*3.3. Previous Works and Future Investigations*

Previous studies have investigated the role of the hydrogeological parameters on the potential impacts induced by UPSH in the underground medium [16,30]. The present investigation goes further and shows how the *K* controls the potential environmental impacts on surface water bodies and on the global *η* of the plant. While the results are useful to understand the system behavior and to define screening methodologies, further investigation should be done regarding not only the influence of other hydrogeological parameters, such as the effective porosity, but also considering heterogeneities in the underground medium. In addition, the importance of the shape and volume of the available underground cavity should be assessed.

The results showed that, under certain circumstances, the discharge of large volumes of the pumped groundwater into surface water bodies would be needed. In addition, the effects on the piezometric head of pumping and discharge may alter the interactions between the groundwater and nearby surface water bodies. Consequently, it would be advisable to investigate how UPSH may influence the e-flows of nearby rivers [51] by considering UPSH plants during the environmental flow assessment processes.

This paper also investigated, for the first time, the relevance of the elevation of the piezometric head in terms of *η* and the environmental impacts. The results showed that the relative elevation of the piezometric head was crucial for the operation of a UPSH plant, and for this reason, it should certainly deserve more investigation in the future.

Finally, we assessed the influence of the water exchanges on the global *η* of UPSH plants. To date, only one study had investigated the relation between groundwater exchanges and *η* [32]. However, this previous investigation only considered the influence of groundwater exchanges on the *η* of pumps and turbines. While the findings of this previous study were relevant, the reported variations of *η* were much lower than those obtained in this paper.

The current investigation went further and considered the variations in the global *η* of UPSH by comparing the energy consumed for pumping water from the underground reservoir and that obtained by discharging water from the upper reservoir. Considering that the *η* of the pumps and turbines depends on the head difference between reservoirs, it is advisable to investigate the effect on their *η* of different positions for the piezometric head as in the present paper.
