Heavy metal concentrations in roadside soils and correlation with urban traffic in Beijing, China

A detailed investigation was conducted to study the heavy metal concentrations in roadside soils of Beijing. The concentrations of Cd, Cu, Pb and Zn showed a decreasing trend with increasing distance from the road while such trend was not identified in As, Cr and Ni. In addition, the concentrations of Cd, Cu, Pb and Zn significantly positively correlated with black carbon (BC) and TOC (p<0.01). The soil samples from West 2nd Ring Road with the highest traffic volume had the highest heavy metal concentrations of the 10 roads, and Pb concentration was significantly positively correlated with traffic volumes (p<0.05). According to the soil guideline values of China, Cd was considered to have considerable contamination in roadside soils, while Cu, Pb and Zn less, but As, Ni, Cr none. The concentrations of heavy metals in roadside soils of Beijing were considered medium or low in comparison with those in other cities; this may be due to the windy and dry climate in Beijing. The heavy metals could move with wind along the wind direction and the soil samples had higher heavy metal concentrations at the downwind direction.