Geochemical Background Of Elements In Bed Sediments From Lower Reaches Of The Xiangjiang River In Hunan Province,China

The Xiangjiang River is one of the rivers that were most seriously contaminated by heavy metals in China.The geochemical background values of elements in the sediments of this mining impacted river are critical for assessment of the heavy metal pollution.Based on the previous geochemical analysis of major and trace elements in bed sediments in Zhuzhou,Xiangtan,Changsha and Xiangyin sections of the Xiangjiang River,this study recognized the elemental geochemical characteristics,screened the heavy metals from the natural and anthropogenic sources,and then calculated the elemental geochemical background values of sediments by means of geochemical and mathematical statistical analysis.Then,the background values of elements were verified by the analysis results of the Community Bureau of Reference（BCR）sequential extraction residual elements and the discrimination results of Pb isotope natural source samples.The results are as follows.The results show that the concentrations of major elements in the sediments of Zhuzhou Section are significantly different from the corresponding major elements in the sediments of Xiangtan,Changsha and Xiangyin sections.The concentrations of Si O₂,Ti O₂,Al₂O₃,Fe₂O₃and K₂O in the sediments of Xiangtan,Changsha and Xiangyin sections are homogeneously distributed（Coefficient of Variation C_V<0.2）,while those of Mn O,Mg O,Ca O,Na₂O and P₂O₅are distributed very heterogeneously（C_V>0.2）.The enrichment factors（EF）suggest that major elements Si O₂,Ti O₂,Al₂O₃,Fe₂O₃,K₂O,Mg O,Ca O,Na₂O,P₂O₅do not show significant enrichment or depletion features in the sediments（EF<2.0,except the depletion of Na₂O）.While,Mn O are significantly higher enriched（EF>2.0）if compared to that of the North American Shale（NASC）.In the vertical direction,the concentrations of major elements in the sedimentary core have no obvious change.In the sediments of Zhuzhou section,the concentration of Si O₂is lower than that of other reaches,while the concentrations of Ti O₂,Fe₂O₃,Ca O,Na₂O are higher than that of other reaches and are enriched.The distributions of trace elements in the sediments of Xiangtan,Changsha and Xiangyin sections of Xiangjiang River are also different from that of Zhuzhou section.The heavy metals such as Co,Ni,V,Cr,lithophile and high strength field elements such as Ba,Sc,Ga,Ge,Rb,Nb,Y and REE have less variation（C_v<0.20）,and they are believed to be homogeneously distributed in the sediments.While,heavy metals Cd,Mn,Cu,Pb,and Zn generally have considerably high variation（C_v>0.25）of concentrations in the sediments.Compared to the NASC,the EF values suggest that trace elements V,Co,Cr,Ba,Sc,Th,U,Sr,Ga,Ge,Rb,Nb,Y and REE do not show significant enrichment or depletion features in the sediments（EF<2.0,except the depletion of Sr）.While heavy metals Cd,Mn,Cu,Pb,and Zn are significantly higher enriched（EF>2.0）.In the sediments of Zhuzhou section,the concentrations of heavy metals such as Cd,Mn,Ni,Cu,Zn,Pb and high field strength elements such as Zr,Hf,Ta are higher than those in Xiangtan,Changsha and Xiangyin section.The principal component analysis（PCA）and Pearson’s correlation analyses suggest that the elements with EF values around 1.0（heavy metals V,Cr,Co,Ni,lithophile and high strength field elements Ba,Sc,Th,U,Cs,Ga,Ge,Rb,Sr,Zr,Hf,Nb,Ta,Y and REE）are hosted mostly within the silicate minerals,and they are of natural sources.While heavy metals Cd,Pb,Zn,Cu,Mn that are significantly enriched in the sediments（EF>2.0）reside mostly within the Fe-Mn oxide minerals,they may include the proportions that were from the anthropogenic sources.The background values of elements from different sources are obtained by different methods.Then,the background values of elements in the sediments obtained in this paper were verified by the analysis results of BCR sequential extraction residual elements,the discrimination results of Pb isotope natural source samples,and the ratios of stable elements.The results show that the background values of elements in bed sediments of Xiangjiang river calculated by the mathematical statistical methods and the geochemical method are reasonable.Based on the natural geological background of Xiangjiang River basin,they can be used to evaluate the heavy metal pollution in the sediments of Xiangjiang River.