IMPACT OF FLUVIAL DEPOSITION ON HEAVYMETAL BURDEN OF ALLUVIAL SOIL OF SELECTED FLOODPLAINS IN ONDO STATE.

Abstract

Fluvial transportation and deposition along floodplains have brought some challenges to the environment because of its potential negative impact on the floodplains soils. Thus, this research is designed to assess the level of some heavy metals and their distribution in the matrix of alluvial soil of some selected floodplains in Ondo State, Nigeria. Soil samples taken along the floodplains of Ala, Owena and Ogbese rivers were subjected to sequential extraction technique given five (5) fractions, including exchangeable (F1), bound to carbonate (F2), bound to Fe-Mn oxides (F3), bound to organic matter (F4) and the residual (F5). Heavy metals (Co, Cr, and Pb) in each fraction were quantified using Atomic Absorption Spectrophotometer. Some soil Physico-chemical parameters including particle size, total organic matter content (TOM), pH, that are known to influence the interaction and nature of metals within the soil matrix were also determined using standard methods. The results for the three floodplains soils showed sand, clay and silt in the range of sand (48.31- 71.50), (2.00 – 34.90) and (6.00 – 36.92) respectively, indicating that the soils are predominantly sandy-clay-loam soil. The soil reaction showed acidic to slightly alkaline condition with pH values ranging from (5.36 – 7.53),while the total organic matter content (TOM) ranged from (2.91 – 25.88%) The heavy metals levels(mg/kg) in the floodplains soils were: Co (3.01 – 7.14); Cr (7.64 – 21.26) and Pb(3.63 - 38.89) for Owena river, Co (3.75 – 19.13), Cr (6.88 – 17.51) and Pb (4.38 – 17.13) for Ogbese river; and Co (7.14 – 25.63); Cr (13.51 – 37.64) and Pb (2.89 – 58.01) for Ala river. The distribution of the metals in the soil matrices showed Co to be predominantly more bound to F5 followed by F3, F4, F1 and F2; Cr to F5 followed by F4, F2, F3 and F1; and Pb to F4 followed by F2, F1, F3 and F5. These show that the presence of these metals are attributed to both anthropogenic activities and lithological process. Correlation study showed that positive correlations (at p<0.05) exist between TOM and Co as well as TOM and Cr in Owena floodplain, TOM and Co, TOM and Cr as well as TOM with Pb in Ogbese floodplain soil, while there exist no correlation in Ala floodplain soil. The pollution load index (PLI) in Owena floodplain soil ranged from(0.56 – 1.29); Ogbese floodplain soil (0.62 – 1.20) and Ala floodplain soil (0.73 – 2.73), with mean values indicating deterioration of Ogbese and Ala rivers floodplains, while Owena floodplain indicates baseline level of the metals. The contamination factor (CF) of the three studied metals in the floodplains soils showed Co (0.34 – 2.91); Cr (0.75– 3.66) and Pb (0.31 – 4.38) indicating low to moderate contamination. The mobility factor (MF) (%) for the three metals recorded low to moderate mobility with Co (2.61 – 41.16); Cr (0.00 – 32.70) and Pb (2.97 – 60.62),while the enrichment factor (EF) for Co, Cr and Pb were 0.66 – 3.59, 0.29 – 1.83 and 0.06 – 5.52 respectively, indicating ‘deficiency’ to ‘minimal enrichment’. This study therefore has provided vital information about the distribution, levels and contamination/pollution status of the studied metals in the floodplains soils useful for proper management of the rivers’ catchment.