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

VOLUME 85 NUMBER 30 27 JULY 2004

EOS,TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 281–288

the early hypothesis that the Monoun gas burst Degassing the “Killer Lakes” originated in the release of huge amounts of pressurized ,previously dissolved in the lower layers of the 95-m-deep and Monoun, [Sigurdsson et al.,1987]. Most probably,the hypothesis holds for the 210-m-deep Lake Nyos, PAGES 281, 285 better understood, thanks to the almost contin- though on a much larger scale. Any exogenous uous scrutiny of the two lakes by scientists. disturbance, or intrinsic instability,may upset A unique humanitarian, scientific, and tech- Lakes Nyos and Monoun occupy the crater the density stratification of the lakes’ water col- nical experiment is currently taking place in a of a supposedly extinct volcano, in a region umn (CO -laden water is denser than pure forgotten mountainous region of West Africa. 2 known by geologists for its numerous soda water), trigger an overturn of the lake, and The experiment is on the verge of reaching its (not thermal) water springs,a common feature subsequently release suffocating carbon diox- operational stage, which consists of the eradi- of old volcanic areas.The region belongs to ide. cation of a “new”natural hazard; one that is the so-called volcanic chain of Cameroon, The term “”was first coined potentially devastating, but which has been which culminates and ends 300 km further by J.-C. Sabroux at a UNESCO Conference on known for only 20 years. southwest at the still-active Mount Cameroon the Lake Nyos disaster (March 1987;Yaoundé, The Lake Nyos catastrophe, which claimed (4000 m asl.). Accumulated evidence strengthens Cameroon) for describing this violent overturn 1800 victims in August 1986, was not unprece- dented. Indeed, 2 years previously,a lethal gas burst,originating from nearby Lake Monoun in the same remote area of Cameroon, killed 37 people—an odd and tragic episode that went almost unnoticed.One had never before heard of Mother Nature asphyxiating human beings and most higher animals on such a scale in a single and brief non-volcanic event. Today,the emotion has vanished,and this tragic episode is almost forgotten.However,the causes and mechanism of these tragedies are now much

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

Fig.1.Carbon dioxide/water jets on Cameroonian lakes. (A) The ~8-m-high steady jet flushing out the 140-mm inner-diameter (Ø) and 65-m- long pipe (l) moored vertically in the middle of Lake Monoun,April 1992.The gas/water volume 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 experiment very close to the operational scale (r = 7 and q = 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). Eos,Vol. 85, No. 30, 27 July 2004 0 isothermal expansion of gas bubbles drive the flow of the gas-liquid mixture upwards, as Sum of the partial pressures long as dissolved gas is available for ex-solution 10 Hydrostatic pressure and expansion. Due to the very limited amount of funds c~ partial pressure which could be allocated to the project, the techniques that are currently used in the off- 20 CH4 partial pressure shore oil industry were definitely not applica- ~ ble in this remote area, which is hardly accessible partial pressure during the rainy season.The participating 30 scientists knew that the entire installation had to be set up virtually by hand, with only the mechanical assistance of a few rubber boats. 40 The breakthrough came with the choice of e._.. the material used for the pipe.With a specific gravity of 0.96 kg/L, high-Oensity polyethylene .c 50 (HOPE) has a nearly neutral buoyancy in water: c. a pipe can be sunk vertically with the help of very little ballast, and retrieved horizontally by 60 blowing air into it.This plastic material is very t! flexible, and not at all brittle. It can be readily assembled in the field from 6-m-long segments 70 electrically soldered end to end.The extensive use of HOPE in natural gas supply networks proves its resistance to weathering and chemical 80 alteration.

