Controlling mechanisms and health risk assessment of high fluoride geothermal water in the Pearl River Delta region, South China.

In the Pearl River Delta (PRD) region, which is one of the most famous geothermal areas in China, there is much use of the abundant geothermal water resources for the purpose of medicinal and therapeutic benefits, cooling of buildings, chemical processing, fish farming, and agricultural greenhouses. However, obviously, the development and utilization level of geothermal water resources in this region is relatively low. The PRD region has seen a rapid growth in economy and population increase since the 1990s [1]. Excess fluoride loading in groundwater and/or geothermal water system is a major concern due to its health hazards and environmental impacts [2], such as fluoride poisoning (dental caries or tooth decay) by high-fluoride groundwater intake [3]. This fluoride enrichment has been commonly attributed to natural factors (e.g., atmosphere, water-rock interactions, evaporation, and hydrodynamics) and anthropogenic sources [4], [5], [6].

High fluoride concentrations in groundwater or geothermal water systems have been noted worldwide, such as the Great Rift Valley of East Africa [7] and Ghana [3] in Africa, Cerro Prieto geothermal field in México [8], sedimentary basins/plains and coastal aquifers [9], [10], [11], [12] in China, Coimbatore district in India [13], [14], Nagar Parkar area in Pakistan [15], Basin and Range basin-fill aquifers and Colorado Plateaus aquifers in the USA [16], coastal aquifers in South Korea [17], and volcanic-sedimentary aquifers in central Italy [18]. Different interpretations are given on the sources of fluoride, evolution for the elevated fluoride groundwater, and possible health risks to humans. Long-term exposure to elevated levels of fluoride has also been shown to pose serious health risks to humans or to have multiple adverse effects on human health in many parts of the world [5], particularly in parts of Africa, China, India, and South America [16]. It is often the case that the exposure to fluoride has been shown to induce a decrease of IQ levels and intellectual functionality among children [19]. It is reported that in China about 26 million populations from 29 provinces have been influenced by high fluoride groundwater or thermal groundwater (particularly in South China) [20], [21]. Despite local-scale distribution of high fluoride groundwater in South China, the prevalence of dental and skeletal fluorosis in children and adults in endemic fluorosis areas has been documented [21].

According to the World Health Organization (WHO), a guideline maximum F^– concentration of 1.5 mg/L for drinking water has been established [22]. Besides, the WHO highlights that countries should establish relevant standards or guidelines based on their local characteristics. The high exposure of the population in the PRD region to fluoride and relatively high fluoride intake from groundwater might pose serious health risks (e.g., endemic fluorosis, either dental or skeletal). Therefore, it is imperative to assess the fluoride hazards and risks in different population groups (mainly infants, children, adult females, and adult males). The Human health risk assessment (HHRA) method proposed by United States Environment Protection Agency [23] can often be used to evaluate health impacts of the pollutants (e.g., fluoride) in groundwater around the world [10], [24]. For instance, health risk assessment of high-fluoride geothermal water in Weihe Basin of North China using this method suggests that infants are the most susceptible population groups and that the development of geothermal water resources would lead to fluoride mixing into shallow groundwater and contamination of drinking water [9].

High F^– concentrations are often observed in aquifers associated with acidic igneous rocks, volcanic rocks, sedimentary deposits, and metamorphic basement rocks [25], [26]. Chemical weathering and dissolution (i.e., leaching) of abundant fluorine-rich minerals could mainly be responsible for F^– enrichment in groundwater. An examination of the speciation of dissolved fluoride and saturation indices of selected mineral phases can also help to determine geochemical factors (principally mineral precipitation and dissolution) influencing F^– concentrations in thermal groundwater [16]. Generally, aquifers with high pH and alkalinity, low Ca²⁺ concentrations, higher temperatures, and longer residence times are in favor of fluoride enrichment, during which desorption of fluoride from clay minerals, competitive adsorption involving OH^– and HCO^–₃, and ion exchange between Ca²⁺ (and/or Mg²⁺) and Na⁺ (and/or K⁺) come into play [6]. Climatic factors (e.g., temperature, atmospheric precipitation) also have a great influence on high fluoride contents in groundwater, particularly in arid and semi-arid region [26], [27], where relatively strong evaporation could precipitate low-solubility Ca-bearing minerals (e.g., calcite) especially in shallow groundwater, lowering the concentrations of Ca²⁺ and enhancing the dissolution of F-bearing minerals (e.g., fluorite), and thereby increasing F^– levels [6].

Far more attention has been given to geothermal water in the PRD region of South China. Detailed studies have been more focused on general groundwater quality [28], [29], [30], major ion chemistry [31], [32], [33], [34], fluids recharge [31], [32], [33], [34], mixing processes [35], water-rocks interactions [36], [37], provenances of major components [36], [37], and geothermometry [32], [38]. However, the mechanism controlling fluoride enrichment and health risk assessment of high fluoride geothermal water remain poorly understood. In particular, the PRD region represents a unique setting and a complex groundwater/hydrogeological system characterized by coexisting of multiple aquifers (i.e., granular aquifer, fissure aquifer, and karst aquifer) [39], rapid urbanization and industrialization [1], [40], and strong seawater intrusion and high salinity in coastal regions [34], [36]. These factors might, to some extent, influence hydrogeological and/or hydrogeochemical processes (e.g., ion exchange, complexation, mineral equilibrium state, evaporation process), but the effects on fluoride behavior in the PRD region have not been studied systematically, with the exception of evidence under consideration by Wei et al. (2021) [38].

Here, we combine new hydrochemical data with previously published major ions and fluoride chemistry of geothermal water in the PRD region into a larger geochemical dataset. By integrating these hydrochemical data across the region, the aims of this study are to investigate hydrogeochemistry of high fluoride geothermal water, to constrain the origins of groundwater fluoride, to identify hydrogeochemical processes controlling fluoride release into groundwater, and to define factors controlling fluoride enrichment. On this basis, the overall assessment of health hazards and risks of fluoride in thermal groundwater to different population groups (i.e., infants, children, adult females, and adult males) has been made. This study provides a comprehensive hydrogeochemical analysis of controlling mechanisms of high fluoride geothermal water and valuable guidance for populations exposed to high concentrations of fluoride in thermal groundwater of the PRD region in South China.