Biological Availability And Resuspension Of Heavy Metals In Coastal Sediments From South Yangtze River Estuary

The intertidal flat in Wusong-Liuhe coastal zone in the Yangtze Estuary is a complex ecosystem influenced both by natural fators and strong human activities.In order to evaluate biological availability of heavy metals in sediments,acid volatile sulfides(AVS) and simultaneously extracted metals(SEM) of 37 surface sediments colleted from Wusong-Liuhe coastal zone in the Yangtze Estuary were determined. This is favourable for evaluating the toxicity level to human;a tidal cycle was observed nearby Chenghang reservoir to study resuspended release process of heavy metals(Pb,Cr and Cd) in sediments under the condition of different tidal dynamics and physical and chemical factors(such as pH and salinity).In addition,combined with results of spot observation,the effects of sediment resuspension for Pb,Cr and Cd at the sediment-water interface exchange at the sediment through different techniques of experimental simulation were discussed.The main results gotten in the article are as follows:1)The average content of Pb,Cr,Cd,Hg and As was 33.1μg·g^-1,56.6μg·g^-1, 0.4μg·g^-1,0.38μg·g^-1 and 40.0μg·g^-1 of sediments in Wusong-Liuhe coastal zone. All heavy metals are above background values of heavy metals of mad flat in Shanghai.The content of all heavy metals exceed background value of heavy metals of soils in Shanghai except Cr.The average of Cd were 0.189μg·g^-1, 0.234μg·L^-1 in sediments and water from upwards Liu river estuary.The content of Cd in sedimens and water exceed the background value of heavy metals and the water of Chenhang reservoir in June,2008,respectively.There probably exists a polluted strip in Yangtze River south estuary.2)The SEM contents of the surface sediments ranged from 0.20μmol·g^-1 to 1.37μmol·g^-1(dry weight),with an average value of 0.74μmol·g^-1;The AVS contents varied between 0.20-0.80μmol·g^-1(dry weight),with an average being 0.23μmol·g^-1 of sediment in Wusong-Liuhe coastal zone.Both total contents of different heavy metal(Pb,Cr,Cd,Hg and As) and SEM are over background value of intertidal flat and soil in Shanghai.This is mainly contributed by waste from industry and daily life.The heavy metals combined with HCl is dominating form existed in study area.In upriver of Liuhe estuary,Cd is major contaminant, Pb and Cr are minor contaminant.3)TOC,water level and medium particle size of the sediment are the main controlling environmental factors in study area.The ratio of SEM to AVS and the difference between SEM and AVS are used to assess the biological availability of sediment SEM content.The result shows that the SEM of the surface sediments at Wusong-Liuhe riparian zone in Yangtze Estuary had moderate toxicity to the aquatic life.Resuspended process of sediment will enhance the release of heavy metals which combined with sulfide and then arouse environmental toxicity.4)During spot observation of tidal cycle,water velocity is the mainly controlled environmental factor for the release of heavy metal(Pb,Cr,Cd) in substrate sludge.In addition,water temperature,NO₃^-,DOC,DIC,NH₄⁺,TN are predominate environmental factors which control dissolved Cd in bottom water, DO,Eh are important environmental factors which control dissolved Cd and Pb surface water.Cl^-is an important environmental factor which control dissolved Pb in bottom water.Eh is an important environmental factor which control dissolved Cr in surface water.DO and NO₃^-are predominate environmental factor which control particled Cr in surface water.DOC is an important environmental factor which control dissolved Cr in bottom water.EC is an important environmental factor which control particled Cr in bottom water.5)Correlation analysis is an important way to weigh the relation between dissolved and particled heavy metals(Pb,Cr and Cd) during ticle cyc.Dissolved Cd has significant positive correlations with dissolved Pb and Cr.Particled Cd and total Cd also have this relation with particle Pb and Cr.It is a forceful evidence that particled Pb,Cr and Cd have similar physical and chemical properties and sources.Water velocity has significant positive correlations with particled Pb and Cr and total Pb,Cr and Cd.Tidal impetus has important effect to particled heavy metals of water.6)In comparision with heavy metals(Pb,Cr,Cd,Cu,Ni,Mn,Fe and Mn) content and physics and chemical parameters of columnar sediments before and after tidal cyc,the bottom mud has concentration effect to Pb and Cr,the change of Cd,Fe and Mn are possibly effected by pore water,the bottom mud has a release of Cu and Ni to water.From the initial assessment,the bottom mud depth caused by perturbation affects probably is(0-5cm) initially during the moist cyclical process.7)Under the protection of water biomembrane,the change of dissolved and particled heavy metals' concentration in water is mainly caused by"the silt effect"of heavy metals.When the change of external environmetal factors is relative stable,dissolved and particled heavy metals will be in short-lived balance condition.Moreover,DOC and DO will also controll the change of heavy metals in individual time.8)In the process of indoor resuspension simulation,in contrast with heavy metals (Pb,Cr and Cd) content and physiochemical parameters(sulfide,moisture content, organic matter) of columnar sediments before and after resuspension experiment, most experiments mostly proved that sediment release heavy metals to water,but the way of resuspending has slight difference:or originates from the deposit after the pore water upward rehydrate body's release;or originates from the deposit pore water;or originates in the water body biomembrane the ferro manganese oxide compound union condition heavy metal release.9)Lab-scale resuspending simulation of heavy metal in Changjiang delta bottom mud discovered that the heavy metals'"the silt effect"has remarkable effect to the heavy metals' migration and transformation activity in water body,under the water body biomembrane existence condition.The change of sulfide content can be a powerful evidence to judge the bottom mud depth disturbed by water momentum.