Continuous monitoring of water chemistry of Lake Nyos using an automated CDT and satellite data

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Lakes Nyos and Monoun (Cameroon) exploded in the mid 1980s killing about 1800 people. At these lakes, temperature, total dissolved ions, and CO2 concentration in the bottom water have been steadily increasing since the last explosions. Thus the lake explosion may recur unless CO_2 concentration is lowered. Currently gas removal is going on using a degassing system that was installed in January 2001 at Lake Nyos and February 2003 at Lake Monoun.In the present investigation, we determined (1) the spatial distribution of CO_2 and variation with time in the total amount of CO_2 in the lakes, and (2) the changes in the physical structure of the lakes, I.e., temperature, chemical composition, and density of the lakes. We made a detailed CTD observation at Lake Nyos along N-S and E-W cross sections. This enabled us to construct a three-dimensional distribution of temperature and chemistry for the first time. It was found that the lakes show strong stratification and that the artificial degassing did not give any substantial change in the chemical and physical structures of the lakes except for enrichment of chemical species in surface water. This was caused by addition of degassed deep water at the surface.The highest ( 350 mmol/kg) is observed below 190 m depth a Nyos. The CO_2 recharge rate in the deep waters is 100 Mmol/yr, at least as high as in the previous ten years. The highest ratios of ^3He/^4He (8.1x10^<-6>) and ^<40>Ar/^<36>Ar (600), suggestive of magmatic affiliation. At monoun, the highest CO_2 concentration of 160 mmol/kg is observed between 65-95 m depth. The lakes may spontaneously explode in the near future if CO_2 saturation is reached at any depth. However, gas explosion at these lakes can occur at any time upon introduction of external forcing such as a landslide.