Traffic-related heavy metal accumulation in soils and plants in northwest China

Pollution caused by traffic activities is increasingly becoming a great threat to urban environmental quality and human health in many municipalities in Northwest China. The Sophora japonica L., a native tree species occurring widely in many regions of Northwest China, was used as a case study to assess the potential effects of traffic pollution on heavy metal accumulation in leaves of S. japonica trees and associated soils. Fifly-four leaf samples and 41 relevant soil samples (0-10 cm) were collected systematically along main trunk roads and at parks distant from main trunk roads in the city of Lanzhou, Northwest China, respectively. Traffic pollution has resulted in significant accumulation of heavy metals in both the roadside leaves and soils, but the pattern and level of accumulation varied remarkably between elements. The nine elements examined can be classified into three groups relating to their responses to traffic pollution. The first group, including Zn, Cd, Hg, Pb and Cr showed greater accumulation in both roadside soils and leaves. The second group, including Co, Ni and As, indicated greater accumulation in the roadside leaves only. The third group included only Cu and demonstrated a greater accumulation in the roadside soils only. Overall, Zn, Cd, Hg, Pb, Cu and Cr concentrations in the roadside soils were higher (8-72%) than those in the park soils, as well as much higher (32-300%) than the background values of the respective elements set for local soils (Lanzhou). Zn, Cd, As, Hg, Pb, Cr, Ni and Co concentrations in the roadside leaves were higher (27-111 %) than those in the park leaves. The differences found among elements in the levels of accumulation suggest that the relative importance of the individual elements contributing to urban environmental deterioration will vary considerably.