Soil Metal Distribution under Different Land Uses of Emerging Mega Cities in Southwest Nigeria and the Associated Ecological Risk

Data on Inventory, distribution and risk assessment of heavy metals in soils of emerging megacities in sub-Saharan African countries is necessary to assess pollution sources and environmental safety. The present study evaluated Ni, Cr, Cu, Cd, Zn, Mn, and Pb concentrations in soils from different land uses in Ibadan and Abeokuta, Nigeria. Samples were collected at 0-20 and 20-40cm depths from crop-farm, industrial site, animal-farm, natural forest, roadside, educational institution, residential estate and markets. Results showed that the depth distribution of the metals were similar, Cu and Pb were concentrated in the top 20cm soil depth at both locations. There were higher concentrations of Ni, Cu and Zn in Ibadan soils while Abeokuta had higher levels of Mn. Copper and Zn contamination were higher in animal farm and industrial land, respectively. Roadside soil had a significantly higher Pb (p 0.05), comparable to forest, educational and residential lands; however, market land use was more acidic. The soil metals accumulated and appeared bound to organic matter, while nickel was positive and significantly correlated with soil Cr, Cu, Zn and Mn while soil Zn and Mn correlated positively with Cr. Contamination/pollution factor across soil depth, locations and land use was in the order Cd Zn Mn Cr Pb Cu Ni. The soils’ contamination indices are high with Cd, moderate with Zn and low for other metals. Cadmium has a high potential ecological risk, while the potential ecological risk indices of the locations were moderate. Generally, the metal concentrations were lower than those of other developed megacities.