*1.2. Hydrogen Reactivity in a Natural Environment*

The impacts expected from a hydrogen leak in the underground environment are linked to the fact that it is a strongly reducing gas acting as a potential electron donor for numerous chemical species: metal sulfides, sulfates, carbonates, oxides (in particular of iron and magnesium), nitrates, ferrous ions, and gases (CO and CO2) [7,8]. The resulting oxidation-reduction reactions can thus modify the chemical composition of water or the mineral composition of aquifer rocks [6]. However, most reactions that occur in the presence of hydrogen require—at least in the laboratory—high temperatures or pressures or the presence of catalysts (Table 1). Under ambient conditions, hydrogen-consuming oxidation-reduction reactions have slow kinetics because hydrogen is not a polar molecule and the H-H bond is difficult to break owing to its elevated binding energy (436 kJ·mol−<sup>1</sup> ). However, Truche et al. [12] showed that the reduction of pyrite by hydrogen could have significant kinetics at low pressure and temperature in the presence of catalysts, in this case clay minerals. According to these authors, catalysts could also be bacteria, other mineral surfaces, or certain metals (iron, carbon steel, stainless steel, copper, nickel, platinum, and palladium). In the case of shallow aquifers, frequently used for water production (wells and boreholes for drinking, agricultural, industrial, or mineral water) or potentially crossed by other types of underground structures (wells, geotechnical foundations, etc.), the presence of metal parts made of iron, steel, or stainless steel could therefore play this catalytic role locally (Table 1).


**Table 1.** Some examples of abiotic reductions due to H<sup>2</sup> under experimental laboratory conditions.

\* Pyrite transformed into pyrrhotite (FeS1+x with 0 < x < 0.125).

In addition, the natural leak analogs, which are the hydrogen emission sites, also show that the surface and subsurface environments affected by these emissions can be significantly altered, even under ambient temperature and pressure: e.g., decrease in sulfates, decrease in oxidation-reduction potential, and increase in pH [9]. However, these are usually biogeochemical reactions that also have slow kinetics [12,21,22]. As such, the laboratory experiment carried out by Berta et al. [22] under ambient conditions lasted 180 days: it demonstrated a biochemical reduction of sulfates and carbon dioxide, a decrease in the calcium concentration, and an increase in the silica concentration concurrent with an increase in pH.
