Wednesday, 21 August 2013

Degassing Nyos

On 21st August 1986, 1700 people in a remote area of Cameroon woke up dead.  They lived close to a small body of water called Lake Nyos  The region is seismically active and Lake Nyos is set on the side of a volcano, indeed it looks like a caldera but is more precisely a maar.  It's only 160 ha in area, and on average 100m deep - Lake Baikal is 150,000x bigger by volume.  I'm prepping my environmental chemistry classes this week.  One of the the odd things that happens in lakes is stratification of the water.  The relationship between solar flux, temperature and water density can sometimes leave the bottom of Irish lakes dark and cold for a long time rather than the whole mass getting mixed up.  In Nyos and a nearby lake called Monoun, because they sit on volcanically active rock, carbon dioxide leaches into the water from the bottom and accumulates in the depths as carbonic acid.  In water, the pressure gets higher the lower you go and the more gas can be dissolved.   Because of stratification of the Nyos water column, this CO2 saturated water sat there until something caused the upper and lower levels to start mixing.  Heavy rainfall or a natural landslip are suspected.  It was like opening a 150 billion litre bottle of fizzy Ballygowan.  As the CO2-laden water was stirred up, the pressure dropped and the CO2 came out of solution as fizz.  This further stirred the water bringing more deep water out of its natural pressurised container and so on.  Nobody saw it but investigations afterwards suggest that the column of bubbly was 100m high above the lake. It generated a tsunami 25m high that stripped trees off one shore.  When the action subsided the lake was 1 meter lower and had turned bright red: iron rich sediments had been stirred up from the bottom and were oxidised over the succeeding days.  On dit que 300,000 tons of CO2 were released in one catastrophic whoomph.

Carbon dioxide gas is much less dense than water but 1.5x denser than air,  so doesn't disperse up into the atmosphere but hugs the ground. Everybode kno that 1 mole of gas fills 22lt.  The molecular weight of CO2 is 48 (call it 50).  So a kilo of the gas fills 400lt and a ton of gas fills 400,000lt.  You can do the intermediate calcs yourself (and check that I haven't dropped a bunch of zeroes - Wikipedia claims of gas escaped) but the investigators estimated that the wall of CO2 was 50m deep and traveled at 50km/h, so of ground was covered much higher than people's noses.  The nature of the terrain (narrow valleys steep sides) meant that the 1700 victims were concentrated in three radiating river valleys and as much as 25km distant from the lake!

It seems that Lake Nyos has only been there for about 400 years and the pumice wall of the maar is quietly eroding from the bottom, so it probably won't be there in 400 years time.  It might seem appropriate to leave the local people sitting on a time bomb - sub-Saharan Africans are beset by so many random horrors (genocide, famine, globalisation, malaria, loa-loa the eye-worm, trypanosomiasis) that one more may not be noticed.

But Michel Halbwachs and colleagues from Université de Savoie, in France came up with a cunning plan.  Why not sink a PVC pipe to near the base of the lake and float the top end on the surface?  If you do that and just start pumping water through the pipe then the gas-dense pressurised water will quickly reach a depth where the water will spontaneously fizz up and you can switch off the pump.  The system will continue to siphon so long as there is CO2 saturated water available.  Вы русских инженеров can have a geek at the technical details, but I'll just clip one picture of the life-saving geyser. Isn't science just so cool! It serves a more useful purpose than the Geneva jet d'eau which is just a raree show for tourists.  Furthermore, with 2 x 500kW pumps pushing water into the sky day and night, le jet costs the taxpayers of Geneva in the region of €800,000 a year.  That pretty much matches the cost of installing the 'appropriate technology' system in Nyos.  You do the geopolitical math!

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