[1] The deep waters of the East African Rift Lake Kivu contain large amounts of dissolved carbon dioxide and methane The release of a fraction of these gases, which could be triggered by a magma eruption within the lake, would have catastrophic consequences for the two million people living on its shore Up to now the safety assessment of the lake was based on the assumption that the gas concentrations in the deep waters are in a steady state with a residence time of 400 years Turbulent transport was regarded as the main pathway of vertical exchange Recent measurements and the analysis of the vertical transport processes in the lake radically change this evaluation The vertical turbulent exchange is negligible, as documented by a spectacular set of several hundred double-diffusive layers Gases are mainly transported out of the deep zones by a slow upwelling with a residence time of 800–1000 years Our results indicate that the methane production within the sediment has recently increased, leading to a gas accumulation in the deep waters and consequently decreasing the heat input needed to trigger a devastating gas release With the estimated current CH4 production, the gas concentrations could approach saturation within this century

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Weak mixing in Lake Kivu: New insights indicate increasing risk of uncontrolled gas eruption

Since the catastrophic releases of CO2 in the 1980s, Lakes Nyos and Monoun in Cameroon experienced CO2 recharge at alarming rates of up to 80 mol/m2 per yr. Total gas pressures reached 8.3 and 15.6 bar in Monoun (2003) and Nyos (2001), respectively, resulting in gas saturation levels up to 97%. These natural hazards are distinguished by the potential for mitigation to prevent future disasters. Controlled degassing was initiated at Nyos (2001) and Monoun (2003) amid speculation it could inadvertently destabilize the lakes and trigger another gas burst. Our measurements indicate that water column structure has not been compromised by the degassing and local stability is increasing in the zones of degassing. Furthermore, gas content has been reduced in the lakes ≈12-14%. However, as gas is removed, the pressure at pipe inlets is reduced, and the removal rate will decrease over time. Based on 12 years of limnological measurements we developed a model of future removal rates and gas inventory, which predicts that in Monoun the current pipe will remove ≈30% of the gas remaining before the natural gas recharge balances the removal rate. In Nyos the single pipe will remove ≈25% of the gas remaining by 2015; this slow removal extends the present risk to local populations. More pipes and continued vigilance are required to reduce the risk of repeat disasters. Our model indicates that 75-99% of the gas remaining would be removed by 2010 with two pipes in Monoun and five pipes in Nyos, substantially reducing the risks.

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Degassing Lakes Nyos and Monoun: Defusing certain disaster

better understood, thanks to the almost continuous scrutiny of the two lakes by scientists. Lakes Nyos and Monoun occupy the crater of a supposedly extinct volcano, in a region known by geologists for its numerous soda (not thermal) water springs, a common feature of old volcanic areas.The region belongs to the so-called volcanic chain of Cameroon, which culminates and ends 300 km further southwest at the still-active Mount Cameroon (4000 m asl.). Accumulated evidence strengthens the early hypothesis that the Monoun gas burst originated in the release of huge amounts of

https://mhalb.pagesperso-orange.fr/nyos/2004EO300001_papier%20EOS%20Nyos.pdf

Degassing the “Killer Lakes” Nyos and Monoun, Cameroon

Large iron isotope fractionation at the oxic–anoxic boundary in Lake Nyos

Evolution of CO2 in Lakes Monoun and Nyos (Cameroon) before and during controlled degassing is described using results of regular monitoring obtained during the last 21 years. The CO2(aq) profiles soon after the limnic eruptions were estimated for Lakes Monoun and Nyos using the CTD data obtained in October and November 1986, respectively. Based on the CO2(aq) profiles through time, the CO2 content and its change over time were calculated for both lakes. The CO2 accumulation rate calculated from the pre-degassing data, was constant after the limnic eruption at Lake Nyos (1986‐ 2001), whereas the rate appeared initially high (1986‐1996) but later slowed down (1996‐2003) at Lake Monoun. The CO2 concentration at 58 m depth in Lake Monoun in January 2003 was very close to saturation due to the CO2 accumulation. This situation is suggestive of a mechanism for the limnic eruption , because it may take place spontaneously without receiving an external trigger. The CO2 content of the lakes decreased significantly after controlled degassing started in March 2001 at Lake Nyos and in February 2003 at Lake Monoun. The current content is lower than the content estimated soon after the limnic eruption at both lakes. At Monoun the degassing rate increased greatly after February 2006 due to an increase of the number of degassing pipes and deepening of the pipe intake depth. The current CO2 content is ~40% of the maximum content attained just before the degassing started. At current degassing rates the lower chemocline will subside to the degassing pipe intake depth of 93 m in about one year. After this depth is reached, the gas removal rate will progressively decline because water of lower CO2(aq) concentration will be tapped by the pipes. To keep the CO2 content of Lake Monoun as small as possible, it is recommended to set up a new, simple device that sends deep water to the surface since natural recharge of CO 2 will continue. Controlled degassing at Lake Nyos since 2001 has also reduced the CO2 content. It is currently slightly below the level estimated after the limnic eruption in 1986. However, the current CO 2 content still amounts to 80% of the maximum level of 14.8 giga moles observed in January 2001. The depth of the lower chemocline may reach the pipe intake depth of 203 m within a few years. After this situation is reached the degassing rate with the current system will progressively decline, and it would take decades to remove the majority of dissolved gases even if the degassing system keeps working continuously. Additional degassing pipes must be installed to speed up gas removal from Lake Nyos in order to make the area safer for local populations.

