Distribution and assessment of trace metal contamination in the surface sediments of the Meliane River and the Coast of the Gulf of Tunis (Tunisia, Mediterranean Sea)

The Meliane River, the second longest and most important river in Tunisia, is one of the major rivers that flow into the Gulf of Tunis. However, it is known by its significant discharges of urban and industrial activities that have seriously affected the quality of the aquatic ecosystem. The highest amounts of Cadmium (Cd), Copper (Cu), Lead (Pb), Zinc (Zn), Nickel (Ni), Chromium (Cr) and Manganese (Mn) in the surface sediments of the Meliane River indicate that the downstream part is the most polluted area. The chemical speciation shows that the majority of the trace metals are bound to a reducible fraction (Zn, Mn, Cr and Ni) and the Pb and Cu are bound to the oxidizable fraction. The Cd is linked to the exchangeable fraction, which indicates its high mobility and toxicity. Both the contamination and enrichment factors values of some trace metals are elevated in the downstream part of the Meliane River. The percentage of the risk assessment code in Cd is 59.8%, which presents a very high environmental risk. Therefore, the spatial distribution of trace elements in the surface sediments is characterized by its high concentration of metals in the downstream part of the Meliane River. These concentrations decrease as a function of change in the physicochemical parameters from the freshwater-saltwater mixing zone to marine stations.