Sources and Distribution of Mercury (Hg) Residues in Sediments and Physico-Chemical Characterization of Waters in the Cotonou Channel and Lake Nokoué, Benin

Abstract

Water is an essential resource for maintaining aquatic life and the quality of aquatic products. However, sediments are the most important place of accumulation of metal residues and their remobilization in the food chain. The objective of this work is to study the water physicochemical quality and the mercury contamination levels of the sediments in the Channel of Cotonou and Lac Nokoué. Temperature, pH, conductivity, salinity and dissolved oxygen have been measured on-situ in surface and deep water. The surface sediments have been sampled and analyzed in the laboratory using the cold vapor technique (CV-AAS) by the Direct Mercury Analyzer (DMA 80) for mercury analysis. The characterization of the sediments through the granulometric fractions shows that the channel is sandy due to marine inputs while Lake Nokoué is essentially of the silt-sandy type. Mercury concentrations in Lake Nokoué sediments (up to 130.72 μg/kg) greatly exceed geochemical natural loads (20-30 μg/kg) and reveal mercury contamination; this is the case along the channel where the distribution of these residues is very uneven due to the dilution observed at the mouth by marine inputs from the Atlantic Ocean. According to the results obtained, the sediments of the two ecosystems are of class 6, extremely polluted sediments (Igeo > 5) showing a significant risk of migration of these residues in the food chain, especially for benthic organisms.