From Experiments to Operational Degassing 90 In March-April 1992 at Monoun, two ballasted 70-m-long polyethylene pipes (50-mm and 100 140-mm inner diameter) were hung one after another on a floating raft anchored to the lake 0 200000 400000 600000 800000 bed [Halbwachs et al., 1993], and successfully primed (Flgure la) . In March 1995 at Nyos, the Pressure (pascals) same experiment was duplicated, with only the larger pipe diameter plunged at a 200-m Fig. 2. The partial pressure profiles in Lake Monoun as of February 2003. The critical situation of depth. Despite some technical difficulties, due the lake can be emphasized since the total gas pressure at -55 m is quite dose to the hydrostatic to the much greater depth and higher gas con- pressure.A slight increase in the dissolved gas concentration should result in over-saturation, trig- tent of the lake, a 21-m fountain was activated, gering the formation of bubbles, which in tum will initiate the "avalanche process" leading to the foreshadowing the operational degassing of overturning of the lake (limnic eruption). The vertical arrow shows the amount of subsidence of the killer lake (Flgure I b ). In all cases, the this critical layer induced by a 12-month degassing (see Figure 3). observed results (in terms of gas and water flow rates, internal pressure, and upward thrust) of a meromictic lake (a lake whose waters do containment of their very particular"weapon could be interpreted by numerical modeling not mix). It takes into account the analogy with of mass destruction; it has been proposed to of the twcrphase flow.The reliability of a remotely the volcanic eruption, also powered by gas make these lakes safer by extracting, in a con- operated immersed control valve for stopping bubbles ascending and expanding in a liquid trolled way, the carbon dioxide stored in them. and starting the flow on request was also (the magma). Limnic eruption, the mechanism The proposal, already mentioned in the high- demonstrated, as well as the basic concept of of which was further refined by Tietze (1987), lights of the UNESCO Conference [Sigualdason, a hanging pipe: a pendulum oscillating freely explains satisfactorily the sequence of events 1989) , came from several scientists, including from a raft anchored by ropes secured to the and consequences of the Monoun and Nyos a team of engineers from the French Ministry lake shore. disasters, and suggests a possible mitigation of of Environment, who dubbed their project the After completion of those preliminary tests, this newly discovered geological hazard. "Orgues de Nyos" ("Nyos Organ Pipes"). the gas content in both lakes continued to rise, In fact, in the years following the gas bursts, The method is no more than a limnic erup- due to persistent gas seepage into the lake. In thorough investigation into the physics and tion brought under control. It is inspired by February 2001, the C02 bottom concentration chemistry of Lakes Monoun and Nyos progres- the industrial process known as "gas lift; by of Lake Nyos was due to reach JO STP ml gas sively revealed that both still contained huge which a liquid (mostly oil) is pumped up by per cubic m of water, as compared to the 7 ml amounts of dissolved carbon dioxide. The mixing it with air or carbon dioxide bubbles, per cubic m in 1995. In Lake Monoun , the Standard Temperature and Pressure (STP) and thus making it buoyant. buildup of an almost-saturated water layer at - volumes are currently of 17 million ml in The basic design consists of a pipe set up 55 m made degassing the lake a matter of Monoun and 280 million ml in Nyos, and the vertically between the lake bottom and the urgency (Figure 2).A fully fledged degassing gas is being added at such a rate----0.38 million surface. A small pump (or preferably, a com- column was successfully assembled, installed, ml/yr [Kling et al., 1994) and 2.8 million ml/yr pressed gas injection), operating close to the and primed at Nyos on 30 January 2001 (F1g- [Kusakabe et al., 2000) into Monoun (below top of the pipe, raises the water in the pipe up ure Id), and a spectacular 50-m-high fountain .{)() m) and Nyos (below -180 m), respectively to a level where it becomes saturated with gas, soared above the lake surface.As expected, - that saturation of a deep-water layer could thus leading to degassing and subsequent this is significantly higher than the fountain be reached within a few years at Monoun and lightening of the water column. Then, the primed in 1995 with a similar pipe, and a few decades at Nyos. biphasic fluid rises to the surface. Once the gas reflects the considerable increase of C02 con- Since it is impossible to guarantee perennial lift is primed, the process is self-powered. centration in the bottom layers of the lake stability of the lakes, and indefinite Above the saturation level, degassing and since the 1995 experiment. Eos,Vol. 85, No. 30, 27 July 2004

