Sources, Fates, and Geochemical Cycling of Mercury in Geothermal Fields: Insights From Mercury Isotopes

• Published in: Geophysical research letters Vol. 51; no. 9
• Main Authors: Pan, Fei, Huang, Jen‐How, Feng, Xinbin
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.05.2024; Wiley
• Subjects: Atmosphere; Cycles; Degassing; Environmental perception; Fluids; Fumaroles; Gases; Geochemistry; Geothermal areas; Geothermal fields; Heat sources; Hot springs; Hydrothermal fields; Isotope composition; Isotopes; Lava; Magma; Magma chambers; Mercury; Mercury (metal); Mercury isotopes; Rain; Rain water; Rainwater; Spring water; China
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL107384

Geothermal fields emit remarkable amounts of mercury (Hg) to the environment. To address the source, fate and geochemical cycling of Hg in geothermal fields, we investigated Hg concentrations and isotopic compositions of hot spring water and fumarole gases from Rehai and Dagejia in SW‐China. Elevated Hg concentrations in fumarole gases (10.0–167 ng m−3) and hot spring water (3.44–84.5 ng L−1) were observed, suggesting that both geothermal fields are of environmental concern. The variation in Δ199Hg (−0.06 to 0.23‰) and Δ200Hg (−0.09 to 0.19‰) in hot spring water supports Hg likely originating from endogenous volcanic degassing and/or rainwater. Negative and nearly zero Δ199Hg in fumarole gases (−0.32 to 0.03‰) supports volcanic degassing and background atmosphere origin. The ranges of δ202Hg in fumarole gases (−0.74 to 0.59‰) and hot spring water (−1.29 to 0.52‰) could reflect limited fluids boiling in geothermal fields. This study, thus, fills an important knowledge gap regarding Hg global cycling. Plain Language Summary A large amount of mercury (Hg) is emitted into the atmosphere through geothermal activities. A knowledge gap exists regarding the source, fate and geochemical cycling of Hg in geothermal fields. Here, we studied Hg concentrations and isotopic compositions in hot spring water and fumarole gases from two large geothermal fields in SW‐China. Our results support that the two fields received Hg from endogenous volcanic degassing and rainwater. Deep magma chambers acted as heat sources, triggering fluid circulation in geothermal areas and entraining Hg from rainwater into geothermal systems. Key Points Rehai and Dagejia geothermal fields are likely of local environmental concern regarding Hg emission Mercury isotopic ratios evidence that geothermal fields could receive Hg from endogenous volcanic degassing and rainwater Geothermal fluid boiling is not common in geothermal fields in SW‐China