Hydrogeochemical impacts of pumped hydro power storage in European open-pit lignite mines.

The need for large-scale energy storage technologies intensifies due to the increasing demand of renewable energy sources. Since the availability of wind and photovoltaic energy is temporarily limited and depending on a highly dynamic weather system, excess energy has to be stored to be available in times of high energy demand. Implementation of pumped hydro power storage (PHS) plants in abandoned open-pit lignite mines is expected to contribute to this issue (Thema & Thema 2019). PHS follows the concept of pumping and releasing water between two reservoirs at a certain hydraulic head difference.

The success of energy storage by PHS in abandoned mines highly depends on the geo- and hydrochemical processes in the reservoirs and the surrounding porous media (Pujades et al. 2018). Possible high acidity, triggered by the generation of Acid Mine Drainage (AMD; Akcil & Koldas 2006), especially by pyrite oxidation, could impact groundwater chemistry and slope stability at the respective sites, and further cause corrosion of critical technical infrastructure (Pujades et al. 2018).

In the scope of the present study, we have investigated the chemical reaction paths by numerical modelling to gain the knowledge required for the conceptualisation of comprehensive reactive transport simulations to undertake environmental risk assessments. Therefore, we considered calcite buffering, mineral dissolution-precipitation balances, heavy metal contamination as well as mixing processes between lake and groundwater. In summary, geochemical impacts potentially occurring with PHS operation under hydrochemical boundary conditions representative for European open-pit lignite mines were investigated and quantified.