Man-made impact on the geochemistry of the Nile delta lakes. A study of metals concentrations in sediments

Abstract

Forty two sediment samples were collected from four River Nile northern delta lakes (Mariut, Edku, Burullus and Manzalah) to asses the man-made impact on their environment during the last 20 years. Due to the continuous discharge of agricultural, industrial and sewage waste waters to these lakes, significant variations in the sediment's nature took place, with fine fractions dominating opposite to the outflowing area of the main drains. Organic carbon was elevated at the southern regions, reaching 10.5 ± 4.5% in lake Mariut. Meanwhile, carbonate increased reaching 90% for samples collected at the lake-sea connection areas. The 0.5 N HCl extractable metals indicated enriched levels of different metals at sites affected by sewage and industrial discharge. This fraction constituted 75 and 81% of the total metals in Lakes Mariut and Manzalah, respectively. Extractable Cd for Lakes Mariut and Manzalah constituted 51 and 53%, respectively, while Ph formed 59% of the total digestable metal in Lake Manzalah. Total metals concentrations especially Cd and Pb were elevated 8 to 70 times than those observed in the same lakes 25 years ago. The conversion of the untreated sewage discharge of Alexandria city to Lake Mariut and significant portion of Cairo sewage to Lake Manzalah during the past 10 years lead to the appearance of elevated levels of contaminants in both lakes. Agricultural discharge showed less impact on Lakes Edku and Burullus. Cluster analysis revealed associations between stations located under same types of stress while the correspondence factor analysis displayed that metals distribution pattern in the Nile delta lakes are governed by organic carbon as well as fine grained size fraction distribution.