THE NYOS-MONOUN DEGASSING PROGRAM:

Preliminary Report of the U.S.-OFDA Technical Project

October-November 1999

9 November 1999, Yaounde, Cameroon

George Kling, University of Michigan, Ann Arbor MI 48109, USA

William Evans, U.S. Geological Survey, Menlo Park CA 94025, USA

Gregory Tanyileke, IRGM, BP 4110, MINREST, Yaounde, Cameroon

Minoru Kusakabe, Okayama University, Misasa, Tottori-ken 682-0192, Japan

Yutaka Yoshida, Yoshida Engineering Company, Tokyo

SUMMARY

            The release of carbon dioxide gas (CO₂) from Lake Monoun in 1984 and Lake Nyos in 1986 resulted in the loss of nearly 1800 lives. This report summarizes the preliminary findings and accomplishments of the joint U.S.-Cameroonian-Japanese technical project and scientific investigation during October-November of 1999. This field work was the first phase of a hazard mitigation project supported by the U.S. Office of Foreign Disaster Assistance (USAID-OFDA). The OFDA project is part of an overall program directed by the Cameroonian Inter-Ministerial Committee for the Degassing of Lakes Nyos and Monoun. The goals of the overall program are to (1) install the equipment necessary for monitoring the safety of the degassing operation, (2) install the pipes needed to degas the lakes, (3) educate the affected populations about the risks associated with these lakes, (4) study the potential for flooding caused by the weak natural dam at Lake Nyos, and (5) study the issues concerned with management and potential use of the CO₂ produced by the operation.

Findings and Accomplishments:

 (I)  The total gas content in the two lakes is very high and is increasing. The total gas pressure at the bottom of Lake Nyos has increased by 10% compared to measurements made in April of 1998. Thus the danger of future lethal gas releases is also increasing, and another gas release could occur at any time in either lake.

 (II)  Monitoring stations were installed on both lakes. These stations continuously monitor the gas pressure at 2 depths near the bottom of the lakes, the water temperature of the lakes at 12 to 14 different depths, and the meteorological conditions at the lakes including air temperature and humidity, wind speed, solar energy inputs, rain, and barometric pressure. This information is required to monitor the progress and safety of the degassing operation. The degassing pipes will be installed in phase two of the project, to begin in the year 2000.

 (III)  We met with local delegations and officials at both lakes to explain the nature of this project. There is an acute need to educate people about the risks associated with the lakes. This education could be initiated by the Inter-Ministerial Committee, in cooperation with local authorities and other agencies such as the Peace Corps and International Red Cross. By working within the local and provincial governmental structure, the affected populations could be effectively sensitized to these hazards.

INTRODUCTION

            The lethal release of large amounts of gas from lakes is very rare. Only three lakes in the world are known to contain high concentrations of dissolved gas in their bottom waters: Lakes Nyos and Monoun in Cameroon and Lake Kivu in East Africa. Only two of these lakes, Nyos and Monoun, are known to have recently released gas resulting in the loss of human life. Two conditions are necessary for CO₂ accumulation in lakes to reach dangerous levels. First there must be an abundant source of CO₂, and second a lake must be strongly stratified, a situation where the bottom and surface waters do not mix, in order to trap the CO₂ in bottom waters. Geological conditions in much of Cameroon are favorable for the generation of CO₂ gas. This gas is discharged harmlessly to the atmosphere in many soda springs found throughout the country. In most of Cameroon's crater lakes, sheltering from the wind and great depth help to create stable stratification. The combination of gas-rich springs feeding into strongly stratified lakes led to the buildup and eventual violent release of gas from the bottom waters of Lakes Nyos and Monoun. Our previous investigations have shown that these conditions do not occur in any other Cameroonian lakes, and no other lakes contain dangerous amounts of CO₂ .

            The gas contents of these two lakes continues to build, supplied by the input of CO₂ from underground springs that discharge into the bottom of the lakes. The trigger for a gas burst is unknown although any disturbance that moves deep, gas-rich water closer to the surface could result in a gas release. As that deep water rises, the weight of water above it (the hydrostatic pressure) decreases, and at some point the dissolved gas pressure will become equal to the hydrostatic pressure. At this point there is nothing to force the gas to remain in solution, and gas bubbles begin to form. This process is identical to the removal of the cap from a bottle of soda -- when the cap is removed there is no more pressure to keep the gas dissolved in the soda, and bubbles are formed. Once bubbles are formed in the lake they rise rapidly and drag the deep water toward the surface, at which point additional deep water is drawn upward and depressurized. This leads to a chain reaction that eventually results in the violent release of enormous amounts of lethal CO₂ gas. Because the gas content of these lakes is currently very high, catastrophes similar to those in 1984 and 1986 could occur at any time.

