Evaluation of Heavy Metal Pollution from Vehicular Exhausts in Soils along a Highway, Southwestern Nigeria

The Lagos-Ibadan Expressway is the busiest inter-state route in Nigeria and one of the largest road networks in Africa. It handles more than 250,000 PCUs (Passenger Car Units) daily. Amount of metal emissions being released daily from vehicles plying this road into the environment (air, soil, plant and water) is great! This study evaluated heavy metal composition in soils resulting from exhausts pollution from vehicular movement along Lagos-Ibadan Expressway. A total of two hundred and seventy-six soil samples were collected at 5, 15 and 25 m away from the edge of the road. The soil samples were collected with hand auger from the surface to a depth of 10 cm during both dry and wet seasons. Heavy metal concentrations were determined using Atomic Absorption Spectrophotometer (AAS). The heavy metal concentrations found in sampled soil during dry season in mg.kg-1 along the study sites revealed the following, Zn (558.03, 21.98), Pb (130.96, 3.64), Cr (24.08, 1.09), Cu (97.43, 4.41) and during wet season Zn (532.51, 5.72), Pb (120.52, 1.85), Cu (79.90, 1.05), Cr (19.82, 0.22). The results indicated a general reduction in heavy metal concentrations in the soil collected during rainy season, when compared with soil collected during dry season. Index of geo-accumulation (Igeo) of sampled soils revealed that contamination ranged from moderate to extreme in all the study locations during dry and wet seasons. Potential ecological risk Index (PERI) of the individual location in the study area ranged from 4.38 to 146.84. However, the total PERI of study area was 1136, which indicated that Lagos – Ibadan Expressway is generally polluted with heavy metals. Fe, Mn, Zn, Pb, Cu and Cr are the major heavy metal pollutants found in the soils during dry and wet seasons. The source of these heavy metals being vehicular exhausts emissions along Lagos – Ibadan Expressway.