A Geochemical assessment of Soils within Oke-Ogun Area, Southwestern Nigeria

Abstract

Soil samples within Oke-Ogun area, Southwestern Nigeria, have been analysed for major elements and trace metals with a view to investigate the lithogenic concentrations and distribution, assess the relative mobility of elements during weathering and understand the heavy metals association using multivariate statistical analytical methods.  Inductively coupled plasma-mass spectrometry (ICP-MS)  analytical results of trace metals in the residual soils indicate  elevated values (ppm) for Pb(66.78), Ni(10.64), Cr(35.17), Se(0.73), V(19.36), Mn(665.41) and depletion for Cu(16.59), Zn(52.85), As(1.18), Sn(1.41), Sb(0.18) when compared with those of underlying rocks. Also Pb, Cr and Cd recorded greater values than the maximum limits repoted for Poland, Russia, U.S.A. and UK. Multivariate analysis shows good to strong correlation between Cu-Pb-Cd-Ni-Co; Zn-V and Mn-Cr associations. Hierarchical cluster analysis on the other hand revealed early cluster associations between Cu-V-Cd-Zn and Ni-Co while Pb and Mn formed subsequently later as two additional clusters. The Principal Component Analysis (PCA) revealed a three-factor association with the first having a loading of Cu-Pb-Cd-Ni-Co-Se at a loading factor > 0.6. The ambiguity presence of Cu in Factors 1 and 2 was resolved using orthogonal rotation (varimax) revealing differing sources for the trace metals. Soil matrix inclusion in the plot revealed that the metal loadings of Factor 1 were associated with the Fe-leachate phase. Weathering indices and metal ratio showed that the freshly weathered soils have marked enrichment in Pb, V, Cr and Co which can be linked to the weathering of amphibolites within the area.

Keywords: Principal Component Analysis (PCA), Weathering, Metal mobility, Soil Geochemistry