Spatial distribution and accumulation of heavy metals in tidal flat sediments of Shanghai coastal zone

Surface and core sediment samples were taken from 15 sites at high, middle and low tidal flats of Shanghai coastal zone on July, 1998. The sediment cores were sliced into 1 cm thick sections. All surface and sliced core sediment samples were dried, ground and sieved, then digested with HF-HC104-HNO3 and made up to a standard volume. Concentrations of Cu. Pb. Zn. Cr, Fe and Mn were determined by a flame atomic absorption spectroscopy (AAS). For the determination of Cd. AAS with graphite furnace was used. The average concentrations of heavy metals in surface sediments of Shanghai coastal zone are arranged in the following order: Zn>Cu>Cr>Pb≫Cd. The concentrations of Cd is almost three orders of magnitude lower than that of other heavy metals and shows no obvious pollution. However, the concentrations of Cu, Pb, Zn and Cr are 2∼5 time higher than their background values and show very serious pollution trend due to the direct discharge of industrial and municipal sewage along the Shanghai tidal flat. Along the south bank of Yangtze estuary, the concentrations of Cu, Zn and Cr decrease from the upper estuary to lower estuary, with the peak at both sites of Shidongkou and Bailonggang sewage outlet. However, there are no obvious changes in the concentrations of Cu, Zn and Cr along the north bank of Hangzhou bay. The average concentrations of heavy metals in the south bank of Yangtze estuary are 50%∼180% higher than that in the north bank of Hangzhou bay, Chongmingdongwangsha bank and Jiuduanzhongsha bank. These results indicate that heavy metals prefer to accumulate in the sediments near sewage outlet. The concentrations of Cu, Zn and Cr in sediments of the study area are closely related with geomorphic units of tidal flat except Pb. They appear to be enriched in the sediments of high flat with a concentration being 30%∼70% higher than that of middle and low flat. This cross-section pattern is mainly caused by tidal hydrodynamic action. However, the violent human activities, such as the building of large engineering projects in the bank can totally destroy this regular pattern. The cross-section distribution of Pb reflects that it is mainly originated from wet and dry depositions of automobile tail gas and industrial dust. The vertical profiles of heavy metals in sediment cores show multi-peak patterns, with the largest peak appearing at the depth of 10∼30 cm. This subsurface enrichment of heavy metals in tidal flat sediments is mainly controlled by the redox cycle of Fe and Mn during early diagenesis near the sediment-water interface. The multi-peak pattern may be related with the windstorm tide which results in unlinear sedimentation.