## 2 Materials and Methods
### Hydrochemical Context  
Chalk is generally composed of high percentage values (60-95%) of calcite (CaCO\({}_{3}\)) [22]. Specifically, the chalk of the Triviferes formation is composed of more than 92% CaCO\({}_{3}\). These chalk geological formations also contain some slightly ferruginous beds, as well as some phosphate nodules [14]. The hydrochemical composition of the groundwater  
Figure 1: View of the Obourg chalk quarries. The red line, intersecting the studied open pit, gives the location of the altimetric profile.  
Figure 2: Modified south-north geological section of the Mons Basin and projection of the Obourg quarry location [21].  
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\({}_{3}\) in the presence of dissolved CO\({}_{2}\). The dissolution of CaCO\({}_{3}\) is governed by a series of acid-base equilibria as follows:  
\[CO_{2}+H_{2}O\ \leftrightarrows\ H_{2}CO_{3} \tag{1}\]  
\[H_{2}CO_{3}\ \leftrightarrows\ H^{+}+HCO_{3}^{-} \tag{2}\]  
\[HCO_{3}^{-}\ \leftrightarrows\ H^{+}+CO_{3}^{2-} \tag{3}\]  
\[H_{2}O\ \leftrightarrows\ H^{+}+OH^{-} \tag{4}\]  
\[CaCO_{3}+2H^{+}\ \leftrightarrows\ Ca^{2+}+H_{2}O+CO_{2} \tag{5}\]  
In accordance with these equilibrium reactions (1-5), most of the dissolved elements in the chalk aquifers' groundwater are Ca\({}^{2+}\) and HCO\({}_{3}^{-}\). The HCO\({}_{3}^{-}\) ion is the predominant form. Other major ions in chalk aquifers' groundwater are Mg [24], Na\({}^{+}\), Cl\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), and Fe\({}^{2+}\)[23; 24; 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 Mn\({}^{2+}\) and Fe\({}^{2+}\).  
These two ions (Mn\({}^{2+}\) and Fe\({}^{2+}\)) are involved in redox reactions, shown below (Equations (6) and (7)), involving the O\({}_{2}\) 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 O\({}_{2}\). The presence of dissolved O\({}_{2}\) leads to oxidation of Fe\({}^{2+}\) and Mn\({}^{2+}\) to FeOOH (goethite, iron hydroxide) and MnO\({}_{2}\) (pyrolusite, manganese oxide), respectively.  
\[4 Fe^{2+}+O_{2}+8 OH^{-}+2H_{2}O\ \leftrightarrows\ Ae(OH)_{3(s)} \tag{6}\]  
\[Mn^{2+}+\frac{1}{2}O_{2}+H_{2}O\ \leftrightarrows\ MnO_{2(s)}+2H^{+} \tag{7}\]  
Concerning the major ions observed in the study site, the presence of magnesium may indicate the existence of dolomite (CaMg(CO\({}_{3}\))\({}_{2}\)) [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 \(\mathrm{meq}/\mathrm{L}\), 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\({}_{2}\) was considered to be zero, and the partial pressure corresponding to the concentration of dissolved CO\({}_{2}\) was 10-2 bar.  
\begin{table}
\begin{tabular}{c c} \hline
**Ion** & **Concentration (mg/L)** \\ \hline Ca\({}^{2+}\) & 142.2 \\ HCO\({}_{3}^{-}\) & 350.1 \\ Mg\({}^{2+}\) & 11.3 \\ Na\({}^{+}\) & 26.2 \\ Cl\({}^{-}\) & 41.4 \\ SO\({}_{4}\)\({}^{2}\) & 111.5 \\ Mn\({}^{2+}\) & 0.82 \\ Fe\({}^{2+}\) & 2.62 \\ \hline \end{tabular}
\end{table}
Table 1: Average concentration of ions in the chalk aquifer of the Mons Basin [26].