Metal speciation in sediments from crude oil prospecting in the coastal area of Ondo State, Nigeria

Information obtainable from metal speciation is far more valuable with respect to bioavailability, toxicity and fate of metals than information from total metal data. Therefore, metal speciation on sediment to assess the bioavailability, fate and mobility of As, Cd, Cr, Fe, Mn, Ni, Pb, and V in the crude oil prospecting area of Ondo state Nigeria was carried out in this study.  Five operationally defined metals species: exchangeable specie, carbonate bound specie, iron/manganese bound specie, organic bound specie and the residual specie were obtained using sequential extraction method.  The concentrations, spreads and relative abundances of the species were used to assess their fate, mobility and toxicity potential.  The crude oil exploration site and the mouth of the estuary had the highest concentrations of these metals compared to other sites.  The high risk species, exchangeable and the organic bound, were the most predominant in the crude oil exploration sites but their concentrations reduced away from these sites downstream.  The residual fraction which was least in the crude oil prospecting site was the most predominant in the farthest distant sites downstream.  There are indications of self-management of the metals in the costal system through favourable biogeochemical reactions that is partitioning the metals from the high risk species to the low risk ones and the non-toxic residual fraction.  It could however be concluded that the sediments from the crude oil prospecting area may serve as non-point sources of metal contaminant to the coastal system, they have higher metal bioavailability and higher toxicity risk potential than the other sites which should be curtained.