Environmental Geochemistry Of Heavy Metals In Sediments In Hejiang River, Guangxi Province

River is an important part of the ecosystem of the surface of the earth, it has the very high ecological service function. With the rapid development of industry and agriculture, river ecosystem degradation is becoming more and more serious. The Hejiang River is one of the largest tributary of the Xijiang, more and more pollutants discharged into rivers directly or indirectly, the resulting environmental pollution accident are increasingly prominent. Therefore, it is vital significance to research comprehensively the distribution of heavy metals in the Hejiang River.We discuss the spatial distribution and the ecological risk of heavy metals based on surveying the content and chemical forms of the nine kinds of heavy metal elements Pb, Zn, Cd, Cr, Cu, As, Fe, Al, Mn from 10 surface sediments and two sedimentary columns. At the same time, we use the isotope dating techniques to calculate the sediment deposition rate and heavy metal deposition flux in order to reconstruct geochemical history of heavy metal elements. At last, we determine the heavy metal pollution sources in the Hejiang River by principal component analysis method. The main conclusion obtained from this topic is as follows:(1) The average content of 10 kinds of heavy metal in surface sediment as follows:Al>Mn>Zn>As>Cu>Pb>Cr>Cd. In addition to Cr, Fe and Al, the coefficient of variation for other elements were greater than 0.5, which shows that most of the heavy metal content changed a lot. Speciation analysis for the surface sediment indicated Zn, Cr, As all is mainly the residual species; variety along the river of Pb, Cu, Mn were obvious; Cd was mainly the Acid-extractable species which had a strong biological effectiveness and existed most serious impact on the Hejiang River.(2) Dating for two sedimentary columns C1 and C2 collected in the Hejiang River by 210Pb isotope dating technique, and obtained average sedimentation rate (0.78cm·yr-1for C1, 0.79cm-yr-1for C2), we can reconstruct 60 (1956-2013 yr) and 80 (1930-2013 yr) sequence of sedimentary history for C1 and C2 respectively. Each heavy metal content in sediment column C1 located in the mining area decreased as time goes on The content change in core C2 are consistent with historical events.(3) The sources analysis of heavy metals in sediment showed that Pb, Zn, Cu, As has significant correlation with Fe and Mn in surface sediment, and has no obvious correlation with Cr, which shows that its source is different from other metals. Cd just has significant correlation with Pb and Zn. The principal component analysis show that the main pollution source is divided into three categories:mining production and agricultural pollution; electroplating industrial point source pollution; rock weathering erosion, natural changes of organic matter degradation. Its source mainly summed up in two kinds including the industrial source and natural sources.(4) Zn, Cr, As in two sedimentary columns were mainly residual species, Cu in sedimentary column C1 and Ni in the sedimentary column C2 were also mainly residual species, which indicated that they didn’t have the potential risk. Cd was mainly Acid-extractable species in two sedimentary columns, which inbdicated that it exist strong potential risks to the environment.(5) Heavy metals through the ecological risk evaluation method and combining with the analysis results show that He Jiang sediment pollution is lighter, the Cd for He Jiang biggest threat.