Roadside topsoil concentrations of lead and other heavy metals in Ibadan, Nigeria

Concentrations of Pb, Zn, Cd, Cu, Cr, Co, and Ni were determined in roadside topsoil collected from locations of varied vehicular traffic densities in the city of Ibadan, Nigeria, with a view to determining the level of contamination and the contribution of traffic density. Levels of Pb, Zn, Cd, and Cu were elevated above background concentrations measured in control areas. Average values (ppm) for all sample locations were Pb - 81 +/- 140; Zn - 48 +/- 37; Cd - 0.55 +/- 0.49; Cu - 17 +/- 17, Cr 22.1 +/- 9.6; Co - 7.9 +/-. 3.8; Ni- 10.5 +/- 9.7. Factors of accumulation of metals in roadsides relative to control sites were highest for Pb. Vehicular traffic was not an important source of chromium, cobalt and nickel, for which roadside concentrations were about those of the control sites. Metal concentrations were poorly correlated with traffic volumes. An average of about 60% of total soil concentration of the metals were determined to be held in bioavailable geochemical phases, of which the highest concentrations were mostly held in either the reducible or oxidizable phase. Levels of the metals in the topsoil were generally lower than the soil quality criteria of some developed countries.