IMPACT OF FLUVIAL DEPOSITION ON HEAVY METAL BURDEN OF SELECTED FLOODPLAINS SOILS IN ONDO STATE

Abstract

One of the major issues of concern relating to floodplains is the deposition of contaminants from anthropogenic sources (such as domestic wastes, agricultural run-off, fishing, industrial effluents discharge and urban run-off), with the potential of carrying both environmental and public health hazard. In order to assess pollution levels of the floodplains, soil samples were collected from three floodplains of Owena river, Ogbese river and Ala river in Ondo State. The soils were analyzed for their physico-chemical characteristictics: pH, electrical conductivity, organic matter, particle size, and the effective cation exchange capacity. The pH ranged from 5.52-7.53 with mean value of 6.43, 5.93 and 7.02 for Owena, Ogbese and Ala alluvial soils, respectively. Electrical conductivity ranged from 9.20 to 85.00 μScm-1 with mean values of 44.05 μScm-1, 30.58 μScm-1 and 41.27 μScm-1 for Owena, Ogbese and Ala alluvial soils respectively. Total organic matter were high with values ranged from 9.43% to 29.10% with mean values of 19.17%, 17.62% and 21.42% respectively for the three alluvial soils. Effective cation exchange capacity ranged from 7.32 to 52.76 cmol/kg with mean values of 21.84 cmol/kg, 18.62 cmol/kg and 20.47 cmol/kg in the analysed soils respectively. The study also investigated the chemical speciation of Cd, Cu, Ni and Zn in the alluvial soil matrix. The heavy metals were quantified using Atomic Absorption 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) < Ni (0.73-7.84 mg/kg) ) < Zn (1.80-21.82 mg/kg), while 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). 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 that both geogenic and anthropogenic sources contributed to the total metal load of the floodplain soils. The contamination/pollution index, enrichment factor, geo-accumulation index and pollution load index were used to quantitatively assess the influences of heavy metal pollution. Pollution/contamination index evaluation showed that the alluvial soils had very slight metal contamination for all the metals except for Cd being found in the range of very severe contamination to slight contamination in Owena and Ala floodplains. Concentration of Cd in Ala