Spatial and seasonal variation of trace elements contamination level of the waters from the hydrosystem Lake Togo-Lagoon of Aného (South of Togo)

The hydrosystem Lac Togo-Lagoon of Aneho is located in a watershed where phosphate mining of trace element-rich phosphorites takes place. This activity generates various types of wastes that are dumped into the environment without prior treatment. In addition, there are contributions of its tributaries after leaching of agricultural and urban soils. All this contributes to the degradation of the water quality of this ecosystem. The aim of this study is to assess the level of trace elements contamination of the waters from the hydrosystem Lake Togo-Lagoon of Aneho and its spatio-seasonal variations. In the dry and rainy season, the physicochemical parameters of waters were measured according to French standards (AFNOR). The trace elements were determined by atomic absorption spectrometer coupled to a hydride and cold vapour generator. The spatial distribution was evaluated using maps which were performed by ordinary kriging interpolation method of ArcGIS 10.2.2 software. The results showed that the average dry season concentrations were significantly higher (p < 0.0001) than those of the rainy season except that of Zn. The average concentrations (in μg/l) observed for Cd: (39.28; 24.63), Pb: (216.82; 66.44), Cr: (197.00; 69.05) and Ni: (100.06; 66.50), respectively, in dry and rainy season were significantly higher than the WHO standards. However, overall trace elements contamination has been noted with polymetallic contamination indices which reached 4.14 in the dry season and 2.41 in the rainy season. This contamination is mainly of anthropogenic origin and comes from the intrusion of seawater loaded with phosphate effluents in the dry season and the leaching of mining and agricultural soils in the rainy season. The results proved that this lagoon ecosystem is polluted by trace elements mainly caused by phosphate mining. Thus, it deserves special attention for its better management for a sustainable development.