Distribution characteristics of fluorine in deep geothermal water in Jizhong Depression and its risk assessment and development utilization suggestions

This paper is the result of hydrogeological survey engineering.

Objective The geothermal resources of Jizhong Depression are rich. The distribution characteristics and risk assessment of fluorine in geothermal waiter are of great significance to the development and utilization of geothermal resources.

Methods This study takes the geothermal water in Jizhong Depression as the research object. The distribution characteristics and enrichment regularity of fluorine in geothermal fluid were studied by analyzing the hydrochemical data of geothermal fluid in sandstone and carbonate reservoirs, and the quality of geothermal fluid was evaluated, and furthermore some suggestions on the development and utilization of geothermal fluid were put forward.

Results The results show that the geothermal fluids in the study area are mainly Na-Cl and Na-Cl-HCO₃ type water, basically alkaline water, with high fluorine content. In particular, the concentration trend of fluorine ion content in the geothermal fluids of carbonate reservoir is 7.5-9.5 mg/L, and the highest is 13.9 mg/L. Weakly alkaline environment, water temperature and water-rock interaction are the main factors affecting fluoride ion enrichment in the study area. The F-concentration in sandstone geothermal reservoir has a significant negative correlation with Ca²⁺ and Mg²⁺ concentrations, while the correlation is not significant in carbonate geothermal reservoir. Fluorine enrichment is controlled by fluorite and other fluorine-bearing minerals. Fluorite saturation index of the geothermal fluids in sandstone reservoirs is all less than 0, while fluorite saturation index of the geothermal fluids in carbonate reservoirs with higher fluoride ion concentration is generally distributed around 0.

Conclusions The high fluorine geothermal fluid in the study area cannot be directly used for drinking water, farmland irrigation and fisheries. It is of great importance to solve the problem of fluoride removal in geothermal tail water at the situation of conducting gradient utilization of geothermal resources in the study area.