## 5 Conclusions  
The hydrogen leak simulation experiment consisted of extracting water from the aquifer, saturating it with hydrogen gas in a tank, and then reinjecting it into the aquifer. The saturation of the water with gaseous hydrogen initially caused several physicochemical and hydrochemical phenomena within the tank and, consequently, in the aquifer:  
* A sharp decrease of the oxidation-reduction potential
* The almost total disappearance of dissolved O\({}_{2}\) and CO\({}_{2}\)
* A slight increase of the pH that induced the precipitation of alkaline earth bicarbonate ions and, accordingly, the decrease of electrical conductivity  
These variations are primarily observed at the main PZ2 BIS monitoring piezometer located 5 m downstream of the injection well and, to a lesser extent, at the PZ3 piezometer located 10 m downstream of the injection well. The other piezometers, located between 20 and 60 m downstream of the injection well, are not significantly affected. Our monitoring results show that, under the experimental conditions, the impact is only significantly measurable up to 10-20 m downstream of the injection well. This demonstrates the utility of closely monitoring the immediate surroundings of a future hydrogen injection well. This surveillance must be applied not only to the groundwater aquifer, but also to all major aquifers located deeper. These results, however, are only valid under the experimental conditions of this test, which consisted of injecting a limited amount of dissolved hydrogen (9 g or 100 L STP) for a short time to simulate a sudden leak. Thus, in the case of a larger and/or longer leak, it is  
Figure 28: Comparative evolution of the ratio of maximum concentrations (C\({}_{\text{max}}\)) to the injection concentration (C\({}_{\text{ini}}\)) of tracers and hydrogen up to 20 m downstream of the injection well. likely that the physicochemical and hydrogeochemical impacts would be greater across both space and time. In addition, bacterial growth could take place and induce biochemical reactions that may consume some dissolved species (sulfates, hydrogen), as observed by [PERSON] et al. (2020). It is also shown that, during this experiment, the rapid transfer of hydrogen through the aquifer and its significant dilution beyond 10 m downstream of the injection well did not allow the development of significant chemical or biochemical reactions: hydrogen behaved here as a predominantly conservative element and is not (or not very) reactive. This experiment is a first test of the impact of a hydrogen leak in a shallow unconfined aquifer. It made it possible to show that there are direct and indirect impacts of the arrival of dissolved hydrogen even in low amounts in an aquifer and, therefore, to recommend the implementation of a monitoring of future underground hydrogen storage sites that takes all of these physicochemical and hydrogeochemical parameters into account: concentrations of dissolved H\({}_{2}\), O\({}_{2}\), and CO\({}_{2}\), pH, electrical conductivity, and oxidation-reduction potential. In the case of long-term leakage, the concentrations of sulfates, nitrates, and bicarbonates must also be monitored. This short time experiment should be considered as a first test intended to fit the injection and monitoring protocols for the effective monitoring of the impacts of a hydrogen leak in an aquifer under geological storage conditions. To be more representative of a real hydrogen leak, it should be supplemented in the future by a continuous leak simulation, lasting several days, the monitoring of which should focus on the relevant parameters previously identified. Conceptualization and methodology, P.G., S.L., Z.P. and E.L.; validation, P.G., S.L., Z.P., E.L., P.d.D. and N.J.; data curation, P.G. and S.L; writing--original draft preparation, P.G. and S.L; writing--review and editing, P.G., S.L., Z.P., E.L., P.d.D. and N.J.; visualization, P.G. and S.L. ; project administration, P.G. ; supervision and funding acquisition, P.G. and S.L.