PHYSICOCHEMICAL QUALITY OF WATER AND SEDIMENTS OF LAKE HLAN IN SOUTHERN BENIN

Lake Hlan is experiencing environmental problems that threaten its existence. This lake is located in the south of Benin and is shared by two communes namely Zogbodomey and Toffo. The lake is a subject of different anthropogenic pressures linked to demographic expansion. This present study aims to evaluate the physicochemical quality of the water and sediments of this lake. To achieve this objective, field and laboratory work was carried out. To understand the impact of pressure on the water quality of the lake in the municipality of Toffo, the different uses of the lake were inventoried among the local population using a questionnaire. Five sites were targeted for water and sediment sampling.The methodological approach took three phases:Documentary research having made it possible to better understand the contours of the subject, data collection through a survey, sampling of water and sediments, laboratory analyzes and interpretation of data collected. The survery resultats indicate that 35% of the population practices agriculture and market gardening, 36,67% practice the transformation of pal nuts into red oil, 23,33% practice fishing 5% practice breeding. As for the results of physicochemical analyzes, we note that water is moderately charged and polluted by ortho phosphates and ammonium, which is justified by activities by the activities carried out around the lake and the input of waste through runoff. Lead and cadmium levels are respectively (21,769 to 39,607 mg/kg) and (1,769 to 3,003 mg/kg). These results are higher than standards for aquatic life and indicate pollution of the lake and constitutes a threat to aquatic species and to humans.The presence of trace metal elements which are beyond standards are due to anthropogenic activities, particularly agriculture, through intensive use of fertilizers and pesticides.