The influence of traffic density on heavy metals distribution in urban road runoff in Beijing, China

The concentrations, potential risk, and distributions of heavy metals in urban road runoff from different traffic density were determined and compared in Beijing, China. It showed that the concentrations of heavy metals in road runoff were strongly influenced by traffic density, resulting in total concentrations of Cu, Zn, Fe, Mn, and Pb in the runoff from highway higher than those from the road nearby campus. The potential ecological risk of heavy metals in the runoff from highway was higher than those from the road nearby campus. The distributions of heavy metals were not influenced by the traffic density. Cu, Zn, Cr, Cd, Pb, and Mn in road runoff transported predominantly in particulate-bound form and the dissolved form mainly distributed in colloidal fraction (1 kDa to 0.45 μm). Traffic density did not change the speciation of heavy metals in the road runoff, in which Cr and Zn mainly expressed in organic colloidal fraction while Fe, Mn, Cd, Pb, and Cu expressed in inorganic colloidal fraction. The traffic activities would contribute to the strong correlations between Fe, Zn, Mn, Cr, and Pb because of the similar sources.