Using
Ecosystem Science to Solve Problems:
The Case of the Killer Lakes of Cameroon
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Dr. George Kling, Department of Biology.
Introduction
Only three lakes in the world are known
to have high concentrations of dissolved gas in their bottom waters: Lake
Nyos and Monoun in Cameroon, and Lake Kivu in East Africa. In August of
1986 Lake Nyos "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.
For more information, visit Dr.
Kling's website on these events.
The Build-Up of Gas
The lakes need to be permanently stratified
so that their bottom waters (the hypolimnion) don't mix with their surface
waters (the epilmnion). Any gas that is input into the bottom waters is
then trapped, until its violent release. Lakes that are deep and protected
from the wind are likely to have strong stratification.
This was not a volcanic event. 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. Gas buildup
was relatively slow over time.
The Release of Gas
Conditions that are required for gas
release include the weakening of the strength of stratification. Stratification
is weakest when the surface waters lose heat, which is a function of decreasing
air temperatures and decreasing solar energy input that occurs during the
monsoon season. This monsoon season occurs in August in Cameroon, and may
explain why both Nyos and Monoun exploded in August.
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.
Recovery
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.
Remediation of the Hazards
Pipes can be placed in the lakes to
pump the gas-rich bottom waters to the surface. Such a plan has been tried
on a small scale.
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.
Summary
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The functioning of these ecosystems
is largely controlled by physical and chemical processes.
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The physical aspects of lake morphology,
lake stratification, and of climate control the buildup and release of
gas.
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The chemical aspects of groundwaters
and dissolved gases control the inputs of gas to the lake, the total amount
of gas that can accumulate, and the effects of the gas on humans and other
organisms.
All materials © 2000 by
the University of Michigan.
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