Chemical Speciation And Ecological Risk Assessment Of Heavy Metals Of Yarlung Zangbo Surface Sediments

Heavy metals have been considered a kind of important water pollutants because of its toxicity, mobility and persistence. Sediment is thought to be the main sources of heavy metals, the contents of heavy metals can be used to assess contamination of the study area. However, the ecological risk of heavy metals is not only related to its content but also depends on the chemical speciation of heavy metals. Along with the population increasing and the influences of industrial and agricultural activities, more and more heavy metals were released into the river system. Finally, sediments will become the source of poison through the biological enrichment.At present, the research of heavy metals in the water mainly focused on the typical polluted rivers and lakes in the east of China and lack of the research of the western river. The Yarlung Zangbo river flows through most densely populated areas of Tibet, and it is treated as the most important water sources of the Tibetan plateau. Although some researches of sediment element contents have been done, the research of chemical speciation has not been found.In this study, surface sediments from the Yarlung Zangbo River were analyzed by means of BCR sequential extraction procedure. Based on the concentration and speciation distribution of these metals, ecological risk of heavy metals was assessed with 6 kinds of different methods. The main results show that:The heavy metals content and the space-time distribution characteristics of sediments:Contents of most heavy metals are relatively close to the background values of soil in Tibet; The contents of As and Cs is much higher than China crustal abundance; The heavy metal concentrations of study area are far lower than those of eastern rivers of China, which mainly comes from the natural environment instead of human activities. On the spatial distribution, the heavy metal contents of the middle part areas are generally higher than other parts; On the temporal distribution, the concentrations of most elements during the non-monsoon are higher than those of monsoon.(2) The chemical speciation composition characteristics of heavy metals. In addition to the elements Cd, Most of heavy metals have the similar distribution characteristics, which is mainly composed of residual fraction (59%-95%); There is a significant positive correlation among elements Co, Ni and Cu, Elements Pb, As and Cs have very significant positive correlation, it is shown that those elements come from the same source. What’s more, there is no correlation among four kinds of fraction in the sediments.(3) Potential ecological risk assessment of heavy metals. Based on the concentrations and speciation of heavy metal, different evaluation methods were used to assess the potential risk. Generally, studied river sediment has a lower potential biological toxicity effect. Given most of heavy metals exist in residual fraction, the sediments of this region pose a low ecological risk to the environment. Furthermore, the water of the rivers were Grade I standard level according to Environmental Quality Standard for Surface Water, indicating that the studied rivers were still very clean, mainly influenced by natural processes instead of human activities.The purpose of this research is to evaluate the water quality and pollution situation based on the contents and forms of heavy metals, and provide certain scientific basis for the safety of drinking water. Additionally, there are more important practical significance for the local residents in the region.