Hydrochemical processes and fluoride enrichment patterns in high-fluoride geothermal water in the Weihe Basin, China.

Fluoride (F) is an essential element for human health, and a moderate fluoride intake reduces the incidence of dental caries and supports bone development [1], [2], [3]. However, fluoride concentrations in drinking water that exceed 1.5 mg/L or fall below 0.5 mg/L can have detrimental health effects [4]. Aquifers with elevated fluoride concentrations are present in numerous regions worldwide. It is estimated that more than 220 million people consume drinking water with fluoride concentrations exceeding the World Health Organization’s recommended limit of 1.5 mg/L and comparable national standards (1 mg/L in China). The countries most severely affected are China, India, parts of Iran, and Sri Lanka [3], [5], [6], [7]. In 2000, well water in parts of China was found to contain fluoride at a concentration of 21.5 mg/L [8], [9], [10].

Geothermal energy is a clean energy source, characterized by a wide distribution, low pollution, and other ecological benefits on a global scale [11], [12], [13], [14]. The high fluoride levels found in groundwater are closely linked to the geographical distribution of geothermal areas, particularly their geochemical and hydrogeochemical characteristics. This is especially true of geothermal water, which is significantly affected by water-rock interactions during the fluid migration process [15], [16]. The migration of fluorine in geothermal fluids is significantly controlled by the deep thermal structure [10], [17], [18]. However, as geothermal resources continue to be developed and utilized, several potential environmental issues have emerged. These issues include the pollution of local shallow water resources; while the long-term, large-scale extraction of geothermal water can lead to a fall in the groundwater level, ground subsidence, and air pollution; moreover, the direct release of underutilized geothermal water can cause thermal and soil pollution. Fluoride is a significant contaminant and high-fluoride concentrations have been widely documented in groundwater, including the Iran, Pakistan, China, South America, Mexico, Nigeria, Brazil, Kenya, and Africa [19], [20]. For instance, the fluoride concentration was determined to be as much as 19.4 mg/L in the Yangbajing geothermal field, on the Tibetan Plateau, and wastewater from geothermal power generation is discharged into the Zangbajiang River, causing high levels of contaminants, including fluoride, downstream of the Zangbajiang drainage outlet and in the adjacent watersheds [21]. In Yellowstone National Park (USA), and other areas influenced by geothermal activity, the drainage dominated by geothermal water often contains anomalously high levels of fluoride [22]. Additionally, more than 60,000 individuals in the Guide Basin were found to be at significant risk of developing fluorosis, primarily due to the long-term exposure to groundwater with elevated fluoride levels [9].

Therefore, from environmental and human health perspectives, it is important to investigate the sources, enrichment patterns, and evolutionary features of fluorine in geothermal systems, to facilitate the geothermal development of sedimentary basins. Fluoride has been identified as a significant constituent of granites, hydrothermal deposits, and alkali granites [23]. When fluorinated minerals, such as fluorapatite (Ca₁₀(PO₄)6F₂), fluorapatite (Ca₁₀(PO₄)6F₂), fluorite (CaF₂), as well as their weathering products, interact with groundwater, water-rock interactions facilitate the gradual release of fluoride ions (F?) into the groundwater system [9], [24]. The principal source of fluoride is the increase in fluorine-enriched crustal deep thermal water through channels such as faults and fracture zones [18].

The Weihe Basin is one of the largest Cenozoic sedimentary basins in northwestern China. It has abundant geothermal resources which have considerable potential for further development [25]. Previous studies of the geothermal system in the Weihe Basin have focused mainly on its heat source and reservoir characteristics [26], [27]. However, insufficient attention has been paid to environmental issues that may arise during the development of geothermal fields. Here we focus on the issue of high fluoride concentrations in deep geothermal fluids in the Weihe Basin. We conducted a comprehensive analysis of the geological conditions, hydrogeochemistry, thermal reservoir temperature of geothermal fields; and the distribution characteristics, enrichment patterns, fluid evolution processes, and other factors influencing fluoride concentrations. Our findings provide a scientific foundation for the environmentally sustainable exploitation of thermal resources in the Weihe Basin and analogous regions elsewhere. They also contribute to mitigating the risk of fluoride poisoning in local drinking water.