Oued Moulouya: vecteur de transport des métaux lourds (Maroc)

Abstract

Les mines qui sont abandonnées avec leurs installations minières et métallurgiques constituent un exemple représentatif du transfert des éléments métalliques dans l’écosystème. Au Maroc, le site minier d’Aouli est drainé par l’oued Moulouya (l’unique oued de la région) d’une longueur de 520 km avec un bassin versant d’une superficie d’environ 50.000 km 2 . A travers ce travail, nous avons déterminé un certain nombre de paramètres chimiques afin de voir la destinée de certains métaux (Pb, Zn, Cu et Cd) issue de ce district minier abandonné. Les résultats ont montré que dans l’eau, dans les matières en suspension et dans les sédiments, il y a une contamination métallique due à la mine d’Aouli situé au niveau de la Haute Moulouya ; tandis qu’au niveau de la Basse Moulouya, l’affluent oued Za contribue à cette contamination par un apport anthropique qui résulte en une augmentation des teneurs en cadmium, plomb et zinc. L’effet de cette contamination externe est également mis en évidence par les caractères géochimiques d’une station témoin situé en amont de l’oued loin de toute influence minière.L’examen des rapports des différents éléments (Cd, Pb et Cu) par rapport au Zn dans les sédiments montre qu’ils sont préservés dans les sédiments en aval (Basse Moulouya). Par contre, au niveau de la Haute et la Moyenne Moulouya, on assiste à une diminution des rapports Cu/Zn et Cd/Zn et une augmentation du rapport Pb/Zn au niveau de la station S3; ce qui reflète l’effet de la mine d’Aouli. L’augmentation des rapports au niveau des stations S1 et S4 sont en relation avec la présence d’algues et qui jouent un rôle important dans la modification des teneurs en Zn dans les sédiments.L’étude de ces rapports a permis de confirmer que dans notre cas aucun effet notable dû aux apports des affluents n’est mis en évidence sur le contrôle des concentrations des éléments dans les sédiments de l’oued Moulouya. Cette particularité permet de suggérer que dans la cas de l’oued Moulouya la dilution physique est plus importante que la mobilisation chimique et permet d’expliquer le déclin en aval des concentrations des éléments métalliques. Mining activity started in Morocco in the 9 th century. Several metals were prospected for but lead was preferentially mined owing to its silver content. The mining history of Morocco can thus be summarized by the history of lead prospecting within the country. Strong mining activity resumed in the 1970s and has been maintained since that time, with lead still being the main metal produced. In 1980 its production reached about 170,000 tons of concentrated lead, which corresponds to 3.5% of the world's production (Wadjinni, 1998). In 1975, the Aouli and Mibladen mines were depleted and in 1986 the Zaida mine was closed. This resulted in large mining districts being abandoned with their mining and metallurgic equipment left on the surface together with all the accompanying waste materials, including tailings. This has created an important source of contamination, progressively releasing trace elements into the environment. The goal of the present study was to characterize the spatial distribution of the heavy metals (Cd, Cu, Pb and Zn) released from the Aouli mine into the Moulouya River. This river, which drains the Aouli, Mibladen and Zaida mines, is a good example of the pollution impact arising from abandoned mining sites. In order to understand the behaviour and the fate of these metals within the only permanent river in this region, contamination levels were measured in three components of this river, namely: water, suspended solids and sediments.Due to the presence of mountainous areas (Rif, Middle and High Atlas), the Moulouya River basin is characterized by variable relief. The river basin also contains high plateaus (the Horsts chain) and low plains (the Missour-Outat El Haj and the Taourirte-Guercif basins). This large area can thus be subdivided into three zones designated by the upper, middle and lower Moulouya basins (Fig.1).The Upper Moulouya basin corresponds to the southwest region of the Oranaise Meseta that is bounded by the High Atlas on the Southeast and by the Middle Atlas on the Northwest. They are composed of two separate Paleozoic massifs (the Bou-Mia and the Aouli). The Paleozoic substratum that outcrops into these massifs consist of pelletic and quartzitic rocks intruded by Hecynian granites and overlain non-uniformly by a Mesozoic cover consisting of Triassic evaporite/clastics and Jurassic and Cretaceous carbonates and shales.The Middle Moulouya basin is separated from the Lower Moulouya basin to the North by the Yacoubat paleo-high that corresponds to an East-West oriented topographic bulge where the river narrows considerably. The outcropped rock consists mainly of Jurassic carbonates and marls, Cretaceous carbonates, marls and sandstones and Tertiary and Quaternary conglomerates, marls and gypsum.The Lower Moulouya basin corresponds to a vast plain extending from the Yacoubat High in the South to the Jbel Mazgout and the Beni-Snassen in the North. This area is underlain by Paleozoic granites, Triassic marls and basalts, Jurassic and Cretaceous carbonates, marls and sandstones and Tertiary and Quaternary conglomerates, marls and carbonates. The Moulouya river basin is characterised by a variable climate, which changes from a Mediterranean type in its low portion to a sub-Saharian type in its median portion and to a continental type at higher elevations.Sampling was carried out during low water level periods. In order to obtain good representative sampling, three samples were taken from each station and were used to measure metal contaminant concentrations in water, suspended solids and sediments. The sediment samples were taken from approximately 5 cm below the surface within the river plain, which is characterized by fine-grained low-energy organic matter-rich sediments. We defined a certain number of pollution parameters within the Moulouya basin, based on the samples taken from eight representative sites. The areal distribution of metal concentrations in water and in suspended solids shows the existence of two main contamination sources: the abandoned Aouli mine and urban waters. Dissolved Zn concentrations varied from 3 to 30 µg·L -1 and the two highest concentrations occurred at station 3 near the Aouli mine and at station 8 located in oued Za near the town of Taourirt (Table 1). Cadmium, Cu and Pb concentrations showed similar trends with high concentrations occurring at stations S3 and S8 for Cd and Cu and at stations S3 and S5 for Pb (Fig. 2). These high levels were explained by the effect of Aouli mine for station S3, the Tindit mineralization area for station S5 and urban waters from Taourirt for station S8. Metal concentrations in sediments reflect the downstream attenuation of the Aouli mine effect (station S3). In fact this station is characterized by high metal concentrations, especially Pb and Zn and to a lesser degree Cd and Cu. For Cu, its concentration in sediments was found to be similar to its concentration in the suspended solids. This is explained by the precipitation of Cu(II) as oxides and hydroxides. The areal distribution of Pb and Cd was similar to that of Zn, suggesting that the Aouli mine is also a source of these metals.By comparing concentration ratios of different metals with respect to that of Zn, we found that all these ratios remain constant downstream from the Aouli mine, with the exception of the Pb/Zn ratio, which increased at station S3 (Fig. 3). This distribution indicates an effect of the Aouli mine (station S3) and to a lesser degree of the Tindit area (station S4). No notable effect of the Moulouya tributaries on the concentrations of metals was detected.