**5. Conclusions**

A test of the combined injection of tracers (organic and ionic) and helium-saturated water was done in April 2019 to assess and optimize the concept of injecting water saturated with hydrogen, planned for later, and monitoring its physicochemical properties.

The test has confirmed the technical feasibility, under field conditions, of saturating a significant quantity of water with a low-solubility gas and injecting it in a controlled manner into a shallow aquifer.

It was possible to properly monitor the propagation of the dissolved gas plume in the aquifer with the means of analysis used. Helium could be detected up to 20 m downstream of the injection well by mass spectrometer analysis of the gas mixture obtained through partial degassing of water samples by mechanical agitation.

Among the five tracers used, uranine and lithium were shown to be the most effective. The first is a colored fluorescent organic tracer, easily and continuously detectable in situ but affected in this specific hydrogeological context (fine matrix and fissure porosity) by a certain retardation factor with respect to the propagation of the water. The second is a colorless ionic tracer, not affected by such a retardation factor, but not detectable in situ. To obtain a cleaner signal, the tracer mass used will be ten times higher when injecting the water saturated with hydrogen.

The temporal modeling of the post-injection evolution of these two tracers reveals, as a function of the distance from the injection well, two distinct hydrodynamic regimes linked to the existence of a multiple porosity, of both matrix and fissure type. These elements will be essential in understanding the transport of the hydrogen plume during the next injection simulation.

The preparation and the conditions of injection of the tracer tank and hydrogen-saturated water tank have been modified to take the results obtained into account: doubling of the number of bubbling outlets in the 5 m<sup>3</sup> tank bubbling device, establishment of a latency period between the two injections, reduction of the flow rate of the second tank.

Finally, the protocol of monitoring has also been modified: the establishment of specific monitoring of the PZ2BIS piezometer with continuous in situ recordings of a maximum of data, adaptation of the sampling schedule to the specificity of each piezometer and increase in the overall monitoring time.

Thus, the adoption of all of these improvements will permit proper execution of the main experiment of injecting hydrogen-saturated water and carrying out the associated monitoring, which will also be preferentially done during periods of high water.

**Author Contributions:** Conceptualization, methodology, and validation, P.G., E.L., S.L., and Z.P.; analysis, S.L., P.G., Z.P., E.L., P.d.D., and N.J.; writing—original draft preparation, S.L., P.G., and Z.P.; writing—review and editing, S.L., P.G., Z.P., E.L., P.d.D., and N.J.; supervision, P.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the French Scientific Interest Group GEODENERGIES in the framework of the ROSTOCK-H project (Risks and Opportunities of the Geological Storage of Hydrogen in Salt Caverns in France and Europe).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
