Evolution of CO2 in Lakes Monoun and Nyos, Cameroon, before and during controlled degassing

Abstract

Evolution of CO 2 in Lakes Monoun and Nyos (Cameroon) before and during controlled degassing is described using results of regular monitoring obtained during the last 21 years. The CO 2(aq) profiles soon after the limnic eruptions were estimated for Lakes Monoun and Nyos using the CTD data obtained in October and November 1986, respectively. Based on the CO 2(aq) profiles through time, the CO 2 content and its change over time were calculated for both lakes. The CO 2 accumulation rate calculated from the pre-degassing data, was constant after the limnic eruption at Lake Nyos (1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001), whereas the rate appeared initially high (1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996) but later slowed down (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) at Lake Monoun. The CO 2 concentration at 58 m depth in Lake Monoun in January 2003 was very close to saturation due to the CO 2 accumulation. This situation is suggestive of a mechanism for the limnic eruption , because it may take place spontaneously without receiving an external trigger.The CO 2 content of the lakes decreased significantly after controlled degassing started in March 2001 at Lake Nyos and in February 2003 at Lake Monoun. The current content is lower than the content estimated soon after the limnic eruption at both lakes. At Monoun the degassing rate increased greatly after February 2006 due to an increase of the number of degassing pipes and deepening of the pipe intake depth. The current CO 2 content is ~40% of the maximum content attained just before the degassing started. At current degassing rates the lower chemocline will subside to the degassing pipe intake depth of 93 m in about one year. After this depth is reached, the gas removal rate will progressively decline because water of lower CO 2(aq) concentration will be tapped by the pipes. To keep the CO 2 content of Lake Monoun as small as possible, it is recommended to set up a new, simple device that sends deep water to the surface since natural recharge of CO 2 will continue.Controlled degassing at Lake Nyos since 2001 has also reduced the CO 2 content. It is currently slightly below the level estimated after the limnic eruption in 1986. However, the current CO 2 content still amounts to 80% of the maximum level of 14.8 giga moles observed in January 2001. The depth of the lower chemocline may reach the pipe intake depth of 203 m within a few years. After this situation is reached the degassing rate with the current system will progressively decline, and it would take decades to remove the majority of dissolved gases even if the degassing system keeps working continuously. Additional degassing pipes must be installed to speed up gas removal from Lake Nyos in order to make the area safer for local populations.

High-resolution modelling of atmospheric dispersion of dense gas using TWODEE-2.1: application to the 1986 Lake Nyos limnic eruption

“…At present, the level of risk at lakes Nyos and Monoun has decreased notably after the successful deployment of degassing pipes in March 2001 and February 2003, respectively. The progressive gas removal resulted in considerable deepening of the level of gas-rich water in a short period of time (Kling et al, 2005;Kusakabe et al, 2008). However, there is still the recognised need to perform a quantitative CO 2 hazard assessment for several reasons, including the possibility of future gas build-up or a breakthrough of the dam built at the northern shore of the lake.…”

δ18O and δD variations in some volcanic lakes on the Cameroon Volcanic Line (West-Africa): generating isotopic baseline data for volcano monitoring and surveillance in Cameroon

“…Afterwards, a different water column structure characterized by two anomalous layers at middle depths developed. That change may reflect the on-going gas removal (Halbwachs et al, 2004;Kusakabe et al, 2008;Issa et al, 2013) and the ground water inflow. In contrast to the surface and middle depths, which alternately experienced isotopic enrichment and depletion during the reference period, the deepest waters (monimolimnion) became heavier over time.…”

“…Based on recent studies, Lake Monoun may release CO 2 approximately every 30-40 years, while Lake Nyos requires a longer interval, which is approximately 100 years (Kusakabe et al, 2008;Issa et al, 2013). Despite the historical and geochemical evidence, few or no records describing similar events among the populations living in the Nyos and Monoun areas have been discovered (Eby and Evans, 2006).…”

“…The killer lakes tragedies spurred researchers to survey crater lakes in connection with this new geologic hazard in locations, such as Italy (Martini et al, 1994;Cabassi et al, 2013), Indonesia (Christenson, 1994) and Costa Rica (CVL7 Workshop). Based on recent geochemical studies, Nyos-and Monountype events are recurrent phenomena (Kling, 1987b;Tietze, 1992;Halbwachs et al, 1993Halbwachs et al, , 2004Schmid et al, 2004;Kling et al, 2005;Kusakabe et al, 2008) and may not be as unique as the extraordinary events associated with volcanic lakes that might have occurred elsewhere in the Cameroon Volcanic Line (CVL) corridor, according to social studies (Shanklin, 1992); in particular, the lake-related maleficent activities that brought death and destruction to some ethnic groups, such as the Bamessi, Kom and Oku in the Oku region, forced these populations to emigrate (Shanklin, 1992;Eby and Evans, 2006). Although the CVL lakes are obviously serious threats, they have rarely been studied (Hassert, 1912;Corbet et al, 1973;Green et al, 1973;Kling, 1987a;Tanyileke et al, 1996) to evaluate the risks for the neighboring populations.…”

Minimal model for double diffusion and its application to Kivu, Nyos, and Powell Lake

“…The first profile shows only the initial phase of the formation of the staircase, while the second and third profiles clearly suggest that the double-diffusive structure rapidly expanded. Unfortunately, corresponding data of CO 2 concentration are not available, and only one profile was measured in January 2003 [Kusakabe et al, 2008]. Such a profile has been used as a reference to define an equivalent salinity S e 5S1ðb CO2 =bÞ C CO2 , where C CO2 is the CO 2 concentration and b CO2 its contraction coefficient (see Table 1).…”