## 3 Results and Discussion
### Non-Dischargeable Volume of Water (Difference between Pumped and Discharge Water)
#### 3.1.2 MIDDLE Scenarios (Influence of k)  
Figure 6 shows the evolution of the non-dischargeable volume of water (a) and the accumulated volume of water exchanges in both directions (i.e., the inflows and outflows) (b) for the MIDDLE scenarios. In this case, the influence of \(K\) was lower than in TOP scenarios and the non-dischargeable volume of water did not evolve proportionally with \(k\). This behavior was related to the elevation of the hydraulic head with respect to the piezometric head in the surrounding medium. Given that the piezometric head was located at a half depth, the hydraulic head in the mine was sometimes higher and sometimes lower, depending on the operation schedule of the plant.  
Therefore, in contrast to the TOP scenarios, similar volumes of water were exchanged in both directions (toward the underground medium and toward the underground reservoir) (Figure 6b). Thus, the inflows and outflows were more equilibrated than in the TOP scenarios, and less water was accumulated at the surface reservoir. In scenario MID-3, non-dischargeable water was not accumulated because the surrounding medium was so permeable that pumping and discharge did not modify the hydraulic head greatly because pumped water is quickly replaced by water from the surrounding medium or discharged water is transferred quickly to it. In addition, when the hydraulic head is modified by consecutive pumping or discharge periods, it returns quickly to the elevation of the piezometric head after the cessation of these periods.  
Figure 5: Volume of water that cannot be discharged into the underground reservoir (**a**) and accumulated difference of water exchanges between the mine and the surrounding groundwater system (**b**) for the TOP scenarios.  
As a result, the hydraulic head never reaches the top of the underground reservoir and water can be always discharged. This behavior is also reflected in Figure 6b where it is possible to observe the high volume of water exchanged in both directions. In scenario MID-4, the volume of non-dischargeable water increased slightly since the water exchanges were constrained by the \(K\) of the surrounding groundwater system, and the top of the chambers was reached occasionally as a consequence of consecutive discharge periods. This is the same reason for which a volume of non-dischargeable water is accumulated in scenario MID-5; however, in this case, the final volume was larger than in scenario MID-4 because the \(K\) was lower, and thus, the water exchanges were more constrained.  
However, the trend changed for scenario MID-6 since the non-discharged volume of water decreased with respect to the scenario MID-5. Contrary to the observed trend in scenarios MID-3, MID-4, and MID-5, the non-discharged volume of water decreased when \(K\) was reduced more than \(10^{-5}\) m/s (i.e., scenario MID-6). In scenario MID-6, the water exchanges were very low (Figure 6b) due to the value of \(k\), and the system response was more similar to that of an isolated underground reservoir. Given that the operation scenarios were designed considering an isolated underground reservoir, the head evolved as expected and the top of the underground reservoir was not exceeded during most of the simulated time. If the \(K\) was reduced lower than \(10^{-6}\) m/s, the non-discharged volume of water would decrease even more.  
Concerning the environmental impacts into surface water bodies, the largest impacts were expected for scenario MID-5; however, they were much smaller than those in the TOP scenarios. Nevertheless, the largest environmental impacts in the underground medium were expected for scenario MID-3. Despite the fact that all the pumped water can be discharged into the underground reservoir, the results showed that water exchanges were higher than in the other scenarios, which indicates that the interaction between the UPSH plant and the surrounding materials, and therefore the impact on the groundwater, was high.