            The natural hazards presented by these lakes are unique in that remediation is possible before a disaster occurs. A proposal for remediation was recently funded for ~$450,000 by the U.S. government's A.I.D. Office of Foreign Disaster Assistance. The plan involves pumping of gas-rich bottom waters to the surface of the lakes through pipes. As the bottom water moves toward the surface the dissolved gas comes out of solution and forms bubbles. These bubbles lower the density of the gas-water mixture and the fluid begins to rise rapidly through the pipe. The energy released during degassing is sufficient to drive the pumping operation without any external power source. Using this approach, a group of scientists headed by the French and including Cameroonians, Japanese, and Americans, has proven that the technology for degassing is reliable during two demonstration projects, one in Lake Monoun in 1992 and one in Lake Nyos in 1995. In addition to removing the gas currently in the lakes, it is possible to prevent future gas buildups using a pipe that continuously flushes bottom water out of the lake. After most of the gas is removed from Lake Nyos, it is also possible to lower the lake level so that the threat of flooding from the weak dam is eliminated.

            Until the lakes are degassed using the method described above, the people living around the lakes are at risk, and increased awareness and education about the risks are needed. In addition, relocation of survivors from the Lake Nyos event has already occurred. However, people are moving back into the evacuated region, and they should be made aware of the risks. There was no relocation of survivors around Lake Monoun; however, the population around the lake has increased in recent years and thus the need for risk education is as important here as it is in the Nyos region.

            This report presents the preliminary scientific results of the first phase of the OFDA project that began on 29 October and ended on 8 November 1999. The first phase was designed to install the monitoring equipment that is required to proceed with the installation of pipes and the degassing procedure. This project is part of an overall program for degassing and hazard mitigation coordinated by a Cameroonian Inter-Ministerial Committee, Chaired by the Prime Minister, head of government. Financial and logistical support was provided by MINREST, the U.S. OFDA and the U.S. Embassy in Yaounde, the Japanese Ministry of Education, Science and Culture, the U.S. Geological Survey, and the U.S. National Science Foundation.

RESULTS AND DISCUSSION

 Gas chemistry

            As long as the lakes contain large quantities of dissolved gas they could potentially release lethal amounts of gas without warning. A key factor in estimating the likelihood of a gas burst is the degree of gas saturation (the ratio of gas pressure to hydrostatic pressure) in the bottom waters. In December 1989, we began using a gas probe which can be lowered to various depths in the lakes to provide an immediate, direct measurement of gas pressure. This probe does not distinguish between different gas species such as CO₂ and methane, but samples collected by other methods will be analyzed in the laboratory to provide this information. Collectively, all these methods have shown good agreement and have established that the gas pressure is mainly due to dissolved CO₂. The graph on the next page shows the results of our gas pressure measurements in Lake Nyos starting in 1989. The gas pressure in bars (1 bar ~ 1 atmosphere) measured by the probe is plotted against the depth of the lake in meters.

            There has been a pressure increase at all depths below 170 m since 1989, with the largest increases occurring in the bottom 20 m of the lake. Gas pressure near the lake bottom at 198 m depth exceeds 14 bar, which represents just over 66% of the saturation value based on an ambient hydrostatic pressure of 21 bars. This is an increase of 10% just since April of 1998. In Lake Monoun gas pressure has increased in waters below 60 m depth and now reaches over 80% of the saturation pressure at that depth. Fortunately, the rate of pressure increase in Lake Monoun is less than the rate of increase in Lake Nyos. The fact that present day gas saturation is less than 100% does not indicate that the lakes are safe. Any large disturbance of the water column such as a landslide, rockfall, or violent storm could trigger a release of the gas remaining in these lakes at any time.  

