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Outgassing of CO2 from a lake after lowering the water level

  1. Apr 12, 2013 #1
    Hi folks!

    English is not my native language so please excuse my bad grammar. I am a geology student working on a project about a lake when an interesting puzzle piece - a newspaper article from 1844 - came up. This is not homework and I don't even know if a solution exists to my question at all, so I hope this is the right forum section. If not, I apologize and don't mind it being moved elsewhere.



    So here it goes:

    The Laacher See is a lake in a volcanic crater with no natural outflow that is always fully saturated with CO2.

    In 1844, the local council people decided to build a canal through the crater wall to lower the water level and claim land for agriculture. During the year (the exact time and speed at which this happened is unkown, most likely 3-5 weeks) the water level was lowered by about 7 meters.


    Now it gets fascinating - a newspaper article from July 1844 from an unnamed eyewitness claims to have seen billowing dirty water, weird smells and sounds, as well as dead animals on the shore. At that time, it was dismissed as complete fantasy.



    The first thought that comes to mind is that the lower water level decreased the pressure at the bottom which in turn decreased the CO2 solubility which created a big CO2 bubble. Like opening a soda can.

    The thing is - something like this happened in 1986, which killed 2000 people near a Lake in Africa. But the situation might be different, in Africa the event was triggered by a sudden landslide, whereas in this case it was more like a few weeks.


    I don't know if my line of thinking is physically sound. If it is, I would like to calculate the amount of CO2 released. The figures are at the bottom of this post. I've tried getting my head around applying the Henri Law of Gases, but I am not very good around non-mechanical physics.

    Any help or suggestions are tremendously appreciated! Maybe we can solve this mystery together.


    Data:
    Depth before lowering: 60.8 meters
    Amount of lowering: 7.48 meters
    Surface elevation (new) is 275 m above sea level.
    The lake is nearly half-sphere-shaped.
    Area before: 3,875,643 sq meters
    Area after: 3,327,903 sq meters (not sure if these last 2 numbers matter)
     
  2. jcsd
  3. Apr 12, 2013 #2
    What was keeping the water saturated with CO2 before the excavation? Was there a source of CO2 within the volcano that was seeping CO2 into the water?
     
  4. Apr 12, 2013 #3
    The Africa scenario was methane gas I recall reading that. Methane entrapment is getting more common with climate change.
     
  5. Apr 12, 2013 #4
    In my judgement, what we are dealing with here is more likely an induced seismic event caused by the perturbation to the stress pattern in the rocks resulting from the drop in subsurface compressive stress associated with the excavation. A seismic event could easily release a large amount of CO2, and the seismicity could easily cause the observed disturbance to the water.
     
  6. Apr 12, 2013 #5

    russ_watters

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    I wonder if the pressure difference due to the lowering if the water level could have contributed.
     
  7. Apr 12, 2013 #6

    rcgldr

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    Homework Helper

  8. Apr 13, 2013 #7
    In the mechanics of water saturated porous materials (poroelasticity), it is the Terzaghi effective stress that determines both the deformational response and the failure mechanics. The Terzaghi effective stress is equal to the total stress, minus the pressure of the pore water. If the water level had been lowered in the lake, the pore water pressure within the subsurface rock would have decreased, and this would have increased the Terzaghi effective compressive stress. This would have been a stabilizing influence tending to reduce the potential for rock failure.

    But, on the other hand, the excavation would have removed rock that is under stress, and thus relieved compressive stress. This would be analogous to what happens in a coal mine when rock is removed. In many cases, the removal of the rock (and the associated removal of compressive stress) results in a buckling of the mine floor.
     
  9. Apr 13, 2013 #8

    SteamKing

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    1. Take a bottle of a carbonated soft drink.
    2. Shake bottle vigorously for several seconds.
    3. Remove cap from bottle and observe.

    Note: perform this experiment outside and while wearing old clothes.
     
  10. Apr 13, 2013 #9
    Methane is lighter than air, so it could never cause a disaster like that.
     
  11. Apr 13, 2013 #10

    Borek

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    Lowering the water level would lower the pressure at the bottom and the water there could get oversaturated with CO2. Sounds reasonable to me.

    Once the bubbling starts water density goes down lowering pressure even more, plus water starts to mix moving the saturated layers up, and dioxide evolves even faster - kind of a positive feedback loop. If I recall correctly that's what happened in Lake Nyos.
     
  12. Apr 13, 2013 #11
    Back to the CO2 scenario. You mentioned that the lake was saturated with CO2, but at what pressure? Ordinary tap water is close to saturated with ~ 400 ppm CO2 in the atmosphere, but this corresponds to a very low pressure, and tap water is pretty much "flat." So if the CO2 scenario is realistic, the equilibrium pressure of the saturated CO2 in the lake had to be pretty high. This means that the concentration of CO2 in the lake water had to be pretty high. Now, back to my original question: where was this CO2 coming from? Certainly, if there were no CO2 seepage into the lake from below, the lake would pretty quickly come to equilibrium with the atmosphere, as CO2 came out of solution and the lake went "flat." If CO2 seepage were entering the lake water from below, the CO2 concentration would rise to a steady state value, in which the seepage rate would be equal to the release rate of CO2 at the air interface. The concentration of the CO2 in the water at steady state would be a function of the seepage rate and the mass transfer coefficient at the air interface. Do you have any idea what the CO2 concentration within the lake could have been at steady state, or what the seepage rate could have been? This is the only way we can begin to quantify what may have happened. If we knew this, we would be able to model the CO2 release.
     
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