*1.1. General Information on Underground Hydrogen Storage*

In 2015, France promulgated the Law on Energy Transition for Green Growth in order to contribute more effectively to the fight against climate change and the preservation of the environment, as well as to strengthen its energy independence [1]. This law aims in particular to develop renewable energy sources, some of which are of a fluctuating or intermittent nature and thereby require significant storage capacity. The underground environment is well suited to large-scale storage, and France already has 78 salt caverns with a capacity to store liquid or liquefied hydrocarbons as well as natural gas [2]. The gradual phasing out of fossil fuels gives rise to the hypothesis that future underground storages of hydrogen (H2) will need to be developed, as already in the United Kingdom or the United States [3]. This technology is likely to be of interest to several European countries: Germany, the Netherlands, Denmark, Poland, France, etc. [4].

This is why the French Scientific Interest Group GEODENERGIES chose to fund a research project in 2017 on the risks and opportunities of the geological storage of hydrogen

in salt caverns, entitled ROSTOCK-H. Among other things, this project focuses on the risks associated with possible hydrogen gas leaks from deep storage. In the event of such leakage, the hydrogen would migrate to the surface and encounter at least one aquifer where the gas would dissolve in the water until saturation [5]. However, considering the very low solubility of this gas (in the order of 1.8 mg·L <sup>−</sup><sup>1</sup> at saturation under surface conditions), there is a risk that the leak flow rate may exceed the dissolution potential for hydrogen in water. In this case, part of the gas would continue its migration to the surface in gaseous form until it encounters an impermeable formation or a void where it can accumulate (mine, underground networks, cellar, underground car park, tunnel, etc.): hydrogen may then cause asphyxiation or, due to its very weak lower flammability limit (around 4%), explosion or fire [2].

To study the risks associated with this new storage technology, particularly in the event of a leak towards the surface, we carried out an experiment to simulate a hydrogen leakage from a possible deep geological storage. To do this, we injected hydrogen into the chalk aquifer, which is a major drinking water resource in the Paris region. Several monitoring devices were set up directly in the aquifer up to 60 m downstream from the injection point, in order to monitor the evolution of the dissolved hydrogen plume and the associated physico-chemical and hydrochemical phenomena. Under the proposed experimental conditions, and due to the brevity of the injection, we did not expect any biochemical reactions that require long time scales to occur. Moreover, additional studies [6–9] pointed out that a sudden injection of hydrogen saturated water into an aquifer increases the dissolved hydrogen content of the water, and consequently reduces its oxidation-reduction potential, as well as the concentration of other dissolved gases (O2, CO2, N2, etc.). The injection of hydrogen was preceded by the injection of tracers (helium and hydrogeological tracers) in order to facilitate the monitoring of its progress underground. This experiment was conducted at the Catenoy (Oise) experimental site in a surface aquifer representative of the carbonated hydrogeological context of the Paris Basin. It followed characterization work for this site, previously carried out during CO<sup>2</sup> leak simulations [10,11], as well as the realization of a baseline and a preliminary injection of helium [6].
