The heavy metals/trace elements contents of sediments from Owalla Reservoir, Osun State, Southwest Nigeria

The heavy metals/trace elements contents of sediment samples from Owalla Reservoir were analyzed every three months in two annual cycles (March 2011 – February 2013). The main aim was to measure concentrations of selected elements in sediment samples, their variations in space and seasons and the level of pollution and/or contamination. The bottom sediment samples were collected with a Van Veen Grab and the elemental analysis in the laboratory was based on air-dried samples following standard methods. The overall hierarchy of heavy metals/trace elements in the sediments of the reservoir was in the decreasing order of concentrations: Fe > Mn > As > Zn > Ni > Co > Cr > Cu > Pb > Cd. The concentrations of the heavy metals did not follow any definite pattern from the upstream-downstream basin, although most of them (Cu, Fe, Mn, Zn, and Co) showed significant differences (P<0.05) in their horizontal variations. Mn, Pb, Co, Fe, Ni, and Zn were significantly (P<0.05) higher at the open water region than in the littoral region. All the elements except Ni did not show significant seasonal variations (P>0.05). Most of the elements in the reservoir sediment have concentrations within the background levels and concentrations defined in environmental regulations and guidelines, except for As and Cd. The contamination factors (Cf) for most metals (Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) suggested low contamination in the sediments (Cf < 1.0). Conversely, the sediments were moderately contaminated with Cd (Cf = 2.41) and very highly contaminated with As (Cf = 19.33).