Bioavailability Dynamics and Ecological Risk Assessment of Heavy Metals in Sediment of Two Confluenced Tropical Rivers in West Africa

The presence of heavy metal in the sediments of tropical rivers is a significant hazard to the ecological quality and human health; however, the relationships of the biomagnification of metals have not been sufficiently investigated in urban West African environments. The current study determined the bioavailability and ecological risk of eight heavy metals (As, Cd, Cr, Fe, Mn, Ni, Pb, Se) in Nworie and Otamiri Rivers sediments, Nigeria. The BCR sequential extraction procedure was used to determine concentrations and distribution among four fractions (F1 -F4 ). The Risk Characterization used the Risk Ranking Index (RAC), the Geo-accumulation Index (Igeo), and the Potential Ecological Risk Index (PERI) and principal component analysis (PCA) supported the process of source apportionment. Findings showed that there was a strong spatial and temporal heterogeneity. The potentially bioavailable factions (F1 + F2 + F3) had maximums downstream at sites affected by urban activity cadmium at 1.95 mg/kg (81 %, SS5), lead at 5.81 mg/kg (72% SS7), and nickel at 5.37 mg/kg (100% SS7). The RAC more than 30% on cadmium showed increased mobility, and enhanced PERI (maximum of 285) on SS5 induced mainly by cadmium (E:241). The PCA revealed that 78% of the variance was explained, PC1 (54 %) linked cadmium and lead and nickel with anthropogenic urban runoff, and PC2 (24%) related geogenic iron and manganese with the remaining fraction F4. The Nworie and Otamiri river systems has a significantly high ecological risk. More studies focusing on the organization of fraction F3 and the following bioaccumulation mechanisms should be suggested to optimize the risk management approaches in this urban-tropical nexus.