Fluoride enrichment in geothermal waters based on H, O, Sr, and Li isotopes of the Taiyuan Basin, North China.

Geothermal systems play a vital role in shaping the hydrogeochemical environment of tectonically active regions (Burns et al., 2016, Zhang et al., 2024a). In these regions, geothermal water typically circulates along deep fault zones, interacting with high-temperature reservoirs and surrounding lithologies (Bai et al., 2023, Wang et al., 2023c). These fluid-rock interactions lead to the mobilisation of elements such as sodium, calcium, and fluoride from the dissolution of minerals like fluorite (CaF₂) and silicates (Sracek et al., 2015, Wang et al., 2021, Mwiathi et al., 2022). The high fluoride concentrations observed in the geothermal waters are closely associated with these geochemical processes. Furthermore, the migration of geothermal fluids along fault zones can lead to overpressure, reducing the effective normal stress on faults and potentially triggering fault slips, thereby potentially contributing to seismic events (Wang et al., 2013, Shi et al., 2015). Therefore, understanding the interrelationships among fluid pressure, fault stability, and earthquake generation is crucial for assessing seismic hazards in geothermal regions.

Stable isotopes (?D, ?¹⁸O, ?⁷Li, and ⁸⁷Sr/⁸⁶Sr) are valuable tracers for understanding groundwater origins, pathways, and geochemical processes controlling fluoride mobilisation (Guo et al., 2009, Guo et al., 2010, Ma et al., 2011, Li et al., 2015, Guo et al., 2020, Qu et al., 2022). Specifically, ?D and ?¹⁸O isotopes provide insights into the sources of geothermal waters and their interactions with reservoir lithologies (Wang et al., 2023b, Wu and Zhou, 2023). Geothermal waters often exhibit enriched in ?¹⁸O and ?D, indicative of isotopic exchange with host rocks at elevated temperatures (Gu et al., 2020, Hao et al., 2020, Álvarez-Amado et al., 2022). This isotopic enrichment suggests prolonged residence times and deeper fluid circulation, which are key factors influencing fluoride enrichment (Nordstrom, 2022, Nakaya et al., 2023).

Lithium isotopes (?⁷Li) are increasingly being used to investigate silicate weathering and water–rock interactions in geothermal systems. Li-isotope fractionation is temperature-sensitive during these interactions, making ?⁷Li a valuable tracer for high-temperature geochemical processes (Martin et al., 2020, Négrel and Millot, 2023, Yan et al., 2023). Elevated ?⁷Li values in geothermal waters compared with those in cold groundwater indicate significant lithium release from silicate dissolution, which is often associated with fluoride-bearing minerals, thereby highlighting the role of silicate weathering in fluoride mobilisation (Sracek et al., 2015, Martin et al., 2020, Chen et al., 2023). Similarly, strontium isotopes (⁸⁷Sr/⁸⁶Sr) are commonly used to trace the sources of dissolved strontium in groundwater and to assess the extent of water–rock interaction (Temizel et al., 2021, Huang et al., 2022b). The elevated ⁸⁷Sr/⁸⁶Sr values further support the hypothesis that fluoride enrichment is closely associated with the dissolution of fluorite and Sr-bearing minerals in geothermal reservoirs (Xie et al., 2013, Wang et al., 2020, Evans et al., 2024, Qiu et al., 2024)

The Taiyuan Basin, located within the tectonically complex North China Craton, is characterised by significant geothermal resources and active faults that influence the movement and mixing of geothermal and non-geothermal waters (Guo et al., 2007, Ma et al., 2009, Huang et al., 2023). Previous studies on groundwater fluoride in the Taiyuan Basin have primarily focused on identifying geochemical controls on fluoride concentrations and assessing associated health risks (Guo et al., 2007, Ma et al., 2011, Xie et al., 2013). Elevated fluoride concentrations are particularly prevalent in geothermal waters, where water–rock interaction and ion exchange processes facilitate fluoride mobilisation (Guo et al., 2007, Ma et al., 2009). However, the application of isotopic tracers such as ?⁷Li and ⁸⁷Sr/⁸⁶Sr to elucidate the source and evolution of fluorides in the region remains relatively limited. Furthermore, the potential links between geothermal fluid circulation, fluoride enrichment, and regional seismicity have not been adequately explored.

This study aims to address these gaps by investigating the hydrogeochemical processes responsible for fluoride mobilisation and enrichment in the Taiyuan Basin using a combined approach of hydrochemical and isotopic analyses (?D, ?¹⁸O, ?⁷Li, and ⁸⁷Sr/⁸⁶Sr). By analysing geothermal and non-geothermal water samples, the study aimed to elucidate the geochemical pathways leading to elevated fluoride concentrations and assess the influence of deep fluid circulation on this process. This study provides valuable insights into the complex interplay between geothermal systems, fluoride enrichment, and tectonic activity within the Taiyuan Basin.