- Past, Present & Future - Title Lakes Nyos and Monoun - Past, Present & Future - M. Kusakabe1, T. Ohba2, Y. Yoshida3, Issa4, S. Djomou4, G. Tanyileke4, and J.V. Hell4 1University of Toyama, Japan, 2Tokai University, 3Yoshida Consulting Engineering Office, 4IRGM, Cameroon First of all, I thank you for inviting me as an invited speaker. I will summarize the measures taken so far to prevent recurrence of limnic eruption at Lakes Nyos and Monoun (Cameroon). Goldschmidt 2016 Conference at Yokohama 1st July 2016

CO2 gas disasters at Lakes Nyos and Monoun As you well know, Lakes Nyos and Monoun in Cameroon suddenly released a large amount of CO2, killing altogether 1800 people in mid-80s. These lakes are volcanic crater lakes located along the Cameroon Volcanic Line. National Geographic Magazine　 （September 1987） Lake Nyos (August 21, 1986), 1746 casualties Lake Monoun (August 15, 1984), 37 casualties These gas disasters have never been recorded in the past. Catastrophic gas release from the lakes was named “limnic eruption”, and this word will be used in this presentation.

Origin of gases Mantle-derived CO2 20Ne/22Ne = 9.9 (11.9) 3He/4He = 5.7 Ratm (~7) 20Ne/22Ne = 9.9 (11.9) 21Ne/22Ne = 0.0303 (0.051) 40Ar/36Ar = 550 (>1650) Isotopic signatures Green numbers: gases from CVL Mantle.

Sublacustrine CO2 recharge model f r o m h w d ( ) W E - 1 5 2 4 6 8 Groundwater Gradual accumulation of CO2 3He/4He = 1 Ra 20Ne/22Ne = 9.80 21Ne/22Ne = 0.0290 40Ar/36Ar = 296 13C = -8 ‰ Sub-lacustrine fluid reservoir A well accepted view of the CO2 recharge model is shown here for Lake Nyos. It is believed that magmatic CO2 is permanently supplied from magma which resides beneath the lake. The CO2 meets with groundwater coming from the surface at a sub-lacustrine level to form a fluid reservoir. Interaction between rocks and CO2-rich fluid produces a source of soda spring, which leaks into the lake bottom. Thus, gradual accumulation of CO2 takes place at the deeper part of the lake. 3He/4He = 5.7 Ra 20Ne/22Ne = 9.9 21Ne/22Ne =0.0303 40Ar/36Ar = 550 13C = -3.5 ‰ CVL mantle 3He/4He = ~7 Ra 20Ne/22Ne = 11.87 21Ne/22Ne = 0.0508 40Ar/36Ar >1650 13C = -3.5 ‰ CO2 gradually accumulates in deep part of the lake. Too much CO2 accumulation led the limnic eruption in 1986. Magmatic CO2 wood and Rubin (1989) (Lockwood & Rubin, 1989)

Pre-degassing evolution of CO2 profiles at NyMo CO2 (mmol/kg) Pre-degassing evolution of CO2 profiles at NyMo Lake (b) Lake Nyos Recently our program is running under the project SATREPS. SATREPS is an abbreviation of Science and Technology Research Partnership for Sustainable Development. The project is a new type of Japan’s Official Development Aid aiming at international cooperation, addressing global issues, advancing science, and capacity development ,sponsored by JICA and JST. This map shows 37 countries where SATREPS is running. - Similar CO2 evolution at both lakes. - CO2 concentration of recharge fluid stayed constant for ~15 years. - Underplating of the recharge fluid from beneath the bottom.

Accumulation of CO2 at NyMo during pre- degassing period These diagrams show the change with time of the total dissolved CO2 in Lakes Nyos and Monoun since their limnic eruption. Gradual accumulation of CO2 at both lakes indicates a steady supply of magmatic CO2 continued with a supply rate of 0.12 Gmol/year and 8.4 Mmol/year for Lakes Nyos and Monoun, respectively. The CO2 concentration at Lake Monoun in 2003 was close to saturation at about 50m depth, implying a possibility of the second limnic eruption in the near future. Close to saturation in 2001- 2003 at Lake Monoun. Gas removal started in 2003.

