Pb et As dans des eaux alcalines minières : contamination, comportement et risques (mine abandonnée de Zeïda, Maroc)

Abstract

Des échantillons d’eau et de matière en suspension ont été prélevés le long de l’oued Moulouya et dans des lacs de carrière au niveau de l’ancien centre minier de Zeïda (Haute Moulouya, Maroc) en vue d’en évaluer la salubrité. Il est en effet important d’établir le degré et les causes éventuelles de dégradation de la qualité de ces eaux, compte tenu de leur usage à des fins tant domestiques qu’agricoles. Des résidus de traitement ont également été échantillonnés dans les haldes abandonnées.L’analyse des distributions et des variations spatio-temporelles des concentrations de Pb et As a permis de mettre en évidence que le centre minier a véritablement un impact sur la qualité des eaux de surface environnantes, malgré le caractère neutre à alcalin du drainage. Aucune ne rejoint les critères de l’Organisation mondiale de la santé en matière de Pb et As dans l’eau potable (10 µg/L), mais près de la moitié souscrit aux normes marocaines (50 µg/L). L’importance de la dégradation varie selon la saison et la localité, et le contraste entre sites a priori non dégradés et sites dégradés n’est pas toujours très prononcé. On observe même des variations à l’inverse des tendances attendues. Les résultats peuvent cependant être réconciliés en tenant compte de l’importance du transport particulaire par rapport au transport dissous au moment et au lieu de l’échantillonnage.Il appert que les minéralisations et les résidus miniers restés sur place peuvent constituer la principale source de pollution des eaux de surface de la région de Zeïda.The Upper Moulouya Basin was the location of extensive lead mining between 1930 and 1985, with three major operations near Aouli, Mibladen and Zeïda. The Moulouya drains about 7.5% of the Moroccan territory and provides drinking and irrigation water to many communities over its more than 500 km path. It is thus important to determine the impact of past mining activities on its water quality, since the mining sites were abandoned with little or no rehabilitation. This paper focuses on the Zeïda area, the uppermost of these mining centres on the Moulouya.About 630,172 t of lead concentrates (40‑70% Pb) were produced between 1972 and 1985 at Zeïda. Lead was mined from carbonate and sulphide mineral deposits (cerussite, 70%; galena, 30%) mixed with barite in stratiform ore bodies hosted by Permo-Triassic arkoses. Mining left 12 Mt and 70 Mt of tailings and wastes in fully exposed piles on each side of the Moulouya, as well as a dozen water-filled open-stopes. Mine drainage is of neutral pH, thanks to the low content of residual sulphide minerals and the availability of carbonate in the tailings and host rock. The river and some quarry-lakes are tapped to fulfill domestic, agricultural and stock-breeding needs. One lake is used to directly feed Zeïda’s water network (pop. 3,000), without any water treatment.The Moulouya, upstream and downstream of Zeïda, and four lakes were sampled twice in 2002 (dry period: February; wet period: April). Temperature, electric conductivity (EC), Eh and pH were measure...

Mineralogy and chemical forms of lead and zinc in abandoned mine wastes and soils: An example from Morocco

“…One of these sites is the Upper Moulouya lead district which contained one of the largest concentrations of lead in Morocco, with a total output of more than 1 million metric tons (Rajlich, 1983). Currently the area is characterized by a low population density (less than 20 inhabitants per km 2 ) and poor economic conditions and only few studies have been undertaken to acquire information about the environment contamination by heavy metals from past mining activities (Bouabdli et al, 2005;El Hachimi et al, 2006;El Khalil et al, 2008). Our interest for the area was also due to the fact that it is crossed by the largest Moroccan river, the Moulouya, which, with its 520 km of length and tributaries, drains approximately 53,500 km 2 in the eastern part of the country, spreading contamination far away from the source up into the Mediterranean Sea where it finally flows.…”

“…According to literature (Bouabdli et al, 2005;El Hachimi et al, 2006;El Khalil et al, 2008), the main metallic contaminants in the studied area are Pb and Zn. Their total content in mine wastes and soils is reported in Table 4.…”

Assessment of trace metals contamination in sediment and surface water of quarry lakes from the abandoned Pb mine Zaida, High Moulouya-Morocco

“…The contact of waters with mining residues rich in clay gives the result of quarry lakes metallically polluted. The waters of the lakes and Moulouya river are used for domestic consumption, irrigation and livestock watering; these waters are also intended for the supply of drinking water without prior treatment in the network of the village of Zaida (5000 inhabitants) (El Hachimi et al 2007).…”

“…The waters of lakes showed a slightly alkaline pH (8.4 < pH < 9.3). According to the literature (El Hachimi et al 2007;Makhoukh et al 2011), this is strongly related to the presence of carbonates that can buffer the study environment. Much of the surface waters that fill the quarry lakes are generally coming from the precipitation that flow and infiltrate into the marno-dolomitic and limestone cover, also in the non-vegetated treatment residues rich in carbonate minerals (deposited on the surrounding), this process makes possible to buffer the water that could have been acidified by the oxidation of the sulfides.…”

Theoretical study of the dissolution kinetics of galena and cerussite in an abandoned mining area (Zaida mine, Morocco)

“…The main ore extracted is lead, mainly carried by cerussite (70%) and galena [7]. The climate in this locality is arid to semi-arid with rainfall of 300mm / year and mean annual temperatures ranging from 18 ° C to 29 ° C, while minimum winter temperatures are in the range of 14 to 21 ° C [2] The studied lake ZA located near the tailing I (mining waste), is filled with surface water with an area of about 50Ha accumulating a volume of water of about 1Mm 3 ; its chemical composition sometimes shows concentrations of metals in lead and Arsenic [1,3]…”