3.1.2. Boundary Conditions

The northern limit of the model corresponds to the interface between the chalk aquifer and low-permeability deposits in contact with the base of the chalk aquifer (Figure 3). This limit is represented by a zero flow Neumann boundary condition (BC). The southern, western, and eastern boundaries were implemented in the model using Dirichlet BCs whose values correspond to the mean values of the piezometric head of the chalk aquifer at these locations [14] (Figure 3). Those boundaries were chosen sufficiently far around the quarry not to interfere with the influence of the PSH operations. The distance and location of these boundaries were determined from the results of a generic study about the impact of PSH using quarries as lower reservoirs [7]. The base of the model corresponds to the lower limit of the chalk aquifer. The "Haine River" is a draining stream along the considered section. It is represented by a potential-dependent flow BC (i.e., Fourrier BC), which only allows groundwater flowing out the model. The value of the conductance

factor, which governs the Fourrier BC, was high enough to keep the piezometric head in equilibrium with the river. Two major groundwater pumping stations in the chalk aquifer were included in the modelled area. They were implemented in the chalk layer of the model using Neuman BCs, with prescribed flow rates equal to 5,234,976 and 883,008 m3/year. A recharge of 300 mm/year was applied to the top of the model.
