Reaction Path Modeling of Water Pollution Implications of Pumped Hydropower Storage in Closed Open-pit Lignite Mines

Pumped hydropower storage (PHS) operations in former open-pit lignite mines can provide substantial additional energy storage capacities, which are urgently needed for the current European energy transition. However, this storage concept requires adequate water quality to mitigate both environmental and technical impairments. Pyrite oxidation, acid buffering, and dilution are crucial processes for predicting water pollution by sulfate, iron, and acidity in the system. A novel reaction path modeling framework based on numerical geochemical batch reactors was developed using PHREEQC to assess the hydrogeochemical processes in the storage reservoirs and their adjacent groundwater aquifers over a hypothetical PHS lifetime. The results of 36,380 simulation scenarios reveal that while PHS operations generally have limited influence on pH, sulfate, and iron concentrations compared to natural processes in commonly flooded pit lakes, certain conditions, such as insufficient buffering capacity or pyrite oxidation by atmospheric oxygen, can cause variability. The pH values stabilize within a range of 1.9 - 7.9 over 20 years of PHS operation time, with carbonate buffering and dilution playing a critical role in mitigating acidification. Elevated acidity and sulfate concentrations, driven by pyrite oxidation and incomplete precipitation, pose potential risks to PHS infrastructure as well as adjacent aquifers and require careful management. These findings highlight the importance of early site-specific geochemical assessments and proactive measures to manage pyrite oxidation and maintain buffering capacity, ensuring the long-term integrity of PHS systems in flooded open-pit lignite mines.