*2.1. Study Site*

Several chalk quarries are present in the Walloon region (Belgium), including active and abandoned ones. The chalk open pit, which was studied in this article, is representative of those quarries. It is located close to the Obourg locality, about 4.5 km north-east of Mons city (South-West Belgium). The study site is composed of five quarries located in chalk geological formations. The two most easterly quarries are still in operation, whereas the three located to the west are no longer used. One of those abandoned quarries is studied to be used for PSH (Figure 1). The considered quarry has a surface area of 0.34 km<sup>2</sup> . The upper reservoir, with a volume of 1 million m<sup>3</sup> (100 × 1000 × 10 m), would be built north of the quarry, close to the E19 motorway. The difference in altitude between the upper reservoir and the quarry would be 40 m as shown in Figure 1.

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 3 of 17

upper reservoir and the quarry would be 40 m as shown in Figure 1.

north of the quarry, close to the E19 motorway. The difference in altitude between the

**Figure 1.** View of the Obourg chalk quarries. The red line, intersecting the studied open pit, gives

**Figure 1.** View of the Obourg chalk quarries. The red line, intersecting the studied open pit, gives the location of the altimetric profile. **Figure 1.** View of the Obourg chalk quarries. The red line, intersecting the studied open pit, gives the location of the altimetric profile. the location of the altimetric profile.

### *2.2. Geological and Hydrogeological Context 2.2. Geological and Hydrogeological Context 2.2. Geological and Hydrogeological Context*

upper reservoir and the quarry would be 40 m as shown in Figure 1.

The Mons sedimentary basin has a syncline-shaped (Figure 2) structure and is composed of Mesozoic and Cenozoic deposits. This basin has been affected by subsidence events since the end of the Paleozoic. The Obourg chalk quarries are in the northern part of the Mons Basin where the Cretaceous chalk formations are exploited. These geological formations are included in the "Chalk Group" and represent a major aquifer system called the "Haine Valley Chalk Aquifer" or the "Mons Basin Chalk Aquifer." The thickness of this aquifer is variable and can reach up to 250 to 300 m. The chalk aquifer is bounded at the north, east, and south-west by schisto-sandstone formations from the Upper Carboniferous [14]. In the South, the aquifer is limited by the Lower Devonian deposits, which include several aquifer systems. The chalk aquifer overlies low-permeability marly geological formations [14]. The chalk aquifer is characterized by a relatively high hydraulic conductivity on a macroscopic scale. This high hydraulic conductivity is the result of the fracture network, consisting of diaclases, stratification joints, and faults. The chalk has a high value of total porosity that can be divided in a matrix porosity, which allows the storage of large quantities of water and a fracture porosity, which allows preferential flows [15–20]. The Obourg quarries are located in the vicinity of drinking water abstraction stations, pumping in the same aquifer, and located about 1.5 km in the south-west direction. This groundwater abstraction complex, using pumping wells, is one of the most important in the Walloon region. The Mons sedimentary basin has a syncline-shaped (Figure 2) structure and is composed of Mesozoic and Cenozoic deposits. This basin has been affected by subsidence events since the end of the Paleozoic. The Obourg chalk quarries are in the northern part of the Mons Basin where the Cretaceous chalk formations are exploited. These geological formations are included in the "Chalk Group" and represent a major aquifer system called the "Haine Valley Chalk Aquifer" or the "Mons Basin Chalk Aquifer." The thickness of this aquifer is variable and can reach up to 250 to 300 m. The chalk aquifer is bounded at the north, east, and south-west by schisto-sandstone formations from the Upper Carboniferous [14]. In the South, the aquifer is limited by the Lower Devonian deposits, which include several aquifer systems. The chalk aquifer overlies low-permeability marly geological formations [14]. The chalk aquifer is characterized by a relatively high hydraulic conductivity on a macroscopic scale. This high hydraulic conductivity is the result of the fracture network, consisting of diaclases, stratification joints, and faults. The chalk has a high value of total porosity that can be divided in a matrix porosity, which allows the storage of large quantities of water and a fracture porosity, which allows preferential flows [15–20]. The Obourg quarries are located in the vicinity of drinking water abstraction stations, pumping in the same aquifer, and located about 1.5 km in the south-west direction. This groundwater abstraction complex, using pumping wells, is one of the most important in the Walloon region. The Mons sedimentary basin has a syncline-shaped (Figure 2) structure and is composed of Mesozoic and Cenozoic deposits. This basin has been affected by subsidence events since the end of the Paleozoic. The Obourg chalk quarries are in the northern part of the Mons Basin where the Cretaceous chalk formations are exploited. These geological formations are included in the "Chalk Group" and represent a major aquifer system called the "Haine Valley Chalk Aquifer" or the "Mons Basin Chalk Aquifer." The thickness of this aquifer is variable and can reach up to 250 to 300 m. The chalk aquifer is bounded at the north, east, and south-west by schisto-sandstone formations from the Upper Carboniferous [14]. In the South, the aquifer is limited by the Lower Devonian deposits, which include several aquifer systems. The chalk aquifer overlies low-permeability marly geological formations [14]. The chalk aquifer is characterized by a relatively high hydraulic conductivity on a macroscopic scale. This high hydraulic conductivity is the result of the fracture network, consisting of diaclases, stratification joints, and faults. The chalk has a high value of total porosity that can be divided in a matrix porosity, which allows the storage of large quantities of water and a fracture porosity, which allows preferential flows [15–20]. The Obourg quarries are located in the vicinity of drinking water abstraction stations, pumping in the same aquifer, and located about 1.5 km in the south-west direction. This groundwater abstraction complex, using pumping wells, is one of the most important in the Walloon region.

