Assessment And Source Analysis Of Heavy Metal Pollution In Sediments And Coastal Soils Of The Main Stream Of The Yangtze River

Heavy metals will cause serious harm to the surrounding environment and human health.With the acceleration of urbanization and industrialization in various regions,heavy metals discharged into the environment by human beings are increasing day by day,which has aroused public concern.The main stream of the Yangtze River runs through China from west to east.At the same time,it has brought many challenges to the surrounding environment.Sediment and soil,as carriers of environmental materials,are also affected by heavy metal pollution.At present,researches on heavy metals in the main stream of the Yangtze River are mostly focused on a single aspect of sediments or soil in a certain area of the Yangtze River estuary,but the heavy metal content in sediments and soil of the total main stream system of the watercourse is not clear.To this end,in this study,50 sampling points were set up in 23 areas from the Tuotuo River,the source of the Yangtze River to the mouth of the Yangtze River,and a total of 108 sediment and soil samples were collected along the main stream of the Yangtze River in two periods(dry and wet periods),measured its heavy metal content,and evaluated its pollution,trying to trace the source of heavy metal pollution.The main studies are as follows:(1)Among the heavy metals in the sediments of the main stream of the Yangtze River,the four heavy metals with the most toxic effect on human body are Pb,Cr,Cd and As.The average content in dry and wet season were 2.40,2.71,13.88,13.97 times and 2.10,2.77,9.97,10.31 times of the Chinese stream sediment background value.The average contents of other heavy metals additionally exceed the background values of stream sediments in China to varying degrees.For example,the average content of Cu,Zn,Ni and Mn in sediments during dry season is 2.03,2.69,2.60 and 1.36 times of the background value,respectively.The average contents of Cu,Zn,Ni and Mn in sediment during wet season were 1.67,1.99,2.48 and 1.15 times of the background value,respectively.Compared with other research results,heavy metals in sediments of the main stream of the Yangtze River have accumulated in the past decade,and some heavy metal elements in local areas are even higher than the level of this study.As China is vigorously promoting the development of the Yangtze River Economic Belt,there are more urban agglomerations along the main stream of the Yangtze River,and the population distribution is relatively more dense.The average content of heavy metals within the sediments of the main stream of the Yangtze River is above that of alternative vital rivers reception and abroad.(2)Among the heavy metals in the soil of the main stream of the Yangtze River,the four heavy metal elements Pb,Cr,Cd and As with the greatest toxic effect on human body have an average content of 1.97,3.69,13.82 and 11.41 times that of China’s soil background value in dry season.The average content of Pb,Cr and As is 2.11,2.24 and6.02 times that in China’s soil background.The content values of other heavy metals also exceeded the background value of Chinese soil to varying degrees.The average contents of Cu,Zn,Ni and Mn in dry season and wet season were 2.74,2.43,2.87 and2.04 times,1.86,2.38,2.15 and 1.32 times of the background value respectively.The average contents of Cr,Cd and As in dry season exceed the screening value of GB15618-2018 soil environmental quality standard,which are 1.13,4.47 and 4.26 times of the standard screening value respectively.In wet season,only the content of as exceeds the screening value of GB15618-2018 soil environmental quality standard,which is 2.25 times of the standard screening value.Due to the continuous development of industry and agriculture along the Yangtze River in recent years,the intensification of human activities,the discharge of industrial sewage and farmland fertilization and irrigation,the content of heavy metals in the soil along the main stream of the Yangtze River has an increasing trend compared with previous studies,but the current content level is at a medium level,and there is still much room for treatment.(3)Using different evaluation method(Single factor index method,Nemerow index method,Geo-accumulation index,Potential ecological risk index method)to evaluate the sediment and soils along the entire Yangtze River mainstream,it is found that As and Cd are two elements that exceed the standard more seriously.The exceeding standard is more serious in different media in different periods.It is necessary to strengthen prevention and control.In terms of pollution level,the upstream Chengdu Chongqing Economic Zone,Yichang Three Gorges section and the urban agglomeration in the middle reaches of the Yangtze River have high pollution degree and high pollution risk.On the whole,the pollution degree and pollution risk in the upper and lower reaches of the main stream of the Yangtze River are relatively low in the middle reaches.This may be due to the dense urban distribution in the middle reaches,which is greatly affected by human activities,and the sparse population distribution in the upper reaches,which is less affected by human activities.In the lower reaches,the regional economic development is relatively rapid,and the treatment technology of heavy metals is more advanced and the management is more strict.Sediments is larger than the soil heavy metal pollution degree,than plentiful water pollution,water pollution situation in severe cases,may be because the sediment as "sink" of heavy metals,it gathers heavy metals from various source,in addition,owing to the plentiful precipitation in the Yangtze river main stream sediment and soil heavy metal has a certain "dilution effect".(4)The source analysis results combined with correlation analysis,cluster analysis and principal component analysis showed that the sources of heavy metals in sediments and soils in the mainstream of the Yangtze River were closely related to industrial activities,coastal factory operations,sewage discharge,agricultural irrigation,and natural sources.Combining with the statistical data in the main stream of the Yangtze River,the sources of heavy metals are known.The upper reaches are mainly natural sources,the middle reaches are mainly industrial and agricultural sources,and the lower reaches are mainly anthropogenic sources.Therefore,we should control the amount of heavy metals into the lake along the coast,monitor the sewage discharge of industrial production bases along the coast,and regularly monitor the agricultural fertilizer and irrigation water,striving to control the heavy metals from the source.