Risk assessment of sedimentary metals in the Yangtze Estuary: new evidence of the relationships between two typical index methods.

The contamination and ecological risks of six metals (i.e., As, Cu, Mn, Pb, Sb and Zn) in sediments of the Yangtze Estuary were assessed using two typical index approaches, (i.e., total content and speciation indices). Special attention was paid to the relationships between the two index methods. The ecological risk of each sampling site was uniformly low to moderate in degree. The contamination levels of these sites were low to moderate according to the speciation index used, while the total content indices indicated a moderate to considerate contamination. A significant positive relationship existed between the two index methods when assessing the risks of Cu, Pb and Zn, while negative correlations and inconsistencies existed for As, Mn and Sb. The correlation behaviors were mainly dependent to three metal factors: the fractional distribution, the enrichment degree and the toxic response factors. Significant correspondences were observed between the total content and the speciation indices when assessing the ecological risks of each sampling site and each metal (Cu, Pb or Zn), while discrepancies were found between the two approaches when assessing the contamination of specific sites in the estuary area.