Potential hydrochemical impacts of pumped hydropower storage operation in two European coal regions in transition: the Szczerców-Bełchatów mining complex, Poland, and the Kardia Mine, Greece

Energy storage plays a vital role in stabilising electric grids incorporating renewable energy sources like wind and solar, which are inherently intermittent. Among the most effective and widely used large-scale energy storage solutions are pumped hydropower storage systems (PHS), known for their high efficiency and capacity. Implementing PHS technology in decommissioned open-pit lignite mines involves creating a high-altitude reservoir and a lower reservoir within the open-pit mine. This system can lead to water exchange between the reservoirs and nearby contamination of adjacent groundwater aquifers from pyrite oxidation products like sulphate leached from open-pit sediments. A quantification of hydrogeochemical impacts resulting from PHS operation is essential to comply with national and EU water quality regulations. In this work a novel reaction path modelling framework was utilised to assess these effects of two potential PHS systems in the Kardia mine (Greece) and the Szczerców-Bełchatów mining complex (Poland). The simulations cover the site-specific evolution of the hydrochemistry of the reservoir water and groundwater during operation. The results show that the impact of the PHS on the pH and sulphate concentrations at the Greek site will be low. Sufficient buffer capacities and impermeable sediments in the reservoir-sediment interface layer limit water quality deteriorations. At the Polish site, the reservoir size of 1.75 bn m³ m 3 and continuous dilution from groundwater reduce the impact of pyrite oxidation from PHS operation. Therefore, from a hydrochemical point of view, the operation of PHS at the two sites will have negligible impacts on the water quality and the technical infrastructure of the facilities.