Heavy metal distribution and assessment in stream sediments of River Orle, Southwestern Nigeria

Ten heavy metals, namely, Ag, As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn were partially extracted using aqua regia digestion method and analysed by ICP-AES from 56 stream sediment samples collected from River Orle, Igarra area, southwestern Nigeria. The analytical results were used to produce geochemical distribution maps for the elements and were subjected to univariate statistical analysis in order to evaluate the distribution and abundance of the heavy metals in the study area. The degree of pollution of these stream sediments by these heavy metals was evaluated by calculating such parameters as enrichment factors (EF), as well as pollution load and geo-accumulation indices (PLI and Igeo). Co, Cr, Cu, Ni, Pb and Zn are widely distributed in the drainage system while the distribution of Ag, Cd, As and Hg is restricted to only parts of the drainage system with Ag and Cd being localized to one sample site each near Epkeshi in the southern part of the study area. Cr and Pb display anomalously high concentrations, each from a site, also in the same locality where Ag and Cd were detected, indicating the likelihood that the four elements, Cr, Pb, Ag and Cd, are genetically related. Calculation of the enrichment factor (EF), pollution load index (PLI) and geo-accumulation index (Igeo) yielded results that indicate that all the 56 stream sediment sites, except one located about 4 km southeast of Epkeshi in the southern part of the Orle drainage system, are practically unpolluted by heavy metals. The relatively high metal concentration of this anomalous site having Pb EF of 62.5, PLI of 1.14 and Pb Igeo of 2.44 signifies Pb pollution. Both natural and anthropogenic sources of the Pb contamination around Epkeshi locality are possible. In conclusion, the levels of concentrations of heavy metals in the study area, in general, do not constitute any serious environmental risk except for Pb which needs to be monitored at only one site in the study area. Therefore the concentration ranges for the different heavy metals in the study area can serve as baseline environmental data against which the degree of pollution of these heavy metals can be evaluated in future.