A unique humanitarian, scientific, and tech-nical experiment is currently taking place in aforgotten mountainous region of West Africa.The experiment is on the verge of reaching itsoperational stage, which consists of the eradi-cation of a “new”natural hazard; one that ispotentially devastating, but which has beenknown for only 20 years.

The Lake Nyos catastrophe, which claimed1800 victims in August 1986, was not unprece-dented. Indeed, 2 years previously, a lethal gasburst,originating from nearby Lake Monoun inthe same remote area of Cameroon, killed 37people—an odd and tragic episode that wentalmost unnoticed.One had never before heardof Mother Nature asphyxiating human beingsand most higher animals on such a scale in asingle and brief non-volcanic event.

Today,the emotion has vanished,and this tragicepisode is almost forgotten.However, the causesand mechanism of these tragedies are now much

better understood, thanks to the almost contin-uous scrutiny of the two lakes by scientists.

Lakes Nyos and Monoun occupy the craterof a supposedly extinct volcano, in a regionknown by geologists for its numerous soda(not thermal) water springs,a common featureof old volcanic areas.The region belongs tothe so-called volcanic chain of Cameroon,which culminates and ends 300 km furthersouthwest at the still-active Mount Cameroon(4000 m asl.). Accumulated evidence strengthens

the early hypothesis that the Monoun gas burstoriginated in the release of huge amounts ofpressurized carbon dioxide,previously dissolvedin the lower layers of the 95-m-deep lake [Sigurdsson et al.,1987]. Most probably, thehypothesis holds for the 210-m-deep Lake Nyos,though on a much larger scale. Any exogenousdisturbance, or intrinsic instability, may upsetthe density stratification of the lakes’ water col-umn (CO2-laden water is denser than purewater), trigger an overturn of the lake, andsubsequently release suffocating carbon diox-ide.

The term “limnic eruption”was first coinedby J.-C. Sabroux at a UNESCO Conference onthe Lake Nyos disaster (March 1987;Yaoundé,Cameroon) for describing this violent overturn

Degassing the “Killer Lakes”Nyos and Monoun, Cameroon

VOLUME 85 NUMBER 30

27 JULY 2004

PAGES 281–288

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EOS,TRANSACTIONS, AMERICAN GEOPHYSICAL UNION

PAGES 281, 285

Fig.1.Carbon dioxide/water jets on Cameroonianlakes. (A) The ~8-m-high steady jet flushingout the 140-mm inner-diameter (Ø) and 65-m-long pipe (l) moored vertically in the middleof Lake Monoun,April 1992.The gas/watervolume ratio r =2.9,and the water flow-rate q= 48 L/s. (Photo: J.Grangeon). (B) The 21-m-high soda fountain at Lake Nyos,March 1995(Ø = 140 mm and l = 200 m): an experimentvery close to the operational scale (r = 7 andq = 62 L/s), including the service platform.(Photo: B.Canet). (C) The February 2001 full-strength,50-m-high jet of Lake Nyos,with Ø =140 mm and l = 203 m (Photo: M.Halbwachs).Due to its ca.0.1 density, the discharged fluid(r = 9 and q = 70 L/s) is harmless and silent,despite the high exit velocity (> 100 km/h).(D) The 8-m-high jet at Lake Monoun,Febru-ary 2003: Ø = 140 mm, l = 73 m, r = 3.2 and q= 50 L/s (Photo: S.Chikhi).

BY MICHEL HALBWACHS, JEAN-CHRISTOPHE

SABROUX, JACQUES GRANGEON, GASTON KAYSER,JEAN-CLAUDE TOCHON-DANGUY,ALAIN FELIX,JEAN-CHRISTOPHE BÉARD,ADELIN VILLEVIEILLE,GÉRARD VITTER, PATRICK RICHON,ALFRED WÜEST,AND JOSEPH HELL

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of a meromictic lake (a lake whose waters donot mix). It takes into account the analogy withthe volcanic eruption, also powered by gasbubbles ascending and expanding in a liquid(the magma).Limnic eruption, the mechanismof which was further refined by Tietze [1987],explains satisfactorily the sequence of eventsand consequences of the Monoun and Nyosdisasters, and suggests a possible mitigation ofthis newly discovered geological hazard.

In fact, in the years following the gas bursts,thorough investigation into the physics andchemistry of Lakes Monoun and Nyos progres-sively revealed that both still contained hugeamounts of dissolved carbon dioxide.TheStandard Temperature and Pressure (STP)volumes are currently of 17 million m3 inMonoun and 280 million m3 in Nyos, and thegas is being added at such a rate—0.38 millionm3/yr [Kling et al., 1994] and 2.8 million m3/yr[Kusakabe et al., 2000] into Monoun (below -60 m) and Nyos (below -180 m), respectively—that saturation of a deep-water layer couldbe reached within a few years at Monoun anda few decades at Nyos.

