## 3 Results
### Other Physicochemical Parameters  
Regarding the electrical conductivity, we note the existence of natural variations within the aquifer, recorded at PZ1 upstream of the injection well: at this piezometer, before injection and up to six days after, the average conductivity is 550 \(\upmu\)S-cm\({}^{-1}\); it then decreases to 492 \(\upmu\)S-cm\({}^{-1}\) after this date. This fluctuation, which is observed upstream of the area affected by the injections, therefore affects the entire aquifer with successive repercussions on all piezometers: at PZ2 BIS, the conductivity thus dropped from 558 to 495 \(\upmu\)S-cm\({}^{-1}\) (Figure 1(a)). On the other hand, a detailed analysis of the first day following the injection again shows the same three successive decreases in concentration already encountered at this piezometer with the previous physicochemical parameters (Figure 1(b)). Figure 19: Comparative evolution of the dissolved oxygen during the dissolved hydrogen injection experiment at PZ1 (20 m upstream piezometer) and at the downstream piezometers: (**a**) PZ3 (10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (60 m). Figure 18: Detailed evolution of the dissolved oxygen and tracer concentrations at PZ2 BIS (5 m downstream) during the first day of dissolved hydrogen injection. The minimal values are reached on the day of injection at 14:30 with 490 \(\upmu\)S-cm\({}^{-1}\), at 15:20 with 564 \(\upmu\)S-cm\({}^{-1}\), and at 16:00 with 502 \(\upmu\)S-cm\({}^{-1}\). As before, this corresponds to the rapid passage of the plumes of water from the two tanks, as well as the slow release into the aquifer of water that has remained trapped in the porous matrix around the injection well. We show below that this correspond mainly to a decrease in the cumulative concentration of the dominant ions (Ca\({}^{2+}\), Mg\({}^{2+}\), and HCO\({}_{3}\)) of this bicarbonate-calcic groundwater: this is interpreted as a consequence of the precipitation of CaCO\({}_{3}\) and MgCO\({}_{3}\) from the degassing of dissolved CO\({}_{2}\) following the saturation of the water with He or H\({}_{2}\). On the other hand, these decreases are immediately followed by a peak in conductivity reaching 744 \(\upmu\)S-cm\({}^{-1}\) for the first, 707 \(\upmu\)S-cm\({}^{-1}\) for the second, and, later (t = 2.80 days), 582 \(\upmu\)S-cm\({}^{-1}\) for the third. These are the highest values recorded since the implementation of baseline monitoring over more than a year ago. These cycles of decrease and increase should therefore be related to the injections conducted: it seems that the observed increases can result from the dissolution of the carbonaceous aquifer rock by the injected water, which became aggressive to calcite, following the degassing of CO\({}_{2}\) induced by the bubbling with other gases. Regarding pH, we note that it is already varying cyclically from 6.8 to 7.4 at PZ1 (upstream) before the injections. As is the case for the conductivity, we assumed that these are natural groundwater fluctuations since they are measured upstream of the injection well, and they affect the entire experimental site up to the most downstream piezometers: at PZ2 BIS, the pH thus varies from 7.0 to 7.7 (Figure 21a). On the other hand, the detailed analysis of the first day following the injection shows, at this piezometer, a weak but sudden increase in pH from 7.2 to 7.4 when the first two tracer peaks passed through (Figure 21b). This behavior is to be linked to the degassing of dissolved CO\({}_{2}\), following the dissolution of He and H\({}_{2}\). The pH then decreased to 6.9 after t = 2.88 days, i.e., for the duration of the passage of the third plume of water, while at the same time the water at PZ1 showed an increase of natural origin. One could possibly interpret this increase as an impact of the slow arrival of hydrogenated water with the main aquifer flow.