**5. Conclusions**

nearby drinking water pumping wells.

intensive enough.

**5. Conclusions** 

The numerical models developed in this study were realistic and representative since they were based on a real quarry that was considered for PSH. The developed numerical models aimed to assess the interactions between the Obourg quarry and the surrounding The numerical models developed in this study were realistic and representative since they were based on a real quarry that was considered for PSH. The developed numerical models aimed to assess the interactions between the Obourg quarry and the surrounding aquifer systems. From a feasibility point of view, an important problem related to the case of the Obourg quarry is the possible interference of the PSH activities with nearby drinking water catchments.

aquifer systems. From a feasibility point of view, an important problem related to the case of the Obourg quarry is the possible interference of the PSH activities with nearby drinking water catchments. Results showed that the fluctuation zone of the piezometric head did not reach these catchments after 14 days of pumping–discharge cycles. Concerning the impacts on the water quality, PSH activities tended to soften the water. The precipitation of minerals containing major chemical elements (MnO2, FeO(OH), CaCO3) in the upper reservoir reduced its hardness, causing a slight increase in pH. These hydrochemical changes were strongly Results showed that the fluctuation zone of the piezometric head did not reach these catchments after 14 days of pumping–discharge cycles. Concerning the impacts on the water quality, PSH activities tended to soften the water. The precipitation of minerals containing major chemical elements (MnO2, FeO(OH), CaCO3) in the upper reservoir reduced its hardness, causing a slight increase in pH. These hydrochemical changes were strongly attenuated in the aquifer and were only visible in the first few meters around the quarry. Concerning the specific case of the Obourg quarry and its local context, simulated results showed that the hydrochemical changes in the groundwater around the quarry should not influence the hydrochemical characteristics of the groundwater abstracted in the nearby drinking water pumping wells.

attenuated in the aquifer and were only visible in the first few meters around the quarry. Concerning the specific case of the Obourg quarry and its local context, simulated results showed that the hydrochemical changes in the groundwater around the quarry should PSH activities thus imply an environmental impact around the quarry through the propagation of the piezometric head fluctuations within the chalk aquifer and the modification of the hydrochemical characteristics of the groundwater, which can reduce its quality. Overall, the system behavior and its consequences are strongly dependent on the hydraulic

propagation of the piezometric head fluctuations within the chalk aquifer and the modification of the hydrochemical characteristics of the groundwater, which can reduce its quality. Overall, the system behavior and its consequences are strongly dependent on the hydraulic properties of the surrounding medium, and therefore, on the geological and hydrogeological contexts in which a quarry is located. The concentrations of chemical elements and the related equilibria were here considered with relatively constraining assumptions to evaluate the evolution of the hydrochemical conditions in the most problematic case. Achieved results thus correspond to some strong hypotheses. The impact assessment on hydrochemistry was thus performed by considering a certain degree of safety in relation, which is relevant considering the presence of drinking water catchments

not influence the hydrochemical characteristics of the groundwater abstracted in the

PSH activities thus imply an environmental impact around the quarry through the

properties of the surrounding medium, and therefore, on the geological and hydrogeological contexts in which a quarry is located. The concentrations of chemical elements and the related equilibria were here considered with relatively constraining assumptions to evaluate the evolution of the hydrochemical conditions in the most problematic case. Achieved results thus correspond to some strong hypotheses. The impact assessment on hydrochemistry was thus performed by considering a certain degree of safety in relation, which is relevant considering the presence of drinking water catchments located near the quarry. The quarry, however, is located in the capture zone of the abstraction stations. In this context, it is important to make sure that the PSH equipment does not emit contaminants, especially regarding any lubricants that may be used.

An equivalent porous media approach was here considered to develop the models. This hypothesis did not consider possible heterogeneities within hydrogeological units. At the scale of the modelled area, this heterogeneity was limited, but it could have an impact on groundwater flow, hydrochemical impact distribution, and related influence zones. For sites with strong heterogeneity, as for example in the case of the karstified limestones, the hypothesis of a homogeneous medium becomes unrealistic. In those cases, the consideration of more complex models would be needed. In the framework of the Obourg quarry model, the regional gradient was neglected. The results of Poulain et al. (2018) nevertheless show that the presence of a regional gradient only has a very slight influence on piezometric head fluctuations and the distance of influence around the quarry. The area around the quarry, where hydrochemical conditions are influenced, is probably not strongly influenced by a realistic hydraulic gradient, although this should be formally evaluated. Finally, the presence of bacteria in the reservoir could also promote chemical reactions during the precipitation of minerals [34]. It would be interesting to assess the impact of organic matter and the bacterial ecosystem present in the groundwater on the hydrogeochemical behavior of the groundwater during the pumping–discharge cycles.

In conclusion, this study highlighted the importance of considering both the hydrodynamic and the hydrochemical aspects of PHS plants. The following points must be considered:


This kind of numerical modelling can be developed for any PSH project, considering in each situation the characteristics of the quarry, the geology, hydrogeology and lithology, and the existing mineral species. For a PSH project using a quarry as a lower reservoir, a

piezometric network system around the quarry would allow the control of the chemical evolution of the groundwater and would ensure a rapid reaction in case of contamination or unexpected hydrochemical changes.

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

**Funding:** This research was supported by the Public Service of Wallonia, Department of Energy and Sustainable Building in the framework of the SMARTWATER project. IDAEA-CSIC is a Centre of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S). 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.
