## 4 Discussion  
Following this test, we can see that the experimental protocol for saturating water with gas and then injecting it into the groundwater is operational, as is the way of monitoring the saturated zone. However, the results obtained lead us to propose a certain number of improvements for the hydrogen injection experiment. Concerning the gas saturation of the water in the 5 m\({}^{3}\) tank, it will not be possible--for safety reasons--to allow hydrogen to bubble all night to obtain maximal saturation at the time of injection, as was done with the helium. Hydrogen is an easily flammable gas requiring the establishment of an ATEX zone and suitable control measures. These measures are difficult to ensure overnight. As a result, the hydrogen bubbling will have to be interrupted in the evening to resume the next morning. This can  
Figure 12: Dissolved helium concentrations in the samples taken at the downstream piezometers PZ2 BIS, PZ3, and PZ4. lead to a delay of several hours in reaching an optimum level of hydrogen saturation in the water in the tank and, consequently, the same delay in all the subsequent operations (injection, monitoring measurements, etc.). To increase saturation kinetics, the number of gas outlets at the bottom of the 5 m\({}^{3}\) tank will be doubled, from 200 to 400. The first 1 m\({}^{3}\) tank will hold the fluorescent and ionic tracers that have been shown to provide the best performances: uranine and lithium. Considering the weakness of the signal obtained during this test, owing to a strong dilution of the tracers in the groundwater, they will be used at a concentration higher than an order of magnitude: 10 g\(\cdot\)L\({}^{-1}\) instead of 1 g\(\cdot\)L\({}^{-1}\). In addition, the water in this tank will also be saturated with helium, to be used as an inert tracer gas to compare its behavior with that of hydrogen, a potentially reactive gas in this aquifer context. The second 5 m\({}^{3}\) tank will be saturated with hydrogen by means of bubbling in a gaseous state during the first day of preparing the materials, as well as during the following morning. The injection of the hydrogen-saturated water will therefore take place at the start of the afternoon. In the test conducted with helium, the two tanks were drained successively, but owing to their respective geometries, the injection rate of the second tank was found to be significantly higher (3.2 m\({}^{3}\cdot\)h\({}^{-1}\)) than that of the first (1.7 m\({}^{3}\cdot\)h\({}^{-1}\)). Following this, the two plumes probably coalesced, which hampered the interpretation of the tracer breakthrough curves, and probably diluted the plume of dissolved gas. To avoid this, we will apply a latency time of \(\frac{1}{2}\) h between the two injections, and the emptying rate of the second tank containing dissolved hydrogen will be retained at less than or equal to that of the first tank containing the tracers. The PZ2 BIS piezometer placed directly downstream of the point of injection provided the best recovery curves and will thus be considered to be the principal monitoring piezometer. As such, given the speed of the response obtained (1.15 h), it must be equipped with a specific monitoring device to provide continuous data acquisition: dissolved hydrogen measurement probe, physicochemical measurement probe, and borehole fluorimeter. This equipment must be available in duplicate to be able to monitor the other piezometers manually. As soon as the tracer signal has disappeared from the PZ2 BIS, the continuously recording borehole fluorimeter will be moved to the piezometers located further downstream (PZ3, PZ4, and PZ5). The piezometers must not all be sampled at the same frequency, but at specific time intervals in function to their distance from the injection well, and the current hydrogeological conditions, by taking particular account of the propagation speed of the fluorescent tracer.