## 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 \(\eta\) of UPSH plants and their associated environmental impacts. Despite the relevant \(\eta\) 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 \(K\) of the surrounding medium drove the groundwater exchanges. Consequently, \(K\) played an important role concerning the \(\eta\) of UPSH and its associated environmental impacts. Thus, \(K\) should be considered in the selection process of abandoned mines when constructing future UPSH plants. * The influence exerted by \(K\) depended on the elevation of the piezometric head with respect to the mine. If the piezometric head was located at a high elevation, high values of \(K\) were harmful for the \(\eta\) and the environmental impacts. When the natural piezometric head was located at the half elevation of the mine, the \(K\) did not affect the \(\eta\) of UPSH nor the environmental impacts over surface water bodies; however, the impact could become important in terms of the groundwater quality, increasing with high values of \(K\). * The elevation of the piezometric head was relevant and must be considered when designing an UPSH plant. The results showed that, for the same values of \(K\), the \(\eta\) was higher, and the environmental impacts over surface water bodies were lower if the piezometric head was located at a low elevation. However, the potential impacts on the groundwater increased, since the outflows from the underground reservoir to the surrounding geological medium increased with low piezometric heads. Consequently, an agreement between the \(\eta\), the environmental impacts into surface water bodies, and those generated in nearby aquifer systems will be needed in order to choose potential sites to implement UPSH. 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 \(\eta\) 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. Conceptualization, E.P. ; methodology, E.P. ; software, P.O. ; investigation, E.P., A.P., P.O., P.G.