## 1 Introduction  
Renewable energies, such as solar or wind, may not be sufficiently efficient since they are intermittent and random, and consequently, their production of electricity is not adapted to the demand [1, 2, 3, 4]. For this reason, they must be combined with energy storage systems (ESSs) [5] that allow for balancing the production and the demand [6]. ESSs are useful to store the surplus of electricity during periods of low demand and to generate electricity when the demand increases. Pumped storage hydropower (PSH) is the most worldwide used EES [7] because it allows for the storage and production of large amounts of electricity [8]. For example, about 95% of the utility-scale energy storage in the United States is FSH [9], and up to 99% in the European Union [10]. PSH plants consist of two reservoirs placed at different elevations (upper and lower reservoirs). The excess of electricity during low demand periods is stored in the form of potential energy by pumping water from the lower to the upper reservoir. Later, during high demand periods, electricity is produced by discharging the water through turbines from the upper to the lower reservoir [11]. Despite its extensive use, FSH has limitations [12; 13; 14], the most important being that a specific topography is required as both reservoirs must be located at different elevations [15]. Consequently, FSH plants can only be installed in relatively steep areas [16]. Underground pumped storage hydropower (UPSH) [17] is an opportunity to increase the capacity of managing the electrical production in areas where a conventional PSH is not possible. In addition, UPSH avoids some of the adverse environmental impacts related to PSH, and to hydropower in general, such as modifying the flow discharge in a river or changing the seasonal flow regime [18; 19]. UPSH uses an underground cavity as the lower reservoir (underground reservoir) and constructs the upper reservoir at the surface [20] or, alternatively, at a shallow depth. While the underground reservoir can be specifically excavated [21], the most inexpensive (i.e., efficient) option can be to take advantage of abandoned underground mines [22; 23]. In addition, there are numerous mines that could be potentially used for UPSH. For example, in France, there are up to 4710 active mines and 101,616 abandoned mines [24], and, in Belgium, there are 964 active mines and more than 5000 abandoned mines [25]. Clearly, not all of these mines are suitable for constructing an UPSH plant; however, the objectives of electricity production and storage could be reached by using only a small portion of them. For example, in France, these objectives could be reached by using 0.1% of the total available mines [26], and, in Belgium, it would be possible to obtain up to e 4896 MWh considering only mines with suitable characteristics for UPSH [27]. However, since mines are generally not waterproofed, it is expected that water exchanges will occur between the underground reservoir of UPSH plants and the surrounding groundwater systems [28]. This fact may entail negative consequences in terms of the environmental impacts [29; 30; 31] and for the efficiency (\(\eta\)) of UPSH [32]. We refer to \(\eta\) as the ratio between the energy used for pumping water from the underground reservoir and the energy generated when water is discharged from the upper reservoir under ideal conditions. Thus, energy losses due to conversion issues are not considered. Recently, researchers observed that water exchanges may progressively fill the underground reservoir, reducing \(\eta\). Occasionally, a volume of pumped water could actually not be fully discharged into the underground reservoir because the latter has been partially filled by underground water exchanges [33]. In addition, this volume of water must then be discharged into surface water systems, which could alter their quality because mine water is often not of an appropriate quality.