Abstract

The total elemental content of soil though may give abundance of element concentration but have been found not to be suitable for prediction of environmental bioavailability and toxicity by scientific community. Surface (0-30cm) and subsurface (60-90cm) soil profile in the Southern Ondo State Nigeria were investigated for spatial distribution, bioavailability and mobility of some essential trace elements (Cu, Fe, Mn, Zn). Their spatial distribution were very similar in both surface and subsurface soil environment indicating that similar geochemical factors may be responsible for their distribution. The North was composed of basement complex while the South was largely undifferentiated sedimentary rock. Higher concentrations of Cu, Fe, Mn and Zn were recorded in the North through the centre of study area to lower concentrations in the South. The spatial concentration of the trace elements may have been influenced by the nature of underlying bedrock type. Cu was potentially bioavailable in both surface and subsurface soil environment considering the fact that >50% of its total concentration were in the nonresidual fraction. Other trace elements were not bioavailable because >60% of their total concentrations were found in residual fractions. The relative risk assessment code of Cu (surface; subsurface) indicated progressive risk (MoF1, MoF2, MoF3) from low (2-10; 1-6) through medium (12-30; 10-21) to high risk (25-40; 21-35) in both surface and subsurface soil environment while Zn (surface) shows similar trend (1-5; 11-21; 22-35) only in the surface soil environment. Other elements show some level of risk to no risk. There is likelihood that Cu and/or Mn may be associated with anthropogenic sources.

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