**4. Conclusions**

This is the first paper to assess the role of hydrogeological features (i.e., the *K* and piezometric head) on the groundwater exchanges that occur in the context of UPSH using abandoned mines and, therefore, on the *η* of UPSH plants and their associated environmental impacts. Despite the relevant *η* issues, those related to potential environmental impacts are paramount importance, as UPSH plants must fulfill the current regulations concerning water bodies. For example, in the European context, the Water Framework Directive [34] states that countries must preserve the "good state" of water bodies.

Considering that, under some scenarios, the water quality may be deteriorated under the influence of UPSH (e.g., if abandoned coal mines are used as underground reservoirs [29]) additional water treatments may be required before releasing the water surplus into surface water bodies. Overall, the final goal of this work was to improve the knowledge about the interactions between an UPSH and the groundwater to complement previous studies developed in this field and, in this way, to contribute to establish criteria for (1) the selection of abandoned mines that are most suitable for UPSH and (2) designing future UPSH plants. The main conclusions of this paper are the following.


The pumping–discharge frequencies must be adapted according to the K and the position of the piezometric head in order to increase the efficiency of UPSH plants. If the hydrogeological parameters are not considered, large volumes of water could be not discharged into the underground reservoir under certain circumstances (i.e., large values K and high piezo metric heads). This would decrease the *η* and increase the environmental impacts on surface water bodies.

The model used in this investigation was purposely simplified to obtain general results applicable to other sites with similar features, which was the primary objective of this work. However, the consideration of a specific mine for constructing a UPSH plant requires site specific, detailed, and realistic numerical models. These models must consider all the characteristics of the site, such as the heterogeneity, the seasonal variations or the presence of fractures and faults. In addition, these models should be in 3D and should simulate the system behavior over a large period of years.

**Author Contributions:** Conceptualization, E.P.; methodology, E.P.; software, P.O.; investigation, E.P., A.P., P.O., P.G. and A.D.; resources, P.O., A.D.; data curation, E.P.; writing—original draft preparation, E.P., A.P., P.O., P.G. and A.D.; writing—review and editing, E.P., A.P., P.O., P.G. and A.D.; visualization, E.P.; supervision, A.D.; project administration, A.D, E.P.; funding acquisition, E.P. and A.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** IDAEA-CSIC is a Centre of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S). This research was funded by the Public Service of Wallonia– Department of Energy and Sustainable Building through the Smartwater project. E.P. was also funded by the Barcelona City Council through the Award for Scientific Research into Urban Challenges in the City of Barcelona 2020 (20S08708).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All analyzed data in this study has been included in the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
