Lagoons of the Nile delta, Egypt, heavy metal sink: With a special reference to the Yangtze estuary of China

Lagoons of the Nile delta are a vital aquacultural base for millions of people in Egypt. Since the 1960s, when the Aswan High Dam was completed, the estuary has changed from high to low turbidity and this has dramatically altered the eco-hydrological environment. In this study we attempt to explore the spatial and temporal distribution of heavy metals (Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) based on 6 short sediment cores recovered from Manzala, Burullus and Edku lagoons on the Nile delta. Radiometric dating indicates that the upper 10–15 cm of the core sediment is post-Aswan Dam. Manzala on the eastern delta coast is severely polluted by almost all metals analyzed in the present study, especially Mn, Pb, Zn and Cd, due to its connection to the city of Cairo, and the direct human input from neighboring megacities, where the petro-chemical industry is thought to be a major source. Although Burullus on the central delta coast has the lowest concentrations of Mn and Pb, there is an increasing trend, implying a linkage to local agricultural sources, and the recently expanding megacities in the central delta plain. Edku on western delta coast seems remote from any major pollution sources, but higher Mn, Pb, and Zn in the upper portion of the lake sediment suggest human influences from Alexandria to the west via the littoral current. The horse-saddle distribution pattern of polluted metals along the Nile coast, as evidenced by the Enrichment Factor (EF), is closely associated with the regulated runoff to the lower delta plain and coast, where extremely low precipitation occurs. This physical setting is certainly prone to concentrating anthropogenic heavy metals in the lagoons. The opposite example is the intensively-cultivated Yangtze estuary in China, where monsoonal precipitation flushes out a huge amount of metals as manifested by the lower EF than that of the Nile.