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One of the many environmental problems in Nigeria is improper disposal of solid waste which can lead to air pollution and the release of leachates into the environment. In this study, the toxicity of leachate from Olusosun landfill in Lagos, Nigeria was investigated using bioassays. Physiochemical parameters and heavy metals of the leachate were analyzed using standard methods. Acute toxicity was performed using Culex pipiens larva and Clarias gariepinus. Antioxidant enzyme activities such as glutathione (GSH), superoxide dismutase (SOD), catalase (CAT) and lipid peroxidation were also determined in the liver and gills of Clarias gariepinus post-juveniles exposed to sub-lethal concentrations of 1/100th and 1/10th of the LC50 respectively. The pH of the leachate was 8.1 and conductivity was 1610 μS/cm. Heavy metal with highest concentration was iron (3.45 mg/l) while cadmium had the lowest concentration of 0.006 mg/l. Toxicity results showed that the LC50 for Culex pipiens larva was 11.52 mg/l and that of Clarias gariepinus fingerlings was 12.58 mg/l. Antioxidant enzyme activities were found in the liver and gills of Clarias gariepinus post-juveniles. These results are of grave public health concern due to the lack of a containment mechanism for the leachate generated in the research area. This finding is critical for monitoring the environmental safety of landfills to protect animals, human health and the environment. Therefore, different stakeholders in waste management in Lagos State should develop policies that will help improve the quality of the environment.
Keywords: landfill, leachate, toxicity, antioxidants, olusosun