THE DEGASSING OF THE LAKE MONOUN
Situation in January 2006
Precise measurements of the concentration in dissolved gas were taken, using the mini-separator technique.
The carbon dioxide still dissolved in the water was measured using a Karat tube.
The methane level was determined using an infra-red analyser.
On this diagram the volumes of water and of gas have been noted at each depth - at five meter intervals - in millions of cubic metres.
Using the gas content data and temperature and conductivity profiles, we have been able to calculate density according to depth
This is the essential parameter which allows us to evaluate the safety parameter anywhere in the lake.
On this diagram we have noted the effect of each parameter on water density : temperature, ion content (salinity) content of dissolved gas and hydrostatic pressure.
The analysis of the dissolved gas content allows us – using Henry’s law - to trace back to the partial pressure of each constituent and thus to the total gas pressure at all levels. This figure shows that the lake is in a critical state : total gas pressure is almost equal to hydrostatic pressure at a depth of 55 m. The slightest disturbance of the lake waters could provoke a gas explosion.
Monoun partial pressure profiles.
The partial pressure profiles in Lake Monoun as of February 2003. The critical situation of the lake can be emphasized since the total gas pressure at –55 m is quite close to the hydrostatic pressure. A slight increase in the dissolved gas concentration should result in over saturation, triggering the formation of bubbles which in turn will initiate the “avalanche process” leading to the overturning of the lake (limnic eruption). The vertical arrow shows the amount of subsidence of this critical layer induced by a 12-month degassing.