Abstract
Lake Mariut is suffering from pollution as it receives industrial effluents and a mix of sewage from different drains, often lacking treatment systems. Sequential extraction procedures were applied for the speciation of heavy metals (Fe, Mn, Zn, Cu and Pb) and P in the lake sediment to evaluate their potential bioavailability. Total concentrations of metals had low values compared with the sediment quality guidelines. The chemical speciation data for Mn and Zn indicate a potential pollution level since the non-residual fractions contain up to 77.6 and 64.9% (respectively) of the total metal content. However, Fe, Cu and Pb are mostly linked to the inert fraction. Risk assessment of metals was carried out using the mobility factor, the contamination factor, the risk assessment code and the enrichment factor. All the analytical approaches indicated the accumulation of pollutants exceeding the adsorptive capacity of Lake Mariut sediments. The organic phosphorus fractions dominated in the sediment accounting for 51.4%, while the most abundant form of inorganic phosphorus was Ca-bound phosphorus. The rank order of P fractions was HCl-P > NaOH-P > BD-P > NH4Cl-P > Res-P. The decrease both in the dissolved oxygen level in the lake water and the Fe:P ratio in the lake sediment resulted in the high contribution of the phosphorus fractions to the overlying water.