## 2 Material and Methods
### Overview of the Monitoring Protocol  
The monitoring protocol for the saturated zone was detailed by [PERSON] et al. [6] and is based on the preliminary injection of helium, because hydrogen and helium have a comparable physical behavior, in particular a very low solubility and a high diffusion coefficient in water. As a reminder, the monitoring protocol includes (Figure 3):  
1. Two physicochemical probes measure temperature, pH, electrical conductivity, oxidation-reduction potential, and dissolved oxygen. One is installed in the PZ2 bis while the other is mobile in order to take measurements at all the piezometers. It should be noted that this last probe had to be replaced on 19 November 2019 at 14:00, which is visible on the graphs as a deviation due to the new calibration (just before the injection of dissolved hydrogen). 2. Two field fluorometers allow on site measure of the water fluorescence. As before, one is installed in the PZ2 bis, while the other is mobile in order to take measurements in all the piezometers by groundwater sampling. 3. Six submerged electric pumps are installed at a depth of 16 m at the PZ1, PZ2 BIS, PZ3, PZ4, PZ5 and PZ6 piezometers. They are used to regularly sample the groundwater in order to perform laboratory analyses of tracers, dissolved gases (CH\({}_{4}\), He, H\({}_{2}\), and H\({}_{2}\)S), major elements (Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\), K\({}^{+}\), HCO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), and NO\({}_{3}\)\({}^{-}\), with detection limits of 0.01-0.05 mg.L\({}^{-1}\)), and minor elements (SO\({}_{3}\)\({}^{-}\), S\({}^{-}\), NO\({}_{2}\)\({}^{-}\), and NH\({}_{4}\)\({}^{+}\), with detection limits of 0.01-0.02 mg.L\({}^{-1}\)). 4. Raman and Infrared (IR) spectrometers installed at the PZ2 TER [25,27] make it possible to analyze the concentration in the groundwater of mononuclear diatomic molecules (H\({}_{2}\), O\({}_{2}\), and N\({}_{2}\) for Raman) and polar molecules (CO\({}_{2}\) and CH\({}_{4}\) for Raman and IR). It should be noted that the data acquired by these devices are not presented here but are the subject of another specific publication to come. Here, we selected Raman and IR technologies because they were already used in other studies, which ensures they suit our needs. New sensing technologies using optical fiber grating platforms are presently developed and tested on lab benches and would be promising for field studies in the close future [28]. Figure 2: Detailed plan view of the experimental site. There are specific analyzers for measuring the gas concentration in the piezometer head spaces and in the gas mixture released from the water extracted from the aquifer. A DRAGER multigas analyzer equipped with a catalytic cell (resolution of 0.1% vol.) and a portable Biogas analyzer equipped with an electrochemical cell (resolution of 1 ppm) are used for hydrogen, while an ALCATEL ASM 122D transportable mass spectrometer is used for helium. 6. Device for extracting and degassing water by mechanical agitation serves to establish the dissolved gas concentration in the groundwater, in conjunction with the gas analyzers detailed above. The migration of the plumes injected into the aquifer is monitored automatically at PZ2 BIS and at PZ2 TER by means of devices installed in situ and manually at the other piezometers by sampling the water. All measuring devices or probes are installed between 15 and 16 m deep, i.e., in the most productive zone of the aquifer under study [10]. During the first week, the monitoring was relatively frequent with 4-15 samples per day depending on the piezometer. During the following weeks, a follow-up with a more spaced measurement frequency was set up, at a rate of 1-3 samples per piezometer and per week. In total, this monitoring made it possible to take 130 water samples to analyze the tracers and chemical elements and to conduct 104 physicochemical measurements.