Distribution Characteristics Of Heavy Metals In Burullus Lagoon Of The Nile Delta, Egypt

The Nile river,located at the northeast of Africa,is the longest river in the world. It originates from Burundi Highland,passes through Burundi,Rwanda,Tanzania, Uganda,Sudan and Egypt,and finally flows into the Mediterranean Sea.The river has brought its tremendous sediments from its upstream to the river coast to form its huge delta system.The Nile delta,accounting for ca.22 000～24 000 km²,covers almost two thirds of plantation of the country.Since Aswan High Dam construction in 1964, it has brought a great of benefits to Egyptian people and agricultural products on the delta plain.It also provides cheap and clean electric power,which helps promote greatly the development of major power-based industries.Since then,industrial wastes containing potentially toxic metals have been dumped into the water networks in the delta plain.On the other hand,the dam has markedly diminished the volume of runoff and sediments reaching the delta-coast through the Nile distributaries.Flood water no longer breaches the Nile delta. Thus,farmer used the fertilizer to accelerate the growth of crops,which inevitably pollutes the soil and water quality of the coastal wetlands.Degraded coastal setting has also influenced the quality of fishes and crops,etc.through heavy metal transfer-food chain role,to harmful for our human society.Burullus lagoon is one of the four lagoons in the Nile coast.The lagoon sediment and sedimentation processes are closely associated with the pre- and post-Aswan high dam construction.To examine the heavy metals of the lagoon sediment in combination with radioactive dating can be helpful to see the history of environmental pollution in relation to pollution sources in response to the recent industrial booming.Our present study is chiefly based on the lagoon sediments taken by three vibrocores(B-1,B-2 and B-3) from the eastern Burullus lagoon.The length of the three vibrocores is:55cm(B-1),75 cm(B-2) and 80 cm(B-3),respectively.The sampling interval is 5 cm for each,totaling forty-two samples.Samples were carried back to the laboratory,East China Normal University,after airproofed in the field. All 42 samples were taken for grain size analysis,total organic matter(TOM) and heavy metals(Cu,Zn,Ni,Al,Fe,Mn,Pb,Cd).Twenty-nine samples were selected for ²¹⁰Pb and ¹³⁷Cs dating.Thus,the present study,incorporating the previous research results,is to focus on the lagoon sediment nature,sedimentation rates and heavy metal distribution.This goal of the study is to examine the spatial and temporal change of heavy metals to evaluate the lagoon environment under the recent human impact.Also given is the comparison to the Yangtze delta case,where there exits fruitful research results that can be shared to help a better Nile coastal administration in the future.Our results can indicate that the upper part(～15 cm) of B-1 sediment is polluted by human activity,which is reflected by grain size verses to TOM.Higher TOM in the upper part of lake sediments do not fit to the coarser grain size,showing pollution from industrial and/or agricultural activities.In contrast,the grain sizes of the upper part of B-2 and B-3 sediments fit well with TOM distribution,implying almost no potential pollution that recorded there.The lower TOM in B-3 probably hints the seawater dilution effect.Of note,the water content of B-1 and B-2 shows higher value at 10-15 cm in the upper part of vibrocore sediments.This probably indicates the post-Aswan dam sedimentation.The radioactive isotope test of ²¹⁰Pb and ¹³⁷Cs reveals very low excess value in the vibrocore sediment,although it can be detected in some samples.Because ²¹⁰Pb_ex and ¹³⁷Cs are mostly derived from the air fallout through precipitation,but, since there is extremely lower precipitation in the Nile delta,this has resulted in the lower content of ²¹⁰Pb_ex and ¹³⁷Cs.Even though,there is a relative poor record of ²¹⁰Pb_ex and ¹³⁷Cs,the 4 samples of B-1 in the upper part(15 cm above) of sediment show weaker,but useful information for sedimentation calculation.The CIC model and associated nuclear test theory used for the present study suggest sedimentation rate of 0.42～0.56 cm/yr for the upper part of B-1 sediment.This helps to delineate pre- and post-dam sedimentation in the lake.The post-dam sediment thickness(～15 cm) of B-1 keeps fairly agreement to the nature of water content.Radioactive isotope B-2 and B-3 does not meet the calculation of sedimentation rate,but on the basis of characteristic,of water content of B-2,the upper part of 10 cm with higher water content can probably infer the post-dam sedimentation rate,estimating at about 0.24 cm/yr.According to the correlation analysis of heavy metals(in the upper part 10-15 cm sediment of B-1 and B-2),clay(＜4μm) and total organic matter(TOM),it is known that Cu,Ni,Al and Fe are significantly positively correlated.The correlation coefficient is 0.886,0.667 and 0.942,respectively.Besides,Cu,Al Ni,Fe are significantly positively correlated with Clay,showing the derivation from physical weathering processes.Mn,Pb,Cd and Fe that are negatively correlated(i.e.-0.229; -0.316;-0.489) are also negatively correlated with Clay(-0.634;-0.364;-0.082),but positively correlated with TOM(0.029;0.135;0.547).This means Mn,Pb and Cd are not derived from the nature,but most likely form human pollution.After normalization to Al,the heavy metals in the superficial sediment of the three vibrocores indicate that all metals except Cu reduces from B-1 to B-3 in a seaward.This can be attributed to B-1 site closer to a sewage outlet by the lake coast.The higher Cu in B2 and B3 surficial sediment can be caused by overuse of CuSO₄ as an algicide in treating and controlling the massive macroalgal blooms in the Nile,the main source of Cu to Nile water,stream and drains,which transfers through the drainage water to the Nile delta lakes.The column distribution of heavy metals after normalization to Al,it is determined that Mn,Pb and Cd above the 15 cm in B-1 and Mn,Pb above the 10 cm in B-2 increase markedly,while heavy metals in B-3 did not see a clear change trend.These,agreeable with the above correlation analysis, reveal Mn and Pb to be the diagnostic pollution metals in the Burrulus lake.The another pollution metal could be Cd,showing its higher value in the upper of B-1,but not in B-2 and B-3,which is probably due to the sites away from the sewage outlet. The diagnostic Mn relates to pollution source of coal burning in the study area,and Pb may come from overwhelming petroleum chemistry,and Cd is most likely derived from metallurgy and waste burning.The background heavy metals in the lake sediment can be summed up by those below vibrocore depth of 10-15 cm in B1 and B2,which can also correlate to sedimentation of pre-Aswan High Dam,before 1964.Pre-dam Burullus lagoon was much more freshwater flooded,with lower industrial pollution.Comparison of heavy metals of pre- and post-Aswan High Dam,it is clearly known that Mn,Pb and Cd are higher than that of background setting,reflectingthe pollution in the last half century.Comparing with the sedimentation rates and heavy metals of in the Yangtze coastal sediment,it is found that the sedimentation rate of the Yangtze can be 2～20 times higher than the Nile delta.This may be caused by much higher sediment supply of the Yangtze at present before damming.Also,it seems that heavier pollution has occurred in the Yangtze coast than the Nile,both by comparing those diagnostic metals to its local background values.This results obviously from different industrial and economical level at the both sides. Using single factor contamination and Nemero index model,it is known that Mn, Pb,Cd are the major polluted metals,while Cu and Zn may remain as local polluted elements.This concludes that the present Burullus lagoon remains in a gently polluted setting.