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

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 measured in the field. Samples were filtered through 0.45 µm membranes. The filtrates were preserved with 4% HNO3 and kept at 4°C until analysis. The filters and their particulate fraction, as well as a composite sample of the tailings, were dried and kept dry until dissolution and analysis. All measurements were performed by ICP-MS and capillary electrophoresis analyses.Lead and arsenic are well above « normal » concentrations in the tailings, at 5,547 g/t and 192 g/t, respectively. These elements are clear threats to population health, since dust from unstabilized tailings can be dispersed by wind and rain waters, contaminating agricultural soils and surface waters, and eventually leading to cases of saturnism or arsenical intoxication in the population. All Pb and As concentrations measured in the waters sampled are above the World Health Organisation criteria for drinking water (10 µg/L for Pb and As). Nevertheless, about half of these measurements meet the Moroccan criteria (50 µg/L). All samples show near neutral or slightly basic pH values (7.2-8.9). EC is also high (> 1,000 µS/cm).As and Pb are largely associated with the particulate fraction (> 80% of total As and Pb), except for As in two lakes (< 40%). Overall, their concentrations are higher in the Zeïda area than upstream in the Moulouya. However, this is a tendency rather than a rule, because the differences are often small (< 50%) and suffer exceptions. For instance, Pb concentrations are 60% lower than the so-called uncontaminated reference station, in two quarry-lakes sampled during the wet period. The occasional lack of significant and consistent contrast between an obviously degraded environment and a pristine site was unexpected. The results were therefore further investigated, in order to identify possible explanations for the apparent discrepancies.Coherency in the data set emerges when one considers the relative importance of dissolved and particulate transport in the various types of environments sampled. Using this interpretation scheme, EC is considered as an indicator of solute transportation, since EC is a function of dissolved ionic components. Total Pb is regarded as an indicator of particulate transportation, since Pb is strongly adsorbed to particulate substrates at the pH observed. Following these assumptions, particulate transport appears to dominate over dissolved transportation in the Moulouya. Total Pb increases by factors of 3.4 and 9.8 from dry to wet periods, whereas EC decreases by 0.7-0.8, as a result of rain dilution. In two of the four lakes, dissolved transport is comparatively more important, since EC does not change significantly and total Pb shows only a small increase from the dry to the wet period. In these lakes, the dissolved input during the rainy period appears to be large enough to keep EC at its previous value, without significant dilution, as opposed to what is going on in the Moulouya. In the two other lakes, both EC and total Pb decrease from the dry to the wet period, pointing to dilution effects greater than either dissolved or particulate mobilization.Apparent discrepancies in the intensity and direction of variations are explained when prevailing modes of dispersion are taken into account. For instance, the enrichment factor of total Pb in the Moulouya, downstream of Zeïda, jumps from 1.1 (dry) to 3.2 (wet), with respect to the reference station. Meanwhile, EC increases only from 1.2 (dry) to 1.5 (wet). The greater increase of total Pb over EC is explained by prevalent particulate transport. In another case, the enrichment of EC with respect to the reference station, in two lakes, increases from 14.2 and 20.4 (dry) to 20.1 and 27.9 (wet) while total Pb enrichment decreases from 2.0 and 2.0 (dry) to 1.6 and 1.3 (wet). Here, the prevalence of dissolved transport in these two lakes, combined with particulate transport at the reference station, allow for a strong increase in the EC parameter, concurrent with a weak increase in total Pb. Finally, in the lakes where both dissolved and particulate transport are presumably minor, total Pb undergoes enrichment with respect to the reference station, in the dry period (by 3.4 and 1.8), whereas depletion characterizes the wet period (0.6 in both lakes). In this case, enrichment is likely the result of evaporation during the dry season, and depletion the result of dilution by rain during the wet period. These two types of lake behave differently because they are located next to residual mineral deposits (likely with more soluble phases), have short travel distances and thus fewer contacts with adsorbing substrates, which is not the case for the other two lakes