【作者】 顾家伟；

【导师】 殷鸿福； 陈中原；

【作者基本信息】 华东师范大学 ， 第四纪地质学， 2011， 博士

【摘要】 尼罗河三角洲属于典型的地中海干燥性气候区域,降水稀少。1964年阿斯旺大坝的建造又拦截了大量的径流和泥沙,导致尼罗河河口从高浊度演变为现今的清水环境。强烈的人类活动向河口地区排放了大量的污染物(有机和无机的),它们只能够通过径流排放至河口—泻湖入海。因此,探讨近半个世纪以来重金属元素在泻湖沉积物中的时空分布、环境变迁,以及与河口环境综合管理等具有十分重要的环境地学科学意义。本文以尼罗河三角洲泻湖(Manzala、Burullus、Edku)为典型区域,在2007—2008年分别采集6根柱状样并高密度采样(1cm/样),选取若干主要重金属元素(Al、Cd、Cr、Fe、Mn、Pb、Cu、Zn等)为研究对象,并辅以三个泻湖典型钻孔(M-1、B-4、E-1)的Z10Pb和137Cs核同位素定年,分析在受自然和高强度人类活动综合作用下的河口湿地系统环境中、沉积速率特征以及重金属元素在沉积物中的时空分布、累积和迁移转化过程；旨在指示阿斯旺大坝建造后,沉积环境对重金属污染元素分布的影响,并对尼罗河三角洲泻湖湿地重金属环境污染进行生态风险评价。另外,本研究试图与长江三角洲的重金属元素时空分布进行对比,探索季风湿润气候条件和高强度人类活动影响下重金属的分布迁移的不同模式。通过以上研究,本文旨在为大河三角洲河口综合环境管理评价和环境修复提供科学依据。在对大量第一手数据资料进行综合分析的基础上,本文主要得到以下几条结论：(1)尽管前人在尼罗河三角洲地区利用210Pb、137Cs放射性测年遇到诸多困难,但本文通过选取合适钻孔进行高分辨率采样,并利用CIC模式获得了尼罗河三角洲泻湖建坝后的沉积速率(0.22-0.27 cma-1)。低沉积速率反映了进入三角洲河口的泥沙极少,它们在泻湖中分布较为平均；阿斯旺建坝是导致泥沙剧减、三角洲泻湖沉积速率极低的主要原因。研究表明,上游来沙减少和海岸保护工程的实施使得泻湖成为一个较为稳定的沉积环境,极少的泥沙得以在此缓慢堆积。本研究中钻孔上层(-10cm)沉积物中保存较好的210Pb记录表明,尽管一些钻孔存在一定程度的生物扰动,但不能否定放射性同位素测年在泻湖中的应用。CRS模式结果显示1980s以来泻湖的沉积速率逐渐增加,其原因可能是湖泊周围城市化、农业围垦,以及高速公路建设等人类活动所引发湖面日益萎缩,使得入湖的物质相对“积聚”。(2)多数重金属(绝对含量和Al归一化数值)在钻孔上部(-10-15cm)沉积物中富集。它们的增加反映了阿斯旺1964年建坝后河口环境污染的历史。重金属水平分布显示,三角洲东西两边的Manzala和Edku湖污染较重,而中部的Burullus较轻。相关性分析结果表明Manzala湖的主要污染元素是Mn、Pb、Cu、Zn和Cd。Manzala湖重金属输入源主要是三角洲东部大城市(包括开罗)的石油化工相关产业的废水,以及Manzala湖周边的Matariya市、Damietta市排放的废水。研究表明,Burullus湖主要重金属污染元素为Mn和Pb,这可能与Burullus湖主要接纳三角洲农业污水有关。但是,近些年中部平原土地利用方式的改变以及一些大城市的兴起(Tanta, Baltim等),使得钻孔上层出现了一定程度重金属富集(Mn、Pb等),也暗示了该湖不容乐观的污染预期。Edku湖的主要重金属污染元素为Mn、Pb和Zn。研究表明Edku湖虽然远离主要工业区,但已经遭受一定程度的污染,其重金属污染源可能来自邻近的、与亚历山大工业废水有关的Abu-Qir湾,污水可以通过潮汐作用回灌Edku湖。(3)对比研究尼罗河与长江三角洲重金属污染分布规律,本文发现尼罗河三角洲的多数重金属含量高于长江三角洲,并且前者的EF值为长江三角洲的～3倍之多。两三角洲的重金属污染分布格局差异明显,尼罗河泻湖在建坝后逐渐成为重金属的聚集盆地；长江三角洲重金属含量向海沿程逐渐降低,巨量季风降水和径流对重金属的扩散能力很强,因而导致污染水平较低。与世界其他地区河口重金属污染程度比较,尼罗河三角洲的重金属污染属于中等水平,而长江三角洲属于中下水平。(4)两三角洲的重金属污染评价结果显示,尼罗河三角洲已经达到“中等”生态风险,长江三角洲为“轻”生态风险。在两三角洲重金属中,Cd的生态风险均为最高。另外,本文从社会经济、自然条件、人类活动(建坝)等三方面对比研究了两地三角洲重金属污染的控制因素。研究表明,阿斯旺建坝后处于干旱气候带下的“封闭型”尼罗河三角洲沉积环境,日益演化成重金属的“沉降池”。相反,长江三角洲的污水排放量(重金属输入)虽远大于尼罗河三角洲,但由于位于季风气候带,长江口的巨量水沙输入所形成的稀释效应要远大于尼罗河三角洲,另外长江口属于强动力沉积“开放型”沉积环境,易于污染物的扩散,以上长江口污染较轻的主要原因。(5)本文对三峡建坝后长江对重金属污染物容量进行了初步评估。结果表明,建坝后如果要维持长江口沉积物环境质量达到海洋环境质量Ⅰ类标准,所需的年输沙量应该达到>-0.7亿吨；考虑到泥沙的季节性分配不均,那么年输沙量可能要维持在＞～1.75亿吨时,冬季长江口沉积物环境质量才能达到上述标准。鉴于近几年(2003-2009)长江输沙量逐渐递减,研究认为对未来长江口的重金属环境污染监测要予以足够重视。更多还原

【Abstract】 The Nile Delta is located in the northeast of Africa under the arid climatic setting, with the precipitation varying from～50-100 mm a-1 along the Nile coast. The Aswan High Dam (AHD) built in 1964 impounded huge amount of water and sediment, which leads to the Nile estuary evolving from high-turbidity to low-turbidity setting. Contaminants (organic and inorganic) derived from intensifying anthropogenic activities are discharged into the Nile lagoons through distributaries and densely-distributed drainage networks, which has caused an adverse impact on estuarine eco-health in the Nile coast. Thus, to examine the temporal and spatial distribution of heavy metals in the lagoon sediment (Manzala, Burullus, Edku) can help a