Tragedy at Lake Nyos, Cameroon: Involving introductory chemistry students in a multidisciplinary approach to investigating CO2(g) budget and hazard mitigation
A chief complaint of introductory chemistry students at Arizona Western College (AWC) is that chemistry is too abstract and being thus does not relate well to the world around them. In contrast, our chief complaint as instructors is that students rarely observe, critically, their world. In order to make chemistry more relevant, and at the same time perhaps a bit more interesting for students, we use a multidisciplinary exercise that showcases chemical concepts and their applications in a larger context. In this case concepts such as molar mass, density-temperature relationships, Henry's Law, and gas solubilities are integrated with physical volcanology, geochemistry, and natural hazard mitigation.
The setting for the exercise is Lake Nyos, Cameroon, site of one of the first documented limnic eruptions in August 1986. This setting was chosen because many AWC science students show a natural affinity or interest in geology and volcanic activity. To tap this wellspring of interest we use the remarkable geological event at Lake Nyos to show clearly the relationship between chemistry and a real world environmental problem that tragically killed nearly 2000 people.
The exercise and its themes are first introduced in lecture (lecture info: twice/week, 85 minutes/lecture) about a quarter of the way into the 14-week semester. A visual presentation uses pad camera projections (on overhead TV screens) of illustrations and photographs from a 1987 National Geographic article on Lake Nyos (1). This semester we intend to supplement the presentation with a video, filmed by Dr. Britt Hill from the Center for Nuclear Waste Regulatory Analyses , of the December 1996 eruption of Cerro Negro volcano in Nicaragua. Following the visual presentation there is a brief description of maar volcanism , which serves as background information necessary for understanding the unusually high CO2(g) concentration in Lake Nyos' waters. The introduction to the Lake Nyos exercise is rounded out with a handout that: 1) reinforces / summarizes the information provided in lecture, and 2) poses a series of problems that the students must solve.
Requirements and Expectations
Upon completion of the presentation and providing students with handouts as described above, students are encouraged but not required to work in teams of two or three. Teams are not assigned by the instructor but rather are chosen by students. A few students choose to work by themselves. The division of labor within the team is decided upon by the members. Students though are clearly told that they will be tested on the Lake Nyos' exercise, especially the calculations.
Each team is to investigate the environmental incident at Lake Nyos, analyze and interpret relevant data, and then develop a principal and alternative hazard mitigation plan. Several textual and electronic resources about Lake Nyos are available. They include: a book on natural disasters (2) , articles from Physics Today(3) and National Geographic (4) , and Lake Nyos' websites [(5), (6), (7)]. Even though articles from scientific journals are not off limits, thus far no students have requested them. Click here for a list of research articles describing Lake Nyos' geothermal and chemical processes. The final job of completing the problem set (calculations) , reporting the results , and devising principal and alternative mitigation proposals is the collective responsibility of the student teams.
Student teams are given one full week to complete the exercise. Approximately 25 minutes are allotted in consecutive lecture classes for students to make inroads on the problem set. Setting aside this time has twofold importance- 1) it gives the instructor an opportunity to consult with student teams and to address any problems / misunderstandings concerning the problem set, and 2) it provides the instructor an opportunity to observe group dynamics and perhaps better understand just how students go about the business of learning. In a nutshell students are given instant and constructive feedback. The introductory chemistry course at AWC has on average 40 students. The Lake Nyos' exercise and other similar exercises that we do provide the instructor(s) with critical insight into how students grow intellectually in the class. Hopefully, we become better instructors because of it.
At the end of the one week period student teams are required to turn in the following:
Calculations' solution page (legible and shows all units).
One to two page written report describing one principal and one alternative method for mitigating hazards associated with future limnic eruptions. Student generated sketches or figures illustrating each mitigation method are required.
Several students have requested additional time to complete the exercise. We aggressively resist giving additional time because most student teams turn in complete, medium to high quality reports within the existing time-window.
Observations & Results
One of the authors (M.C.) introduced the Lake Nyos' exercise about a quarter of the way into the 14-week semester to introductory chemistry classes, a one semester course, in Fall 1997 and Spring 1998. Students listened attentively to the lecture and were unusually animated during the question / answer period that immediately followed. Interestingly, normally reticent students were generally as inquisitive and as likely to ask questions as the normally more outgoing, inquisitive students.
Since the concept of molar mass is still relatively new, there is a fair amount of tete-a-tete about the calculations. The mathematics prerequisite for introductory chemistry is Beginning Algebra. The metric system and exponential notation are discussed the first week of class. Surprisingly, students had more difficulty converting km3 into m3 than with molar mass calculations. But, based on our qualitative observations and classroom interaction with students, most teams complete the problem set without too much difficulty or instructor help. We have also observed that questions that are rarely asked in class are asked within and among the teams. This is encouraging.
