Study On The Fractions And Risk Assessment Of Heavy Metals In Sediments Of Xiangjiaba Reservoir Area

Heavy metals and sediments have different levels of pollution in rivers of different sizes worldwide,but the effects of river sediments are more pronounced,as their concentration,accumulation and migration make it more difficult to monitor and treat heavy metals in sediments.Xiangjiaba reservoir is the last rung of Jinsha River terraced reservoir group,which has important influence on the accumulation and spatial and temporal migration of heavy metals in the lower reaches of Yangtze River.In this paper,the content and morphological characteristics of Cu,Zn,Pb,Cr,Cd,Ni,Fe and Mn heavy metals in Xiangjiaba reservoir are analyzed.The pollution level was evaluated objectively by multifactor evaluation and ground cumulation index evaluation.The following results were obtained:(1)Under the influence of human activities and natural factors,the total amount of heavy metals in some sections of sediments in Xiangjiaba reservoir exceeds the soil background values in this area.Based on the lower limit values of heavy metals in sediments published by Taiwan Environmental Protection Agency,four heavy metals,Zn,Cd,Ni and Cr,can be calculated to be higher than their normal contents.The content of the other heavy metals was at a low level,and there was little difference between the sections.At the intersection of tributary and main stream,the content of some heavy metals is obviously increased,which indicates that there may be enrichment.(2)Morphological experiments showed that Cd and Cu were mainly in the extractable state,and the remaining heavy metals were mainly in the residue state.Cd and Cu are mainly extractable acetic acid and have high bioavailability,so Cd and Cu are more harmful to the environment than other heavy metals.(3)Ecological risk assessment of heavy metals in sediments of Xiangjiaba reservoir area was carried out by means of geological accumulation index method and multifactor index method.The results of both evaluations show that the overall pollution level of sediment deposited in Jiaba reservoir area is relatively light,but Zn and Cd heavy metals provide the main pollution factors and are at a higher risk level.(4)In addition to the significant positive correlation between Mn and p H in sediment heavy metals,there was a weak correlation between other heavy metals and the physico-chemical properties of water bodies and sediments;Sediment heavy metals Cr,Cu and Cd are mainly exogenous inputs from anthropogenic activities,Mn is mainly exogenous input from geotechnical weathering,and Zn is strongly associated with phytoplankton and is an endogenous source of water.There is a weak correlation between heavy metals and the exchangeable nitrogen and bioavailable phosphorus in sediments,which shows that there are complex synergistic,inhibitory and antagonistic relationships between heavy metals and nitrogen and phosphorus in sediments.