| Summary: | This paper presents the results of an experiment to simulate a sudden and brief hydrogen leak from a potential deep geological storage site. A 5 m<sup>3</sup> volume of groundwater was extracted, saturated with hydrogen, and then reinjected into the aquifer. Saturating the water with hydrogen caused a decrease in the oxidation-reduction potential, the dissolved gas content (especially O<sub>2</sub> and CO<sub>2</sub>), the electrical conductivity, and the concentration of alkaline earth bicarbonate ions and a slight increase in pH. These changes are observed until 20 m downstream of the injection well, while the more distant piezometers (from 30 to 60 m) are not significantly affected. During this experiment, no indicators of the development of chemical or biochemical reactions are observed, because of the rapid transfer of the dissolved hydrogen plume through the aquifer and its significant dilution beyond 10 m downstream of the injection well. Here, hydrogen behaved as a conservative element, reacting very slightly or not at all. However, this experiment demonstrates the existence of direct and indirect impacts of the presence of hydrogen in an aquifer. This experiment also highlights the need to adapt the monitoring of future underground hydrogen storage sites.
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