| Urban runoff pollution is becoming a serious environmental problem in the context of rapidly urban expansion and no-point pollution control in China. In this study, the mechanism of street dust washoff processes and the estimation method of pollutant loads in the runoff with street dust were discussed based on investigation of spatial distribution and pollution characteristics of street dust, linkage between street dust and runoff pollution, and rainfall simulation experiment. The findings are as follows:The obvious differences in environmental background and management level between urban and rural regions significantly affected street dust distribution and pollution characteristics. The investigation showed the proportion of finer grain size and concentration of Pb in street dust were the highest in Urban, as for concentration of Pb on the road, the ranking order from high to low was urban (70.5±11.8mg/kg)>urban village(66.7±12.8 mg/kg)> satellite town (56.3±9.5 mg/kg)>township(51.2±6.6 mg/kg)>village(38.6±3.2 mg/kg), while unit area mass and heavy metal load were higher in urban village and village than other places, unit area Pb mass on road and residential district were 4.87±1.45mg/m2,6.94±9.32 mg/m2; 4.74±4.53 mg/m2,6.33±6.34 mg/m2 respectively.Land use types were key factors affecting particle size distribution of street dust heavy metal contamination in street dust and runoff. The study showed that the concentration of heavy metal in street dust and runoff were highest in traffic road in all of land use types in this study, as well as the percentage finer grain size particulate(smaller than 105μm,47.8%,81% respectively, while there was no significant of particle size distribution in runoff among all the land use types.A series of rainfall simulation experiments were conducted in order to study street dust washoff processes influenced by different diverse particle constitutes of street dust. It was found that the smaller grain size fraction of street dust was more rapidly washed from the cement ground than from the asphalt floor, while bigger one is not significant. This result indicated that roughness was very important for runoff behavior of finer particle. There is clearly higer mobility of small grain size better than big one in both impervious ground. The quantity of per unit area of street dust will not influent the rate of washoff.Based on rainfall simulation experiments, the method of estimating heavy metal loads in runoff at the large scale was established. Imperious surface information was picked up with GIS, then categorize and calculate the area of different tapes, at last according to the ratio of washoff obtain urban-suburban-rural gradient washoff potential pollution load. The result show that washoff potential pollution load of Pb at rainfall intensity 25mm/h,40mm/h,80 mm/h were 2319.4kg.2693.6kg,3287.1kg in 1h in Bejing.This study can provide the basis for deeply understanding the pollution characteristics of street dust and its pollution mechanism through urban runoff, which is helpful to build methodological foundation of estimating pollutant loads from street dust in urban runoff events at large scale. As well as indicate that effective to solve the problem of water-environment with measures of control source of runoff pollution-street dust. The findings imply that different strategies should be considered among different urban and rural regions for practices of controlling runoff pollution, such as first flush control will be taken in urban and satellite town, and improvement of clean technology on impervious ground will be carried out in urban village and village.
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