*1.2. Risks Associated with Underground Hydrogen Storage*

If there is a leak coming from a deep geological reservoir, the gas will migrate to the surface. In most cases, it will encounter at least one aquifer before reaching the surface [6]. If the leakage rate exceeds the dissolution potential of hydrogen in the groundwater, which is of the order of 2 mg·L −1 at surface conditions, which is low compared to other gases (11 mg·L −1 for dioxygen, 24 mg·L −1 for dinitrogen, 2500 mg·L −1 for carbon dioxide), part of the hydrogen will continue its migration to the surface. Hydrogen is then likely to accumulate in a confined underground area near the reservoir (cellar, underground car park, urban underground network, tunnel, etc.) where it will become a risk factor for explosion, fire, or asphyxiation. Indeed, hydrogen is a highly flammable gas with a very wide explosive range of between 4% and 75% at ambient pressure and temperature [7].

In the event of a potential hydrogen leak, the aquifer, therefore, represents the last warning barrier on the path of migration to the surface [6]. By transporting information from upstream to downstream, the aquifer constitutes a very favorable environment for the implementation of an integrated monitoring system immediately downstream of a deep storage site. As dissolved hydrogen is not normally present in water, detecting it within an aquifer will indicate a potential leak. This could be manifested as a direct detection (H<sup>2</sup> dissolved in water) or indirectly by means of the effects caused by this strongly reducing gas: decrease of the oxidation-reduction potential, decrease in the content of other dissolved gases in the water (mainly N2, O2, and CO2), and oxidation-reduction reactions, for example [8–14]:


The literature shows that, under normal pressure and temperature conditions, the reduction of nitrates and sulfates cannot take place except in the presence of a catalyst such as iron, copper, nickel, or platinum [8–14]. However, the frequent use of stainless steel, which contains iron and a significant amount of nickel (up to 20%), in the metal casings of a large number of water boreholes (for drinking water, mineral water, etc.) and hydrocarbon wells inevitably brings some of these catalysts into contact with the groundwater.
