## 3 Results
### Oxidation-Reduction Potential  
At the PZ2 injection well, the baseline showed an average oxidation-reduction potential of +192 mV prior to the injections [6]. When the measurements are resumed with the reopening of the well the day after the injections (t = 1.01 days), the oxidation-reduction potential is still low, with a value of +94 mV and it did not regain its initial value until t = 2.8 days onwards (Figure 14a).  
Figure 12: Comparative evolution of dissolved H\({}_{2}\) and tracer concentrations at the different piezometers: (**a**) PZ1 (20 m upstream) and PZ2 BIS (5 m downstream); and (**b**) PZ1 and PZ3 (10 m downstream) and PZ4 (20 m downstream).  
Figure 13: Detailed evolution of the dissolved H\({}_{2}\) and tracer concentration at PZ2 BIS (5 m downstream) during the day of injection.  
At PZ2 BIS, the oxidation-reduction potential decreased from +154 mV prior to the injections to a minimum of \(-\)139 mV during the injection (Figure 14b).Despite the shift in the values due to a difference in the calibration of the measuring probes, it is noted that the water remained reductive for at least 1.8 days and did not return to its normal value until after 2.8 days.  
Again, at PZ2 BIS, a detailed examination of the first day of the experiment shows the existence of two successive minima (see circled 1 and 2 in Figure 15):  
1. The first is not very marked (\(-\)22%) and reached +120 mV at 14:15, i.e., 15 min after injection from the first tank containing helium and hydrological tracers. It corresponds to the passage of less oxidizing water, probably because of its deoxygenation induced by the introduction of dissolved helium.
2. The second is very clear (\(-\)190%) and it reached \(-\)139 mV at 15:45, i.e., 55 min after injection from the second tank containing dissolved hydrogen. This drop is in fact synchronous with the second tracer peak.  
Due to the persistence of reducing conditions in the aquifer, in particular because of the release of the hydrogenated water stock present in the chalk's porous matrix in the immediate surroundings of the injection well, the oxidation-reduction potential remained at  
Figure 14: Comparative evolution of the oxidation-reduction potential and the tracer concentration at PZ1 (20 m upstream), PZ2 (injection well), and PZ2 BIS (5 m downstream): (**a**) PZ1 and PZ2; and (**b**) PZ1 and PZ2 BIS. The gap in the redox potential values corresponds to the change of probe.  
low values for the first day after the injection at PZ2 BIS. It then increased in regular fashion at a mean speed of +59 mV-day\({}^{-1}\) as a result of three mechanisms acting in unknown proportions: (i) dilution of the plume injected into the aquifer; (ii) partial degassing of the hydrogen; and (iii) potential chemical or biochemical reaction between the hydrogen and certain elements present in the water or the aquifer rock.  
For the piezometers located further downstream, we consistently noted the existence of fluctuations in the oxidation-reduction potential during the first two days (Figure 16). These are in fact artifacts arising from the increased frequency of measurements, which demonstrate intraday fluctuations that were not visible during the looser monitoring on the other days. Apart from this, there is no evidence of an impact of the injection of hydrogenated water on the oxidation-reduction potential once the distance downstream of the injection well reaches 10 m.  
Figure 16: Comparative evolution of the oxidation-reduction potential and the tracer concentration at the other downstream piezometers: (**a**) PZ3 (10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (60 m).  
Figure 15: Detailed evolution of the oxidation-reduction potential and the tracer concentration at PZ2 BIS (5 m downstream) during the day dissolved hydrogen was injected.