## 4 Discussion
Compared to the baseline state, the concentration of dissolved O2 also decreased at that time, as well as those of the dominant ions (Ca2+, Mg2+, and HCO3-), while the pH increased slightly. In the piezometers located further downstream, i.e., at a distance of 10 m or more from the injection well, no impact of this type is measured in the aquifer: only the tracers show a weak presence which clearly reflects the passage of the plumes of water injected from the tanks. In addition, no piezometer showed significant variations, other than natural ones, in the concentrations of the oxidized and potentially reactive NO3- and SO42- ions or in any of their expected metabolites (NO2-, NH4+, SO3-, and S2-). It should be specifically noted that no element likely to play a catalytic role (especially metals) came into contact with the groundwater on the site. The behavior of the aquifer during the passage of the third tracer plume is also interesting. This plume corresponds to the slow release of the water stock that has remained trapped in the poorly permeable zone at the base of the injection well PZ2 or in the porous matrix of its immediate surroundings. Thus, at PZ2 BIS, the oxidation-reduction potential remained low throughout the first day following injection, although it regularly increased at an average rate of +59 mV\(\cdot\)day-1 due to three possible mechanisms: (i) the dilution of the plume injected into the water table; (ii) the presumed degassing of the hydrogen; and (iii) the potential chemical or biochemical reaction of the hydrogen with some elements present in the aquifer rock or in the groundwater. Because the hydrogen is injected under conditions of undersaturation with respect to the aquifer (hydrostatic pressure > saturation pressure of the 5 m3 tank), and no impact of the injection of hydrogen on the concentration of nitrates and sulfates could be demonstrated, the dilution of the injected plume seems to be the principal mechanism. Again, at PZ2 BIS, the dissolved O2 concentration of the aquifer remained low until at least 2.80 days after the start of the injections. During this same period, the pH and the concentration of dominant ions dropped below their initial values. Regarding the conductivity, the decreases observed during the passage of each injected plume of water are followed by brief increases before returning to the initial values. The decreases correspond to the passage of undersaturated water due to the precipitation of calcite and dolomite within the tanks, linked to the degassing of CO2 due to the bubbling of He or H\({}_{2}\). The observed increases however could represent a brief renewal of the carbonaceous aquifer rock dissolution following the passage of more aggressive water due to its lower concentration of dominant ions (Ca\({}^{2+}\), Mg\({}^{2+}\), and HCO\({}_{3}\)\({}^{-}\)). According to lithium breakthrough curves and the corresponding calculated dilution factors, the theoretical hydrogen concentration should have reached a maximum of around 0.70 mg\(\cdot\)L\({}^{-1}\) at PZ2 BIS and 0.002 mg\(\cdot\)L\({}^{-1}\) at PZ3. The values observed of 0.63 and 0.002 mg\(\cdot\)L\({}^{-1}\), respectively, correspond well with this hypothesis. During this experiment, the hydrogen therefore behaved mainly as a conservative tracer (Figure 28). It can also be estimated that, at the more distant piezometers, the maximum concentration of dissolved H\({}_{2}\) must have been lower than the analytical detection threshold, which meant that this element could not be detected, nor could its possible impact on the aquifer.