Heavy metal contamination of roadside topsoil in Osogbo, Nigeria: its relationship to traffic density and proximity to highways

The levels of lead, cadmium, copper, nickel and zinc were determined in the roadside topsoil in Osogbo, Nigeria, with the view to determining the effect of traffic density and vehicular contribution to the soil heavy metal burden. The levels of the metals at the high density roads were significantly higher than the corresponding levels at the medium and low traffic density roads. The average levels of Pb, Cd, Cu, Ni, and Zn in all road locations at a distance of 5 m from the roads were 68.74±34.82, 0.60±0.31, 21.19±11.34, 8.38±2.40, and 42.45±17.68 mg/kg, respectively. Pb, Cd, and Cu were of average levels of 92.07±21.25, 0.76±0.35 and 27.69 mg/kg, respectively at a distance of 5 m from the road at high traffic density roads, while the levels of Ni and Zn averaged 9.65±2.61 and 56.27±11.58 mg/kg, respectively. There was a rapid decrease in the level of the metals with distance, with the metal levels at a distance of 50 m from the road almost reaching the natural background levels of the metals at the control sites. The levels of the metals were also determined at the four major motor parks and at the seven mechanic workshop settlements. The levels of the metals at the motor parks and mechanic workshops were far above the levels at the control sites. The levels of Pb, Cd, Cu, Ni, and Zn at the motor parks were 519±73.0, 3.6±0.8, 37.9±12.5, 17.3±4.6 and 71.9±11.3 mg/kg, respectively, with the levels of Pb, Cd, Cu, Ni, and Zn at the mechanic workshops averaging 729.57±110.93, 4.59±1.01, 116.6±26.8, 30.21±9.40, and 88.74±28.11 mg/kg, respectively.