Comprehensive risk assessment of heavy metals in lake sediment from public parks in Shanghai

Rapid urbanization has caused potential pollution of heavy metal in Shanghai. A comprehensive pollution study of heavy metals (Cd, Cu, Pb, and Zn) in 35 lake surface sediments from city parks in the four different urbanized areas of Shanghai was conducted. Intensive human activities caused moderate enrichment of the four metals in highly urbanized areas, especially Cd with the significant enrichment in the central urban core area. However, the levels of the four metals in all the sediments were lower than the corresponding consensus-based Probable Effect Concentration, indicating adverse effects not to occur frequently. The integrated pollution assessments of multiple heavy metals also suggested low ecological risk and 15-29 percent probability of toxicity in most of sediments. The metal speciation analysis showed that Cd, Cu, Pb, and Zn were dominated by the non-residual fractions and thus they have high mobility and bioavailability, indicating significant anthropogenic sources. According to the Risk Assessment Code, Cd had the highest bioavailable fraction and represented high or very high risk, followed by Zn with medium or high risks in most of samples, while no or low risk was found for Cu and Pb at most sites because they were dominated by reducible and residual fractions. Correlation analysis showed that chemical fractions of heavy metals were prone to transform among each other if environmental conditions changed. Therefore, in view of anthropogenic inputs and speciation distribution, heavy metals with very high bioavailability at very low total levels and those with low bioavailability at very high total levels should not be ignored.