Mobility and bioavailability of trace elements in the Touch catchments(Haute-Garonne, France)
Résumé
In the Touch catchment (South-West of France), samples of sediment and water were collected to evaluate the mobility and bioavailability of some potentially toxic elements (Cr, Co, Ni, Cu, Zn, As, Pb and Cd) for ecosystems. In addition to these trace elements, certain compounds of the dissolved phase (pH, major ions, and dissolved organic carbon) were also studied. The results show two groups of trace elements in the Touch aquatic environment. We distinguish the elements associated with more than 50% in the residual phase (Cr, Zn, Ni and Cu) and those whose non residual fractions are dominant more than 60% (As, Cd, Co and Pb). In the non-residual phases, the trace elements have a significant association with the iron oxides, with the exception of Cd which is controlled to about 50% by the carbonate phases. The first two phases of the sequential extraction (water soluble and exchangeable) are insignificant in the pollutants controlling. Arsenic despite its very high proportion of non-residual phase (about 90%) is relatively less mobile than certain elements such as Cd, Co and Pb that have non residual proportions less important. Trace elements more stable in relation to physico-chemical conditions prevailing in the Touch aquatic environments are Cr, Zn and As, and most mobile are Cd, Co and Pb. The availability of these elements is particularly sensitive to the Physico–chimical conditions of this environment. Their presence in solution increases with the concentrations of inorganic ligands such as chlorides and organic ligands (DOC), while it decreases with increasing pH and alkalinity (in particular Cd and Pb because of their relative abundance in the carbonate phases).
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