Coastal reclamation mediates heavy metal fractions and ecological risk in saltmarsh sediments of northern Jiangsu Province, China

Coastal reclamation has created enormous extra land for the rapidly growing economy, but it has also caused serious environmental pollution problems and threatened the sustainable development of coastal areas. However, there are few studies focusing on the distribution patterns, geochemical speciation and ecological risks of heavy metals along the land-to-sea belt, as well as the differences between reclamation and non-reclamation. Here, we collected 69 sediment samples from four sediment types along the land-to-sea sampling belts in the reclaimed and non-reclaimed tidal flats of Jiangsu, China. Geochemical speciation and contents of heavy metals were determined to investigate their spatial distributions, ecological risks and effect factors. Results showed that As, Cd, Cr and Ni in the sediments posed considerable or moderate ecological risk according to the Ontario guidelines and sediment quality guidelines (SQGs) of USEPA, but they were lower than the SQGs of China. Higher geoaccumulation index and potential ecological risk index suggested that the sediments were moderately to heavily polluted by Cd and As. Generally, reclaimed sediments exhibited higher metal pollution levels. Additionally, reclaimed areas showed a unimodal pattern of metal content along the direction of land-to-sea, suggesting that Spartina alterniflora could accelerate the deposition and accumulation of metal pollutants caused by reclamation, and ultimately control the transfer of terrigenous metals to marine environment. We found that residual fraction was the dominant geochemical fraction for the metals determined. Reclamation processes have changed the composition of heavy metal fractions, especially Cd, Pb, Zn, and Ni. Approximately 20% of Cd existed in the acid extractable/exchangeable fraction and posed medium ecological risk according to the risk assessment code. The principal component analysis and correlation matrix further indicate that organic matter and particle size of sediment could be the major factors regulating the metal distribution, and Cd and Zn might be anthropogenic sources.