No increasing risk of a limnic eruption at Lake Kivu: Intercomparison study reveals gas concentrations close to steady state

Abstract

Lake Kivu, East Africa, is well known for its huge reservoir of dissolved methane (CH 4) and carbon dioxide (CO 2) in the stratified deep waters (below 250 m). The methane concentrations of up to~20 mmol/l are sufficiently high for commercial gas extraction and power production. In view of the projected extraction capacity of up to several hundred MW in the next decades, reliable and accurate gas measurement techniques are required to closely monitor the evolution of gas concentrations. For this purpose, an intercomparison campaign for dissolved gas measurements was planned and conducted in March 2018. The applied measurement techniques included on-site mass spectrometry of continuously pumped sample water, gas chromatography of in-situ filled gas bags, an in-situ membrane inlet laser spectrometer sensor and a prototype sensor for total dissolved gas pressure (TDGP). We present the results of three datasets for CH 4 , two for CO 2 and one for TDGP. The resulting methane profiles show a good agreement within a range of around 5-10% in the deep water. We also observe that TDGP measurements in the deep waters are systematically around 5 to 10% lower than TDGP computed from gas concentrations. Part of this difference may be attributed to the non-trivial conversion of concentration to partial pressure in gasrich Lake Kivu. When comparing our data to past measurements, we cannot verify the previously suggested increase in methane concentrations since 1974. We therefore conclude that the methane and carbon dioxide concentrations in Lake Kivu are currently close to a steady state.

Intrusions of sediment laden rivers into density stratified water columns could be an unrecognized source of mixing in many lakes and coastal oceans

“…Below 50 m the temperature increases with depth due to geothermal heating, and reaches temperatures of 26°C at the lake bed at 450 m depth. The lake is meromictic due to a strong chemical stratification, with a strong chemocline at 250 m depth, below which the waters have high concentrations of dissolved methane and carbon dioxide (Bärenbold et al ., 2020). The resulting density stratification has a pycnocline where Δ ρ ~ 1 – 2 kg m −3 , which is approximately twice as strong as in a typical temperate dimictic lake.…”

How dangerous is Africa’s explosive Lake Kivu?

“…If that trend were to continue, the deeper layers would reach saturation by 2090, triggering an eruption. In 2020, however, data in another paper 5 -with Schmid as co-author -suggested the gas had not increased after all. This reassured many researchers, but the findings remain controversial.…”

Consequences of “natural” disasters on aquatic life and habitats

“…The lake biodiversity is likely supported by its higher altitude if compared to other African lakes with the same characteristics, and its greater depth, thus determining cooler temperatures at the surface that may support a stronger stratification between the oxic and predominantly autotrophic (Borges et al 2014) mixolimnion up to 70 m and the deep monimolimnion rich in CH 4 and CO 2 . From data of recent studies, it seems that methane and carbon dioxide concentrations in Lake Kivu are currently close to a steady state (Bärenbold et al 2020). Tsunamis Tsunamis are massive and powerful waves that are most commonly generated by earthquakes, and less commonly by submarine (or terrestrial) landslides or aquatic cosmic impacts.…”