Evolution of CO2 in Lakes Monoun and Nyos, Cameroon, before and during controlled degassing

Since the catastrophic CO2 eruption in 1986, Lake Nyos has been investigated in detail by several research groups. However, no signs of double-diffusive convection were observed before December 2002, when a set of 26 well-mixed layers with thicknesses of 0.2-2.1 m and sharp interfaces in between were discovered at 53-74 m depth. Such pronounced steps are a characteristic feature of double-diffusive convection of the diffusive regime. A temperature time series measured at 62 m depth indicates that the double-diffusive convection started in the second half of March 2002. The trigger was most probably the cooling at the top of this layer caused by relatively strong seasonal convective mixing down to 52.5 m depth during the dry season in February 2002. The heat fluxes calculated by the heat budget method and the thicknesses of the layers agree within the uncertainties with the values expected from the double-diffusive flux laws. The heat fluxes increased by an order of magnitude since the establishment of the double-diffusive convection and reached values comparable to the heat input by a source of warm and CO2-enriched water to the deepest zone of the water column. In contrast, the CO2 fluxes caused by double diffusion Lire negligible compared to the input by this source. Because the double-diffusive heat fluxes were higher in the upper layers of the staircase compared to the lower ones, the temperature gradient between 60 and 75 m depth approximately doubled from March 2002 to December 2002, whereas the total dissolved solids gradient remained almost constant during this period. Consequently, this process is reducing the stability of the staircase and could potentially lead to a complete homogenization of this zone within a few years. It cannot be excluded that a similar double-diffusive event could have been the trigger of the CO2 eruption in 1986. (C) 2004 Elsevier Ltd. All rights reserved.

Double-diffusive convection in Lake Nyos, Cameroon

To prevent the recurrence of a disastrous eruption of carbon dioxide (CO2) from Lake Nyos, a degassing plan has been set up for the lake. Since there are concerns that the degassing of the lake may reduce the stability of the density stratification, there is an urgent need for a simulation tool to predict the evolution of the lake stratification in different scenarios. This paper describes the development of a numerical model to predict the CO2 and dissolved solids concentrations, and the temperature structure as well as the stability of the water column of Lake Nyos. The model is tested with profiles of CO2 concentrations and temperature taken in the years 1986 to 1996. It reproduces well the general mixing patterns observed in the lake. However, the intensity of the mixing tends to be overestimated in the epilimnion and underestimated in the monimolimnion. The overestimation of the mixing depth in the epilimnion is caused either by the parameterization of the k-epsilon model, or by the uncertainty in the calculation of the surface heat fluxes. The simulated mixing depth is highly sensitive to the surface heat fluxes, and errors in the mixing depth propagate from one year to the following. A precise simulation of the mixolimnion deepening therefore requires high accuracy in the meteorological forcing and the parameterization of the heat fluxes. Neither the meteorological data nor the formulae for the calculation of the heat fluxes are available with the necessary precision. Consequently, it will be indispensable to consider different forcing scenarios in the safety analysis in order to obtain robust boundary conditions for safe degassing. The input of temperature and CO2 to the lake bottom can be adequately simulated for the years 1986 to 1996 with a constant sublacustrine source of 18 l s−1 with a CO2 concentration of 0.395 mol l−1 and a temperature of 26 °C. The results of this study indicate that the model needs to be calibrated with more detailed field data before using it for its final purpose: the prediction of the stability and the safety of Lake Nyos during the degassing process.

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Gregory Tanyileke

Papers

Double-diffusive convection in Lake Nyos, Cameroon

Development and sensitivity analysis of a model for assessing stratification and safety of Lake Nyos during artificial degassing