Bioavailability of heavy metals in urban surface dust and rainfall-runoff system

A sequential digest was used to examine the speciation of particulate-associated heavy metals in multi-media environment of surface dust and rainfall-runoff system. Within the Shanghai central district, different environment medium in four sites were sampled including street dust, runoff suspended particles, gully pot sediment and river sediment during April 2006. The result shows that in the study area, heavy metal concentrations of surface dusts are significantly higher than the Shanghai soil background values and the nonpoint runoff pollution of Pb, Cr and Ni are serious while Cd, Cu and Zn pollution degree relatively light. In the multi-media transport process, the order of heavy metal bioavailability is Zn > Ni > Cd > Cu > Pb > Cr. For Cr, Zn and Cu, the dominated chemical forms of the four different environmental media remain the same phase of residual, carbonates and organic fractions respectively. For Ni, the main fraction of surface dust is associated with residual form, while the other three media become associated with carbonate fractions. For Cd, the surface dust is mainly associated with carbonates, while runoff particles mainly with labile fractions. The dominated chemical form of Pb also changes from Fe/Mn oxides phase to organic phase. The runoff particles contain the highest percentage of the labile fraction (F1+F2), and the mean value of transporting ratio of the runoff suspended particles equals to 1.74, indicating that in urban runoff water, the high bioavailability of the heavy metals and the potential toxicity effect deserves our attention greatly. In gutter inlet and rivers deposit components, the low percentages of the labile fraction and the higher content of residual fraction reduce the environmental risk of the heavy metals and act as the sink of these elements.