Characteristics of heavy metal pollution in road runoff in the Nanjing urban area, East China

An extensive field survey was conducted in four types of road area to study heavy metals in road runoff. Eleven rainfall events were monitored from February 2011 to March 2012, which were classified into four categories according to the relationship between peak of the rainfall amount, rainfall duration, and average rainfall intensity. Runoff samples were collected from overpass sections, college areas, residential areas, and road sidewalks. Heavy metal concentrations were obtained to investigate the outflow laws governing heavy metals in runoff. The concentration fluctuations of seven heavy metals were monitored to assess the influence of rainfall characteristics on metal concentrations. To estimate the impact of heavy metals on the water environment, the event mean concentrations (EMCs) were determined to describe the overall pollution degree of heavy metal in runoff, and then the EMC values of heavy metals in runoff were compared with surface water environmental quality standard thresholds. The results indicate that the EMC values of heavy metals varied widely in different rainfall fields and under the same rainfall at different sampling points. Average rainfall intensity has a significant impact on the EMC of heavy metal outflow, followed by maximum rainfall intensity and rainfall amount.