| Heavy metals in urban road sediments, the important carrier of pollutants, may be washed off and finally end up in receiving water bodies during rainfall events. Further more, the heavy metals will threaten urban water systems. Heavy metals in sediments have potential risks to human health due to their non-biodegradable and persistent characteristics. Though a great number of studies fouced on heavy metals in urban road sediments, there still exist some disadvantages in related research. Little information on heavy metals releasing into rain water is available, especially in the contribution of pollution load to the storm runoff under rainfall condition.Therefore, this study focuses on the heavy metals(Cu, Zn, Pb, and Cd) in road sediments in Beijing. The present status of related research are systematically investigated, and then this paper analysis the distribution characteristics of particles. The concentrations of heavy metals in sediments with different grain size are also discussed. In addition, this study analysis the chemical fraction of heavy metals in urban road dusts using partial sequential extraction with the Tessier procedure and the apparent mobility and potential metal bioavailability. Finally, this study addresses on the variation of heavy metals during the rainfall events, and the quantitative analysis is monitored to identify the contribution of heavy metal pollution load to storm runoff.Main results of this research are listed below:(1) Most of the sediments are in medium and small size, while percentage by weight of fine particles(< 300 μm) accounted for about 70%. The results demonstrate that the main influences on the composition of particles are the traffic density, extensive construction and dry period. Moreover, the accumulation of particles is affected by the roughness and material of road significantly.(2) The amount of heavy metal content was the largest in particles with small size(< 300 μm) among all samples. Specifically, the percentage of Cu, Zn, Pb and Cd in these particles was 74.7%, 55.5%, 56.6% and 71.3%, respectively. With the grain size increasing, the metal content will decrease in general. The results of the chemical fractionation patterns of heavy metals in sediments reflect different change patterns. The high percentage of Cu is measured in the organic fraction and Zn partitioning is dominated by the carbonate and Fe-Mn oxide. Pb is preferentially bound to Fe-Mn oxide, residual and organic bound while exchangeable and carbonate fraction of Cd was the highest among all these four metals. Besides, the percentage of unresidual will decrease with the particle size increase.(3) The values of these metals(Cu, Cd, Pb) are higher than the background concentrations in the soil of Beijing, especially of Pb and Cd. The calculated results of geoaccumulation index and Hakanson potential ecological risk index present that the metals’ pollution is at the same level. The values of these two indexes decrease in the order of Cd > Pb > Cu > Zn. Further more, this study indicates that vehicle traffic is the main source of heavy metals in urban road sediments and coal combustion is one of the important sources of Pb.(4) The results demonstrate the release abilities of heavy metal in road dusts in the order of Cd>Pb≈Cu >Zn by runoff erosion under rainfall events. In addition, the contribution rates of pollution load of heavy metal in road sediment accounted for storm runoff pollution load are as as follows: Cu:0.72- 22.21%, Zn: 4.05- 15.61%, Pb: 0.71-16.83%, Cd:2.07-35.16%. The ratio(di) of the average rainfall intensity and the dry period length has a strong correlation with the contribution rates of pollution load of heavy metal in urban road sediments. |
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