Fluoride distribution characteristics and water–rock–temperature genesis mechanism of geothermal water from Lushan Hot Spring, western Henan, China.

Fluoride is an essential trace element for the human body and a common trace element in groundwater. Proper fluoride intake is beneficial to human health, but excessive intake threatens health (Podgorski and Berg, 2022). For example, a fluoride concentration of less than 1 mg/L can prevent dental fluorosis and promote bone development; however, the long-term ingestion of fluoride concentrations greater than 1.5 mg/L often leads to dental fluorosis, while a daily fluoride intake greater than 6 mg/day can induce chronic diseases such as skeletal fluorosis and fluoride poisoning (Shahid et al., 2010). Fluoride mainly enters the human body through drinking water and skin contact. Therefore, the World Health Organization stipulates that the fluoride content in drinking water should not be higher than 1.5 mg/L (WHO., 2017), and according to China’s Sanitary Standards for Drinking Water (State Administration for Market Regulation., 2022), the fluoride content in drinking and living water should not exceed 1 mg/L.

In hot spring geothermal water, fluoride is commonly found due to water–rock interactions and temperature influences. Trace amounts of fluoride in bathing water can have medical and health benefits, such as treating acne, eczema, and other skin diseases. However, when the fluoride concentration exceeds 1 mg/L, it becomes high-fluoride geothermal water. Prolonged skin contact may result in mild symptoms such as erythema and swelling, and severe cases may lead to local bone demineralization or aseptic necrosis (Dong et al., 2024; Guo and Fang, 2022; Cong et al., 2025). Therefore, studying the fluoride concentration and distribution characteristics in hot spring geothermal water is of great significance for the development, utilization, and treatment of hot spring geothermal water.

According to reports, more than 20 countries around the world are facing issues with excessive fluoride levels in groundwater or geothermal water. As a result, the distribution characteristics and sources of high-fluoride water have attracted significant attention from scholars, both domestically and internationally. Liu Yu et al. conducted a study of four typical endemic fluorosis areas in the Zhangcheng region of Hebei Province, China, exploring the impact of the geological environment on high-fluoride groundwater (Liu and Lin, 2024). Their study found that fluoride in groundwater mainly originates from fluoride-rich bedrock, ore deposits, and soil, while the fluoride content in atmospheric precipitation and surface water is relatively low. Sun Hongli et al. collected 30 samples of geothermal and surface water from the high-temperature geothermal area of Tibet (Sun et al., 2015). Their study indicated that the temperature of the geothermal water was linearly correlated with the fluoride ion content, and fluoride is more easily enriched in alkaline environments. Chae et al. analyzed the distribution and sources of fluoride in groundwater in southern South Korea (Chae et al., 2007). Their results showed that the fluoride in the groundwater primarily originates from the dissolution of fluorite in the bedrock. Yang Na et al. studied the groundwater in a typical high-fluoride area in northern Anhui, speculating that the main reason for the increase in fluoride concentration in deep groundwater was the dissolution of fluoride minerals such as fluorite, topaz, fluorapatite, mica, and amphibole (Yang et al., 2017).

It has been well established that the formation mechanism of fluorine in groundwater is due to long-term water–rock interactions, where fluoride minerals in the rock undergo complex chemical reactions, including dissolution and precipitation, adsorption and desorption, ultimately causing the fluoride in the minerals to enter the groundwater (Yuan et al., 2024; Wang et al., 2023b). The fluoride content, solubility, and occurrence form in different minerals vary, directly influencing the fluoride concentration in geothermal water (Thomas et al., 1977). Previous studies of high-fluoride groundwater or geothermal water have mainly focused on the patterns of fluoride in water and speculated on its evolutionary process and key influencing factors from a water chemistry perspective. From the perspective of the water–rock system, these studies traced the carriers and forms of fluoride occurrence, revealing the formation of fluoride during long-term interactions between water and rocks. In addition, the formation of fluorine in geothermal water is related to the dissolution and release of rocks, but also influenced by temperature. Therefore, it is essential to explore the genesis mechanism of fluoride in geothermal water from the water–rock–temperature perspective, integrating the distribution characteristics of temperature and the fluoride content in rocks, mainly fluoride-containing minerals, and fluoride occurrence forms.

The Lushan geothermal hot spring area in Henan Province, China, is located in the Checun–Lushan Xiatang fault geothermal belt. From west to east, it exposes five major hot springs: Shangtang, Zhongtang, Wentang, Xiatang, and Jianchang, with temperatures decreasing from Shangtang to Jianchang (Li., 2022). The region is rich in geothermal resources, with stable water extraction that continuously serves the surrounding residents in terms of heating, bathing, and tourism, providing support for local economic development. With the increasing exploitation and utilization of geothermal water, the groundwater runoff alternation rate has accelerated, causing changes in the chemical composition of the geothermal water. In recent years, excessive fluoride levels have been found, which have affected the drinking value of the geothermal water. Therefore, this paper focuses on the Lushan geothermal hot spring area, where we collected water samples from different aquifers and rock samples from the geothermal water-bearing layers. Through the comprehensive use of Piper Trilinear Diagram, ion relationship diagrams, X-ray diffraction (XRD), scanning electron microscopy (SEM), and other methods, we have analyzed the concentration and distribution characteristics of fluoride in the region’s geothermal water. From the water–rock–temperature perspective, we reveal the genesis mechanism of high fluoride content in the geothermal water of the study area. The research findings of this work provide fundamental support for the comprehensive utilization of geothermal water resources and the treatment of high-fluoride geothermal water.