Characteristics Of Heavy Metals Pollution In Wu River Wetland

The study used four acid digestion(HCI-HNO3-HF-HCIO4) method, BCR sequential extraction method and Tessier five-step method to determine the content and speciation of the elements Cu、Zn、Pb、Cr、Cd、As and Hg in the Sediments of Wu River wetland. The content of the 7 kinds of heavy metals in the water and the various physical and chemical properties of the wetland soil and water environment was also determined. Based on the above experiments, the study analyzed the content characteristics, spatial and temporal distributions, pollution sources, speciation character and its impact factor. Further measures included the enrichment factor, the Nemerow pollution index, pollution load index, geoaccumulation index and potential ecological risk index, for a comprehensive assessment of the degree of wetland pollution and its ecological risk and providing scientific suggestions for the treatment of urban sewage by constructed wetland. The results showed that:(1)According to the content, Cu,Zn,Cd,As in the water was light pollution or pollution-free, but there was significant Pb and Hg pollution in the the dry season and the normal season. And the pollution of Cr was serious in the wet season.The variation coefficient of Zn,Cr in the dry season and Hg,Cr,Cu,Zn in the normal season was high, indicating a high degree of spatial variability. And the each heavy metal element was relatively evenly distributed in the wet season. The content of each heavy element in the overlying water showed the similar patterns. In the surface sediments,the pollution of Cu,Zn,Cd,As,Hg was serious, and the highest content of heavy metals in the water appeared in the wet season; as for the horizontal distribution, its variation coefficient was relatively small compared with its content in the water environment;only in the dry season, the variation coefficient of Cu and Zn was higher, so there may be point source pollution.(2)About the vertical distribution(0~30 cm) of heavy metals in the sediments,the characteristic of Cu was not obvious. The content of Zn decreased as sediments depth increased, and Hg showed the opposite trend. Pb and As showed an increasing trend after the first decrease, with much fluctuation and attained a minimum at depths of 6 cm, 10 cm. The difference was that the change of As was relatively gentle under the depth of 10 cm and Cr,Hg showed much fluctuation and peaked in 10 cm,4cm respectively.(3)Through principal component analysis, cluster analysis and correlation analysis, we could find that the main source of heavy metals pollution in the water was the discharge of industrial and agricultural production waste water. In addition,municipal sewage, garbage piling up and improper application of pesticides was the important source of pollution, too. The main source of Cu,Zn,Pb,Cr and Hg in the water was industrial and agricultural production sewage, but Cd and As was also affected by garbage, pesticides and fertilizers. The most important source of heavymetals in sediments was also man-made sources, such as industrial and agricultural sewage. And Cu,Zn,Pb,Cr,As,Hg were mainly derived from this. Besides those,agricultural fertilizers, pesticides and garbage could also result in Cd, As contamination; natural background values had a certain impact on the Cu, Zn, Hg;physical, chemical and biological activities could make a significant effect on the Pb,Cr, Hg and other poor stability elements.(4)During the three water period(dry season, wet season and normal season), in the sediments, Cu,Zn,Pb,As mainly existed in residual state forms. Cr was mainly in the forms of residual and organic fraction, and Cd,Hg was mainly in the extractable form. By comparing the Tessier and BCR method, the study found that the advantage of BCR method in analyzing the active metals was obvious, and the BCR method was more suitable during analyzing elements of Cu,Pb,Cr.(5)Through correlation analysis and regression analysis, the particle size,magnetic susceptibility, pH, Eh, the degree of eutrophication and the total amount of the metal element content could both affect the chemical speciation of heavy metal elements.(6)Pollution of heavy metals and its ecological risks in the wetland were evaluated and the result showed that: the degree of the pollution was normal season>wet season>dry season; Pb and As had a low enrichment and their pollution was slight, while the enrichment degree of Hg was higher and its pollution was serious. Meanwhile Zn, Cu, Cd, Cr also showed significant pollution to varying degrees; the potential ecological risk of heavy metals in Wu River wetland showed wet season>normal season>dry season; the risk of Cu,Zn,Cr was slight, while Hg and Cd was higher than other elements. In addition, the CEF values of all heavy metals were greater than 1, indicating that the impact of the disturbance by human was obvious.