Spatial and seasonal variation of trace elements contamination in the sediments of a tropical lagoon ecosystem: the Lake Togo-Lagoon of Aneho complex (southern Togo)

摘要

The Lake Togo-Lagoon of Aneho complex is located in the area of the phosphorite mining which produces different types of waste that are dumped directly into the surrounding environment without treatment. In addition, the contributions of its tributaries after leaching agricultural and urban soils need to be considered. This increases the risk of contamination of the hydrosystem, especially the sediments in trace elements. The objective of this study is to evaluate the level of trace elements contamination and their spatial and seasonal variation in the sediments of the Lake Togo-Lagoon of Aneho complex. The sediment physicochemical parameters were determined in line with French standards. The trace elements were determined using an Atomic Absorption Spectrometer coupled to a hydride and cold vapour generator. The spatial and seasonal variation was assessed using Ordinary Kriging Interpolation method. The average content of trace elements in sediments in mg/kg was as follow: Cd: 0.75, Pb: 13.26, Cr: 50.63, Ni: 36.05, Cu: 10.90, Hg: 0.04, As: 3.88, Zn: 47.38 and Mn: 766.74 in the dry season. In rainy season, the average concentration was: Cd: 0.46, Pb: 9.50, Cr: 27.43, Ni: 21.36, Cu: 9.61, Hg: 0.05, As: 5.23, Zn: 39.21 and Mn: 910.08. These concentrations showed a strong spatial variation. Significant seasonal variations were obtained for conductivity, organic matter as well as for Cd, Pb, Cr, Ni, Hg and As. The sediments were found to be weakly to moderately contaminated and enriched by trace elements with a higher contamination for Cd. The sediments may have low to moderate adverse effects on aquatic living organisms. This trace element contamination is of anthropogenic origin through the phosphorite mining and the agricultural and urban effluents. Therefore, this lagoon ecosystem warrants regular monitoring for better understanding to inform protection of the resource.