Hydrogen gas in circular depressions in South Gironde, France: Flux, stock, or artefact?

Abstract Numerous circular depressions have been observed in the Saint-Magne area of South Gironde, France. To understand if the formation of the depressions is linked to hydrogen generation as noted elsewhere, surface gas measurements were made in the vicinities of these depressions by drilling a 12 mm-diameter hole to 1 m depth and inserting a thin perforated tube, which was used to pump and analyse the soil gases on site via a portable gas analyser. Hydrogen concentrations reached 3.5 %, prompting the installation of a permanent monitoring system to verify the hydrogen flux. After more than 1 year of continuous monitoring, including measurements of dissolved hydrogen concentrations in the surface aquifer, no hydrogen flux was detected in the soil or the surface aquifer. Additionally, soil thermal desorption experiments did not release any hydrogen, indicating that no stock of hydrogen was present in the soil. Two different protocols used to make the holes were compared: one using a drill and the other using a tube and hammer, which was less destructive. Comparisons of gases sampled using both techniques indicate that drilling the 1 m hole led to in situ gas generation. Additional soil crushing experiments indicated that drilling could provide enough energy to crack the organic matter naturally present in the soil by a milling action or excessive heating. Hydrogen and alkenes were artificially produced when the rock was crushed, both of which are indicators of drill bit metamorphism. Therefore, hydrogen was artificially generated in the soil during drilling. A robust method that can assure the repeatability of these measurements is paramount to guarantee that hydrogen concentrations obtained from organic-rich soils are not biased by an artefact, as observed here. This paper is the first to discuss this technique commonly used in native hydrogen studies and warns against drill usage when measuring subsurface hydrogen concentrations.