0 rapid COz..induced corrosion in an oxygen- 175 Year 2001 lree environment (formation of siderite FeC03, April 16th at the expense of ) observed on the col- umn in operation at Nyos for 2 years.An 8-m- May 15th high jet (F1gure le) has been gushing forth at 50 180 Monoun since February 2003, the operational July 2 3th parameters of the degassing column falling perfectly within the uncertainty range of the September 14th numerical two-phase flow model already vali- December 14th dated by the more powerful jet at Lake Nyos 185 (seeTable 1).

'Toward Eradication of a Natural Hazard 150 Figure 3 presents the data bearing on the 190 response ol limnological key parameters to the current degassing-a clue to the validity of the process, and a unique opportunity for 200 limnologists and oceanographers to study the response of a stratified, double-diffusion sys-

22 23 24 25 26 24 24.5 25 tem, to the disturbance induced by pumping, T~ature (°C) in a 150-m-high convection-free water column. The experience gained over 12 years through successful trials, and extrapolation of the two- phase flow model, show that five 140-mm pipes at Nyos and three l()().mm pipes at Monoun are 20 Lake Nlonoun needed for an operational scheme of degassing the lakes, down to a level that would rule out the possibility of a limnic eruption (within 4 December 17th, 2002 years at Nyos and 2 years at Monoun) . Such a 40 background paves the way for installing with- out delay the remainder of these operational I- pipes at each lake. 60 Meanwhile, several hundred kg of iron 11- March 24th, 2004 which eventually precipitates as iron Ill hydrox- I ide Oimonite) -pouring onto the lake surface 80 on a daily basis [Bernard and Symonds, 1989) raises some ecological questions that should be addressed by biologists, and parti-<:ularly, by specialists of anoxic environments, before 100 scaling up the current degassing experiments. 20 21 22 23 24 25 Such an issue is more sensitive at Monoun, Terrpirahre (OC) where fishing is an economic resource for local villagers. The current degassing program is relevant to Fig. 3. Evolution of the Nyos and Monoun thermal profiles since the onset of the degassing (with several newsworthy scientific issues, including a 20-month disruption at Nyos).1b take full advantage of the sensitivity offered by the CTD probes (Sea-Bird Electronics, Inc.), the reference depth is the lake's bottom, not the lake's surface. the controversial engineering proposals to The subsidence of the thermal stratification, as a response to the degassing pipe operation, is dump carbon dioxide in the deep ocean (the clearly visible in both lakes. More precisely, the subsidence of the thermocline at stake is consis- Nyos disaster being generally presented as a tent with the water flow rate through the pipes pumping at -203 m at Nyos and -73 m at deterrent to such projects); and even the Monoun. Since the gas-rich layer subsides by the same depth, the effect of the degassing on the methane/mass extinction hypothesis. The safety of the lakes is conspicuous, especially in the case of Monoun (see Figure 2). Nyos crater lake is also an example of the crypto-volcanism contribution to the carbon cycle, despite the fact that the current C02 Three years later, the powerful soda spray from France, through an INMARSAT satellite recharge rate, ii evenly distributed over the flat jet-stable and safe--composed of 90% car- link.A Web cam operating from an ancillary bottom of the lake, corresponds to a daily flux bon dioxide and 10% water (volume/volume) instrumentation barge, and using the same density of 0.01 STP ml/m2; i.e., 10 times less is still spouting up from the vertical pipe sunk satellite link, puts the finishing touch to this than the daily C02 seepage over a similar sur- into the lake.The lower end of the pipe taps instrumentation scheme (http://perso. face area at Mammoth Mountain, Long Valley water at -203 m. The process has been almost wanadoo.fr/mhalb/nyos/). Our current data Caldera, California. continuous, apart from the short periods of complement the monitoring effort carried out Moreover, the Nyos degassing project is a time needed for maintenance. Given a steady by the foreign and Cameroonian teams bench test for the still more ambitious endeav- state C02 flow-rate of 50,000 STP ml per day, through survey and sampling missions [e.g. , or of extracting methane from , East the recharge rate of the lake is now safely offset. Evans et al., 1994; 1bnyileke et al., 1996; Schmid Africa, a bonanza for the lake's riparian coun- The instrumentation of the degassing assembly etal.,2003). tries. F1nally, for those whose concern is the permits the continuous monitoring of some The basic design of the degassing column mere 4% addition, by lull-rate degassing, to the limnological parameters at the pipe inlet, and (including safety equipment) at Monoun was annual C02 emissions from fossil fuel combus- of the two-phase flow regime, together with its similar to the Lake Nyos installation. On the tion in Cameroon, it has been proposed to use potential instabilities. It also permits the con- other hand, the pipe inner diameter was only the 500,000 tons of high.grade (C2H~ < 2 trol and remote operation of the functioning 100 mm, and all metallic parts were made of ppbv), isotopically labeled ( 1 ~-lree) carbon of the degassing platform from Yaounde, or stainless steel, in order to get rid of the extremely dioxide of Lake Nyos for studying, in a large Eos,Vol. 85, No. 30, 27 July 2004

