Heavy metals pollution origin in the bed sediments of Aswan high dam reservoir: lithogenic or anthropogenic sources

Abstract

Heavy metals are present in aquatic sediments as a result of chemical leaching of bed rocks (Lithogenic source), and discharge of urban and industrial wastewaters (Anthropogenic sources). It is important to provide information about the source of heavy metals pollution because high levels of heavy metals can often be attributed to anthropogenic influences, rather than natural enrichment of the sediment by geological weathering (Lithogenic influences). This study was initiated with the objective of determining the origin of heavy metals pollutants in the bed sediments of Aswan High Dam Reservoir (AHD) and to evaluates their environmental risk in reservoir water and its potential use (benefits and risks) in agricultural land. Total metals concentration, the element Enrichment Factor (EF) and the sequential extractions of heavy metals (speciation) were determined in addition to pH, organic matter content and grain size distribution. The results indicated that the total metal contents varied in the following ranges (in mg/kg): Al (1165-18000), Cd (0.62-2.17), Co (11-47), Cr (12-63), Cu (80-261), Fe (3027-36651), Mn (93-1908), Ni (11-58), Pb (7-29) and Zn (98-208). The enrichment factor for all metals in sediment was found to be low and the heavy metals speciation revealed that the exchangeable and carbonate bound fractions are entirely insignificant. The results revealed no anthropogenic contribution of heavy metals and the chemical weathering is a primary factor that controls the concentrations of metals in the bed sediments of the Reservoir. The results indicated that metals in the bed sediments of the reservoir did not represent environmental pollution risk and are derived from lithogenic (natural) processes (i.e weathering and soil formation). The bed sediments of the reservoir were found in the alkaline range and are promoting heavy metals precipitation. Thus it was recommended to use these sediments in soil amendment.