Maria Luisa Moreira Dos Santos

Accurate field sampling of natural hydrogen (H2) is crucial to avoid artifacts that may result in data misinterpretation and inaccurate assessments of hydrogen fluxes. Hydrogen of anthropogenic origin can be generated during sampling due to processes such as the metamorphism of drill bits, which produces excessively heat, leading to the cracking of organic matter—a phenomenon first discussed in the field of natural hydrogen by Halas et al. (2021). Additionally, the corrosion of drilling/sampling steel through aqueous reactions, and mechanoradical processes associated with the dissociation of silicates during drilling, are two other potential sources of artifacts.

This study presents a comprehensive protocol for natural hydrogen sampling, developed to ensure robustness in field measurements and to prevent the production of anthropogenic hydrogen by better understanding its generation. Extensive laboratory tests were conducted to evaluate the effect of different factors on the production of anthropogenic hydrogen, including soil types and humidity, field sampling methodologies, and drilling times. Laboratory simulations of sandy, organic, and rocky soils were carried out in 92 x 10 cm columns, with both near-dry and humid soil conditions tested to assess the impact of moisture on artificial hydrogen generation. Additionally, three different techniques for the installation of the sampling system were compared: drilling with simultaneous installation of the probe (drill bit coupled at the probe tip), drilling a hole followed by probe installation, and a sliding hammer method. The duration of the drilling process was also varied to understand its effect. The tests revealed significant differences in anthropogenic hydrogen creation depending on the sampling methodology and soil conditions. Notably, more than 1000 ppm of hydrogen (exceeding the saturation limit of the gas detector – GA 5000) were measured in the laboratory under the combination of some of the variables. The production of anthropogenic hydrogen was repeatedly confirmed in various concentrations and scenarios.

These findings provide valuable insights into the factors contributing to the generation of artifacts, aiding the development of a protocol for natural hydrogen sampling in fieldwork. By adhering to best practices, researchers and field technicians can significantly reduce the risk of artifacts in hydrogen data, leading to more accurate assessments of natural hydrogen fluxes.

The co-authors and their affiliation are :

- Geneviève Bordeleau : Institut National de la Recherche Scientifique (INRS-ETE)

- Stephan Séjourné (INRS-ETE; Enki GeoSolutions)