Quantitative Decoupling of Dominant Hydrochemical Processes in Coastal Geothermal Systems: Insights into Fluoride Enrichment via Stable Isotopic Tracers and PMF Model

Geothermal energy is a critical low-carbon resource in the global carbon neutrality transition, but expanding exploitation has raised growing concerns over geothermal fluid quality degradation and fluoride-related public health risks in the tectonically active coastal region of Guangdong, South China. However, the hydrochemical mechanisms governing fluoride enrichment remain poorly constrained, and conventional qualitative analytical approaches cannot quantitatively disentangle the superimposed effects of multiple subsurface geochemical processes. Based on 20 geothermal groundwater samples, this study integrates hydrochemical characterization, stable hydrogen and oxygen isotope tracing, and positive matrix factorization (PMF) modeling to quantitatively identify dominant hydrochemical processes and decipher the genetic mechanism of fluoride enrichment. The results demonstrate that the geothermal groundwaters belong to Cl–Na hydrochemical facies, with temperatures ranging from 60 °C to 96 °C and total dissolved solids (TDSs) varying from 560 mg/L to 9862 mg/L. Water–rock interaction dominates hydrochemical evolution: congruent dissolution of halite and other evaporite minerals serves as the primary source of bulk salinity, while bidirectional cation exchange on clay mineral surfaces substantially modifies the ionic assemblage. Stable isotope compositions (δD: −47.2‰ to −39.0‰; δ18O: −7.3‰ to −5.2‰) confirm a dominant meteoric recharge origin, with notable positive 18O shifts in multiple samples reflecting prolonged deep water–rock interaction with silicate host rocks. Recharge elevations are estimated at 239~682 m, delineating the northwestern medium–low mountain zone as the primary recharge area. Fluoride concentrations (2~13 mg/L) universally exceed the drinking water standard, and their enrichment is governed by a coupled geochemical feedback mechanism: hydrolytic weathering of fluor-bearing silicates releases structural fluoride, while widespread calcite precipitation scavenges aqueous Ca2+, weakens the common-ion effect, and promotes fluorite dissolution. The PMF model quantitatively resolves three geochemically meaningful controlling factors with clear process implications. These findings advance the mechanistic understanding of fluoride geochemistry in coastal granitic geothermal systems within the western Pacific tectonic belt, and provide a robust scientific basis for sustainable geothermal resource development and public health risk management.