Temporal variation of a CO2 profile at lake Monoun Steady upward shift of the CO2-profile Note that CO2 concentration at 50 m was close to saturation in 2003. Saturation might have been reached in less than a few years after 2003, if no removal of gas was made. Permanent degassing system was installed by French team at the lake in 2003.

Gas removal by NMDP 45 m 8 m Lake Monoun, February 2003 To reduce a risk of the second limnic eruption, an international project “Nyos and Monoun Degassing Project (NMDP)” was initiated, and device to remove gases in the lake was installed in early 2000s. These are the photographs of the fountains in the initial phase of degassing operation at both lakes. Lake Nyos, February 2001 Lake Monoun, February 2003 Gas removal with a gas self-lift technique.

The amount of dissolved CO2 over the last 30 years Degassing ceased Accumulation of CO2 resumed Degassing stared Natural CO2 recharge Degassing with 3 pipes By 2010, ~93% of gas was removed, and the degassing system lost its gas self-lift capability. This resulted in gas re-buildup, suggesting additional remediation is needed. with 3 pipes Degassing This slide shows how dissolved CO2 has been effectively removed from the lakes during the degassing operation. At Lake Monoun, approximately 90% of dissolved CO2 was removed by 2011. However, the degassing system stopped working, because CO2 concentration became too low to sustain gas-fountaining due to the loss of gas self-lifting capability. After 2011 we observed gas re-buildup at Lake Monoun, although the amount of accumulated gas is far below a risky level. Degassing continues smoothly at Lake Nyos. Currently 60% of the maximum amount of gas still remains undegassed. Installation of 2 additional pipes accelerated degassing, but we need to wait for another 5 years or so to attain maximum degassing. After that, a situation similar to what we observed at Lake Monoun or resumption of gas-buildup would come out. For this reason, it is imperative to keep monitoring the lakes. Approx. 60% of gas was removed by 2014. It may require ~5 years for full gas removal. After that, gas re-buildup will resume.

Deep water removal system at Lake Monoun 14 December 2013 Recently we installed a deep water removal system at Lake Monoun. The principle of the system is schematically shown here. An existing degassing pipe was utilized to accommodate a rotary pump driven by solar power inside the degassing pipe. The photograph shows a whole view of the system in operation. The system can pump deep water at a rate of approx. 100 tons a day. This pumping rate is equivalent to CO2 removal rate of about 3.3 Mmol/yr. Since the natural CO2 recharge rate has been estimated to be 8.4 Mmol/yr, we need 2 more systems to compensate the natural CO2 recharge.　This system was designed and constructed by Y. Yoshida using a fund given by our currently running project. Daily pumping rate of water is ~100 m3/day with a CO2 removal rate of ~ 3.3 Mmol/yr. Two more systems are required to compensate the natural CO2 recharge of ~8.4 Mmol/yr.

Decoupling of CO2 and He in Lake Nyos deep water CO CO2/3He ratio in Nyos deep water --160160 Upper mantle CO2 and 3He are decoupled below and above the chemocline. The decoupling may have been caused by underplating of the later recharge fluids from the bottom that are characterized by higher C/3He ratios. . 4 8 1 2 6 - 9 7 C / 3 H e ( x ) Chemocline, 2001 We measured a C/3He profile in Lake Nyos deep water in 2001. The C/3He ratio was found to shift to higher values below the chemocline in 2001. We interpreted that the recharge fluid characterized by higher C/3Heratio underplated the deepest water and pushed it upward. Water above the chemocline may represent pre-eruption water that has lower c/3He ratios.

