Contamination intensity and origin of trace metals in the bottom sediments from the Sebou basin (NW Morocco)

Due to increasing anthropogenic activities, trace elements remain a major concern for river water quality, particularly when waste effluents are not always constraint. The intensity of the contamination, the origin and the distribution of these elements were not frequently investigated, particularly in carbonated areas from the southern countries of limited water resources. In this context, the present study aimed at understanding the geochemistry of trace elements in bottom sediments from the Sebou basin, representing 1/3 of the surface water resources of Morocco. Four spatial sampling campaigns have been carried out in 2018 and 2019 during contrasted hydrological periods. Total and non-residual concentrations (EDTA extraction) of trace elements (As, Cd, Co, Cr, Cu, Ni, Pb, Zn) were measured in the fine fraction (< 63µm), as well as some physico-chemical parameters.

The order of abundance of the elements was as follows: Zn>Cr>Cu>Ni>Pb>Co>As>Cd. Several sediment indices of contamination and of toxicity risk were used to assess sediment quality. The results showed that 70% of the samples were naturally enriched in trace elements, specially As and Ni, as well as Cd and Pb, except in some stations. On the opposite, the most enriched elements were Cr, Zn and Cu. Chromium presented an enrichment higher than 5 and toxicity risks at some stations, such as downstream the Fez city known for its important tanneries activities. A multi-variate analysis of the datas evidenced the strong link between the identified natural elements (As, Co, Ni) with clays, Fe, Al oxides, whereas elements (Cd, Cu, Cr, Pb, Zn) mainly originating anthropogenic activities (industrial and domestic wastes, homemade, and agricultural practices), were linked to phosphorus, to a lesser extent to particulate organic carbon.