Potential impacts of new underground land use practices on groundwater quality in the context of the energy transition
Résumé
The evolution of our societies towards an energy transition, where the use of hydrocarbons will be reduced to a minimum, implies the development of renewable energy sources and probably the geological storage of excess CO2. One of the features of these energy sources is their intermittent nature (wind, solar), for which it will be necessary to develop storage solutions. The properties of the underground environment make it a preferred storage location: low land use, increased security, high storage capacity (under pressure). In the near future, the underground may therefore be required to play a major role in the storage of hydrogen, compressed air (CAES), water (Underground Pumped Hydro-Energy Storage), heat or CO2. The works presented here deal with disturbances likely to be induced by these new modes of underground use both in the deep aquifers, which may be the place of storage, and in the subsurface aquifers in case of leakage of deeper storage in saline, crystalline or clay formations. The geological storage of CO2 should have the strongest impact because it will be done preferentially in deep aquifers in which the CO2 dissolution will induce a water acidification which could modify the water-rock interactions. Hydrogen can also be stored in deep aquifer or preferentially in saline cavity. In the event of a leak towards the surface, this gas could disturb the overlying aquifers hydrogeochemistry by reducing some oxidized mineral species (eg nitrates or sulphates). This is studied in the current ROSTOCK-H project, which concerns the risks associated with hydrogen storage in saline cavity, including the leaks impacts. Compressed air will also be stored in saline cavity and could, in the event of a leak, oxidize certain reduced mineral species (iron, manganese) present in deep anoxic aquifers. Finally, heat storage may also enhance the dissolution or precipitation of certain minerals and induce corrosion or clogging of storage facilities but also the most draining areas of aquifers, thus affecting their permeability. These new ways of using the subsoil, therefore, need to come with their development by studying in advance the potential risks, and by proposing suitable prevention and mitigation methods that must be directly integrated into the operating modes of these storage facilities.