Lake Kivu Water Chemistry Variation with Depth Over Time, Northwestern Rwanda

Abstract

The water of East African rift lakes contains large amounts of dissolved chemicals such as carbon dioxide, methane greatly and others like  phosphate, silicate, Sulfate, Sulfide, Iron, Ammonia, Alkalinity etc. Lake Kivu is a large, deep rift basin lake located in the western branch of the East African rift zone that contains a methane gas deposit of great economic interest with two main sources: Inorganic carbon dioxide CO₂ + 4H₂ = CH4 + 2H₂O and Organic methanogenesis CH₃COOH =CH₄ + CO₂. Lake Kivu is a stratified, meromictic lake bordering Rwanda and the Democratic Republic of the Congo (DRC). The lake has a surface area of 2,370 Km2, a volume of 580 Km³ and a maximum water depth of 485 m. To characterize the vertical variation of Lake Kivu water chemistry, 8 samples of water were collected using Niskin bottles in Lake Kivu near Gisenyi town. Water samples were therefore collected at different depths: 0 m, 40 m, 90 m, 240 m, 290 m, 340 m, 340 m, and 390 m. Hatch kits were used to analyze  water chemistry of samples taken of Sulfate, Sulfide, Iron, Ammonia, Alkalinity, Silica, PO₄,andphosphorus.The results revealed that alkalinity  increases in the monimolimnion part due to the precipitation of calcium carbonate in the upper levels of the water column and dissolution in the monimolimnion. The conductivity, dissolved oxygen, temperature and pH weremeasured by CTD Sonde. Water column data from these studies showed increasing concentrations with depth. The divide between the mixolimnion and monomolimnion is estimated at a depth of around 40 m. Higher amounts of silica observed closer to the shoreline is likely a result of an influx of siliciclastic sediment and increased silica with depth is likely a result of the dissolution of diatoms below the photic zone.

Keywords: Monimolimnion, mixolimnion, water stratification, chemicals agents.