## 2 Material and Methods
### Experimental Protocol  
Figure 4 shows a view of the experimental site (Catenoy, France). The experiment consisted of extracting 5 m\({}^{3}\) of groundwater from PZ2, saturating it with hydrogen gas in a tank (Figure 5), and then injecting this hydrogenated water into the aquifer through the same piezometer. Another 1 m\({}^{3}\) tank contained groundwater with tracers to monitor the propagation of the injected plume.  
Figure 3: Various devices installed at the injection well and the near downstream monitoring wells.  
The tracers were selected during the preliminary test carried out in April 2019 [6]. They consisted of a neutral gas (helium) and two tracers (uranine and lithium chloride). These tracers were not added to the 5 m\({}^{3}\) tank in order to avoid affecting the dissolution of hydrogen, a gas that is very poorly soluble naturally, with a solubility at saturation of around 1.8 mg\(\cdot\)L\({}^{-1}\) under surface pressure and temperature conditions (compared with that of other gases generally present in groundwater: 11 mg\(\cdot\)L\({}^{-1}\) for dioxygen, 24 mg\(\cdot\)L\({}^{-1}\) for nitrogen, and 2500 mg\(\cdot\)L\({}^{-1}\) for carbon dioxide).  
The two tanks were filled on 18 November 2019 from 10:00 to 15:30 using groundwater. The bubbling of helium gas in the first 1 m\({}^{3}\) tank began immediately after filling it and continued overnight until the next day at 14:00, which made it possible to achieve complete saturation of the water with helium. The tracers were then dissolved in the water of this tank at the concentration of 10 mg\(\cdot\)L\({}^{-1}\) each. These tracers were lithium in the form of LiCl (a conservative but a colorless tracer, only detectable by water sampling and laboratory analyses a posteriori) and uranine (a less conservative tracer but colored and detectable in situ, in real time, by fluorimetry). The water from the first tank was injected by gravity into PZ2 on 19 November 2019 from 14:00 to 14:20 at a rate of 3 m\({}^{3}\cdot\)h\({}^{-1}\).  
The second tank, with a volume of 5 m\({}^{3}\), contained the hydrogenated water. This is pure hydrogen from two compressed gas cylinders (50 L and 200 bars each). The bubbling of hydrogen at a flow rate of approximately 20 L\(\cdot\)min\({}^{-1}\) started immediately after filling the tank and continued until 21:30. It was then interrupted in the evening, for safety reasons, to resume the next morning at 06:30 until 15:10. This made it possible to reach a concentration of dissolved hydrogen of 1.76 mg\(\cdot\)L\({}^{-1}\), or 95% of water saturation by hydrogen under the average temperature and hydrostatic pressure conditions within the tank (Figure 6). The total quantity of hydrogen dissolved in this tank was approximately 9 g or 100 L under Standard Temperature and Pressure (STP). H\({}_{2}\)-saturated water was injected below the water table, i.e., a slight undersaturation with respect to hydrostatic conditions to prevent or limit H\({}_{2}\) degassing.  
Figure 4: View of the experimental site during the experiment.  
Figure 5: View of the injection device.  
The physicochemical parameters of the water were monitored during this operation. During the saturation of the water with hydrogen, the oxidation-reduction potential increased from +148 mV to -224 mV and the concentration of dissolved oxygen dropped from 7.21 mg\(\cdot\)L\({}^{-1}\) to 0.74 mg\(\cdot\)L\({}^{-1}\) (Figure 7a). At the same time, the pH increased from 6.95 to 7.51, while the electrical conductivity remained stable around 552 \(\mu\)S\(\cdot\)cm\({}^{-1}\). The temperature of the water in the tank regularly decreased from 11.5 \({}^{\circ}\)C to less than 9 \({}^{\circ}\)C, under the effect of nocturnal cooling. The water from this tank was then injected by gravity into PZ2 on 19 November 2019 from 14:50 to 17:20, i.e., 30 min after completing injection of the first tank. The average injection flow rate was 2 m\({}^{3}\).h\({}^{-1}\).