Heavy metals have been recognized worldwide for their toxic effects in man and wildlife. The Ogba River in Benin City, Nigeria, has been reported to be contaminated to varied levels by heavy metals as a result of anthropogenic impact. However, there is paucity of scientific data on the chemical fractionation of such metals in sediment which serves as a repository for such contaminants. Specifically, the chemical fractionation of Cadmium (Cd), Copper (Cu), Lead (Pb) and Zinc (Zn) in sediment was thus determined via a five-stage stage (i.e. Exchangeable, Carbonate, Fe-Mn oxide, Organic/sulphide and Residual) sequential extraction procedure while levels of metals were quantified by Atomic Absorption Spectrophotometric technique in order to evaluate the potential ecological risk of these heavy metals. Results from the study revealed that Cd and Pb were predominantly associated with the Fe-Mn oxide fraction of sediment while Cu and Zn were chiefly associated with the residual fraction of sediment. The mean concentrations (mg/kg, dry weight) of the heavy metals in sediment were 0.91±0.18, 7.83±1.57, 4.88±0.98 and 10.84±2.17 for Cd, Cu, Pb and Zn respectively while the environmental risk factor (ERF) revealed that Cd posed a potential threat to aquatic organisms including fish by having a sub-zero value of -14.12. Contamination factors were 14.29, 0.76, 2.52 and 1.46 for Cd, Cu, Pb and Zn respectively, indicating that contamination was low for Cu, high for Cd and moderate for Zn and Pb. Sequel to findings from the research, it was concluded that relevant environmental regulatory agencies should carry out routine monitoring of the sediment from Ogba River in order to ensure a mitigation in the present contaminated status.
Key Words: Heavy metals, Chemical fractionation, Contamination, Sediment, River