Water And Stream Sediment Heavy Metal Environment Of The Wei River In Xi'an Section Geochemical Studies,

Urban river is a complex dynamic system interacted by human and natural environments, especially when receiving large amounts of anthropogenic contaminants from nearby urban areas and industrial sites. Among the various contaminants, heavy metals are of particular concern due to their environmental persistence and biogeochemical recycling and ecological risks. Sediments are not only main places of heavy metals accumulating but also potential pollution sources in water environment. Therefore, it is necessary to research the contents, distribution characteristics and chemical fractionations of heavy metals in sediments. The results would make a great contribution to understand the heavy metal resources, assessing pollution status and the affection to ecosystem, and have a great significance to environmental protection of urban river.Xi'an section of Wei River is regarded as the research area in this paper. Based on full field survey, collecting samples and experiments, the physio-chemical characteristics of surface water including pH value, conductivity, dissolved oxygen (DO), chemical oxygen demand (COD), biochemical oxygen demand in five days (BOD₅) and fluorides and sediments including pH value, magnetic susceptibility, particle size, loss on ignition and major elements are for the first time studied. The contents with different particle size and total contents of Cu, Mn, Pb, Zn, Ni, Cr, Fe and As are determined by X-ray fluorescence spectrometry (XFS) in sediments and the spatial distributions of all heavy metals are analysed. Chemical fractionations of Cu, Mn, Pb, Zn, Ni, Cr and Fe in surface sediments are studied by BCR sequential extraction method and determined by flame atomic absorption spectrometry (AAS). Factors influencing heavy metal elements concentration and chemical fractionations are discussed through the correlation and cluster analysis. These provided a scientific reference for reasonable development and utilization of water resources of Wei River, constituting the measures of pollution control and the watershed management planning.The main conclusions in this study are as follows:(1) The surface water is genera in Xi'an section of Wei River. The averages of pH value, conductivity, dissolved oxygen, chemical oxygen demand, biochemical oxygen demand in five days and fluorides are 7.129, 528μs/cm, 0.29mg/L, 85.1mg/L, 22.6mg/L, 0.4601mg/L, respectively. The average pH value of sediment is 8.604, so it is the alkaline sediment. The sediment texture type is silty sand. The average clay content is 10.050%, and the average silty sand content is 50.395%, and the average sand particle content is 39.555%. The average loss on ignition of sediments is 3.563%. Low-frequency magnetic susceptibility, high-frequency magnetic susceptibility and frequency magnetic susceptibility are 50.7×10^-8m³/kg, 46.2×10^-8m³/kg and 2.36%, respectively. The average contents of K₂O, Na₂O, CaO, MgO, Al₂O₃, Fe₂O,. SiO₂ and Ti are 2.26%, 1.86%, 6.64%, 2.09%, 11.20%, 3.69%, 58.52% and 3593.3mg/kg, respectively.(2) The average contents of Co, Cr, Fe, Cu and Pb in surface sediments from Xi'an section of Wei River are higher than the environment background, respectively. The average contents of As, Mn and Ni are lower than the environment background, respectively. The average content of Zn is similar to the environment background. The horizontal distribution of all heavy metals in surface sediments is of twists and turns. As, Cu, Mn, Pb, Zn, Ni and Fe in surface sediments have the similar horizontal distribution characters. The cumulative values of sections are in the following order: section 4>section 6>section 3>section 2>section 5>section 1. The higher regions of most heavy metal elements in surface sediment are the middle and lower reaches of Wei River.(3) The experimental results of sieving indicate that content distributions of heavy metals in surface sediments with different particle size from Xi'an section of Wei River have some differences. The heavy metals mainly accumulate at 150～250μm, 250～450μm, 450～900μm and >900μm.. The rests are fairly little.(4) The vertical distribution of heavy metals in core sediments from Xi'an section of Wei River is of twists and turns, and do not form a certain feature because of the complex hydrological environment and the different human activity from the coasts. As, Cu, Mn, Zn, Ni and Fe in core sediments have the similar vertical distribution characters, which shows that these heavy metals are of the same material sources or the similar remigration behaviors after depositing. The vertical distribution characters of Pb, Co and Cr are unconspicuous.(5) In surface water from Xi'an section of Wei River, the main fractionations of Cu and Zn are soluble species, whereas the main fractionations of Mn, Pb, Co, Ni, Cr and Fe are particulate species. In surface sediments from Xi'an section of Wei River, the main fractionation of each heavy metal is residual form. The content of "labile" fractionation for Pb is more than "stable" fractionation. The "stable" fractionations of other heavy metals are in the ascendant.(6) The statistical data showed that there are significant correlations among soluble fractionations, particulate fractionations, total contents for Cu, Mn and Fe in surface water. Significant correlations also existe between total content and soluble fractionation, particulate fractionation, respectively for Zn in surface water. There are also significant correlations between particulate fractionations and total contents for Co, Ni and Cr in surface water. For Pb in surface water, there is a significant correlation between total content and DO. For Zn in surface water, there are significant correlations between soluble fractionation and COD, BOD₅, fluoride, respectively. For Co in surface water, there are significant correlations between pH value and soluble fractionation, total content, respectively. For most heavy metal elements in sediments, there are significant correlations between fractionations and low-frequency magnetic susceptibility, high-frequency magnetic susceptibility, clay, silty sand, loss on ignition, K₂O, Na₂O, MgO, Al₂O₃, Fe₂O₃, SiO₂, Ti, respectively, so are total contents. The above phenomenon reflects main factors of influencing tomitsumi and fractionations for heavy metals. There are significant correlations among As, Cu, Mn, Pb, Zn, Ni, Fe, but there is not significant correlation between Co and other heavy metal elements in sediments, so is Cr. For Cu in sediments, significant correlations existe among oxidable fractionation, residual fractionation, total content. For Mn in sediments, significant correlations also existe among acid extractable fractionation, reducible fractionation, residual fractionation, total content. For Pb in sediments, there are also significant correlations between acid extractable fractionation and residual fractionation, at the same time between reducible fractionation and total content. For Zn in sediments, significant correlations also existe between reducible fractionation and acid extractable fractionation, total content, respectively. For Co in sediments, there is significant correlation between residual fractionations and total contents, so is Cr. For Ni in sediments, significant correlations also existe between total content and acid extractable fractionation, residual fractionation, respectively. For Fe in sediments, there are also significant correlations among acid extractable fractionation, residual fractionation, total content, at the same time between reducible fractionation and oxidable fractionation.(7) The source analysis of heavy metal elements shows that As, Cu, Mn, Pb, Zn, Co, Ni and Fe in sediments from Xi'an section of Wei River may be mainly come from natural sources, however, Cr in sediments may be mainly come from pollution due to human activities, which is decided bydistribution characteristics of heavy metals in sediments in the time span.