Heavy metals in road-deposited sediment and runoff in urban and intercity expressways

Road-deposited sediments (RDS) accumulated on road surfaces are an important environmental medium that often provides transport for heavy metals. In order to understand the characteristics of heavy metals in RDS, we investigated the size distribution and heavy metal (i.e. Cd, Cr, Cu, Ni, Pb and Zn) concentrations in RDS and road runoff in typical expressways of suburban and rural area in China. The results demonstrated that RDS particles from all impervious expressway surfaces were predominantly coarse particles (>250 $\mu$m, 58.4% ± 3.8%) and the RDS on all expressways contained more heavy metals than background soils. Furthermore, the concentrations of Cd, Cr, Cu, Ni, Pb and Zn increased with traffic volume, suggesting that these heavy metals are mainly derived from traffic activities. Moreover, it is found that although heavy metals were concentrated in fine grain fractions, the total loading is highest for the coarsest particles due to their large mass percentage. Additionally, it reveals that the heavy metal concentrations in road runoff of suburban expressway are higher than that in urban expressway. Our findings enhance the understanding of expressway pollution, and imply that for controlling runoff pollution from RDS, transportation agencies should be concerned with both fine and coarse particles and take cleaning methods into account.