As scientists we are most interested in the written reports, i.e. what ideas do students come up with to solve the stated problem(s). The reports, which consist of how the students would mitigate hazards associated with future limnic eruption, were usually quite good and reasonably well written.
The mitigation proposals that students devised for preventing a recurrence of the tragic events of 1986 ranged from good, common sensical methods to downright ludicrous. We put no restrictions on how "far-fetched" an idea maybe; it's an idea free-for-all. One student suggested shipping the offending gas, CO2 , into space. Based on the quality of the reports and ideas a significant majority of the student teams put serious thought into their mitigation proposals while, not surprisingly, a few did not. Most mitigation proposals involved using reasonable engineering / scientific methods but the possible weaknesses and implications associated with each method were either underdeveloped or not discussed entirely. Listed below are some representative student solutions put forth in the mitigation proposals.
Tap CO2-rich basal waters with siphoning tubes for continuous venting of CO2(g) into the atmosphere
Add CaO to react with the CO2 and thus remove the gas
Use either submerged or surface pumps to force convection in the lake- one student group suggested providing local villagers with ski-doos to whip surface waters into a froth and promote diffusion across the shallow epilimnion
Draw off CO2(g)-rich basal waters for use in a carbonate drink plant constructed on the lakeshore; Dr. Pepper was the drink of choice
Drain the lake
Drain the lake and fill it with a plug of concrete, thereby sealing its floor (As you can see we probably need to introduce environmental sensibility.)
Use large fans hardwired to CO2(g) sniffers to disperse the gas upon ejection
Permanently evacuate the population around the Lake Nyos area affected by the 1986 eruptions
Engineer gas channels (gas ducts) to drain off CO2(g) just as the river valleys channeled the gas released in 1986
Provide all local villagers with oxygen tanks and masks
It was required that both the principal and alternative mitigation proposal must be accompanied with an annotated illustration. We believed that this would in turn encourage students to better visualize and thus refine their ideas. We do not know conclusively to what degree this was effective, but several students commented that illustrating their ideas made the problem seem more real. Both authors though believe that having students draw out what they are thinking is without question beneficial to the learning process. An additional benefit is that the illustrations are frequently quite good which we believe to indicate that students put considerable time into creating them and took the Lake Nyos' exercise seriously.
In regards to the calculations, most student teams established that basal waters of Lake Nyos will become overpressured (PCO2(g)/Patmosphere > 1.0) again in approximately 21 years, thereby creating conditions ripe for another limnic eruption. This narrow time-window reinforces the urgency for developing effective mitigation efforts and makes the exercise that much more tangible and real. More importantly though, it emphasizes the importance of and relationship between calculations (based on reasonable assumptions and concrete, replicable data) and policy / decision making with respect to oftentimes unique and complex problems. For example, a few student teams incorrectly calculated (large positive relative error) the approximate time Lake Nyos will again become overpressurized. Based on these erroneous calculations and assuming the data reliable it was pointed out that the Lake Nyos' limnic eruption would have occurred with loss of life. Students realized that such seemingly harmless mistakes could potentially have tragic consequences and that the tragic consequences could have been prevented.
Student Skills
Besides being a reasonably effective arena in which to integrate chemical concepts into larger a real world, environmentally based problem, the Lake Nyos' exercise provides an opportunity to introduce new skills and to hone students' pre-existing skills. Piggybacking did not seem to be much of a problem. Listed below are skills required for student groups to successfully complete the exercise.
organizing data
converting within the metric system
interpreting / analyzing data
writing equations
technical report writing
visualizing solutions to complex, multivariable environmental problems
working in teams
We frequently ask if students take the quantitative and qualitative lessons learned in this or other similar exercises and apply them to unrelated problems encountered elsewhere. At this time we have no effective means, other than the quality of the reports, to determine how well students learn or improve upon the skills above.
Future Changes
Our students resist compiling data tables. This semester a recommended table format will be used. There are two reasons for this. First, it impresses upon the students the importance in science of developing effective data cataloging. Second, it facilitates discussions between students and instructors during the in-class consultations.
Mission Accomplished?
Did the Lake Nyos' exercise turn students on to science? We doubt that this single exercise drastically altered students' perceptions about science. It takes more than an occasional exercise such as Lake Nyos to develop and sustain in introductory chemistry students a high level of interest in science. If one of the chemical education community's goals is to convince students that science is indeed worth pursuing as a career, it likely will take an assortment of such exercises interspersed throughout the semester before we begin to reach a larger population of students.
