Spatial And Temporal Distribution And Ecological Risk Assessment Of Heavy Metals In Sediments Of The Yangtze Estuary And Hangzhou Bay

By analyzing the temporal variation of heavy metal content in surface sediments of the Yangtze River estuary,Hangzhou Bay and offshore waters from 2003 to 2019 in spring and summer as well as observing the evolution of heavy metal pollution status,this study intends to obtain more targeted methods for helping heavy metal pollution management when managing heavy metals in the aquatic environment,and to promptly identify heavy metals that pose potential risks to the ecological environment.This paper also investigates the spatial distribution pattern of heavy metals in the surface sediments of the investigated sea area and identifies their possible sources and influencing factors that cause heavy metal pollution,so as to provide a reference for the prevention of heavy metal pollution.The main conclusions are as follows:(1)The average concentration trends of six heavy metals in the survey years were Zn>Cu>Pb>As>Cd>Hg,and the average values of six heavy metals were higher than the environmental background values except for Pb and As.Cu,Zn,Cd,and Hg exceeded 6.40%,9.36%,138.46%,and 17.65% respectively.The annual average content of Cu,Hg,As and Cd exceeded the quality standard of Category I of Marine Sediment Quality(GB18668-2002)in some years.Combined with the analysis of sites,71.26% of the sites met the quality standard of the first class of heavy metals.27.17%of the sites met the second quality standard.(2)Cu,Zn,Cd,Hg,and As showed an overall decreasing trend over time,and Pb showed no significant trend.the significant interannual variations of the six heavy metals indicated that these heavy metals mainly came from the variable human activities in the area,and their contents were sensitive to changes with the emission or control of pollution sources.the key mutation years for Cu,Hg,and As were 2007,2007,and The key mutation years for Cu,Hg,and As were in 2007,2007,and 2008,respectively,and the mutation points for the remaining heavy metals were not obvious.(3)The trend of interannual variation of total heavy metals and the discharge of heavy metals into the Yangtze River in the previous year is relatively consistent.The same regularity exists between the input of sediment from the Yangtze River estuary and total heavy metals in surface sediments before 2011,indicating that the impact of anthropogenic discharge of heavy metals on the environment needs to lag for some time before it manifests itself.(4)Heavy metals were correlated with hydrological conditions such as p H and dissolved oxygen to different degrees,and correlated with the adsorption and degradation of heavy metals in the mud-water phase.The results of correlation analysis and principal component analysis show that the sources of most heavy metals in sediments have some similarity.On the first principal component(PC1),Cu,Cd,Hg,As and Zn elements have high loadings,mainly from various industrial and agricultural wastewaters.Pb has high positive loadings on the second principal component(PC2),mainly from traffic pollution and ship transportation compound pollution.(5)The spatial distribution of heavy metal pollution is mainly concentrated in coastal sites.Land-sourced substances have obvious influence on the distribution of heavy metals.Anthropogenic discharge of industrial and domestic wastewater has an obvious contribution to the pollution of heavy metals.The spatial distribution of heavy metals is also related to hydrodynamics.Under the influence of the Yangtze River runoff and the rivers entering the sea,the human-polluted sediments are mainly concentrated at the confluence of estuaries and currents,and the influence of human activities decreases seaward.(6)The ground accumulation indices of six heavy metals show that they have been in a pollution-free state in recent years from the individual annual average values of the survey years.The potential ecological risk evaluation results show that although the contents of Cu,Zn and Pb are higher than those of Cd,Hg and As,while Cu,Zn and Pb have a lower potential ecological risk index than Cd,Hg and As.The ranking of the potential risk levels of the six heavy metals is Cd>Hg>As>Cu>Pb>Zn,and the contributions of Cd and Hg to the total potential ecological risk index reach 48.76%and 29.09%,respectively,so that these two metals were the main heavy metals polluting the aquatic environment.