Recent changes in CO₂ concentration in deep water at Lake Nyos

Monitoring of deep water CO₂ concentrations is essential to evaluate the risk of recurrence of limnic eruptions (Kling et al. 2005; Kusakabe et al. 2008; Yoshida et al. 2010). The CO₂ concentration along the vertical water column of Lake Nyos was measured using the method described in Yoshida et al. (2010). We collected deep water through a long plastic hose, a miniature version of the degassing pipe, which was deployed to the bottom as described in Nagao et al. (2002) and Tassi et al. (2009). The two-phase flow in the hose was separated into gas and liquid using a separator. The volume of each phase was accurately measured with a gas flow meter and a glass measuring cylinder. This method has two advantages: easy handling and data acquisition on site. The main disadvantage of this method is that it is difficult to apply to water in which the CO₂ concentration is not high enough to keep continuous gas self-lifting. It is possible to measure the CO₂ concentration of shallower lake water by using a potable pump; however, monitoring at the deep layer is required to evaluate the effect of the degassing system.

We started the CO₂ measurement from the deepest water, moving gradually to shallower levels by pulling the hose upward. We took six measurements from January 2009 to March 2014 at Lake Nyos. In a deep layer, where the chemocline develops (c. 180 m), we measured the CO₂ concentrations at an interval of 0.5 or 1.0 m, and at an interval of 5 or 10 m in water shallower than 170 m. Table 1 shows the results. The CO₂ profiles are shown in Figure 2, which indicates a sharp decrease in CO₂ concentrations, especially in the deepest water. Since the bottom water has been continuously transported to the surface, the CO₂ profiles subsided with time. The CO₂ concentrations shallower than 170 m were also reduced, but the shape of the CO₂ profiles has not changed with time, suggesting that a stable structure of mid-depth water was maintained during degassing. From the changes in the profiles over time we can estimate a rate of profile subsidence. As shown in Figure 3, the CO₂-rich water in the deepest zone subsided by 1–3 m between January 2009 and January 2011, by 5–8 m between January 2011 and December 2011, and by 3 to 4 m between December 2011 and March 2012. Based on these results, we obtained the average rates of subsidence for those three periods to be 1–3 m a⁻¹, 5–8 m a⁻¹ and 12–16 m a⁻¹, respectively. Installation of additional degassing pipes in early 2011 and adjustment of location and intake depth of the pipes (IRGM, unpublished report) contributed to the increase in the rate of subsidence. The rate, however, will not continue in the future because the bottom water removal rate will become low following the reduction of CO₂ concentration. This phenomenon has already been observed in the decreasing height of the degassing fountains of both lakes (Figs 1 & 4).

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Fig. 2.

Changes with time in the CO₂ profiles at Lake Nyos.

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Fig. 3.

Enlargement of the CO₂ profiles of the bottom layer at Lake Nyos.

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Fig. 4.

Comparison of fountain height in 2005 and 2007 at Lake Monoun.

The effect of degassing can be evaluated from the change in the amount of remaining CO₂ in the bottom water. We calculated the amount of CO₂ removed during the above period using the respective profiles and the volume of water below 180 m. We used the bathymetry of Kling et al. (2005) for this calculation. Table 2 shows the amount of CO₂ removed and the annual removal rate of CO₂ between 180 and 210 m depth. The amount of dissolved CO₂ decreased from 2.97 Gmol in 2009 to 1.78 Gmol in 2014. The average removal rate decreased sharply after March 2012. This resulted from decrease in CO₂ concentration of the bottom water as shown in Figure 2.

