China's Yangtze Estuary: I. Geomorphic influence on heavy metal accumulation in intertidal sediments

Five intertidal sites along the coast of the Yangtze Estuary, China were examined for concentrations of heavy metals including Cu, Zn, Pb, Ni, Mn and Fe in bulk surficial and core sediments. Differences in heavy metal concentrations are apparent between the sites, which are dependent on site-specific metal inputs and sediment grain size. With the exception of site A (Shidongkou), where the highest concentrations of Cu, Zn, Pb occur due to direct pollutant input from the nearby sewage outlet, heavy metal concentrations at other sites are largely determined by particle size characteristics. Clay-rich sediments, together with a downstream location relative to the sewage outlets, result in elevated concentrations of heavy metals at site C (Donghai). Within the intertidal zone at site C, the vegetated, upper marsh zone exhibits higher heavy metal concentrations in comparison with the bare mudflat. Monthly sampling (May to September) in the Scirpus marsh at site C records a temporal variation in heavy metal concentrations. Both of them can be related to the spatial and temporal variability of sediment grain size. A geomorphic understanding of the heterogeneity of sediment grain size is, therefore, vital to the assessment of sediment pollution in intertidal sediments. The present study reveals that the levels of heavy metals found in the Yangtze Estuary are relatively moderate compared with other estuaries in Asia and Europe. This is related to both the diluting effects of the huge volume of water and sediment in the estuary and a shorter period of industrialisation.