Evaluation of heavy metals pollution of Nokoue Lake

Toxic metals (cadmium, lead, copper and arsenic) assessment was carried out in water and sediments samples of Nokoue Lake. Except for water, a group of sediment samples was mineralized before the analysis with atomic absorption spectrophotometer. The second group of sediment samples was processed for texture analysis. The results revealed that for the physico-chemical characteristics, the pH is from 6.5 to 8.5. Four kinds of textures were identified: mud, sand, sandy mud and muddy sand. Heavy metals mean concentrations in water rose from 0.01 to 10.1 mg/L for lead and 5.84 to 8.47 for arsenic. In water, copper showed low concentrations in all sites and the values of cadmium were below the detectable limits. The mean concentrations in the sediments were: Cd (0.56 to 22.07 mg/L); Pb (3.2 to 13.94 mg/L); Cu (11.51 to 54.39 mg/L) and As (ND to 436.81 mg/L). Besides, the pattern of trace elements accumulation according to textures revealed that sandy mud and sand sediments recorded high concentrations of arsenic, mud registered high concentrations of cadmium and the sediment combining sand and mud registered high concentrations of copper. Besides, the correlation between heavy metal concentrations and pH except for arsenic.Key words: Nokoue Lake, pollution, heavy metal, texture

Fractionnement Géochimique Des Eléments Traces Métalliques (Etm) Dans Les Sédiments Du Delta De L’ouémé Au Bénin

Due to the inherent damages of trace elements to living organisms, the amount of bio available metal is considered to be a quantity that conditions the becoming of aquatic ecosystems. The aim of this work is to evaluate the proportion of Pb, Cu and Cd that are biologically available in the superficial sediments of the Ouémé estuary as well as the influence of environmental factors on the becoming of these metals. Fractionation of the metals was performed according to the sequential extraction method and their measurement was done by inductively coupled plasma mass spectrometer (ICP-MS). Physicochemical parameters evaluated are: granulometry, pH, cation exchange capacity (CEC), organic carbon (Corg), iron, aluminum and calcium oxides (Fe2O3, Al2O3 and CaO). Risk indices related to metal fractionation were evaluated followed by statistical processing in R 3.3.2. It appears that: 39.44% of the Cd are preferentially bound to the exchangeable fraction, 37.25% of the Pb are for the reducible fraction and 47.47% of the Cu for the oxidizable fraction. Risk levels are in the low to very high range. The richness of materials is revealed as a factor limiting the bioavailability of metals while CaO facilitates the bioavailability in high risk sites. Liquid and solid wastes from the Dantokpa market are the potential sources of metals