Fluoride in geothermal water: Occurrence, origin, migration and environmental impact.

Geothermal water, a naturally occurring resource, is garnering increasing attention due to its burgeoning potential in renewable energy generation and direct-use applications like heating and bathing (Fridleifsson, 2001). However, the geochemical composition of geothermal water, encompassing various ions and trace elements, has raised environmental and health concerns (Bundschuh and Maity, 2015; Mott et al., 2022). Among these constituents, fluoride, a common presence in geothermal water, is of particular interest due to its dual role as both a beneficial and potentially harmful element.

Fluoride, when present in drinking water in small quantities (<0.5 mg/L), is essential for human health as it contributes to bone mineralization and helps prevent dental caries. It actively contributes to bone mineralization and plays a crucial role in the prevention of dental caries. Nevertheless, excessive intake of fluoride (>1.5 mg/L), can lead to fluorosis, affecting teeth and bones, and potentially causing health issues like kidney stones, thyroid function impairment, and reduced cognitive development in children (Ozsvath, 2009).

High concentrations of fluoride in geothermal water are reported worldwide, e.g., Argentina (Chiodi et al., 2019; Hudson-Edwards and Archer, 2012; Peralta Arnold et al., 2017; Varekamp et al., 2009), America (Deng et al., 2011), China (Guo, 2012; Guo et al., 2009, Guo et al., 2014; Huang et al., 2023), Chile (Munoz-Saez et al., 2018, Munoz-Saez et al., 2020; Tassi et al., 2010), France (Sanjuan et al., 2010), Indonesia (Delmelle and Bernard, 1994; Rahayudin et al., 2020), Iceland (Arnórsson et al., 1983a, Arnórsson et al., 1983b; Björke et al., 2015; Gudmundsson and Arnórsson, 2005; Kaasalainen and Stefánsson, 2012), and New Zealand (Christenson, 2000). Fluoride in geothermal water ranges from 0.01 to ~6000 mg/L, varying widely due to hydrogeological conditions (Nordstrom, 2022). While geothermal water with high fluoride concentrations has potential medical applications (Mel’nichuk and Khodasevich, 2015), it is often unsuitable for drinking, frequently exceeding the World Health Organization’s recommended drinking water guideline value (1.5 mg/L, WHO, 2011). Fluoride levels in geothermal water typically exceed those in other groundwater and surface water sources (Nordstrom, 2022). The presence of fluoride in geothermal water carries environmental implications (Abiye et al., 2018; Guo, 2012; Guo et al., 2014; Huang et al., 2023; Wang et al., 2021), particularly in regions where geothermal water is discharged without treatment which will hinder the development and utilization of geothermal resources in the area. High fluoride levels can be toxic to aquatic life, accumulate in soils, potentially impact plant growth, and enter the food chain (Ozsvath, 2009). Given the extensive use of geothermal resources, intensive research into the source and distribution of fluorine in geothermal water is crucial for effective resource management, minimizing environmental impacts, and safeguarding public health. However, despite the global prevalence of high fluoride concentrations in groundwater influenced by geothermal activity, the underlying mechanisms remain incompletely understood (Wang et al., 2021).

This review is dedicated to providing a comprehensive overview of the current state of knowledge regarding fluoride in geothermal water. Based on extensive fluoride data, the distribution of fluorine in different types of geothermal systems and different types of geothermal water is described. We summarized the sources, mobilization, environmental impacts, and treatment of the geothermal fluoride. Additionally, combining insufficient current research, we identified current challenges and introduced the future perspective on managing fluoride in geothermal water. This work contributes to a better understanding of the chemical behavior of fluorine in geothermal water and the fate of fluorine after geothermal water discharge, providing valuable insights for the sustainable and safe utilization of geothermal resources.