西藏麦地卡湿地表层沉积物元素分布特征及其污染风险评价

Distribution and pollution risk assessment of trace elements in the surface sediments of Mitika wetland,Tibet

青藏高原是我国天然湿地分布最广泛的区域。其中,位于藏北高原的麦地卡湿地是目前西藏集国际重要湿地和国家级自然保护区为一体的重要湿地。以麦地卡湿地内主要水系表层沉积物为研究对象,首次对其中As、Co、Cd、Cu、Pb、Hg、Cr等15种元素的分布特征及其主要控制因素进行了分析,并采用地累积指数法对其在湿地环境中的污染情况进行初步评价。结果表明,除Cd、Hg和Li以外,麦地卡湿地内主要水系表层沉积物中各被测元素在水体pH介于8.66~9.28时,整体含量较相关区域背景值低。大部分被测元素在湿地内具有相似的空间分布规律:除As和Hg以外,其他元素自上游到下游其平均含量均升高1~2倍。As的平均含量在下游升高近5倍,Hg则基本保持不变。流域内土壤母质风化和土壤侵蚀等自然过程是控制麦地卡湿地内主要水系表层沉积物中大部分被测元素分布及含量水平的主要因素,因此麦地卡湿地水环境仍基本处于其本底状况。但是,污染物的远距离输送并随冰川融水进入湿地水环境对As、Cd和Hg在湿地流域内分布的影响不可忽视。地累积指数结果初步表明Cd和Hg的含量对湿地生态环境具有轻度污染风险。加强麦地卡湿地各环境介质的监测和背景信息研究刻不容缓。

The Qinghai-Tibet Plateau is the home to most natural wetlands in China.Among,the Mitika wetland located in the northern Lhari County,Nagqu City,is one of the most important wetlands in Tibet Autonomous Region,which integrating internationally important wetlands and national nature reserves.In this paper the distribution characteristics of 15 elements(including As,Co,Cd,Cu,Pb,Hg,Cr,et al.)in the surface sediments of major river(Miti Tsangpo)system in the Mitika wetland was studied,major controlling factors of the distribution was also identified.Geo-accumulation index was applied for pollution risk assessment.The results showed that,although the wetland river system had a pH range of 8.66 to 9.28,except Cd,Hg and Li,most measured elements had a concentration values lower than that of soil background of the studied areas.Those measured elements had a similar tendency along the river course in the wetland.Namely,except As and Hg,the contents of all elements in the lower reaches of the wetland were 1~2 times higher than that of the wetland s source area and,in the same trend,As was 5 times higher,however,Hg remained almost the same level.Natural processes,e.g.rock weathering and soil erosion in the wetland was the major controlling factors governing the distribution of the measured elements in the surface sediments of the Mitika wetland.Despite the results of eco-accumulation index indicated that Cd and Hg,particularly effected by long rang transported pollutants and melted through glaciers into the wetland,posed a minor pollution risk to the wetland ecosystem,the aquatic environment of the Mitika wetland remained in undisturbed regarding to the human activities.