## 4 Discussion  
[PERSON] [3] pointed out in 2015 that few articles have been published on the scientific aspects of hydrogen behavior in geological structures. Since then, studies on the impact of hydrogen leaks in an aquifer are still very rare. These include the experimental laboratory work of [PERSON] et al. [22] and an ongoing experiment by the same team at the University of Kiel (D) on a shallow aquifer, the results of which have not yet been published. The results obtained by [PERSON] et al. [22] are however not extrapolatable to our case: they concerned a long contact period (six months) between hydrogen under pressure (2-15 bars) and a reconstituted aquifer medium. Under these specific conditions, the reduction of sulfates, the production of acetate, the precipitation of calcite, and, consequently, an increase in pH and a decrease in electrical conductivity were observed. These parameters were considered as potential targets of a monitoring network covering a hydrogen gas storage site. However, it has been shown that these modifications resulted from the development of a hydrogenotrophic microbial community including sulfate-reducing bacteria. In our experiment, several physicochemical and hydrogeochemical phenomena occurred in the hydrogen saturation tank, and then, following the injection of the hydrogenated water, at the piezometers near downstream to the injection well. Some of these phenomena are the result of the injection of hydrogenated water (drop in the redox po  
Figure 27: Comparative evolution of the ammonium ion concentrations at all piezometers before and during the dissolved hydrogen injection experiment. Figure 26: Comparative evolution of the concentrations of sulfates, nitrates, and tracer at PZ1 (20 m upstream) and PZ2 BIS (5 m downstream) during the week following the injection of dissolved hydrogen. tential dissolved hydrogen in water), but some others are the results of the preliminary dissolution of helium and tracers (mainly, deoxygenation of water and degassing of CO2). At PZ2 BIS, the main monitoring piezometer located 5 m downstream to the injection well, such variations are synchronous with the passage of the first uranine peak (fluorescent tracer), which signaled the arrival of the hydrogen-free (helium-saturated) water from the first tank. Once injected into the aquifer, this first water therefore caused the same phenomena as those observed in the tank, but at a lower intensity as a result of its dilution: A drop in the oxidation-reduction potential, the dissolved O2 concentration, the electrical conductivity and the concentration of dominant ions (Ca2+, Mg2+, HCO3-). In addition, the water from this tank also contained an excess of Cl- ions, linked to the presence of LiCl as an ionic tracer: it thus caused an increase of Cl- ions in the aquifer. Since this ion is particularly conservative, it makes it possible to calculate that the dilution factor of the first injected plume must have reached 60% during the passage of the tracer peak at this piezometer. The dissolution of hydrogen in the second tank also resulted in the same phenomena related to the degassing of dissolved O2 and CO2: the dissolved O2 concentration dropped and the pH increased as a result of the degassing of dissolved CO2. This must have also induced the precipitation of some of the dominant ions (Ca2+, Mg2+ and HCO3-) in the water of the tank. The presence of dissolved hydrogen at a concentration close to saturation, additionally caused a significant drop in the oxidation-reduction potential, which made the water in this tank highly reducing: this is the only real impact directly linked to the saturation of water with hydrogen. Shortly after its injection into the aquifer, the oxidation-reduction potential of the groundwater thus fell at PZ2 BIS, making reductive the initially oxidizing groundwater. This drop coincides with the passage of the second tracer peak induced by the injection of water from the second tank.