Evans, WC., L D. Wh ite, M. L.Tuttle, G.W Kling, G. Tany- Table I. Technical and Experimental Data Pertaining ileke, and R. L.Michel (1994),Silc years of change to the Current Operational Degassing of Monoun and Nyos Lakes. at Lake Nyos, Cameroon, yield dues to the past and cautions for the future, Geochem.1, 28, 139-162. UNIT MONOUN NYOS Halbwachs, M., J. Grangeon, J.C.Sa broux, and A Villevieille (1993), Purge par auto-siphon du gaz Depth of the subsiding thermocline m 55 185 carbonique dissous dans le lac Monoun (Camer- oun): premiers resultats experimentaux. C.R.Acad. 5 Surface area at the said depth m' I.Ox JO' 4.30xl0 Sci. Fbris, 316(11), 483-489. Kling, G. W, WC. Evans, M. L. Tuttle, and G. Tanyileke, Rate of subsidence m/y 8 3.5 (1994) Degassing of Lake Nyos, Nature, 368, 405-406. Volume-rate of subsidence m'ly 8.0xl O' l.51xl0' Kusakabe, M., G. Z. Tanyileke,S.A. McCord, and S.G.Schladow (2000) , Recent pH and C02 profiles Water flow-rate through the pipe m'ly 9.0xlO' 2.00xlO' at Lakes Nyos and Monoun, Cameroon: implica- tions for the degassing strategy and its numerical Balance m'ly < 1.0xlO' 4.9xl0' simulation,1 Vblcanol. Geotherm. Res., 97, 241 - 260. Mousseau, M., Z. H Enoch, and J. C. Sabroux (1997), Calculated gas recharge (see caption) STPm'/y <6.5xl0' 6.93xl0' Controlled degassing of lakes with high CO, con- tent in Cameroon: an opportunity for ecosystem Estimated gas recharge [Kusakabe et al., 2000) STPm'/y 2.80xl0' CO, enrichment experiments in Plant Responses to Elevated CO,, edited by A. Raschi, F. Miglietta, Estimated gas recharge [Kling et al., 1994 ) STPm'/y 3.8xl05 R. Tognetti, and P.R. van Gardingen, pp. 4~5 . Cam- bridge University Press, U.K. Schmid, M.,A. Lorke,A. Wuest, M. Halbwachs, and Without water recharge, the pipe outflow is unaccounted for by the thinning down of the lowermost G. Tanyileke (2003), Development and sensitivity hypolimnion (the layer of water lying below the thermocline): hence, the calculated gas recharge assumes analysis of a model for assessing stratification and that a balancing amount of saturated water is continuously entering the lake, at the lake 's floor level. safety of Lake Nyos during artificial degassing, Uncertainty on the volume rate of subsidence is much higher at Monoun than at Nyos. Therefore, the Ocean Dynamics, 53, 288-30 I. previous estimate ofgas recharge at Monoun (3.8xlO' m'ly) can be considered to fall within the interval Sigurdsson, H., J. D. Devine, F. M.Tchoua,T.S. Presser, of confidence. M. K.W Pringle, and WC. Evans (1987), Origin of the lethal burst from Lake Monoun, Cameroon, scale FACE or other experiment, the ecological Partial funding of the successive experiments 1 Vblcanol. Geotherm. Res., 31, 1-16. impact of elevated C02 in tropical ecosystems at Nyos and Monoun was provided by the Sigvaldason, G. E. ( 1989), International conference [Mousseau et al. , 1997) . Cameroon government (JRGM,MINREST,and on