| 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. |
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