Seasonal variation of surface sediments composition in Mondego River estuary.

Concentrations of major (Al, Si, Ca, Mg, Fe), minor (Mn), and trace elements (Zn, Pb, Cr, Cu, Ag, Cd, Hg) and organochlorine compounds (PCB congeners, pp'DDT, and metabolites) were determined in 24 samples of surface sediments (0-5 cm) collected along the Mondego River estuary in two periods: February and August 2003. All sediment samples showed low levels of contamination reflecting the weak industrialization of the region. Higher incorporation of elements (Mg, Fe, Zn, Cr, Cu, Cd, Hg) and DDT was registered in muds deposited in the inner part of the south channel. Sediments of the rest of the estuary are coarser (mean Si/Al ratio around 11) and showed much lower Me/Al ratios. Concentrations of Mn, Pb, Ag, and PCB showed no geographical distribution tendency. However, Pb, Cu, Ag, Zn, Cd ratios to Al and PCB concentrations were higher in coarser sediments collected in February than in August. In addition, the lower chlorinated CBs (tri + tetra-CB) showed a higher proportion in sediments collected in February, due to its higher mobility and low contamination in the area. The results obtained in this river-dominated estuarine system, with weak local contamination sources, indicate that chemical composition of surface sediments reflects the diffuse source of contaminants associated with the rainy season.