湘江长潭株段河床沉积物重金属污染源的铅同位素地球化学示踪

摘要

本次工作利用等离子质谱仪(ICP-MS)对湘江长沙-湘潭-株洲河段河床沉积物重金属含量和铅同位素组成进行了系统的分析.结果表明,河床沉积物富集V、Cr、Mn、Co、Ni、Th、U、Cu、Pb、Zn等多种重金属,其中Cu、Zn、Pb等重金属于株洲段沉积物中达高度至极度富集.沉积物重金属的富集特征暗示沉积物存在较严重的重金属污染.沉积物铅同位素组成以相对富Th铅为特征,具明显的人为源和自然源铅,其206Pb/207Pb值在1.1723至1.1855之间,206Pb/208Pb值在0.4760至0.4786之间.其中自然源铅主要为花岗岩铅,而人为源铅则为铅锌矿矿石铅和燃煤铅构成,即沉积物铅同位素组成为花岗岩铅、铅锌矿矿石铅、燃煤铅组成的三元混合铅.铅同位素比例估算显示,长沙、湘潭段沉积物铅锌矿矿石源铅比例平均依次为23％、32％,燃煤源铅比例平均依次为47％、23％,自然源铅比例平均依次为30％、45％;株洲段沉积物的自然源铅比例较低,平均为17％,而矿石铅和燃煤铅的比例较高,平均依次为34％和49％.V、Mn、Co、Cu、Zn、U等其他重金属与Pb一样,长沙、湘潭段沉积物人为源比例小于67％,株洲段沉积物人为源比例达70％.因此,湘江长潭株段河床沉积物重金属污染问题值得引起高度重视.%The concentrations of heavy metals in riverbed sediments and the lead isotopic compositions of the sediments from the Changsha,Xiangtan and Zhuzhou sections of the Xiangjiang River in China were analyzed using ICP-MS techniques.The results show that sediments from these sections are enriched in heavy metals V,Cr,Mn,Co,Ni,Th,U,Cu,Pb and Zn,among which metals Cu,Zn and Pb are found to be extremely highly enriched in those sediments from the Zhuzhou section of the river.The enrichment of such heavy metals in the sediments may suggest that these sediments are contaminated by the heavy metals.Lead isotopic analysis results show that the sediments are characterized by the enrichment of Th-radiogenic lead,which is composed of lead contributed from both natural sources and anthropogenic pathways.Bulk sediments of the river have 206Pb/207Pb ratios ranging from 1.1723 to 1.1855,and 206Pb/208Pb ratios from 0.4760 to 0.4786.Such Pb isotopic composition suggests that the natural source lead may be contributed mostly by granitic rocks through weathering taking place in the upper river reaches,while the anthropogenic lead may be contributed by the Pb-Zn ores operated and smelted in the areas and from coal combustion occurring in the cities distributed in the lowermost river areas.Thus,lead in riverbed sediments from these three sections of the river is believed to be contributed by the three end-member sources:granitic rocks,Pb-Zn ores,and coals.The proportions of lead contributed by these three sources are then calculated using a three end-member model.The results show that the average proportions of lead contributed by Pb-Zn ores for riverbed sediments from the Changsha and Xiangtan sections are 23％ and 32％,respectively,and those from coal combustion are 47％ and 23％ respectively.The proportions of lead contributed by granitic rocks for sediments from these two sections are 30％ and 45％,respectively.Sediments from the Zhuzhou section have a relatively low proportion of lead contributed by the granitic rocks,with an average of 17％ in proportion,but have a higher proportion of lead contributed by Pb-Zn ores and coals,with an average of 34％ and 49％,respectively.Other heavy metals such as V,Mn,Co,Cu,Zn and U have similar proportions of anthropogenic and natural source contributions in the sediments to that of lead.They generally have an anthropogenic proportion of less than 67％ for riverbed sediments from the Changsha and Xiangtan sections,but a higher proportion up to 70％ for those from the Zhuzhou section.Therefore,it is necessary to highlight the heavy-metal contamination in the riverbed sediments from the Changsha,Xiangtan and Zhuzhou sections of the Xiangjiang River for environmental protection.