Environmental Biogeochemistry Of Heavy Metals In Intertidal Flat Of The Yangtze Estuary

The intertidal flat in coastal areas of the Yangtze estuary is a complex ecosystem influenced both by natural factors and strong human activities. The distribution, accumulation, transport and fate of heavy metals such as Cu, Pb, Fe, Mn, Zn, Cr and their ecological effects in water-sediment-organisms of such an ecosystem were analysed. Sources and transport channels of heavy metals in intertidal flat of the estuary were discussed. Based on which, the source fluxes and retention of heavy metals were quantitatively estimated. This is favourable for evaluating the intertidal flat contribution in accumulating heavy metals to the whole estuary. The intertidal flat in eastern shore of the Chongming Island was selected for a typical area to construct mass balance for heavy metals. Moreover, environment quality assessment and ecological risk assessment of the heavy metals in intertidal sediments were performed for the whole estuary. The main conclusions can be summed up as follows.(1) Concentrations of heavy metals in intertidal sediments of the Yangtze estuary lie between the mid to lower range for Chinese estuarine and coastal sediments indicating that the Yangtze Estuary receives low but appreciable heavy metal contaminant inputs. Assessment results show that the highest pollution of heavy metals lie in XP (Xu Pu), SDK (Shi Dong Kou) and LG (Lao Gang), but the pollution level of these areas belongs to mid pollution range, in which Zn contribute the most. Moreover, mid potential ecological risk of heavy metals in intertidal sediments is only found along the southern coast of the estuary, in which Cu is the most toxic element. Sewage outlet at SDK and main rivers flowing to the estuary such as Huangpu River are the main pollutant input channels and the hydro-oxides of iron and manganese are the main carriers when pollutants enter the water and are transported. Influenced by the fresh water and the tidal current, most of the heavy metals are deposited in the southern region and some even deposited in the upper areas of the estuary following the flood-tide current. Extreme weather conditions such as storm tide can cause heavily erosion of the intertidal sediments in short term, but may not largely influence the mass balance for heavy metals in intertidal sediments in a long time scale.(2) Cu, Fe, Zn and Cr in intertidal sediments of the Yangtze estuary are mainly in residual fraction, while the major chemical association of Mn is carbonates, with the content up to 48%. The carbonates, residuals and Fe-Mn oxides of Pb have similar contents in the sediments.Concentrations of non-residual heavy metals can be ordered as follows: Mn>Pb>Zn>Cu>Cr>Fe. In the non-residuals, the carbonates of Mn, Pb and Zn have high contents than the other elements and the Fe-Mn oxides have the highest content in Pb. Contents of the organic matter fraction were higher for Cr, Cu and Pb, with the contents up to 9%. The percentages of reducible Cu, Pb, Fe, Mn, Zn, Cr and Al in total concentrations are respectively 26%, 36%, 7%, 49%, 32%, 14% and 1.6%. This order is similar to that of the non-residuals, showing that the bioavailability of heavy metals can be represented by the concentrations of reducible heavy metals available to HA(hydroxylamine hydrochloride in 25% (v/v) acetic acid) to a large extent.(3) The transport and transformation of heavy metals in intertidal sediments are influenced by many factors such as the early diagenetic process, plant growing and so on. Concentrations of metals show surface or sub-surface elevation in mudflat sediments with weak hydrodynamic conditions, which are mainly caused by sediment early diagenetic process. However in salt marsh sediments, the concentrations of heavy metals at the depth of 5-15cm decrease gently due to the absorption of plant roots in summer and autumn and increase gradually to reach broad sub-surface maxima due to the metal release from the decomposition of plant roots at the same depth range in winter and spring. The biogeochemical activities of Cu and P...