Chemical Speciation and Distributon of Heavy Metals in Alluvial Soils of Ondo State

Abstract

Floodplains are very important for agriculture, since it allows planting of arable crops throughout the year. In order to asses pollution levels of the floodplains due to contamination from flooding,soil samples were collected from six locations across each of the floodplain of Owena river, Ogbese river and Ala river in Ondo State. The study investigated the chemical speciation of Cd, Cu, Ni and Zn in the alluvial soil matrix due to deposition of metals as result of inundation of the floodplains. The heavy metals were quantified using Atomic Absorbtion Spectrophotometer. The concentration of heavy metals in  exchangeable fraction  are in the order of Ni (BDL-0.26 mg/kg) < Cd (BDL - 0.41 mg/kg) < Cu (0.08-1.33 mg/kg) < Zn (0.27-7.08 mg/kg), in carbonate fraction: Ni (BDL-0.53 mg/kg) < Cd (BDL – 1.83 mg/kg< Cu (0.13-3.02 mg/kg) < Zn (0.64-11.88 mg/kg), in Fe-Mn Oxide fraction: Cd (BDL-0.21 mg/kg)  < Ni (BDL-0.53 mg/kg)  < Cu (0.68-3.03 mg/kg) < Zn (3.05-17.32 mg/kg), In organic fraction: Cd (BDL-0.15 mg/kg) < Cu (0.51-6.00 mg/kg) < Ni (0.73-7.84mg/kg ) < Zn (1.80-21.82 mg/kg), In residual fraction: Cd (0.20-1.28 mg/kg)  <  Cu (1.18-6.19 mg/kg)  <  Zn (2.77-13.97 mg/kg) < Ni (1.54-13.59 mg/kg). Cd and Ni were most abundant in residual fraction, Cu was found to be predominantly associated with organic and residual fractions while, Zn was  predominantly bound to oxidizable and organic fractions The mean concentration of the heavy metals in the floodplains soil were of the order of Zn > Cu > Ni > Cd. The speciation study futher revealed both geogenic and anthropogenic sources contributed to the total metal load of the floodplain soils.