Evaluation of Metal Contamination in Water, Sediments, and Crassostrea rhizophorae from Water Bodies in Southern Benin

Abstract

Wetlands, particularly the lagoon complex “Lake Nokoué-Lagune de Porto-Novo”, are under severe threat from anthropogenic activities, notably the use of agricultural chemicals. To assess pollution in these ecosystems, a sampling campaign was conducted, involving the collection and analysis of 36 water, sediment and oyster samples. The study focused on three trace metals: lead (Pb), cadmium (Cd) and mercury (Hg), analyzed by atomic absorption spectrometry at the Environmental Geochemistry Laboratory of the University of Lomé, using the flame technique (SAA iCE 3000 SERIES THERMO FISHER) for lead (Pb) and cadmium (Cd) and the flameless technique (SAA iCE 3000 SERIES + VP100) for mercury (Hg). The average concentrations recorded are concerning, exceeding accepted standards. The mean concentrations of cadmium, lead and mercury were 0.0047 mg/L (Cd), 0.390 mg/L (Pb) and 0.0535 µg/L (Hg) in water; 0.2415 mg/L (Cd), 46.439 mg/L (Pb) and 0.2498 µg/L (Hg) in sediments; and 2.329 mg/L (Cd), 3.850 mg/L (Pb) and 0.0479 µg/L (Hg) in oysters from the Ouémé, all significantly above the permissible limits. The stations in Littoral and Atlantic show high concentrations of lead and mercury in water, reaching 0.3775 mg/L (Pb) and 0.0405 µg/L (Hg) in Littoral and 0.110 mg/L (Pb) and 0.1356 µg/L (Hg) in the Atlantic. Sediments are less contaminated, but oysters exhibit elevated levels of cadmium and lead, 2.374 mg/L (Cd) and 3.845 mg/L (Pb) respectively. These results indicate alarming contamination, particularly in oysters, posing a risk to human health and the ecological balance of the lagoon complex.