Hydrogen gas (H₂) artificially generated during soil-gas sampling - Technical report

Alexandra Orban, Stefan Iulian Sfabu, Roxana Valentina Moga, Marius Muresanu, Alexandru Iulian Lupulescu, Ildiko Melinda Martonos, Calin Baciu, Giuseppe Etiope

Surface gas geochemistry methods, including soil gas measurements, are widely used for natural hydrogen (H₂) exploration. It is known that H₂ can be artificially generated by mechanical friction during installation of soil-gas probes. However, it is frequently considered that artificial H₂ is produced only by drill bit rotation, although there are no systematic studies about its generation during soil-gas probe insertions by less invasive probing techniques, such as manual hammering. Accordingly, the H₂ detected in the soil by portable sensors or gas chromatography is often cursorily interpreted as an indication of geological H₂ emanation. The aim of this report is to provide a demonstration of artificial H₂ generation during soil gas probe hammering in a control site, within an urban park in Cluj-Napoca (Romania), where natural H₂ is absent. Soil-gas measurements were performed throughout an area of about 1850 m² and, in more detail, in a site where small pebbles were buried in the soil to simulate a soil with higher strength. Two hammering techniques were tested: (a) sledgehammer and (b) sliding hammer. Four permanent probes were also installed to monitor H₂ over time, unaffected by mechanical friction. Portable electrochemical sensors (Dräger X-am 8000) were used to measure H₂ and CO, which is a proxy for artificial gas generation. We measured H₂ concentrations ranging from below detection limits (10 ppmv) to 385 ppmv (associated with 50 ppmv CO), using the sledgehammer technique. The less invasive sliding hammer method produced up to 200 ppmv H₂. We observed that the H₂ concentration was not stable, decreasing to zero ppmv after 2-3 minutes using the sliding hammer technique, and at least 4 minutes with the sledgehammer. The permanent probes never showed H₂. The test is documented by videos available at https://engagelab.granturi.ubbcluj.ro/media/. This study confirms that artificial H₂ can be generated by any type of soil-gas technique implying mechanical friction within the soil, a phenomenon that should be considered in exploration activities. More exhaustive research, including soil analyses in wider areas, in the absence of natural H₂, is in progress.