Character And Assessment Of Heavy Metals In The Sediments From Five Biggest Freshwater Lakes

Sediment is warehouse of heavy metal pollutants and is also the potential heavy metal pollutant source that influences environment. The heavy metal of water is moved with particles in water and then accumulated and deposited in sediment. The toxic heavy metal can be released into water again by various ways and thus causes secondary pollution.Five biggest freshwater lakes in China (Boyang Lake, Dongting Lake, Taihu Lake, Hongze Lake, and Chaohu Lake) are selected as objects of study in this research, and the concentration and fraction characteristics of six heavy metals (Cu, Cd, Pb, Zn, Ni and Cr) in lake sediment are taken as main study content. By using potential ecological risk index, index of geoaccumulation and AVS-SEM methods, the pollution conditions of heavy metal in the sediment of five lakes are evaluated. The results of the study are in the following:(1)The pollution status of heavy metal in the sediment of Poyang Lake presents the tendency that high in south, low in north. The pollution of heavy metal is most serious in the sediment near the outlet of Rao River, and next is near the outlet of Boyang Lake. The five rivers in the south have great influence on the distribution of heavy metal in the sediment. Cu, Ni, Pb, Cr are mainly in residual fraction and then are in iron and manganese oxide fraction. Zn is mainly in residual fraction and next is in exchangeable and carbonate fraction. Cd is most existed in the exchangeable and carbonate fraction.(2) The section from Lu Jiao to Cheng Lingji located in the downstream of east Dongting Lake is the most seriously polluted and the total pollution of the 20th point is most serious. The variation tendency of concentration of Cu, Ni, Zn and Pb in the sediments of sub-lakes is:East Dongting Lake>Cheng Lingji>East Dongting Lake>South Dongting Lake. Cu, Ni, Zn, Pb, Cr are mainly in residual fractions. Among three extractive fractions, Cu is mainly existed in exchangeable and carbonate fraction, organic and sulphide fraction. The proportion of three effective fractions of Ni is well-distributed. The main fraction of Zn is exchangeable and carbonate fraction, and next is Iron and manganese oxide fraction. The Iron and manganese oxide fraction is also the main fraction of Pb. Cr is mainly formed in the organic and sulphide fraction. The main form of Cd is exchangeable and carbonate fraction.(3) Meiliang Wan, Gonghu Wan and Zhulan Wan in the north of Tai Lake are most seriously polluted by heavy metal in the sediment of Tai Lake, and the pollution in Xuhu Wan and central lake area is comparatively low. The pollution of Ni, Pb, Cd in the whole lake is serious, but low in the Cr pollution. Zn and Cr is mainly in residual fraction. In changeable and carbonate fraction, the proportion of Cr is greatest and next is Cu and Pb. The propotion of iron and manganese oxide fraction of Pb is highest (approximately 40%), but the proportion of Cu, Ni, Zn, Cd is low. The organic and sulphide fractions of each metal are low.(4) The pollution of Cu, Ni, Zn, Pb and Cr in Hongze Lake is mainly distributed in the east and north of lake, and the pollution of Cd is mainly in the west of lake. Pb is the most seriously polluted heavy metal in the lake. Cu, Ni, Zn, Pb, Cr are mainly in residual fractions. Among three extractive fractions, Cu is mainly existed in exchangeable and carbonate fraction, organic and sulphide fraction. The main form of Ni is organic and sulphide fraction, and the other two forms are even in proportion. The main form of Zn is iron and manganese oxide fraction, and next is exchangeable and carbonate fraction. The iron and manganese oxide fraction is also the main form of Pb. Cr is existed in the organic and sulphide fraction. The main form of Cd is exchangeable and carbonate fraction.(5) The west area of Chao Lake is most seriously polluted. The pollution of Cu and Ni in the whole lake is low and Zn and Pb in the west of lake show obvious accumulation. The contents of Cr among each point vary a lot and only the content of Cd in the central lake meets primary standard of earth environment quality. Cu, Ni, Pb, Cr are mainly in residual fractions, and next are iron and manganese oxide fractions, the proportion of changeable and carbonate fraction, organic and sulphide fraction are low. Zn is mainly in residual fraction and next is changeable and carbonate fraction. Cd is most existed in changeable and carbonate fraction, and next is iron and manganese oxide fraction.(6) It can be discovered by using potential ecological risk index method that the potential ecological risks of Cu, Ni, Zn, Pb and Cr in five lakes are slight (Low Risk), but the potential ecological risk of Cd is large and the contribution of total potential ecological risk index of Cd is greatest. Also it can be known by using index of geoaccumulation method that index of geoaccumulation of Cd in five lakes are higher than that of other metals, indicating of more serious pollution, and other metals exist pollution to various extents (Lightly polluted). The biological effectiveness of heavy metal in the outlet of Poyang Lake is comparatively high. It is likely to cause bad influence on the water species and human. And the heavy metals in the sediment of other lakes have no biological effectiveness for bottom dweller.