Spontaneous limnic eruption hypothesis - 2 1 5 3 4 Process 1 (limnic eruption): The CO2 profile before the 1986 limnic eruption may have been similar to the “Before 1986” curve. A large proportion of dissolved CO2 was released to the atmosphere at the time of eruption which may have been ~14 Gmol. Process 2: CO2 gradually accumulated as we observed. Process 3: If CO2 recharge from depth continues at the same rate and CO2 concentration, the CO2 profile “before 1986” may be attained in approx. 100 years. CO2 saturation Before 1986 Depth 2001 1986 Processes 1,2 and 3 may be repetitive. CO2 (mmol/kg)

Conclusions Currently the amount of gas dissolved in Lakes Monoun and Nyos has been drastically reduced. Since the amount of “explosive” in the NyMo bombs is now low, the risk of recurrence of limnic eruption can be said low. However, gas re-buildup resumed after the degassing system stopped working at Lake Monoun. The same situation will be realized at Lake Nyos several years later. Since recharge of CO2 will continue, monitoring of the lakes should be continued. Based on the observed data, a spontaneous limnic eruption hypothesis was proposed. It implies a repetitive nature of limnic eruption. In the previous slides, we stressed the importance of continuous monitoring of the lakes. Scientific work on these issues have been mainly done by foreigners. Since the Lakes Nyos and Monoun gas disasters took place in Cameroon, lake monitoring should be carried out, in principle, by Cameroonian scientists and technicians. To support their independent work for monitoring of the lakes, we are performing an international project called SATREPS to build their capacity.

Decoupling of CO2 and He in Lake Nyos deep water Depth (m)

Profiles of 3He/4He ratios at Lake Nyos in 1988, 2001 and 2011 v (Sano et al., 1990, Aka et al., 2004, Nagao et al., 2010)

Summary (1) Temporal change in the amount of CO2 dissolved in Lakes Nyos and Monoun for the last 30 years. Pre-degassing change in the CO2 amount indicated gradual accumulation of the gas in the lakes with the rates of 8.4 Mmol/yr at Monoun and 0.12 Gmol/yr at Nyos. 3. Degassing operation worked well. By 2010, a majority of gas was removed from Lake Monoun. The risk of recurrence of the limnic eruption is low. The degassing system lost its gas self-lift capability resulting in gas re-buildup at Lake Monoun. 4. We installed a pilot system for deep-water removal at Lake Monoun in 2013 to avoid gas re-buildup. A similar system will be necessary at Lake Nyos when degassing with the current system come to the end.

Summary (2) A “spontaneous gas exsolution hypothesis” was proposed for the cause of the limnic eruptions at NyMo. If this hypothesis is correct, a limnic eruption can happen at every ~100 years interval if the CO2 recharge rate stays unchanged. Decoupling of CO2 and He was found in deep water at Lake Nyos. We interpret that recharge fluid with high CO2/3He ratios underplated the pre-eruption water that had low CO2/3He ratios. CO2/3He ratios in the fluid derived from a single magmatic source can vary by a factor of 3 in a time scale of 10~20 years.

Thank you for your attention! 1988 ~ 30 years later ・・・・ 2015

CO2/3He at Mammoth Mountain, California 3He/4He CO2 He (Sorey et al., 1998)

(OFDA (USAID), Cameroon Government, French Government, and UNDP) NMDP (OFDA (USAID), Cameroon Government, French Government, and UNDP) To reduce a risk of the second limnic eruption, “Nyos and Monoun Degassing Project (NMDP) was initiated. This diagram schematically shows the degassing system which was designed and constructed by Michel Halbwachs. Long plastic hose was deployed to the depth of 205m. Once CO2-rich deep water rises above the CO2-saturation depth in the pipe, gas bubbles form and the 2-phase flow rushes to the pipe mouth to form a fountain.

Change with time in CO2 profiles during degassing at NyMo Nyos Monoun Pre-degassing profile Pre-degassing profile Change with time in CO2 profiles during degassing at NyMo is shown here. Degassing of the lakes went on smoothly, and the CO2 profiles at both lakes subsided with time. At Lake Nyos, the highest CO2 concentration at the bottom is currently reduced to about 160 mmol/kg, and a CO2-rich layer near the bottom has gone. At Lake Monoun, the CO2 profiles subsided and the highest CO2 concentration at the bottom is currently reduced to about 70 mmol/kg. However, it is noted that the 2012 profile rose slightly compared to the 2011 profile which was the minimum observed. The 2013 profile shows a further rise. These observations may indicate that Lake Monoun re-started accumulation of CO2 after the lake lost the gas self-lifting capability in 2009. The 2013 profile has a different shape compared to those of previous years. It is important to make sure that the CO2 increase at Lake Monoun is taking place.