Study On Environmental Geochemistry Of Heavy Metals In The Surface Sediment In Lanzhou Open-ditch Drainages

In recent years, with the continuous development of urban construction and the improvement living standard of people, the important of a good urban environment and restoration the ecological environment of the urban water was realized by many city govemments.So the city sewage rivers’comprehensive harnessing and ecological restoration work was brought up on the agendaThe non-processing or non-compliance emissions of industrial wastewater and domestic sewage from the city are the important source of urban river pollution. Among so many pollutants, heavy metals have drawn greater attention due to its persistence in the environment, the biogeochemical cycle and the potential ecological threat. The sediment is an important part of the water body. It has dual functions of the reservoi and secondary sources of heavy metals It is very important to investigate the contents and distribution of heavy metals in sediments, so we can explore the sources of heavy metals pollution, evaluate pollution status, predict the impact on the entire ecosystem, and restore the river environment.Lanzhou is located in the upper reaches of the Yellow River, and is in the narrow valley of "two mountains and one river", Because of its unique geographical position, Lanzhou has many city ditches; which are mostly seasonal and have lower water yields.As a heavy industry city, the ditches have been seriously polluted for accepting the large amounts of sewage and dumping of garbage in long term. Meanwhile, sewage was directly introduced into the Yellow River, which causes serious water pollution of the Yellow River section of Lanzhou.In the present study, the contents and distribution, sources, removal and enrichment of seven heavy metals (Cu, As, Co, Ni, Zn, Cr and Pb) were analyzed for the surface sediments of Lanzhou open-ditch drainages. Two assessment methods were applied to assess the pollution and potential ecological risk of heavy metals. The main conclusions were summed up as follows:(1) The average of pH value in the surface sediments of Lanzhou open-ditch drainages is8.16, so it is alkaline sediments. The sediment is mainly composed of silty sand and clay. The average proportion is8.47%for clay,53.93%for silty sand, and37.60%for the sand particle. But there are also exceptions.Take Qi Li He ditch as an example, sand particle is the major component of the sediment.The average content of organic matter in sediments is4.5%.The sediments’element component similar to Lanzhou loess.(2) The average contents of Pb、Cr、Cu、Co、Zn and As in the surface sediments from Lanzhou open-ditch drainages were higher than average of the Lanzhou loess geochemical elements and were higher than the background values of Gansu soil. As for the average ontents of heavy metal elements in the study area, the descending order is:Zn>Cr>Cu>Pb>Ni> As>Co. Due to the impact of the surrounding environment, there is difference for each element in different positions(3) The results of correlation analysis showed that:Heavy metals’content is not significantly related to the sediments’pH; High organic matter and fine particle size in favor of removal and enrichment of heavy metals in sediments. Cu, Pb, Ni, Co and As show a significant positive correlation with major elements of K2O, MgO, Fe2O3and Al2O3, and show a negative correlation with Na2O and SiO2; Zn and Cr have weak correlation with major elements. This indicates that the heavy metals in sediments are closely related to sediment mineral composition and the surrounding environment.(4) The source analysis of heavy metal elements shows that As, Cu, Ni, Pb and Co in sediments from Lanzhou open-ditch drainages may be mainly come from natural sources.However, Zn and Cr may be mainly come from pollution due to human activities.The main sources are Loess and industrial wastewater, domestic wastewater, solid waste, car exhaust and atmospheric dust. Human activities and production activities have direct influence on the enrichment of heavy metals in surface sediment.(5) The order of geo-accumulation index (Igeo) of Lanzhou open-ditch drainages is Zn(0.35)>Pb(0.11)>Cr (-0.14)> As(-0.26)>Cu(-0.29)>Co(-0.50). All the Igeo averages are less than1.00, which indicates a pollution level of non-polluted to medium pollution. Zn and Pb are under light pollution conditions, while Co, Cu, As and Cr are under non-polluted conditions. According to the index of potential ecological risk, and on the basis of single element pollution index C(?) average value, the heavy metals are in order of Zn>Pb>Cr>As>Cu>Co, which is consistent with the results of the geo-accumulation index. The consequence of the average potential ecological risk index (Er1) for heavy metal was As>Pb>Cu>Co>Cr>Zn, which shows that these heavy metals have low potential ecological risk, As and Pb were main factors of potential ecological risks in the research areas. On the whole, the comprehensive potential ecological risk index (RI) of the sediments presents a low degree. The potential ecological risk indices of the sediments for the different sampling area could be ranked in the following order:Drainage ditch>Lannigou>Dashagou>Qilihe.The results indicated that the surface sediments in open-ditch drainages at Lanzhou has been rich in quantities of heavy metal pollution, which is harmful to the Yellow River if those pollutants are directly.discharged into the river So the relevant departments should do their best in the monitoring and governance.