Since it is impossible to guarantee perennialstability of the lakes, and indefinite

containment of their very particular “weaponof mass destruction,” it has been proposed tomake these lakes safer by extracting, in a con-trolled way, the carbon dioxide stored in them.The proposal, already mentioned in the high-lights of the UNESCO Conference [Sigvaldason,1989], came from several scientists, includinga team of engineers from the French Ministryof Environment, who dubbed their project the“Orgues de Nyos”(“Nyos Organ Pipes”).

The method is no more than a limnic erup-tion brought under control. It is inspired bythe industrial process known as “gas lift,”bywhich a liquid (mostly oil) is pumped up bymixing it with air or carbon dioxide bubbles,and thus making it buoyant.

The basic design consists of a pipe set upvertically between the lake bottom and thesurface. A small pump (or preferably, a com-pressed gas injection), operating close to thetop of the pipe, raises the water in the pipe upto a level where it becomes saturated with gas,thus leading to degassing and subsequentlightening of the water column.Then, thebiphasic fluid rises to the surface.Once the gaslift is primed, the process is self-powered.Above the saturation level, degassing and

isothermal expansion of gas bubbles drive theflow of the gas-liquid mixture upwards, aslong as dissolved gas is available for ex-solutionand expansion.

Due to the very limited amount of fundswhich could be allocated to the project, thetechniques that are currently used in the off-shore oil industry were definitely not applica-ble in this remote area,which is hardly accessibleduring the rainy season.The participating scientists knew that the entire installation hadto be set up virtually by hand, with only themechanical assistance of a few rubber boats.The breakthrough came with the choice ofthe material used for the pipe.With a specificgravity of 0.96 kg/L, high-density polyethylene(HDPE) has a nearly neutral buoyancy in water:a pipe can be sunk vertically with the help ofvery little ballast,and retrieved horizontally byblowing air into it.This plastic material is veryflexible, and not at all brittle. It can be readilyassembled in the field from 6-m-long segmentselectrically soldered end to end.The extensiveuse of HDPE in natural gas supply networksproves its resistance to weathering and chemicalalteration.

From Experiments to Operational Degassing

In March-April 1992 at Monoun,two ballasted70-m-long polyethylene pipes (50-mm and140-mm inner diameter) were hung one afteranother on a floating raft anchored to the lakebed [Halbwachs et al., 1993], and successfullyprimed (Figure 1a). In March 1995 at Nyos, thesame experiment was duplicated, with onlythe larger pipe diameter plunged at a 200-mdepth. Despite some technical difficulties,dueto the much greater depth and higher gas con-tent of the lake, a 21-m fountain was activated,foreshadowing the operational degassing ofthe killer lake (Figure 1b). In all cases, theobserved results (in terms of gas and waterflow rates, internal pressure,and upward thrust)could be interpreted by numerical modelingof the two-phase flow.The reliability of a remotelyoperated immersed control valve for stoppingand starting the flow on request was alsodemonstrated, as well as the basic concept ofa hanging pipe: a pendulum oscillating freelyfrom a raft anchored by ropes secured to thelake shore.

After completion of those preliminary tests,the gas content in both lakes continued to rise,due to persistent gas seepage into the lake. InFebruary 2001, the CO2 bottom concentrationof Lake Nyos was due to reach 10 STP m3 gasper cubic m of water,as compared to the 7 m3

per cubic m in 1995. In Lake Monoun, thebuildup of an almost-saturated water layer at -55 m made degassing the lake a matter ofurgency (Figure 2).A fully fledged degassingcolumn was successfully assembled, installed,and primed at Nyos on 30 January 2001 (Fig-ure 1d), and a spectacular 50-m-high fountainsoared above the lake surface.As expected,this is significantly higher than the fountainprimed in 1995 with a similar pipe, andreflects the considerable increase of CO2 con-centration in the bottom layers of the lakesince the 1995 experiment.

Fig.2. The partial pressure profiles in Lake Monoun as of February 2003.The critical situation ofthe lake can be emphasized since the total gas pressure at -55 m is quite close to the hydrostaticpressure.A slight increase in the dissolved gas concentration should result in over-saturation, trig-gering the formation of bubbles,which in turn will initiate the “avalanche process” leading to theoverturning of the lake (limnic eruption).The vertical arrow shows the amount of subsidence ofthis critical layer induced by a 12-month degassing (see Figure 3).

