from 'doing science' to 'applying science' is not easy. It requires a much
broader understanding of the natural 'system', which includes not only
the underlying science but various social and political aspects as well."
For more information, visit Dr.
Kling's website on these events.
The natural hazards arise when enormous amounts of dissolved CO2 gas trapped in the bottoms of these lakes is released suddenly. In August of 1986 Lake Nyos in Cameroon "exploded", releasing up to 1 km of CO2 and killing about 1700 people up to 26 km away from the lake. A smaller gas burst from Lake Monoun in August of 1984 killed 37 people. There are several aspects of these disasters that we need to understand, and they include the build-up of the gas, the release of the gas, and the remediation of the hazards.
Natural hazards created by the lethal
release of large amounts of gas from lakes are very rare. Only three
lakes in the world are known to contain high concentrations of dissolved
gas in their bottom waters: Lakes Nyos and Monoun in Cameroon and Lake
Kivu in East Africa. Only two of these lakes, Nyos and Monoun, are
known to have recently released gas resulting in the loss of human life.
These were not volcanic events. The
gas that was released was stored in the bottom waters before the event.
The gas originated from magma about 80-90 km below the lake, and rose through
the fractured rock beneath the lake until it got to the surface and dissolved
in groundwater. The groundwater was then injected into the lake, as evidenced
by the similar chemistry between groundwater and lake water. The gas buildup
occurred relatively slow over time, rather than the gas being injected
all at once such as might occur in a volcanic event.
Both events also occurred in the mid-1980's, and if the gas was built up slowly over time then the probability of both events happening so close together is very small. This is probably explained by the long-term trends of climate in Cameroon -- lower than normal temperatures and higher than normal rainfall in the mid-1980's.
The gas contents of these two lakes continue to build, supplied by the input of CO2 from underground springs that discharge into the bottom of the lakes. The exact trigger for a gas burst is unknown although any disturbance that moves deep, gas-rich water closer to the surface could result in a gas release. As that deep water rises, the weight of water above it (the hydrostatic pressure) decreases, and at some point the dissolved gas pressure will become equal to the hydrostatic pressure. At this point there is nothing to force the gas to remain in solution, and gas bubbles begin to form. This process is identical to the removal of the cap from a bottle of soda -- when the cap is removed there is no more pressure to keep the gas dissolved in the soda, and bubbles are formed. Once bubbles are formed in the lake they rise rapidly and drag the deep water toward the surface, at which point additional deep water is drawn upward and depressurized. This leads to a chain reaction that eventually results in the violent release of enormous amounts of lethal CO2 gas. Because the gas content of these lakes is currently very high, catastrophes similar to those in 1984 and 1986 could occur at any time.
Both lakes continue to increase their
levels of gas in the bottom waters. This buildup provides evidence for
the hypothesis that the gas is input slowly over time rather than injected
during a volcanic event. Nyos and Monoun could become saturated with gas
(that is, the gas pressures at the bottom will equal the hydrostatic pressure)
in 10 to 30 years, respectively, if the gas input rates remain constant.
The energy released during degassing is sufficient to drive the pumping operation without any external power source. Such a plan has been tested on a small scale, and continuous degassing began in 2001. In addition to removing the gas currently in the lakes, it is possible to prevent future gas buildups using a pipe that continuously flushes bottom water out of the lake.
Until the lakes are degassed using the method described above, the people living around the lakes are at risk. Relocation of survivors from the Lake Nyos event has already occurred. However, in part because these people have now been refuges from their homes for 14 years, they are moving back into the evacuated region, and the number seems to increase each year. There was no relocation of survivors around Lake Monoun, but the population around the lake has increased in recent years, and new farmlands have been occupied very close to the lake shore. There are many social, cultural, and political aspects of the degassing plan and how it will affect the local populations.
In addition to the gas hazard, at Lake Nyos, the spillway or natural dam that holds back the upper 40m of the water column is weak and is eroding rapidly. When the dam fails it will produce a flood that could affect 10,000 people in the floodplain below. Any attempt to lower the lake level could result in release of gas that is contained in the sediments, due to the decrease in overlying water pressure, which could trigger another event.
As mentioned above, the third lake in the world known to contain dangerous concentrations of dissolved gas is Lake Kivu on the border of Rwanda and Democratic Republic of Congo in East Africa. This is a giant lake, containing 1000 times more CO2 in its bottom waters than does Lake Nyos, and with millions of people living around the shoreline this is the world's largest ticking time bomb. The situation at Lake Kivu is also different because there are nearby active volcanoes that could destabilize the lake and trigger a gas burst. There are actually volcanic vents that form seamounts on the bottom of the lake, so, it is just a matter of time before the bomb explodes. To compound the problem the methane pressure has been increasing, and if the rates continue the total gas pressure will reach saturation in about 100 years.
Although it’s probably hopeless to try and degas the lake with the small pipes used in Nyos and Monoun, and the cost would be prohibitive, there is one possible solution for this hazard. It turns out that the methane stores in the lake rank as the 5th largest natural gas deposit of any country in the world. The idea is to place offshore platforms with pipes to extract the gas, just like at Nyos, but to separate the methane from the CO2 and send it across the lake to a power plant on shore. The first of several power plants will produce 35 MW, which will double the current power output of Rwanda. And that power will be 5 times cheaper than the diesel they now burn for power, and can it be exported to other countries in E. Africa. Thus this time bomb in the lake may actually be a salvation for this very poor and developing part of the world.
Applying Science to Help Society
The scientific evidence for the causes
of these natural disasters is now well established. But there are other aspects
to consider, which include potentially differing points of view of
the scientists compared to the local population compared to the government.
Our basic assumption is that reducing the danger of these hazards is good,
but people must be first educated about the risks involved and then they
need to decide for themselves what level of risk they are willing to tolerate. For example, the people living around earthquake-prone areas such
as San Francisco must accept the dangers of the area, and they balance those dangers with their desire to live there. In Africa there may be less mobility for populations living near hazards, but the main thing that science can do in this arena is to inform people about the risks. Of course there are additional
cultural and political issues that surround almost all decision-making
aspects of environmental problems or natural hazards, and these must be
dealt with in the same careful and comprehensive way in which we do our
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