Fractionation profile, mobility patterns and correlations of heavy metals in estuary sediments from olonkoro river, in tede catchment of western region, Nigeria

•Sediment cores studied from upstream, middle-stream and downstream of the river estuary.•The high percentage of Mn, Cd and Pb in the bioavailable forms.•Metals contributed from Geogenic as well as Anthropogenic sources.•Metal fractionation showed most of the metals in bioavailable fraction with the exception of Cu and Cd.•Bioavailability of Cd and Pb poses potential risk to the estuarine system if the sediment gets distributed.

This work was carried out to determine the concentrations, bioavailability and mobility of heavy metals in the sediments of Olonkoro River, Tede, Oyo State, Nigeria. The river sediments were sampled at ten selected locations and the samples were analyzed for some heavy metals to determine the concentration, speciation and distribution pattern of the metals. The sediments were obtained specifically at upstream, middle-stream and downstream of the river. In general, the total metal concentrations were in the order: Fe > Zn > Mn > Cu > Pb > Cd in sediment samples and for Tessier's sequential fractionation, manganese was found to be more in the exchangeable phase, Fe in the Residual, Zn in carbonate-bound metal while Pb was greater in the exchangeable form and Cu was found to be more concentrated in the organic phase. Percent of bioavailable and non-bioavailable proportions of heavy metals for Mn, Fe, Cu, Zn, Pb, and Cd in the sediment samples varied between 12.92-87.08, 11.57-88.43, 0.29-99.71, 10.76-89.24, ND-100 and ND, respectively. This clearly indicates that with the exception of Cu and Cd the metals are highly bioavailable leading to their having a higher impact on the target system, subsequently leading to contamination of the water. The high percent bioavailability is an indication of poor retention in the residual geochemical form partly due to saturation of the sediment (river). The pH value was found to be basic but within the WHO standards throughout all locations. The high percentage of Mn, Cd and Pb in the bioavailable forms suggested the need to keep close surveillance on these metals because of their high toxicity.