Three years later, the powerful soda sprayjet—stable and safe—composed of 90% car-bon dioxide and 10% water (volume/volume)is still spouting up from the vertical pipe sunkinto the lake.The lower end of the pipe tapswater at -203 m.The process has been almostcontinuous, apart from the short periods oftime needed for maintenance. Given a steadystate CO2 flow-rate of 50,000 STP m3 per day,the recharge rate of the lake is now safely offset.The instrumentation of the degassing assemblypermits the continuous monitoring of somelimnological parameters at the pipe inlet, andof the two-phase flow regime, together with itspotential instabilities. It also permits the con-trol and remote operation of the functioningof the degassing platform from Yaounde, or

from France, through an INMARSAT satellitelink.A Web cam operating from an ancillaryinstrumentation barge, and using the samesatellite link, puts the finishing touch to thisinstrumentation scheme (http://perso.wanadoo.fr/mhalb/nyos/). Our current datacomplement the monitoring effort carried outby the foreign and Cameroonian teamsthrough survey and sampling missions [e.g.,Evans et al., 1994; Tanyileke et al., 1996; Schmidet al., 2003].

The basic design of the degassing column(including safety equipment) at Monoun wassimilar to the Lake Nyos installation. On theother hand, the pipe inner diameter was only100 mm, and all metallic parts were made ofstainless steel,in order to get rid of the extremely

rapid CO2-induced corrosion in an oxygen-free environment (formation of siderite FeCO3,at the expense of iron) observed on the col-umn in operation at Nyos for 2 years.An 8-m-high jet (Figure 1c) has been gushing forth atMonoun since February 2003, the operationalparameters of the degassing column fallingperfectly within the uncertainty range of thenumerical two-phase flow model already vali-dated by the more powerful jet at Lake Nyos(see Table 1).

‘Toward Eradication of a Natural Hazard

Figure 3 presents the data bearing on theresponse of limnological key parameters tothe current degassing—a clue to the validityof the process, and a unique opportunity forlimnologists and oceanographers to study theresponse of a stratified, double-diffusion sys-tem, to the disturbance induced by pumping,in a 150-m-high convection-free water column.

The experience gained over 12 years throughsuccessful trials, and extrapolation of the two-phase flow model,show that five 140-mm pipesat Nyos and three 100-mm pipes at Monoun areneeded for an operational scheme of degassingthe lakes, down to a level that would rule outthe possibility of a limnic eruption (within 4years at Nyos and 2 years at Monoun). Such abackground paves the way for installing with-out delay the remainder of these operationalpipes at each lake.

Meanwhile, several hundred kg of iron II—which eventually precipitates as iron III hydrox-ide (limonite)—pouring onto the lake surfaceon a daily basis [Bernard and Symonds,1989]raises some ecological questions that shouldbe addressed by biologists, and parti-cularly,by specialists of anoxic environments, beforescaling up the current degassing experiments.Such an issue is more sensitive at Monoun,where fishing is an economic resource forlocal villagers.

The current degassing program is relevant toseveral newsworthy scientific issues, includingthe controversial engineering proposals todump carbon dioxide in the deep ocean (theNyos disaster being generally presented as adeterrent to such projects); and even themethane/mass extinction hypothesis. TheNyos crater lake is also an example of thecrypto-volcanism contribution to the carboncycle, despite the fact that the current CO2

recharge rate, if evenly distributed over the flatbottom of the lake, corresponds to a daily fluxdensity of 0.01 STP m3/m2; i.e., 10 times lessthan the daily CO2 seepage over a similar sur-face area at Mammoth Mountain, Long ValleyCaldera, California.

Moreover, the Nyos degassing project is abench test for the still more ambitious endeav-or of extracting methane from Lake Kivu, EastAfrica, a bonanza for the lake’s riparian coun-tries. Finally, for those whose concern is themere 4% addition, by full-rate degassing, to theannual CO2 emissions from fossil fuel combus-tion in Cameroon,it has been proposed to usethe 500,000 tons of high-grade (C2H4 < 2ppbv), isotopically labeled (14C-free) carbondioxide of Lake Nyos for studying, in a large

Eos,Vol. 85, No. 30, 27 July 2004

Fig.3.Evolution of the Nyos and Monoun thermal profiles since the onset of the degassing (witha 20-month disruption at Nyos).To take full advantage of the sensitivity offered by the CTDprobes (Sea-Bird Electronics, Inc.), the reference depth is the lake’s bottom,not the lake’s surface.The subsidence of the thermal stratification,as a response to the degassing pipe operation, isclearly visible in both lakes.More precisely, the subsidence of the thermocline at stake is consis-tent with the water flow rate through the pipes pumping at -203 m at Nyos and -73 m atMonoun.Since the gas-rich layer subsides by the same depth, the effect of the degassing on thesafety of the lakes is conspicuous, especially in the case of Monoun (see Figure 2).

