Lake Nyos Science and a Killer Lake
Background • Lake Nyos lies in an old volcano crater (maar) in Cameroon, a country of Africa. • Mt. Cameroon, an active volcano, is located 300 km away. • What can you conclude about the climate in Cameroon?
Lake Nyos, the Village • The village of Lake Nyos is located at the base of the volcano that holds the lake.
Lake Nyos, the Inhabitants • The village was home to 1,800 people and some 3,000 cattle.
One summer night... • On the night of August 21, 1986, Lake Nyos quietly killed all but a few of the inhabitants of the village. • The last victim killed lived 27 km downstream from the lake.
The following day... • The normally placid lake showed signs of a violent event. • The water turned brown. • Vegetation as far as 80 m from the shoreline sustained significant damage.
Aftermath • Only a few individuals survived the night.
For several years, scientists puzzled over this catastrophe...
MISSION: Work in groups to come up with possible explanations for this event. TIME: 10 minutes. Yer noggin. RESOURCES: Be prepared to present your explanation to the class at the end of the 10 minutes.
What Didn’t Happen • Although a few witnesses claim to have observed flashes of light, loud noises, and some victims received what were tentatively classified as burns, the event at Lake Nyos did not involve a volcanic eruption. • It is also unlikely that the event involved a “phreatic eruption”--an eruption driven by the vaporization of groundwater without the ejection of magma.
A Limnic Eruption So what caused this catastrophic event to occur in a peaceful lake? • The eruption at Lake Nyos was powered by CO2 gas bubbles, ascending and expanding within the lake water. • What “everyday experience” involves a similar phenomenon (on a smaller scale)?
The Valley Below • As the lake released clouds of CO2, the gas spilled down the mountain- side and into the valleys. • Imagine that you were in the village of Lake Nyos on the night of August 21, 1986 and that you woke up in the middle of the night. What would you have seen / felt?
Spring Water, Naturally Carbonated... • Since the lake is a maar, it is most probable that a cluster of warm springs, rich in CO2, feed the lake through the bottom. • Where does the CO2 come from?
Compositional data for Lake Nyos, 1992. How Much is Too Much? • Do the two layers contain the same concentration of CO2? A lake can dissolve a volume of CO2 more than 5 times its water volume. • Are the layers saturated with CO2? • How many gallons of H2O and CO2 are contained in each layer? (1 dm3 = 1 L; 1 gal. = 3.79 L) • It is believed that the lake emitted 20,000,000,000 gal. of CO2 during the event in 1986. Do you think the lake emptied itself of enough CO2 in 1986 to be safe in 1992?
All Things Being Unequal... • What quality of the lake might explain why more CO2 is dissolved in the lower layer? • What is the significance of the above diagram? • What is a thermocline, and how does it relate to lake turnover? Would Lake Nyos undergo lake turnover?
The sun warms the surface of the lake. Warm springs feed the bottom of the lake. A well-labeled axis says 1,000 words. • Given the info in the graph, would you argue that Lake Nyos is thermally stratified? • Why do you think it is or isn’t stratified? • What do you think causes the variation in temperature?
A Landslide Victory? Since the lake is relatively thermally stable, it is unlikely that simple lake turnover caused the eruption. The current belief is that a landslide triggered a chain reaction...
Fountain soda, anyone? • The chain reaction involved nucleation of CO2 bubbles as they rose through the lake. • This could only occur if [CO2] was greater at the bottom. Why? • But we still haven’t answered why [CO2] is greater at the bottom of the lake...
CO2 concentration H2O Density CO2 saturation NEW! Dannon Yogurt: [CO2] on the... • Based on the data presented in this graph, what do you think most influences the solubility of CO2 in Lake Nyos? • HINT: If you dove to the bottom of the lake (not recommended), and you brought a balloon from the surface, what would the balloon look like at the bottom?
CO2 concentration H2O Density CO2 saturation Actually, it’s safe…trust me. • The graph below displays the relative safety of the lake water as it relates to depth. • Given water at depth A, the safety coefficient is calculated by integrating the difference in density from depth A to the depth at which the [CO2] of layer A exceeds the [CO2] saturation point. • This function estimates the amount of energy necessary to lift a parcel of water up to its eruption point. • In short, the further to the right the red line goes, the less likely that layer will spawn a limnic eruption. • However, keep in mind that the bottom layer of the lake has the highest density (more energy is needed to lift it). • This may seem counter-intuitive, given that the bottom of the lake has the highest CO2 concentration. Even though the bottom layer is the safest...
