Occurrence Speciation,Source Characteristics And Ecological Risk Assessment Of Heavy Metals In Sediments Along The Yangtze River Estuary And Adjacent Sea Areas

Heavy metal elements have been concerned and vigilant for a long time due to their high toxicity,persistent occurrence and strong concealment.With the development of industry and economy,heavy metals polluted by human activities entering into the ocean increases gradually through surface runoff,atmospheric deposition and offshore discharge.Through certain physical and chemical processes such as adsorption,complexation and precipitation,a number of metals accumulate in the sediments,resulting in damage to the marine ecological environment,and even indirectly affecting human health through the food chain.The Yangtze River estuary is one of the most prosperous and densely populated regions of China.The ecological environment of its adjacent sea area has been polluted by heavy metals seriously.In this study,7 metals were used as target compounds in the surface sediments of the Yangtze River Estuary and adjacent sea areas.Moreover,the distribution and occurrence characteristics of heavy metals in the coastal areas of the Yangtze Estuary and adjacent sea areas were investigated based on the data of heavy metal content and speciation components in sediments.Correlation analysis and cluster analysis were used to analyze the sources and pollution characteristics of heavy metals.Finally,sediment quality reference method(SQG),risk quotient method(HQ),secondary phase and primary comparison value method(RSP)and risk assessment codification method(RAC)were used to explore the pollution degree and potential ecological risk of heavy metals to the sedimentary environment,in order to provide scientific theoretical basis for environmental protection and management in this area.Specific results of the studies are listed as bellows:(1)The general trend of particle size distribution along the Yangtze Estuary and its adjacent sea area is that the coastal area is coarse while the adjacent sea area is finer than the coastal region.Of which,the upstream coast is thicker the downstream(Pudong coast).The adjacent sea area is relative thick in the north and thin in the south.(2)The profile trend of heavy metals in surface sediments along the Yangtze River Estuary is: Zn > Cr > Pb > Cu > Ni > As > Cd.There was significant variation of spatial distribution.The mean concentrations of Cu,Cr,Pb and Cd in the south coast were higher than those in the north coast,and the mean concentrations of Cu,Cr,Pb and Cd were25%-150% higher than the background values,of which Cd was strong accumulated by sediments,indicating these metals were greatly influenced by human activities.The concentration of heavy metals in the sediments of the Yangtze Estuary showed a decreasing trend from offshore to offshore: Zn>Cr >Ni>Pb>Cu>As>Cd.The average concentration of most metals(Pb,Zn,Cd,Cr,As)was higher than its background value,and the exceedance of As is the most obvious.All heavy metals meet the national Marine sediment quality standard I except Cd and Cr(standard II).(3)In the surface sediments along the Yangtze River Estuary,Cd dominated the largest proportion of active heavy metals,mainly in the dissolved form of weak acid,with an average content of 54.17%,while the active heavy metal content of Cu and Pb was50%-65%,and mainly in reducible form.Cr,Zn,As and Ni were mainly in the residual state(F4)with high stability.Secondary phases of Zn,Pb and Cd were the main forms in the surface sediments of adjacent sea areas.The mass fraction of the secondary phase of Cd was the highest(74.2%),while the occurrence forms of the secondary phase of Cr and As were 15.05% and 11.67%,respectively.Among them,the main form of Cd is Fe-Mn oxide binding state.The primary phase of Cu,Cr,As and Ni was the main occurrence form.(4)The result of correlation analysis suggested that As showed significantly positive correlation the Cu,Cr,Ni and Zn in the surface sediments of the coastal area of the Yangtze River Estuary,of which,strongest with Zn,while Cd is only correlated with Pb to a certain extent.TOC showed a good correlation with all metals,and the most significant correlation with Zn,Cd and As.The strong correlation between Cu,Pb,Cr,Zn and Ni in the surface sediments of the adjacent sea area indicated that their sources were similar,which may all derive from mining,smelting and printing.Cd showed no significant correlation with other elements except As,indicating that Cd is likely to be an independent pollution source.The surface sediment samples along the Yangtze River Estuary can be divided into four categories.S4-S6 stations were mainly affected by the unnatural factors and presented the highest concentration.The surface sediment samples in the adjacent sea area can be divided into five categories.S11-S13 stations were also classified into one category due to its proximity to the area with intensive human activities.Therefore,the regions with greater human influence may show more similar accumulation of heavy metals.(5)The results showed that the bioeffects of seven heavy metals(Cu,Pb,Cr,Cd,Zn,As,Ni)in the sediments along the Yangtze Estuary and adjacent sea areas were likely to be toxic to aquatic biota(probability was 21%),based on sediment quality criteria,risk quotient,secondary/primary facies and risk assessment codification.In terms of the total content of heavy metals,As and Ni in the coastal areas of the Yangtze Estuary were at moderate risk,which may be related to the unique industrial activities(electroplating and cold rolling)in the coastal areas.The biohazard risk of As in the adjacent waters of the Yangtze Estuary was high.Due to the large content of active state,Cd reached the level of moderate to severe pollution in the whole region.