## 3 Results and Discussion  
### Site Prospection  
Processed tailings of the Cu-Au Mine of Guelb Moghrein Akjoujt are conveyed through a pipeline as a pulp (70% solid content) to the retention ponds (Figure 6). The tailings are discharged by alternating between spigots and by a controlled rotating aerial method that ensures sequential drying of the tailings. This method prevents the stacking of the tailings and the settling of water in the ponds. The ponds are surrounded by protective dykes. Water is drained toward a pumping system, laid down at the center of the TSF, to be reused as water at more than 20% in the flotation circuit. Advanced oxidation has been observed through the TSF1, TSF2 and TSF3 tailings of Guelb Moghrein; as the tailings progressively desaturate, in a relatively dry pond, the oxidation of the tailings becomes more significant. However, progressive dewatering results in the formation of highly oxidized tailings and highly hardened crusts. Figure 7 shows that the depths of tailings oxidation vary within the same pond and from one site to another. Sulfide tailings produced by the mineralogical treatment of base and precious metals have several characteristics similar to the reactions that take place in the tailings storage facilities under climatic conditions. In reactive mine tailings, iron sulfides, the most common of which are pyrite (FeS\({}_{2}\)) and pyrrhotite (Fe\({}_{1-\chi}\)S), oxidize, and they produce acidity in contact with water and air [11; 13]. The neutralization of acidity by the dissolution of alkaline minerals generally produces secondary mineral phases [9; 20; 21]. A characteristic associated with the neutralization process observed across tailings facilities is the precipitation of secondary minerals [20], forming hard pans, which are physically dense and mechanically hard crusts under semi-arid climatic conditions [25]. These hardpans, typical of sulfide mine tailings, have long been the subject of several research works dealing with their geochemical behavior. The oxidation product of these layers are composed mainly of oxyhydroxides, hydroxysulfates, arsenates, sulfates and halides. These secondary minerals significantly affect the geochemical properties of the tailings behavior because they can retain dissolved metals by co-precipitation or adsorption. They also form a resistant barrier to prevent water infiltration and oxygen diffusion, which are the two main concomitant agents of acid mine drainage [26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 39; 60]. Figure 6: Mode of tailings disposal in the TSF3 (**A–C**). Figure 7: Oxidation of tailings and formation of highly indurated crusts. (**A**) TSF1 (317 cm); (**B**) TSF2 (38 cm) and (**C**) TSF3 (53 cm). Numerous phenomena have been observed in the TSF tailings of the Guelb Moghrein mine regarding the hydrogeotechnical behavior associated with the arid climate. Particle size and mineralogical characteristics promote the rapid formation of cracks (Figure 8A) in the tailings initially saturated with water under the effect of self-weight. Thus, as the tailings become desaturated, numerous cracks would form on the surface of the TSF tailings at the Cu-Au mine of Guelb Moghrein, which is similar to other gold mines around the world such as the Bulyanhulu in Tanzania [61; 62] and the Manitou tailings in Canada. Excessive evaporation leads to the removal of the hydraulic barrier and induces oxygen diffusion, which consequently causes the likely production of AMD [47; 63; 64]. However, in more arid countries, such as Akjouij in Mauritania, Western Australia and Bulyanhulu in Tanzania, the precipitation of salts (Figure 8B) on the surface during evaporation results in the development of thin and thick salt crusts, which can significantly reduce the rate of evaporation. Therefore, this phenomenon could impede excessive desaturation and minimize the entry of oxygen and therefore prevents the production of AMD [62; 65].