Characterising the Thermal and Chemical Stratification in Nigerian Deep Reservoirs and Its Implications for Water Quality Management

This study characterises the thermal and chemical stratification dynamics in three deep Nigerian reservoirs, Kainji, Jebba, and Shiroro, all located in Niger State and the Niger-Kogi State border area, examining the implications of stratification for water quality management in drinking water supply and hydropower applications. Thermal stratification in large tropical reservoirs creates conditions for hypolimnion oxygen depletion, internal phosphorus loading from anoxic sediments, and hydrogen sulphide release, all of which degrade raw water quality for treatment plants and can affect aquatic ecology. Depth-profile measurements of temperature, dissolved oxygen, pH, electrical conductivity, turbidity, and redox potential were conducted monthly over eighteen months at three stations per reservoir. Sediment core samples were analysed for organic matter content, iron, manganese, and phosphorus. Water quality at the drinking water offtake levels was monitored against operational treatment plant performance. Results confirm pronounced seasonal stratification at all three reservoirs during the April to October warm season, with thermocline establishment between 8 and 15 metres depth and hypolimnion dissolved oxygen depleted to below 1 milligram per litre at maximum stratification. Internal phosphorus loading from anoxic sediment is estimated at 0.12 to 0.38 milligrams per square metre per day during stratified periods. Manganese release from anoxic sediments is identified as a significant operational problem for the Kainji water treatment plant. Destratification aeration trials at Jebba show measurable improvement in hypolimnion oxygen. The study recommends selective withdrawal management at all three reservoir offtakes. Keywords: reservoir stratification, thermal stratification, water quality, Nigerian reservoirs, hypolimnion.