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scale FACE or other experiment, the ecologicalimpact of elevated CO2 in tropical ecosystems[Mousseau et al., 1997].

Conclusion

In the years to come, limnologists,biologists,environmentalists, volcanologists, and manyother scientists should not miss the opportu-nity to study the impact of the degassing onthe lakes’ physico-chemistry, on the nearbyhuman communities, and on the ecologicalequilibrium. Conversely, this scientific interestwould attract in its wake the bilateral andinternational funding badly needed to main-tain the current installation at a safe level ofoperation, and to scale it up to a point atwhich the threat of another limnic eruptionwill be wiped out.

It is our responsibility as Earth scientists tobring to completion the Nyos and Monoundegassing project that we initiated in theaftermath of the catastrophe, and that wehave been carrying on persistently in a con-text in which Africa, suffering so badly fromother plagues, finds that its killer lakes aresinking into oblivion.

Partial funding of the successive experimentsat Nyos and Monoun was provided by theCameroon government (IRGM, MINREST, andother departments), the Office of Foreign Dis-aster Assistance (OFDA) of the U.S.Agency forInternational Development, the EuropeanCommunities, the Fondation Gaz de France,the International Union of Technical Associa-tions and Organizations (UATI),and the FrenchMinistère des Affaires Etrangères.The entireproject was kick-started by the Délégation auxRisques Majeurs, a department of the FrenchMinistère de l’Environnement.

The Cameroon government participated alsoin the financial and logistical support, togetherwith the French Embassy in Yaoundé, whosefunding is particularly significant in the cur-rent Monoun degassing experiment. Finally,extensive fieldwork spread over 15 years wouldnot have been possible without the unfailinghelp of Cameroon technicians, workers, andthe local population.

References

Bernard,A.and R.B.Symonds,(1989),The significanceof siderite in the sediments from Lake Nyos,Cameroon. J.Volcanol.Geotherm.Res., 39, 187–194.

Evans,W. C., L. D.White, M. L.Tuttle, G.W. Kling, G.Tany-ileke, and R. L. Michel (1994), Six years of changeat Lake Nyos, Cameroon, yield clues to the pastand cautions for the future, Geochem. J., 28,139–162.

Halbwachs, M., J. Grangeon, J. C. Sabroux, and A.Villevieille (1993), Purge par auto-siphon du gazcarbonique dissous dans le lac Monoun (Camer-oun): premiers résultats expérimentaux. C.R.Acad.Sci. Paris, 316(II), 483–489.

Kling, G.W.,W. C. Evans, M. L.Tuttle, and G.Tanyileke,(1994) Degassing of Lake Nyos, Nature,368,405–406.

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Mousseau, M., Z. H Enoch, and J. C. Sabroux (1997),Controlled degassing of lakes with high CO2 con-tent in Cameroon: an opportunity for ecosystemCO2 enrichment experiments in Plant Responsesto Elevated CO2, edited by A. Raschi, F. Miglietta,R.Tognetti, and P. R. van Gardingen, pp. 45–55, Cam-bridge University Press, U.K.

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Sigurdsson, H., J. D. Devine, F. M.Tchoua,T. S. Presser,M. K.W. Pringle, and W. C. Evans (1987), Origin ofthe lethal burst from Lake Monoun, Cameroon,J.Volcanol.Geotherm.Res., 31, 1–16.

Sigvaldason, G. E. (1989), International conferenceon Lake Nyos disaster, Cameroon, 16-20 March,1987: conclusions and recommendations,J.Volcanol.Geotherm.Res., 39, 97–107.

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Author Information

Michel Halbwachs, Université de Savoie,Chambéry, France; Jean-Christophe Sabroux,IRSN,Saclay,France; Jacques Grangeon, Université deSavoie, Chambéry, France; Gaston Kayser (retired),Euratom, Cadarache, France; Jean-Claude Tochon-Danguy,Alain Felix, and Jean-Christophe Béard, DataEnvironnement, Chambéry, France; Adelin Villevieille,UATI,Paris,France;Gérard Vitter,ENSEEG,Grenoble,France;Patrick Richon,CEA,DASE,Bruyères-le-Châtel,France; Alfred Wüest, EAWAG, Kastanienbaum,Switzerland; Joseph Hell, IRGM,Yaoundé, Cameroon