长江源区Cd地球化学省与主要水系的Cd输出通量

中国正在进行的多目标区域地球化学调查成果显示,长江流域存在贯穿全流域的Cd地球化学异常,因此研究该异常的成因及查明长江主要支流Cd的输出通量已成为科学研究的焦点。长江源区水系包括通天河干流及沱沱河、楚玛尔河、尕尔曲、布曲、当曲、口前曲、聂恰曲支流。采用1样/km2的密度采集水系沉积物样品对长江源区进行地球化学填图,在沱沱河—尕尔曲和扎曲—聂恰曲流域分别圈出规模巨大的Cd、Pb、As地球化学省,其他地区Cd、As、Pb均为正常的地球化学背景区。研究发现沱沱河—尕尔曲Cd、Pb、As地球化学省内岩石中Cd、Pb、As含量明显高于土壤和水系沉积物,表明岩石在风化形成土壤和水系沉积物过程中已向水体释放了大量的Cd、Pb、As。通天河干流原水中Cd、Pb、As的平均质量浓度分别为0.144、17.62、2.59μg/L,沱沱河原水中Cd、Pb、As的质量浓度为0.187、19.89、2.33μg/L,聂恰曲原水中Cd、Pb、As的质量浓度为0.144、6.37、2.33μg/L,较其他支流中的含量高近1个数量级,表明源区原水中Cd、Pb、As含量主要受地质背景控制。年输出通量计算显示,由沱沱河输入到通天河中的Cd、Pb、As年通量分别为0.155、16.47和1.93t/a,楚玛尔河输入到通天河中的Cd、Pb、As年通量分别为0.015、0.77、0.35t/a,尕尔曲—布曲输入到通天河中的Cd、Pb、As年通量分别为0.095、10.31、2.72t/a;由通天河输入到金沙江的Cd、Pb、As年通量分别为2.48、290.7、42.3t/a,表明长江源区Cd地球化学省是长江沿江Cd地球化学异常带形成的重要物质来源。对原水和清水(过0.45μm滤膜)中Cd、As、Pb分析显示,长江源区水体中Cd、Pb主要以悬浮物形式迁移,As则以可溶态形式迁移。

The Cd geochemical province in the source area of the Yangtze River and the output fluxes of Cd for its major water systems

The results of the multi-purpose geochemical investigation in China,which is still in progress,show that Cd geochemical anomalies are spreading through the whole Yangtze River valley.Reasonably,the origin of the anomalies and Cd output fluxes for the major tributaries of Yangtze River have become key points of scientific studies.The water system in the source area of Yangtze River includes Tongtian River as well as tributaries of Togton River,Qumar River,Gar Qu,Bi Qu,Dam Qu,Kouqian Qu and Nyaqa Qu.The geochemical mapping has been performed with dense sampling of stream sediments at 1 sample/km2,and a giant Cd-Pb-As geochemical province and a Cd province have been outlined in the basins of Togton River-Gar Qu and the Za Qu-Nyaga Qu respectively,whereas for the rest of the area just show normal background of Cd,Pb and As.It has been found out through the study that the contents of Cd,Pb and As in the rocks in the Togton River-Gar Qu geochemical province of Cd,Pb and As are significantly higher than those in soils and stream sediment,showing that a large amount of Cd,Pb and As were released to water bodies during the forming of soils and stream sediments by weathering.The contents of Cd,Pb and As in the raw water of the trunk stream of Tongtian River are 0.144,17.62 and 2.59 μg/L,respectively;while they are 0.187,19.98 and 2.33 μg/L for the raw water of Togton River and 0.144,6.37 and 2.33 μg/L for the raw water of Nyaga Qu,respectively.They are almost one order higher than those for the rest of tributaries,indicating that the contents of Cd,Pb and As have been controlled by geological process.Based on calculations,the year outputs of Cd,Pb and As from the Togton River to Tongtian River are 0.155,16.47 and 1.93 t,respectively,while they are 0.015,0.77 and 0.35 t from Qumar River to Tongtian River,0.095,10.31 and 2.72 t from Gar Qu-Bi Qu to Tongtian River and 2.48,290.7 and 42.3 t from Tongtian River to Jinsha River,respectively.All these show that the Cd geochemical province in the source area of Yangtze River is an important material source of Cd geochemical anomalies along Yangtze River.Analytical results of the raw water and the filtered water(by using 0.45 μm filtering film)indicate that the migrations of Cd and Pb in the water body in the source area of Yangtze River have been mainly in the form of suspended substances,whereas for As it has been in the form of soluble substances.