated to the enrichment of fluorine in the geothermal water. The fluorine concentration has significant positive correlations with the Na+, Cl- and TDS concentrations in the geothermal water. In addition, it has significant positive correlations with K+ and SO42- concentrations and significant negative correlations with Mg2+ and HCO3- concentrations in the geothermal water in the Yishu fault zone. The cation exchange effect in the geothermal water of the central Shandong uplift zone is stronger than that in the Yishu fault zone, and the reaction strength of Na+ is significantly stronger than that of Mg2+. The geothermal field of the central Shandong uplift and Yishu fault zones are both fracturing types. The thermal storage lithologies are limestone, limestone in the thermal alteration zone and ansanite in the broken belt, with strong water-rock interactions. The provenance of fluorine in the geothermal water is the dissolution reprecipitation of fluorine-containing minerals, and the fluorine concentration in the geothermal water is further increased by cation exchange and other water-rock interactions. High temperature and sodium-rich have a great influence on fluorine enrichment. Conclusion This research provides a reference for the exploitation and utilization of geothermal resources.