## 1 Introduction  
The development and use of renewable energies involve the necessity to temporarily store the energy because of their intermittence [1]. In this context, pumped storage hydropower (PSH) appears an efficient way to store and produce large amounts of electricity that can be used in combination with intermittent renewable energies [1, 2]. PSH consists of two large water reservoirs, with a difference in elevation between them. The general principle is to pump water from the lower reservoir to the upper one when the demand of electricity is low. Later, during peak demand, water stored in the upper reservoir is discharged into the lower reservoir through turbines to produce electricity. The flexibility of this system allows the regulation of the supply and demand of electricity at the daily scale. Usually, the reservoirs are artificial, which has the advantage of offering a certain freedom in terms of plant sizing, volumes, and/or location. However, these constructions are also extremely expensive. In addition, in countries with a gentle relief, such as Belgium, potential locations allowing a significant difference in level between the two reservoirs are sometimes difficult to find. Therefore, the possibility of using old flooded mines and quarries for constructing PSH plants [3] and regulating local power grids [4; 5; 6] has been considered. The implementation of these systems would allow a better management of intermittent local renewable energy production by coupling them with wind and/or photovoltaic systems [2]. The flooded mines and quarries are in continuous interaction with the water contained in the surrounding aquifer systems, both quantitatively and qualitatively. Therefore, pumping and discharging large volumes of water into the lower reservoir can (1) modify the hydraulic head in the quarry and the adjacent aquifer systems impacting on the environment [7] and on the system efficiency [8] and (2) alter the hydrogeochemical balances, and thus, the water quality [9]. The induced hydrodynamic and hydrochemical changes must be compatible with the natural functions observed and established on and around the PSH site. Similarly, these changes should not negatively affect other activities undertaken near to the quarry, such as drinking water pumping stations. Thus, preliminary hydrodynamic and hydrochemical studies must be developed to avoid unexpected and undesired impacts. To date, few studies have been carried out about water hydrochemistry issues in former quarries that are now flooded. Most of them are mainly focused on the presence of heavy metals in the extraction zone [10]. The issue of groundwater pollution, in the context of rock or mineral extraction sites, is better studied in the context of underground mines [11] since many studies have been focused on problems related with mine water acidification [12] as well as other more specific problems. However, investigations focused on hydrogeochemical issues in the specific context of PSH using quarries have not been conducted. Only [8] and [9] developed a numerical study focused on hydrochemical issues related with PSH. However, these previous investigations are limited since they were based on deep, underground coal mines. The results show that the presence of pyrite and calcite significantly influences the evolution of the hydrochemical properties of the water contained in the mine during the pumping-discharge operations. These previous works are exclusively numerical and based on hypothetical cases. To date, very few case studies of PSH using underground works have been investigated. One of them is [13], which investigated the possibility of recycling some USA underground iron mines for PSH. In this context, the main objective of this study was to quantify and assess the hydrodynamic and hydrochemical impacts induced by PSH using a quarry as the lower reservoir. This objective was reached by developing two numerical models.