Assessment of pollution by heavy metals in sediments of river Oyi and its tributary, southwestern Nigeria

Abstract

The sediments of Rivers Oyi and Wara were sampled in order to assess the pollution status of these rivers by heavy metals. Thirty stream sediment samples were collected along channels of Rivers Oyi and Wara, bounded  by Latitudes 8°39'N and 8°50'N and Longitudes 5 00'E and 5°09'E. The samples were analyzed for eight heavy metals, namely, Co, Cu, Cr, Fe, Mn, Pb and Ni after they had been partially extracted using aqua-regia digestion and analysed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The geochemical data obtained were then subjected to univariate statistical and correlational analyses. Enrichment Factor (EF),  pollution load index (PLI), geo-accumulation index (Igeo) as well as USEPA sediment quality guidelines were evaluated in order to assess the degree of pollution of the stream sediments. All the analysed heavy metals were widely distributed in the drainage system. The ranges in the concentrations of the elements were:  2.52-50.88 ppm for Cu, 1.20-12.80 ppm for Co, 5.60-41.80 ppm for Cr, 0.28-1.72 % for Fe, 89-619 ppm for Mn, 2.46-19.34 ppm for Pb, 4.80-70.60 ppm for Zn and 1.40-12.30 ppm for Ni while the mean concentrations were as follows: 10.5 ppm for Cu, 5.07 ppm for Co, 20.82 ppm for Cr, 0.83 % for Fe, 248.80 ppm for Mn, 5.79 ppm for Pb, 15.06 ppm for Zn and 6.29 ppm for Ni. Pearson correlation coefficient produced strong positive correlation between Cu-Zn, Fe-Co, Mn-Co, Co-Cr etc. The enrichment factor (EF), pollution load index (PLI) as well as the geoaccumulation index (Igeo) results indicated that the rivers/streams were practically unpolluted with respect to heavy metals. The slightly high values of some elements could be due to anthropogenic sources. In conclusion, the levels of concentrations of heavy metals in the stream sediments of the study area did not constitute any serious environmental risk.

Keywords: Pollution, Enrichment Factor, Mineralization, Lithology.