While closely studying the Fontaines Salées site in Saint-Père-sous-Vézelay to understand nitrogen degassing along the western fault of the Morvan, natural hydrogen gas (H₂) was also detected at certain locations. Where it was measured, the concentrations remain relatively low but can locally reach notable levels—up to 700 ppm.
Due to its higher solubility compared to helium and nitrogen, H₂ is likely to diffuse more broadly through the near-surface formations. This suggests an upward migration mechanism primarily driven by diffusion through clay layers, possibly combined with variable biological production or consumption depending on soil aeration. The exact mechanisms behind its presence remain difficult to pinpoint.
However, the fact that H₂ is found at multiple localized points along the fault—which vertically cuts through nearly 150 meters of clay—and in non-negligible concentrations, may also point to an advective transport mechanism. Some of the H₂ observed here could originate from the radiolysis of water directly within the granite, which contains several ppm of uranium and thorium.
Congratulations to Emmanuel Léger for leading this study and coordinating the entire team! @GEOPS @université Paris-Saclay @45-8 @Université de Pau @Université d’Orléans @University of Glasgow
Link https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GC012021
orcid.org/0000-0001-6961-2177