Evaluation of Water and Sediment Pollution by Heavy Metals, Agricultural Pesticides, and Hydrocarbons in Lake Nokoué, Benin

Abstract

Lake Nokoué, the largest source of fish products in southern Benin, is unfortunately threatened by advanced chemical pollution. The objective of this study is to assess the level of contamination of the lake's water-sediment matrix by trace metals, agricultural pesticides, hydrocarbons, and polychlorinated biphenyls. Ten (10) study stations were selected for water and sediment sampling in August 2024. The The measurements of trace metal elements (TME) were performed using an atomic absorption spectrophotometer, and the pesticides, hydrocarbons, and polychlorinated biphenyls were measured using gas chromatography. R and QGIS software were used for data processing and to produce the various maps. The average TME levels obtained in the water were 0.004 mg/L, 1.36 mg/L, 0.167 mg/L, 0.001 mg/L, 0.002 mg/L, and 0.001 mg/L for lead, copper, cadmium, arsenic, zinc, and mercury, respectively. All are below the standard except for cadmium. The levels of pesticides and total hydrocarbons obtained in the water are all above the standard and are 0.08 mg/L; 2.47 mg/L; 0.66 mg/L, 1.96 mg/L; 0.06 mg/L and 0.07 mg/L respectively for glyphosate, atrazine, flubendiamide, spirotetramat, total polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPHs). However, the levels found in sediments for the three types of pollutants are below the standard and are 0.298 mg/kg; 0.006 mg/kg; 2.408 mg/kg; 0.010 mg/kg; 0.133 mg/kg; 5.899 mg/kg; 0.462 mg/kg; 0.556 mg/kg; 0.144 mg/kg and 0.156 mg/kg respectively for cadmium, lead, copper, mercury, glyphosate, atrazine, flubendiamide, spirotetramat, PAHs, and PTS. Furthermore, polychlorinated biphenyls (PCBs) are not present in either matrix. Principal component analysis (PCA) shows that trace metals (TM), pesticides, polycyclic aromatic hydrocarbons (PAHs), and total hydrocarbons (THC) present in water and sediments originate from the same sources. In terms of water pollution, the stations at Vékky, Zogbo, and Gogomey are more contaminated with TMMs, while those at Aguégué, Sô-Ava, and Agbato show higher levels of pollution from hydrocarbons and pesticides (glyphosates). In terms of sediments, the Gogomey and Ganvié stations are heavily polluted with TME, those of Totchè, Ganvié1 and Aguégué with PAHs and TOH, and Zogbo and Epkè with pesticides. In light of the results obtained, this lake appears to be a problematic area for water contamination by trace metals (TM), pesticides, PAHs, and PHTs.