## 4 Results  
### Hydraulic Head Fluctuations in the Quarry and Water Exchanges with the Aquifer  
Figure 6 shows the simulated hydraulic head variations in the quarry and water exchanges between the quarry and the aquifer as a function of time and pumping-discharge operations. Positive and negative values for the exchange flowrate relate to groundwater that flowed towards the quarry and water that flowed from the quarry to the chalk aquifer, respectively. These exchange flow rates were globally inversely correlated to the hydraulic head in the quarry. When the hydraulic head in the quarry increased as a result of a water discharge from the upper reservoir, the exchange rates decreased to negative values, reflecting that water flows from the quarry to the aquifer. The opposite behavior was observed during the pumping phases when the hydraulic head decreased in the quarry. During no-activity phases, exchange rates were positive when the hydraulic head in the quarry was located at a lower elevation than the surrounding piezometric level in the aquifer, and vice-versa. During no-activity phases, the hydraulic head in the quarry continued to vary slightly since water continued to be exchanged between the quarry and the chalk aquifer until an equilibrium was reached. For example, at the end of a pumping period, the hydraulic head in the quarry was lower than the surrounding piezometric level in the aquifer. Consequently, the chalk aquifer was drained by the quarry. The exchange rates were positive but decreased slowly, and the hydraulic head in the quarry increased slowly. In the long term, the average elevation around which the hydraulic head fluctuates tended to be equal to the elevation of the hydraulic head in natural conditions. This fact was better observed in the regular scenarios than in the random one because of its more chaotic behavior (Figure 3). This phenomenon of gradual increase in the hydraulic head occurred because the simulated scenarios started with a pumping phase. During this initial phase, the hydraulic head in the quarry fell below the elevation of the hydraulic head in natural conditions. As a result, groundwater inflows from the chalk aquifer to the quarry  
Figure 6: Hydraulic head variations in the quarry (red line) and flowrate exchange between the quarry and the chalk aquifer (blue line). Positive and negative values for the exchange flowrate relate to groundwater that flowed towards the quarry and water that flowed from the quarry to the chalk aquifer, respectively. contributed to increase the hydraulic head. This behavior differed to that expected in the case of an isolated lower reservoir where the hydraulic head remained constant and was not influenced by the groundwater exchanges. During discharge phases, when the volume of water previously pumped is discharged into the quarry, the hydraulic head in the quarry increased above its natural elevation, and thus, water was forced to flow from the quarry to the chalk aquifer. However, total water inflows from the aquifer to the quarry were generally greater than losses, as long as the average hydraulic head was below the natural level. Water inflows and outflows were not equilibrated until the hydraulic head oscillated around its natural elevation. This behavior must be considered, especially if there are constraints concerning the maximum hydraulic head to be reached in the quarry such as stability problems related with the elevation of the hydraulic head. Moreover, the gradual increase in the average hydraulic head was also critical because it may affect the balance between energy stored and produced by the system since the efficiency of the pumps and turbines depends on the difference in the hydraulic head between the quarry and the upper reservoir [8]. Table 3 summarizes the results from the simulations regarding the quarry-aquifer exchanges and the hydraulic head fluctuations in the quarry. The amplitude of the hydraulic head fluctuations was about 3 m, despite the large upper reservoir considered.