Evaluation of Leachate Contamination from Olusosun Landfill on Surrounding Surface Water Bodies, Lagos State

This study evaluates the extent and character of leachate contamination from the Olusosun landfill in Lagos State, South West Nigeria, on adjacent surface water bodies, including drainage channels and receiving streams. Olusosun is the largest active landfill in West Africa, and its unlined design facilitates the migration of leachate containing organic compounds, heavy metals, and pathogens into the surrounding environment. Water samples were collected from seven surface water stations upstream and downstream of the landfill boundary, alongside direct leachate samples from exposed seepage points. Parameters measured include chemical oxygen demand, biochemical oxygen demand, ammonia nitrogen, total suspended solids, pH, electrical conductivity, and concentrations of cadmium, mercury, lead, and iron. Leachate samples exhibit extremely high chemical oxygen demand values exceeding 25,000 milligrams per litre and ammonia nitrogen above 800 milligrams per litre, consistent with actively decomposing waste. Surface water stations immediately downstream of the landfill boundary show significantly elevated heavy metal concentrations and biological oxygen demand compared to upstream control stations, with lead and cadmium exceeding Nigerian surface water quality standards. The contamination gradient dissipates progressively with increasing distance but remains detectable up to 1.2 kilometres downstream. Seasonal variation shows heightened leachate generation and lateral migration during the wet season. The findings highlight an urgent environmental remediation requirement and the need for engineered leachate collection and treatment systems. Recommendations include installing a perimeter interception drain and constructing a leachate treatment facility. Keywords: leachate, landfill, surface water contamination, heavy metals, Lagos.