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Article

Heavy metal and rare earth element geochemistry in the mining influenced sediments of Mejerda watershed (Northem Tunisia).

SAHNOUN Olfa, ADDED Ayed, Laridhi Ouazaa Nejia, LARABI KASSAA Samia, JORON Jean Louis, ABDELJAOUED Saâdi, 2007

Revue Méditerranéenne de l'Environnement, 2, 2, 1, Juillet 2007

Résumé

In order to assess the impact  of industrial activity mining, seventy two superficial sediments were collected from Mejerda river, tributaries and mining residues, in March 2002, October and November 2003. Studies have been based on trace metals and rare earth elements (REE) in the fraction < 63 µm. These elements were analysed by the neutron activation method. The results were expressed by binary diagrams, spidergrams and curves showing the concentration evolution against distance. NASC normalised REE display fiat spectra illustrating the detrital or lithologie component of the sediment, resulting from the heritage of Mejerda watershed  substratum.  The homogeneity of these spectra reveals a unique source of sediment which corresponds probably to the Hoggar shield. Binary curves allowed ranging the elements in three groups: detrital, metallogenic and biogenic.NASC normalised metals allowed to quantify the mining impact on the Mejerda river sediment.The spidergrams show that Sb, Cs, Ag. As and Zn have high enrichment factor (EF >1000) in the mining residues, while in tributaries and  mejerda river, it is either, relatively  high (Sb, Cs, As), or have a value near to  1 (Zn, Ag, Pb). Moreover, the Mejerda  river sediments are enriched with Cd and Br. Obviously, the latter are related to the triasic evaporates and carbonates, while the other elements have a metallogenic source.Zn, Ag and probably Pb are remobilised with iron in the mining residues, then are transported towards the sea. This study allowed understanding the contamination processes by metals in the rivers.