Lake Kivu - Chemistry

Chemistry

Lake Kivu is one of three known exploding lakes, along with Cameroonian Lake Nyos and Lake Monoun, that experience violent lake overturns. Analysis of Lake Kivu's geological history indicates sporadic massive biological extinction on millennial timescales. The trigger for lake overturns in Lake Kivu's case is unknown but volcanic activity is suspected. The gaseous chemical composition of exploding lakes is unique to each lake; in Lake Kivu's case, methane and carbon dioxide due to lake water interaction with a volcano. The amount of methane is estimated to be 65 cubic kilometers (if burnt over one year, it would give an average power of about 100 gigawatts for the whole period). There is also an estimated 256 cubic kilometers of carbon dioxide. The methane is reported to be produced by microbial reduction of the volcanic CO₂. The risk from a possible Lake Kivu overturn is catastrophic, dwarfing other documented lake overturns at Lakes Nyos and Monoun, because of the approximately two million people living in the lake basin.

Cores from the Bukavu Bay area of the lake reveal that the bottom has layered deposits of the rare mineral monohydrocalcite interlain with diatoms, on top of sapropelic sediments with high pyrite content. These are found at three different intervals. The sapropelic layers are believed to be related to hydrothermal discharge and the diatoms to a bloom which reduced the carbon dioxide levels low enough to precipitiate monohydrocalcite.

Scientists hypothesize that sufficient volcanic interaction with the lake's bottom water that has high gas concentrations would heat water, force the methane out of the water, spark a methane explosion, and trigger a nearly simultaneous release of carbon dioxide. The carbon dioxide would then suffocate large numbers of people in the lake basin as the gases roll off the lake surface. It is also possible that the lake could spawn lake tsunamis as gas explodes out of it.

The risk posed by Lake Kivu began to be understood during the analysis of more recent events at Lake Nyos. Lake Kivu's methane was originally thought to be merely a cheap natural resource for export, and for the generation of cheap power. Once the mechanisms that caused lake overturns began to be understood, so did awareness of the risk the lake posed to the local population.

An experimental vent pipe was installed at Lake Nyos in 2001 to remove gas from the deep water, but such a solution for the much larger Lake Kivu would be considerably more expensive. No plan has been initiated to reduce the risk posed by Lake Kivu. The approximately 500 million tonnes of carbon dioxide in the lake is a little under 2 percent of the amount released annually by human fossil fuel burning. Therefore the process of releasing it could potentially have costs beyond building and operating the system.