Natural hydrogen (H2) in soil and spring water gas from the NW Pyrenean foreland, France

In July 2023, we conducted a measurement campaign to examine soil gases from 8 locations and gases from 16 natural springs in the northwestern foothills of the Pyrenees, near Biarritz (France). The aim was to test whether natural hydrogen gas (H₂), formed by the hydration of mafic minerals in an exhumed mantle body beneath the Pyrenees, migrates to the surface along deep-rooted faults.

In-situ measurements of soil gas compositions were carried out using various methods and devices: a landfill gas analyzer GA 5000, a Testo 316 detector, a Dräger Multiwarn X-am 8000 and a portable mass spectrometer “miniRUEDI”. The concentrations of H₂ in the soil measured with these devices were in the range of 0 – 2,000 ppmv.

Further measurements of gas samples collected from springs and degassed water samples in the GFZ laboratories showed H₂ concentrations between 5 - 4,300 ppmv. The spring with the highest H₂ concentration also shows very high (65.1 vol.%) CH₄, 1.7 vol.% CO₂ and +12.7 ‰ δ¹³C (CH₄) at -12.8 ‰ δ¹³C (CO₂).

At GFZ, we also carried out noble gas analyzes on several gas and water samples using a VG 5400 noble gas mass spectrometer. The idea behind this is that when mantle minerals hydrate, noble gases released from fluid inclusions should also migrate to the surface, along with the natural H₂. However, measured noble gas isotope ratios hint at a crustal origin of both the noble gases and the natural H₂. Additional sampling and analysis are planned to verify these first insights.