Decreasing activity of degassing systems

Decreasing activity of the degassing systems can be understood visually from decreasing fountain height. Figures 1 and 4 compare the fountain height at different times at lakes Nyos and Monoun, respectively. The fountain height of Lake Nyos was 45 m at the beginning of the degassing operation (31 January 2001), then gradually lowered year by year, and came to only 4.6 m in March 2014. We checked inside all the degassing pipes and confirmed that the lowering of fountains was caused not by obstacles such as scaling or collapse of the pipes. This suggests that the CO₂ concentration was effectively decreasing. Figure 5 shows the changes with time in the CO₂ concentration at 206 m (intake depth of degassing pipes) and the amount of CO₂ remaining in the bottom water (180–210 m deep). During degassing with a single pipe (before 2011), the amount and concentration of CO₂ decreased slowly. The decrease rate was naturally accelerated after installation of two additional degassing pipes in early 2011. Figure 5 also suggests that the decreasing rate of the amount of the remaining CO₂ will gradually slow down in harmony with the decreased CO₂ concentration, and eventually the degassing at Lake Nyos will cease in the near future as was the case at Lake Monoun (Kusakabe 2014). Once the degassing fountains have ceased, they cannot restart spontaneously. This means CO₂ will buildup again at Lake Nyos due to continued natural recharge of magmatic CO₂, implying an ongoing risk of recurrence of limnic eruption. The best way to avoid this risk is to remove the bottom water artificially. Since a power line is unavailable and not easy to access, especially in the rainy season at Lake Nyos, a system that works without external power is required, preferably a solar powered pump system, as proposed by Yoshida et al. (2010, 2013). This system needs to be less expensive and free of human intervention.

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Fig. 5.

Changes over time in the amount and concentration of CO₂ at Lake Nyos. Open circles and open triangles denote CO₂ concentration at 206 m depth (left y axis) and the amount of CO₂ below 180 m (right y axis), respectively.

Solar powered pumping system

Figure 6 shows the solar powered pumping system (maximum output of the solar power system is 320 W and maximum flow rate of the pump is 2.8 m³ h⁻¹) installed at Lake Monoun. In order to reduce the total cost of installation of the system, we used an existing degassing pipe which had already lost its function. The intake depth of the pipe was 99 m, very close to the bottom. As its inner diameter is only 100 mm, we chose a small rotary water pump with an outer diameter of 74 mm. Since the pump does not need high electric power, we used four small solar modules. The size of each module was 120 cm by 54 cm in dimension and the maximum output of each module was 80 W. We set up the solar modules on an aluminum frame that sits on a new raft. The new raft was connected to the old raft which had been used for the degassing pipe. Naturally, electric power is supplied to the pump only in daytime. A gas self-lift was induced by the up flow of bottom water in the pipe after the system started working, and the outflow did continue even at night. With this system CO₂-rich bottom water can be efficiently uplifted to the surface. In March 2014, we measured the daily removal rate of deep water. It was 25 m³ d⁻¹. Flow rate of a gas self-lift observed by eye-measurement is about five times as much as the flow through the pump. The overall flow rate was estimated to be more than 100 m³ d⁻¹ if water removal by gas self-lift is included. Based on this rate and the CO₂ concentration of deep water measured in March 2014 (90 mmol kg⁻¹ in bottom water, as shown in Fig. 7), the annual removal rate of CO₂ was calculated to be 3.3 Mmol a⁻¹. Since this removal rate is half of the natural recharge rate (8.2–8.4 Mmol a⁻¹, Kling et al. 2005; Kusakabe et al. 2008), it is advisable to install 2–3 additional systems at Lake Monoun to compensate the natural CO₂ recharge. We confirmed that the output of the solar modules, pumping rate, conditions of electric controller and other parts were all satisfactory in December 2014, indicating the perfect performance of the system. The water removal rate also stayed unchanged. This means that the system is robust and can keep working for a long time. Another important point is that the system does not need complicated maintenance and fuel, an important factor that any system should have in a remote area in Cameroon like lakes Nyos and Monoun.

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Fig. 6.

Solar-powered deep-water removal system installed at Lake Monoun. (photo: M. Kusakabe.)

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Fig. 7.

CO₂ profile of the bottom water at Lake Monoun in March 2014. The CO₂ concentration of the bottom water was too low to induce a spontaneous self-lifting flow before 2013.

Conclusions

The amount of dissolved CO₂ in lakes Nyos and Monoun was significantly reduced as a result of the degassing operation since the early 2000s. Detailed measurement of CO₂ concentrations at Lake Nyos by Yoshida et al. (2010) showed that the CO₂ concentration of the deepest water is decreasing to less than half of the maximum value of 350 mmol kg⁻¹. In the near future, however, the current degassing pipes are expected to stop functioning due to lowered gas self-lift capability as seen at Lake Monoun. This would lead to the renewed buildup of CO₂ as long as the natural recharge of CO₂ continues, and to a potential risk of recurrence of limnic eruptions. We have proven that the solar-powered deep water removal system installed at Lake Monoun is robust and keeps working without any problem. This means that the risk of limnic eruptions can be avoided if we install a deep water removal system at Lake Nyos after cessation of self-degassing.