**Figure 2.** Modified south–north geological section of the Mons Basin and projection of the Obourg **Figure 2.** Modified south–north geological section of the Mons Basin and projection of the Obourg quarry location [21]. **Figure 2.** Modified south–north geological section of the Mons Basin and projection of the Obourg quarry location [21].

### quarry location [21]. *2.3. Hydrochemical Context*

*2.3. Hydrochemical Context 2.3. Hydrochemical Context*  Chalk is generally composed of high percentage values (60–95%) of calcite (CaCO3) [22]. Specifically, the chalk of the Trivières formation is composed of more than 92% CaCO3. These chalk geological formations also contain some slightly ferruginous beds, as well as some phosphate nodules [14]. The hydrochemical composition of the groundwater

is mainly explained by the water–rock interactions and, in particular, by the different alteration processes inducing dissolution/precipitation reactions. In the chalk aquifer, the chemistry of the groundwater is related to the dissolution of CaCO<sup>3</sup> in the presence of dissolved CO2. The dissolution of CaCO<sup>3</sup> is governed by a series of acid–base equilibria as follows:

> *CO*<sup>2</sup> + *H*2*O H*2*CO*<sup>3</sup> (1)

$$H\_2\text{CO}\_3 \leftrightharpoons H^+ + H\text{CO}\_3^-\tag{2}$$

$$\rm{HCO\_3^- \leftrightharpoons } H^+ + \rm{CO\_3^{2-}} \tag{3}$$

$$H\_2O \leftrightharpoons H^+ + OH^- \tag{4}$$

$$\rm CaCO\_3 + 2H^+ \leftrightharpoons Ca^{2+} + H\_2O + CO\_2 \tag{5}$$

In accordance with these equilibrium reactions (1–5), most of the dissolved elements in the chalk aquifers' groundwater are Ca2+ and HCO<sup>−</sup> 3 . The HCO− 3 ion is the predominant form. Other major ions in chalk aquifers' groundwater are Mg 2+, Na<sup>+</sup> , Cl−, SO<sup>4</sup> <sup>2</sup>−, and Fe2+ [23–25].

Table 1 summarizes the average concentration of ions in the groundwater of the chalk aquifer of the Mons Basin. Note that a great disparity in concentrations can be observed depending on the location. Groundwater in the study site has high concentrations of iron and manganese, which under reducing conditions are present in the forms Mn2+ and Fe2+ .

**Table 1.** Average concentration of ions in the chalk aquifer of the Mons Basin [26].


These two ions (Mn2+ and Fe2+) are involved in redox reactions, shown below (Equations (6) and (7)), involving the O<sup>2</sup> dissolved in the water. The aeration of the water during the pumping-turbine cycles in the upper reservoir induces the increase in the equivalent partial pressure of O2. The presence of dissolved O<sup>2</sup> leads to oxidation of Fe2+ and Mn2+ to FeOOH (goethite, iron hydroxide) and MnO<sup>2</sup> (pyrolusite, manganese oxide), respectively.

$$4Fe^{2+} + O\_2 + 8OH^- + 2H\_2O \stackrel{\cdot}{\rightleftharpoons} 4Fe(OH)\_{3(s)}\tag{6}$$

$$\rm Mn^{2+} + \frac{1}{2}O\_2 + H\_2O \leftrightharpoons MnO\_{2(s)} + 2H^+ \tag{7}$$

