Hydrochemical changes induced by underground pumped storage hydropower and their associated impacts

Underground pumped storage hydropower (UPSH) using abandoned mines is an alternative system to manage electricity production in flat regions. Water from an underground reservoir is pumped to a surface reservoir to store electricity in the form of potential energy. Later, water is discharged through turbines into the underground reservoir to produce electricity when demand increases. During this operation, the water hydrochemistry continuously evolves. It varies in order to reach chemical equilibrium with the atmosphere (in the surface reservoir) and with the surrounding porous medium and groundwater (in the underground reservoir). The hydrochemical variations may lead to reactions in the reservoirs and in the surrounding porous medium, causing potentially negative consequences for the environment and the system efficiency, especially when pyrite is present in the surrounding porous medium. In this case, pyrite oxidation leads to a decrease in pH and the precipitation of goethite or schwertmannite in the surface reservoir. The decrease in pH is mitigated when calcite is present in the porous medium. However, other concerns may arise, such as slight increases in pH, the precipitation of ferrihydrite and calcite in the surface reservoir, and the oxidation of pyrite and dissolution of calcite in the surrounding porous medium. Understanding the pH variations and the precipitation/dissolution of minerals is of paramount importance in terms of the environmental impacts and system efficiency. For this reason, this work investigates the main hydrochemical changes and their associated consequences when abandoned deep mines are used for UPSH by means of numerical modelling. The main objective is to highlight the importance of considering hydrochemical aspects when designing future UPSH plants.