Safe Is a Relative Term …it’s dissolving more and more CO2, and thus the increasing need to de-gas the lake.
Degassing Complexified • One year after the event, the French Delegation aux Risques Majeurs proposed a degassing method based on off-shore oil production techniques. • The problem was that the proposal was expensive, and CO2 ain’t no cash crop.
In 1990, he did just that. Degassing Simplified • Enter: Michel Halbwachs, professor at the Centre de Recherches Volcanologiques, University of Savoie, France. • I think he said something like, “Hey man, it’s ‘facile.’ Just take some plastic pipes and stick ‘em in the water.”
Once you prime the pipe with a pump... …it creates a self-siphon- ing system. “We have to intubate.” • If the water is most dense on the bottom of the lake, why does it move to the top of the pipe? • The current degassing strategy is no more complicated than taking a plastic (HDPE) pipe and extending it to the bottom of the lake. Hmmm...
You the boss. • Although Michel Halbwachs orchestrated the first degassing “field” test, the lake research was conducted by a multi-national team from the start, including scientists from France, Japan, the U.S., and Cameroon. • Given that you are an internationally renowned gas expert (not the gastrointestinal kind), the team of Lake Nyos scientists is seeking your advice on an important question: To best avoid catalyzing a limnic eruption, how deep should the self-siphoning pipes be placed?
101 Ways to Use a 55-gal. Drum • Given the success of the 1990 self-siphon test, the research group received funding for a larger-scale test in 1995. • The diagram above shows the surface component of the pipe assembly.
Steady! The pipe surface raft being hauled out to the middle of the lake...
“Don’t worry. I’ve got a 2nd thumb.” Cutting and assembling the pipe...
Government workers? “Man, I forgot my Gore-Tex boots.” Feeding the pipe into the water...
McFly: “You are my density.” • If HDPE “almost” floats in water, then why is the pipe floating in the picture below? • Scientists selected high-density polyethylene as the pipe material for many reasons. One reason was because HDPE almost floats in water. • Let’s say you have sample of HDPE that occupies a volume of 10. mL. Predict what the mass of this sample will be.
Oxygen tanks. Just in case... Even though the risk of an eruption was very low, scientists didn’t want to take chances.
Why is it safe to use a motor in the lake? Just one step back, and... Easy does it... Once primed, the system took care of itself.
2001 Expedition • In 2001, the research team beefed up the apparatus. • 120 ft. “geyser.” • 90% of geyser energy is lost to friction. • Assuming a “frictionless environment, how high would the geyser go? • About as high at the Empire State Building.
Mo’ money. • With more funding, the research team has been able to add more features to the degassing system. • This includes automatice shut-off mechanisms, and...
Mo’ money, Mo’ money ...a satellite link for remote monitoring and remote control.
Mo’ money, please. • Scientists estimate that the current degassing rate (single pipe) is barely keeping up with the CO2 input from the springs. • They hope to get more funding to de-gas the lake completely.
Ecological Impact • Not as bad you would think. • Biggest impact is from bringing anoxic water from the bottom of the lake to the surface. • However, the single pipe only brings about 1.2 Mm3 to the surface per year. • This gets diluted more than 50-fold in the upper 45 m of the lake due to mild mixing. • Run-off brings in about 24 Mm3 freshwater each year more dilution. • In addition, single pipe discharge of CO2 is equivalent to the output of a 1.2 MW coal-fired plant without all the other acid-rain producing pollutants. • There has been talk of studying CO2 impact on local ecosystem.
Business, or pleasure? Just when you thought it was safe... • The top layer of Lake Nyos is held in place by a natural dam. • Geologists have studied this dam, and found that it’s in bad shape.
Dam it, Jim! • The upper unit is eroding away. • What would happen, if the upper unit gave way?
What the heck is that? • There are many proposals on the table for how best to address this.
Sources • Degassing the Killer Lakes: Lake Nyos and Monounhttp://perso.wanadoo.fr/mhalb/nyos/index.htm • LAKE NYOS and LAKE MONOUNThe Killer Lakes of Cameroon, West Africahttp://www.biology.lsa.umich.edu/~gwk/research/nyos.html • Photo Gallery:http://shanklin.intrasun.tcnj.edu/cameroon/