Concerning the major ions observed in the study site, the presence of magnesium may indicate the existence of dolomite (CaMg(CO3)2) [22]. The presence of sulphates and iron may be explained by the result of pyrite and marcasite oxidation, although these minerals were not observed extensively in the aquifer. Iron oxidation benches are however regularly visible within the chalk layers. Groundwater near the Obourg quarry was characterized by an alkalinity of 26.5 meq/L, which means that it is relatively difficult to change the pH of the solution. The pH value was equal to 7.24. The presence of dissolved O<sup>2</sup> was considered to be zero, and the partial pressure corresponding to the concentration of dissolved CO<sup>2</sup> was 10–2 bar.

### **3. Model Development** *3.1. Groundwater Flow Model*

**3. Model Development** 

mations.

### *3.1. Groundwater Flow Model* 3.1.1. Model and Spatial Discretization

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 5 of 17

iron may be explained by the result of pyrite and marcasite oxidation, although these minerals were not observed extensively in the aquifer. Iron oxidation benches are however regularly visible within the chalk layers. Groundwater near the Obourg quarry was characterized by an alkalinity of 26.5 meq/L, which means that it is relatively difficult to change the pH of the solution. The pH value was equal to 7.24. The presence of dissolved

3.1.1. Model and Spatial Discretization The numerical models were developed using the finite difference code Modflow [27].

The numerical models were developed using the finite difference code Modflow [27]. The modelled area was equal to 32.8 km<sup>2</sup> . Two hydrogeological units were represented in the numerical flow model (Figure 3): (1) the chalk aquifer that covered the whole modelled area, reaching a thickness of 300 m in the southern part of the model, and (2) a shallower aquifer located in the southern area and overlying the chalk aquifer. This shallower aquifer is made up of marine and fluvial–alluvial sediments from the Cenozoic age. They consist of sand, silt, and clay, structured in alternating permeable and low-permeable formations. The modelled area was equal to 32.8 km². Two hydrogeological units were represented in the numerical flow model (Figure 3): (1) the chalk aquifer that covered the whole modelled area, reaching a thickness of 300 m in the southern part of the model, and (2) a shallower aquifer located in the southern area and overlying the chalk aquifer. This shallower aquifer is made up of marine and fluvial–alluvial sediments from the Cenozoic age. They consist of sand, silt, and clay, structured in alternating permeable and low-permeable for-

**Figure 3.** Plan view and 3D view of the hydrogeological model. The red circle highlights the open pit chalk quarry where PSH operations were studied. **Figure 3.** Plan view and 3D view of the hydrogeological model. The red circle highlights the open pit chalk quarry where PSH operations were studied.

The modelled area was discretized using irregular rectangular cells. The dimensions of the cells were 10 × 10 m in the quarry and increased by a factor of 1.05 in the direction of the boundaries. The mesh was divided vertically in three layers of cells representing the hydrogeological units. The first layer represented the shallower aquifer, made of marine and fluvial–alluvial sediments, whilst the two lower layers represented the chalk aquifer. The central layer was characterized by a constant thickness of 40 m, corresponding to the thickness of the quarry. The Obourg quarry, which was used for PSH, as well as the other quarries, were therefore contained in this layer. This configuration allowed the expected variations of the water table in the quarry and in the aquifer to be contained in the central layer. This enabled us to avoid numerical problems related with the saturation and desaturation of the cells. The lower layer was characterized by a variable thickness, ranging from 10 m in the north to 300 m in the south, forming a beveled layer. The modelled area was discretized using irregular rectangular cells. The dimensions of the cells were 10 × 10 m in the quarry and increased by a factor of 1.05 in the direction of the boundaries. The mesh was divided vertically in three layers of cells representing the hydrogeological units. The first layer represented the shallower aquifer, made of marine and fluvial–alluvial sediments, whilst the two lower layers represented the chalk aquifer. The central layer was characterized by a constant thickness of 40 m, corresponding to the thickness of the quarry. The Obourg quarry, which was used for PSH, as well as the other quarries, were therefore contained in this layer. This configuration allowed the expected variations of the water table in the quarry and in the aquifer to be contained in the central layer. This enabled us to avoid numerical problems related with the saturation and desaturation of the cells. The lower layer was characterized by a variable thickness, ranging from 10 m in the north to 300 m in the south, forming a beveled layer.