            Since 1989, the increases in gas pressure in Lake Nyos have been approximately constant with time as indicated by the following graph. A similar graph for Lake Monoun will require some laboratory analyses. As shown in the graph for Lake Nyos, the rates of gas pressure increase are greatest from 190 m to 198 m and are equal to about 0.66 bar per year. If this rate of gas increase continues, the gas pressure will reach 100% saturation at these depths in about 11 years. Although the gas will form bubbles and probably trigger another disaster once the level of 100% saturation is reached, it is important to note that (1) the actual rate of increase in the future may be either slower or faster than this prediction, and (2) a gas disaster may be triggered before 100% saturation is reached.

Water temperature and chemistry

            The continuous temperature profile in Lake Nyos (following graph) shows warm water in the surface layer due to heating by the sun. The temperature minimum is near 40 m depth, and below this depth the temperature increases all the way to the bottom. This temperature increase is due to the input of warm, gas-rich fluids near the lake bottom. The same general pattern occurs in Lake Monoun with a temperature minimum at ~15 m.

            Chemical measurements made in the field include alkalinity, pH, dissolved oxygen, and conductivity. Most chemical analyses will be performed later in the laboratory. Selected results of field measurements are shown in Table 1. Alkalinity concentrations (as HCO₃^-) in both lakes are generally similar to those from previous years, except in Lake Nyos the alkalinity in surface waters has decreased. The pH and conductivity in Lake Nyos surface waters have also decreased. These changes in surface water chemistry mainly result from dilution of lake water by rainfall inputs and deepening of the surface waters since the degassing event. The chemistry is gradually becoming more similar to that of surface water measured in 1985 before the event.

          The overall water chemistry and the structure of the water column is best represented by the conductivity profile with depth. Conductivity is a direct measure of the dissolved mineral content in the water, and together with temperature it is a measure of the density of the water. There is a distinct chemocline (sharp increase in mineral content) that is present at 45 m depth and has been present there since 1993. In addition, there is a second chemocline at about 180 m depth. The largest increases in gas content have been found below this deeper chemocline.

TABLE 1. Selected Data on Water Chemistry for Lakes Nyos and Monoun.

(A). LAKE NYOS -- Water Chemistry, November 1999

(B). LAKE MONOUN -- Water Chemistry, November 1999

COORDINATION of DEGASSING EFFORTS

            In late October of 1999 the Cameroonian government formed an Inter-Ministerial Committee to coordinate all degassing efforts on Lakes Nyos and Monoun. The Committee has representatives from various Ministries, and will receive input from the scientific and technical community that has worked on these lakes since 1986. This community has held a series of meetings to discuss the best methods for degassing. In 1990 an international meeting of scientists who had participated in research on Lakes Nyos and Monoun was held in Nancy, France. At this meeting a consensus was reached regarding the causes of the gas disasters and the strategy for reducing the hazards by gas removal. A second international meeting was held in 1992 in Paris, France, and included not only scientists but also engineers interested in the specific technologies of degassing. Again, a consensus was reached on the scientific explanations for these events and on the technical means of a degassing project. Since that time, all of the field demonstration projects carried out in Cameroon and all of the proposals for degassing projects presented to various agencies have followed the general consensus reached during these two meetings. In addition, in 1997 an international committee was established to help coordinate and advise the degassing efforts. This committee, named the NMDP Advisory Committee (Nyos-Monoun Degassing Project), is composed of 8 members from six countries (G. Kling, Chairperson, and G. Tanyileke, Cameroonian representative). This report will be provided to the Advisory Committee, and may be viewed at the Web site 'http://www.umich.edu/~gwk/research/nyos.html' or by contacting G. Kling at email address: [email protected]. This web site will also be linked in the future to the web site of MINREST.

TIMETABLE

            The first phase of the OFDA project was to install monitoring equipment at Lakes Nyos and Monoun. This has been accomplished at both lakes. The second phase of the OFDA project will be to install degassing pipes in Lakes Nyos and Monoun - this phase will begin next year (2000). This phase will be closely coordinated by the Cameroonian Inter-Ministerial Committee, and will involve several aspects in addition to the placing of pipes in the lakes. These aspects include: (1) road improvements for the routes to both lakes, especially Lake Nyos; (2) safety precautions at the work sites and notification and potential temporary evacuation of people near the lakes; (3) construction of storage and observatory facilities at the lakes, including a science center; (4) education of local populations concerning the risks of living near these lakes, and (5) support and training of Cameroonian scientists to participate in the overall degassing program and to maintain the project into the future.