Water and gas chemistry at Lake Kivu (DRC): Geochemical evidence of vertical and horizontal heterogeneities in a multibasin structure

Waters and dissolved gases collected along vertical profiles in the five basins ( Main, Kabuno Bay, Kalehe, Ishungu, and Bukavu) forming the 485 m deep Lake Kivu ( Democratic Republic of the Congo) were analyzed to provide a geochemical conceptual model of the several processes controlling lake chemistry. The measured horizontal and vertical variations of water and gas compositions suggest that each basin has distinct chemical features produced by ( 1) different contribution from long circulating fluid system containing magmatic CO2, responsible of the huge CO2(CH4)-rich reservoir hosted within the deep lake water; ( 2) spatial variations of the biomass distribution and/or speciation; and ( 3) solutes from water-rock interactions. The Kabuno Bay basin is characterized by the highest rate of magmatic fluid input. Accordingly, this basin must be considered the most hazardous site for possible gas outburst that could be triggered by the activity of the Nyiragongo and Nyamulagira volcanoes, located a few kilometers north of the lake.