VERTICAL MIGRATION OF SOME SELECTED HEAVY METALS ALONG ILESA-AKURE ROAD SIDE SOIL

Abstract

Environmental pollution of heavy metal from automobiles has attained much attention in the recent past. The present research was conducted to study heavy metal contamination in roadside soils of Ilesa-Akure highway. Roadside soil samples were collected from 5 locations namely; Ilesa, Erin-ijesa, JABU, Ilara-mokin and Akure at the depths of 0-5cm, 15-20cm, 35-40cm and 55-60cm and at the distances of 5m, 30m, 60m, and 110m. The soil samples were analyzed for five heavy metals which include zinc, lead, copper, chromium and nickel, both in the bulk sample and clay fraction. Their concentrations and distributions in the four different road verge zones (5m, 30m, 60m and 110m) were determined. Samples taken at distance of 30m, 60m and 110m were collected mainly from area where there are no traffic and industrial activities. A total of forty- four (44) soil samples were collected. Soil physicochemical properties such as pH, organic matter content and Cation Exchange Capacity (CEC) were determined. Also, basic geotechnical analysis was carried out, coupled with the determination of heavy metal contents at various depths and distances from the road. The result of pH revealed that the soil samples were slightly acidic, while the pH values increased down the soil profile (from 0-5cm to 55-60cm) at the range of 3.48 – 4.94. Also, the CEC increased from the depth of 0-5cm to 55-60cm in the range of 7.61 to 15.29 cmol/kg. On the other hand, soil organic matter content decreased as the sampling depth increased from 0-5cm to 55-60cm and it ranged between 0.49 to 9.97 g/kg. The highest mean concentration of Zn was found in the clay fraction in the value of 83.66 ppm at 5m from the road, while the least was 3.44 ppm in the bulk sample at 110m. The highest mean concentration of Pb was 1.54 ppm at 5m from the road in the bulk sample, while the least was 0.09 ppm in the bulk sample at 110m. Chromium highest mean concentration was 42.06 ppm at 5m in the clay fraction, and least was 0.34 ppm in the bulk sample at 110m. Copper highest mean concentration was 25.61 ppm in the clay fraction at 5m, while the least was 0.63 ppm at 110m in the bulk sample. Nickel highest mean concentration was 18.92 ppm in the clay fraction at 5m, while the least was 0.16 ppm in the bulk sample at 110m. Zinc and Lead exhibited a significant decrease with depth from 0-5cm to 55-60cm. On the other hand, Chromium and Nickel showed significant increase with depths (0-5cm to 55-60cm). Contrary to other metals investigated, Copper showed no significant difference with depth at all. Quantitative assessment of the heavy metal contamination using geo accumulation indices (Igeo) and pollution indices (PI) indices showed that the selected heavy metals were in the order of Cu < Pb< Zn< Ni <Cr for both Indexes. From the result of Atterberg limit test, the percentage clay fraction ranged between 13.89% to 26.31%, while the activity ranged between from 0.30 to 1.38. Clay minerals present in the soil were Illite and Kaolinite. It was found that the concentrations of the metals at the different locations/depths and at various distances from the roadside were lower than their DPR target values. In conclusion, concentration of heavy metals along the Ilesa-Akure highway is gradually moving towards target value provided by DPR; hence the soil would still function optimally as the area is gradually experiencing sub-urbanization/urban sprawl.