better Nile coastal administration in the future.Our present study is chiefly based on the lagoon sediments taken by six short sediment cores (Ml-1, M-2, M-3, B-4, E-1, E-2) from three Nile lagoons (Manzala, Burullus, Edku). This paper uses radiometric analysis (210Pb and 137Cs) of short sediment cores (M-1, B-4, E-1) with high-resolution sampling (1-cm interval) to trace sedimentation rates in the Nile Delta lagoons, particularly since the AHD. Meanwhile, the heavy metals (Al, Cd, Cr, Fe, Mn, Ni, Pb, Cu and Zn) are analyzed in all 6 sediment cores to reveal the temporal and spatial distribution of heavy metals, so as to evaluate the ecological risks of the lagoon environment under the recent human impact. Also given is the comparison of heavy metal contamination between the Nile Delta and Yangtze Delta, where lots of fruitful results from previous studies have been incorporated in the present study, to discuss the environmental impacts linked with the intense human activities (including Aswan damming and Three Gorges damming) under the different physical (arid/monsoon) and societal setting of two rivers’catchments.The main conclusions can be summed up as follows:(1) Though few satisfactory results are obtained in the Nile Delta using 210Pb dating, partly due to poor and low fallout of radionuclide in many previous studies, the present study with high-resolution sampling (1-cm interval) demonstrates the clear identification of declining trend in 210Pbex in about 10 cm of the upper-core sediments from the lagoons of Manzala and Edku, accompanied by two spikes of 137Cs in cores from the lagoons of Burullus and Edku. These findings illustrate average post-dam sedimentation rates ranging from 0.22-0.27 cm a-1 in the lagoons (CIC model), in contrast with those found previously based on low-resolution sampling. The lower sedimentation rates in the lagoons are a consequence of a dramatic reduction in riverine sediment load to the coastal area as a result of the damming. Although widespread erosion occurs along the open estuarine coast, the lagoon setting remains calmer than before due to coastal diking and freshwater regulation in the delta plain in the past decades. This provides the possibility of continuously preserved radiometric records in the less-bioturbated lagoon sediments.Dating individual layers using the CRS model reveals increasing sedimentation rates in Manzala and Burullus since the 1980s, which can largely be explained as a consequence of the reduction in lagoon area due to intensifying reclamation.(2) Spatial distribution pattern of polluted metals shows the highs in the eastern Manzala and western Edku lagoons, but an obviously low in the central Burullus lagoon. The temporal pattern of the most heavy metals identifies the increasing trend with time, especially in the upper sediments of<10-15 cm. Radiometric analysis of the core sediment can confirm chronologically the post-Aswan dam sedimentation of the upper sediment sections (above 10-15 cm).The