Factors Influencing the Partitioning of Metal Residues (Lead) in Nokoué Lake, Benin

Abstract

The impacts on housing and living species (fish, shrimp) related to the disposal of untreated sewage by municipalities and discharges of solid waste was studied by assessing the whole Organic Matter (OM) of the sediments, Chemical Oxygen Demand (COD), Suspended Materials (SM), nitrogen pollution indicators (nitrate-NO3 -, nitrite- NO2 -, ammonium-NH4 -), phosphated (ortho-phosphate ions- PO4 3-), sulphide (sulphate- SO4 2-, sulphite SO3 2-, sulphides S2-) of water and total Pb contents of water and sediments of Lake Nokoué. The organic matter content of sediments (19600 to 17164 mg / kg) and COD (130 to 1880 mg/L) of water are very high and indicate an increase in organic pollution of the lake compared to in previous years (Youssao, 2011, Mama et al., 2011). The strong variations in pH are dependent on the contributions of marine origin (basic to neutral) and the mineralization of the organic matter (pH < 7) in the areas of human settlement resulting in a greater or less mobility of the residues of metals in making sulphur available in its reduced form (sulphide and sulphite). The assessment of pollution indicators in the central part of the lake shows that it is heterogeneous in terms of source and impact of pollution. The sediment Pb concentration (Pbsed) / concentration of Pb in water (Pbw) ratio, represented by Pbsed/Pbw ratio shows that lead is 200 to nearly 1000 tiSM more concentrated in sediments than in the water showing a predominance of the organic form of Pb in the ecosystem of Nokoué lake. Indeed, organic lead residues are massively released into this ecosystem by oil traffickers who use motorized canoes to transport their products from Nigeria.