## 3 Results
### Tracing of the Injected Water  
PZ2 remained sealed for 24 h after the injections to limit the risk of hydrogen degassing. When it was reopened, despite the leaching induced by the injection of 5 m\({}^{3}\) of hydrogenated water that followed the injection of 1 m\({}^{3}\) of tracer-holding water, the tracer concentrations still reached 208 g\(\cdot\)L\({}^{-1}\) for lithium, 117 g\(\cdot\)L\({}^{-1}\) for uranine, and 3.9 g\(\cdot\)L\({}^{-1}\) for helium (Figure 9). Depending on the tracer, this represents 0.3-2% of the concentration injected the day before. Since the injection induced an overpressure, this phenomenon is probably due to the retention of part of the tracers within the chalk's porous matrix (which is of micrometric dimension), and then to their slow release when the natural flow of the aquifer resumed. This phenomenon has previously been demonstrated at other sites where the chalk shows dual porosity [29; 30]. Thus, in the injection well, the tracer concentration remained higher than the background noise for 12.9 days for lithium and helium and until the end of the monitoring (t \(\geq\) 49.9 days) for uranine, on which date the analyses still showed the presence of 0.29 g\(\cdot\)L\({}^{-1}\) of this tracer. This retention phenomenon therefore generated a decreasing but clearly significant background noise for about two weeks after injection. At PZ2 BIS, 5 m downstream, three successive tracer plumes were observed (Figure 10b):  
1. A first plume caused a significant peak (see circled 1 in Figure 10b). It started at 14:30, i.e., 0.5 h after injection of the tracers, and peaked at 14:45 at a concentration of 1678 g\(\cdot\)L\({}^{-1}\) for lithium and 1188 g\(\cdot\)L\({}^{-1}\) for uranine (a signal which saturated the fluorimeters). Due to an initially inadequate measurement interval scheme, no helium was detected in this first plume that corresponds to the passage of water from the tracer tank. During this peak, which lasted approximately 1 h, the tracer concentration reached 17% of the lithium injection concentration and 12% of the uranine injection concentration, i.e., a dilution factor of 5.96 and 8.42, respectively. 2. A second more intense and longer plume lasted about 3.5 h, synchronous with the injection of the hydrogenated water (see circled 2 in Figure 10b). It peaked at 15:00,  
Figure 8: Piezometric monitoring of the experimental site: (**a**) piezometry from the start of monitoring; and (**b**) piezometry before and after the injections. i.e., approximately 10 min after the start of injection, at a concentration of 3913 ug\(\cdot\)L\({}^{-1}\) for lithium, 2667 ug\(\cdot\)L\({}^{-1}\) for uranine (again inducing saturation of the fluorimeters), and 160 ug\(\cdot\)L\({}^{-1}\) for helium. The tracer dilution factors reached 2.56, 3.75, and 9.28, respectively. This plume is interpreted as the result of leaching of the tracers contained in the porous matrix of the aquifer rock induced by the arrival of water at a slight overpressure from the second tank. 3. In the third, weaker and more spread-out plume (see 3 in Figure 10b; note the logarithmic axes), uranine was detected by the fluorimeter. It peaked at 189 ug\(\cdot\)L\({}^{-1}\) on 20 November 2019 at 02:45 (t = 0.531 days) and decreased very slowly, finally lasting about a month; 0.24 ug\(\cdot\)L\({}^{-1}\) of uranine remained at t = 26.8 days, lithium and helium having disappeared by t = 12.9 days. When the concentration peak passed through, the dilution factor reached 52.9 for uranine, the only tracer measured using the recording fluorimeter (because the peak occurred in the middle of the night), which must have corresponded to a dilution factor of about 36 for lithium and about 130 for helium. As regards the other downstream piezometers, a peak of tracers is clearly noticeable at PZ3 (10 m downstream) and PZ4 (20 m downstream) at t = 2 days with respective concentrations of 9.75 and 9.08 ug\(\cdot\)L\({}^{-1}\) for uranine (Figure 11a).