results of cluster analysis in the upper sediment sections shows that Manzala is thought to be the most seriously polluted region, chiefly featured with the metals of Mn, Pb, Zn, Cu and Cd, etc. This lagoon is directly linked with the Great Cairo metropolitan city, and also surrounded by several regional large cites, where pollution sources seem linked with petroleum and chemical refining processes. Burullus of the central coast, although remaining relatively low in the metal concentration, sees an incremental Mn and Pb. Agricultural fanning and rising cites with over million populations are suggested as the local pollution sources. Edku of western Nile coast featured with polluted metals of Mn, Pb and Zn is being threatened by the pollution sources associated with Alexandrian wastewaters, via littoral current transport, although there is lower intensity of industrial activities.(3) The concentrations of most heavy metals gathered from previous studies show that the values of Nile Delta are much higher than that of Yangtze Delta, and EF in Nile Delta is～3-times that of Yangtze Delta, though the Yangtze Delta has lower background values. When compared with other estuaries in the world, heavy metals contamination level in the Nile Delta is roughly at the medium degree and the Yangtze Delta is quite moderate. The reason might be that the two deltas have been subjected to only a short period of industrialization (30-50 years).(4) The evaluation results of heavy metals contamination of the two deltas show that the Nile Delta has attained the middle-level ecological risks, while the Yangtze Delta stays at low-level ecological risks. In addition, Cd is the metal that poses the highest ecological risks on the both deltas.The comparison study is undertaken to reveal the controlling mechanism for the different heavy metals contamination issues in Nile Delta and Yangtze Delta, which is referring to the complex impacts of natural and anthropogenic forces. Under the arid climatic setting, the delta lagoons with enclosed sedimentary setting gradually turn into a’sink’for metal loading since AHD. For the Yangtze River, though the total load of the Yangtze wastewater is seemingly greater than that of the Nile, owing to favorable monsoon climate in the drainage basin, dynamic strong runoff into the seas plays a critical role in diluting the concentration of heavy metals in Yangtze estuary.(5) The preliminary capacity evaluation for heavy metals of Post-TGD in the Yangtze Estuary concludes that the minimum of sediment load of～70×106t a-1 should be guaranteed so as to keep the Yangtze Delta up to the Environmental Quality of State Marine Sediments (I). Taken into account the uneven seasonal distribution of Yangtze sediment discharge, the minimum of～175×106t a-1 is also suggested. Considering the decreasing trend of Post-TGD sediment load in recent years (2003-2009), the closely long-term watch on the tendency of heavy metal pollution in the Yangtze delta is strongly recommended.更多还原