Heavy metals of surface sediments in the Changjiang (Yangtze River) Estuary: Distribution, speciation and environmental risks

Terrigenous sediment in estuarine environments serves as a major sink for heavy metals sourced from land, and thus has been widely used for the investigation of anthropogenic pollution and environmental risk. This study presents the concentrations and chemical speciation of eight heavy metals (Zn, Cu, Ni, Pb, Cr, Hg, As, Cd) in 34 surface sediment samples collected from the Changjiang (Yangtze River) estuarine area, which aims to investigate the potential sources and environmental risks of heavy metals. The concentrations of these metals in inner estuary are overall higher than those in coastal and offshore areas, with the highest values occurring in the South Branch, South Channel, and North Hangzhou Bay. Although the Changjiang river input is the dominant source of these heavy metals, the local contamination and variations in sediment characters especially the grain size and particulate organic matter account for the heterogeneous compositions of these heavy metals in these estuarine sediments. The sequential extraction results suggest that most of these heavy metals, except Cd, are bound in the geochemically inert residual fractions, and primarily determined by the Changjiang input from natural weathering sources. The results of geoaccumulation index (Igeo) and enrichment factor (EF) suggest that these heavy metals do not show obvious enrichments and contamination in this region, while the higher bulk concentrations of Cd in the estuarine and offshore sediments are predominantly inherited from the Changjiang sediments, although the residual Cd in the crystal lattice of minerals reflects the average Cd content in the upper continental crust. The extra Cd adsorbed to the river sediments is strongly bound to exchangeable-, carbonate-and Fe-Mn oxides-associated phases, which may be chemically reactive during estuarine processes and thus has high environmental risks under enhancing human activities.