Lake Nyos disaster,Cameroon, 16-20 March, other departments), the Office of Foreign Dis- 1987: conclusions and recommendations, aster Assistance (OFDA) of the U.S. Agency for 1 Vblcanol. Geotherm. Res., 39, 97-1 07. Condusion Tanyileke, G. Z., M. Kusakabe, and WC. Evans (I 996), International Development, the European Chemical and isotopic characteristics of fluids In the years to come, limnologists, biologists, Communities, the Fondation Gaz de France, along the Cameroon volcanic line, Cameroon, environmentalists, volcanologists,and many the International Union of Technical Associa- 1 African Earth Sci.• 4, 433-441. other scientists should not miss the opportu- tions and Organizations (UATl),and the French lietze, K. (I987) , Cause and mechanism of the disas- nity to study the impact of the degassing on Ministere des Alfaires Etrangeres.The entire bous gas bwst fiom Lake N)OS (Caml!IOOTl/ 1986), 59 the lakes' physico-chemistry, on the nearby project was kick-started by the Delegation aux pp.+ tables and figures, Bundesanstalt fiir Geowis- senschaften und Rohstoff, Hannover, Germany. human communities, and on the ecological Risques Majeurs, a department of the French equilibrium. Conversely, this scientific interest Ministere de !'Environnement. The Cameroon government participated also would attract in its wake the bilateral and Author Information international funding badly needed to main- in the financial and logistical support, together tain the current installation at a sale level of with the French Embassy in Yaounde, whose Michel Halbwachs, Universite de Savoie, operation, and to scale it up to a point at funding is particularly significant in the cur- Chambery. France; Jean-Christophe Sabroux, IRSN, which the threat of another limnic eruption rent Monoun degassing experiment. F1nally, Saday,France;Jacques Grangeon, Universite de Savoie, Chambery. France; Gaston Kayser (retired), will be wiped out. extensive fieldwork spread over 15 years would Euratom, Cadarache, Prance; Jean-Claude Tochon- It is our responsibility as Earth scientists to not have been possible without the unfailing help of Cameroon technicians, workers, and Danguy,Alain Felilc, and Jean-Christophe Beard, Data bring to completion the Nyos and Monoun Environnement, Chambery. Prance; Adelin Villevieille, degassing project that we initiated in the the local population. UATI, Paris, Flance; Gerard Vittei;F.N.5EEG, Grenoble, aftermath of the catastrophe, and that we Flance; Patrick Richon, CEA, OOE, BruyeresleChatel, have been carrying on persistently in a con- References Prance; Alfred Wuest, EAWAG, Kastanienbaum, text in which Africa, suffering so badly from Bema!d,A. and R. B.Symonds, (1 989), The significance Switzerland; Joseph Hell, IRGM,Yaounde, Cameroon other plagues, finds that its killer lakes are of siderite in the sediments from Lake Nyos, sinking into oblivion. Cameroon.1 Vblcanol. Geotherm. Res., 39, 187- 194.