Distribution Characteristics And Environment Risk Of Trace Elements In Surface Sediments Of The Yangtze Estuary And Adjacent Waters

Based on the data of major elements(Al,Ca,Fe,K,Mg,Mn,Na,P,Si,Ti),trace elements(As,Ba,Cd,Co,Cr,Cs,Cu,Ga,Hf,Mo,Nb,Ni,Pb,Rb,Sb,Sr,Sn,Ta,Th,Tl,U,V,W,Z n,Zr)and rare earth elements(REE)in the surface sediments of the Yangtze River estuary and its adjacent waters(29.5-32°N,120-127.5°E),We have systematically analyzed the distribution characteristics of different trace elements in sediments,and try to figure out the sources of trace elements in surface sediments,as well as the key controlling factors affecting their distribution.Moreover,the pollution levels and potential ecological risks of different trace elements in the study area are calculated.We finally draw the following conclusions:The distribution of trace elements in the surface sediments of the study area can be classified into four types.The first type is a "dual-core" shape,which refers to most of trace elements and REE.The element content shows increase tendency as the distance from the shore increases.A high value area can be observed in the south branch of the Yangtze River estuary,the mouth of Hangzhou Bay,and the muddy area outside of the mouth of Yangtze River,then the high value area outward decrease sharply,another high value area appears in the muddy area of the northeastern part of the study area,where the elements content are slightly lower than that in the high value area near the estuary,thereafter forming a dual-core feature.The dual-core high-value type,can be further divided into the following sub-types:sub-type ?:The element content distribution is affected by the hydrodynamic conditions,in the contour map the near-shore high value is greater than the offshore high value.And these two areas have a southeast-facing lingual decline process;sub-type ?:The contour map shows that the offshore contents of high values in the northeast are slightly higher than the coastal high values,which may indicate the differences in geochemical behavior between elements and the difference of deposition environment.The second type of distribution is "solo-core" shape,the distribution trend of this type is that the near shore of estuary has a high content,with the increase of the distance from the shore,the content gradually decreases.The antimony has a distinctly high value in the south branch.Cadium has a distinct high values in the Hangzhou Bay may related to anthropogentic contamination;The third distribution type is "gradient",and the content of strontium has a significant trend of increasing outward the estuary.In the process of increasing,the contour line gradually changes from the tongue to the strip;The fourth distribution type is "mult-core".The distribution trend of element content is low in the south branch of the Yangtze River estuary and Hangzhou Bay,but there is a high value area in the north of the Yangtze River estuary.Meanwhile,in the middle and northeast regions of the study area find high-value area,they show a complex high-low-high-low-high staggered distribution from west to east.Further cluster analysis and correlation analysis showed that there may be three main sources in the surface sediments in study area.The first material source is related to the transport of run-off of the Yangtze River and the coastal current,corresponds to sub-type ? of dual-core type and multi-core type(including Hf,Zr,U,Ta,Th,Nb and?REE).The second source is marine autogenous source,mainly represented by strontium,which is closely related to calcium,and exhibit gradually increase with increasing distance from the shore.The remaining trace elements are mainly transported by the Yangtze River runoff,and have significant correlation with aluminum.Their distribution were controlled by grain size effect under the turbulent hydrodynamic conditions.In addition,the redox environment in the bottom may have some influence on their deposition.Among them,some elements may have anthropogenic input,distribution features is dual-core and single-core.The principal component analysis of trace elements shows that the main controlling factors affecting the trace element distribution are the material source,particle size distribution of sediments,and the redox state of the elements.According to the evaluation results of index of geoaccumulation,the indices of elements in the 6 stations of Z10,Z16,Z18,Z19,Z20 and Z22 are greater than 0,are evaluated as mild pollution(mainly affected by Er,Sb,Tm,T1).These stations are mainly located near the estuary of the South Branch of the Yangtze River,the Hangzhou Bay and the mouth of the Hangzhou Bay.The potential ecological risk assessment for the nine elements(including As,Cr,Cd,Cu,Zn,Mn,Pb,Sb,T1)indicated potential ecological risks in the above regions are very high,need constant attention.Especially we find the potential ecological risks of Sb are very higt near the estuary of the South Branch of the Yangtze River.