To that end though both authors have constructed the chemistry curricula around applications and have drawn from numerous disciplines outside their expertise in the quest to create chemistry courses that appeal- without sacrificing scientific content- to as many students as possible. Since time is the limiting factor, this means that some traditional topics are not discussed in detail. One of the authors (SD) hadn't the foggiest idea about geochemical processes or maar volcanism prior to the Lake Nyos' exercise. And neither did students. But now they do.
Student Comments
Based on the student comments below the Lake Nyos' exercise and our relentless emphasis in lecture on chemistry's ubiquitous role in all things great and small, wild and wonderful have in some unknown but important way caught our students' attention. For those students whom we quote science seems to be no longer just a bunch of disjointed facts and obscure mathematical equations. Hopefully, this heightened sense of awareness leads to bigger and better things such as a decision to choose science as a career. Below is a small sample of representative student comments made about the Lake Nyos' exercise and about other class exercises used during the semester that relate chemistry to everyday events.
"This class has increased my enthusiasm for learning chemistry by increasing my curiosity about how / why things in the physical world exist as they do."
"This class has taught me a lot about my surroundings. What I liked best about the class is how you talked about the world and how chemistry is involved. Those are interesting facts that catch everyone's attention, help us learn, and we can use."
"The best thing about this class is that I saw the world around me as going through chemistry processes."
"Even though I don't like any science of any sort, this class has really changed my view. The reason being is that this class has many real-life examples and it has broadened my outlook on chemistry. Of course, with the help of many life pertaining examples and very deep thought problems it's challenging and I like that."
"It had not occurred to me before how much of a role chemistry plays in my everyday life."
"Didn't realize that chemistry involves many things that we apply in our life."
"I like the fact that the professor made us think about our surroundings."
"I watch the Learning Channel more often."
The unedited comments above lead us to believe that what we are currently doing and what we intend to do is important, catches and holds most students' attention, and has contributed positively to introductory chemistry students' experience(s). We won't reach all of course, even if we develop some spectacular, "knock your socks off" exercises. It's an admirable goal but realistically unachievable at the freshmen level since most students are taking the course to fulfill a general education science requirement and will pursue non-science careers. So the obvious goal is to build a modicum of both scientific literacy and appreciation in our students. The Lake Nyos' exercise is a good means to achieve that end result and besides it's fun for chemistry students to learn a little about geochemical processes and volcanism.
Conclusion
We believe that the Lake Nyos' and similar exercises in development compensate for what we believe is the chief deficiency in chemical education at the freshmen level- applying fundamental chemical concepts to other scientific disciplines and engineering without sacrificing content. Perhaps the most important lesson we have learned so far (after much trial and error) is that introductory chemistry students are attracted to "big picture" science- that is, science that applies on a grand scale to the student's world. The Lake Nyos' exercise is "big picture" science, yet does not suffer from science education anorexia, i.e. emphasis solely on conceptual understanding with the exclusion of the quantitative component that is so vital to scientific exploration.
The Lake Nyos' exercise can be appropriately modified for more advanced courses like general chemistry. For example, one could incorporate a more detailed examination of the water chemistry as a function of depth and partial pressure of CO2(g), investigate the distribution of carbon isotopes in the surroundings, characterize isotopic reservoirs in the Earth's crust and mantle near the lake, and predict changes in salinity as a function of CO2(g) absorption. In short, the exercise can be tailored to fit a variety of chemical topics without eliminating fundamental content.
We welcome and will gratefully acknowledge any constructive criticism that leads to the improvement of the Lake Nyos' exercise. The Lake Nyos' handout is found immediately below. Works Cited and Lake Nyos' journal articles are found immediately after the handout.
On 26 August 1986 a massive jet of gas and water erupted from Lake Nyos, a remote lake in the highlands of northwest Cameroon, which is located in west central Africa (Fig. 1). The water-gas spout reached a height of over 100-m above lake level. Dense carbon dioxide (CO2) gas released during the eruption swept downslope asphyxiating 1746 people and several thousand cattle (Fig 2). The majority of victims were found within 3-km of the lake, but deaths were reported at distances up to 10-km (6 miles) from the lake.
Lake Nyos rests in a volcanic crater called a maar. Maars are inverted volcanoes that form from a series of powerful explosions that occur when magma (temperature ~1100 o C) encounters cooler groundwater (Fig. 3). The resulting steam explosions excavate a crater; a historical example is the Ukinrek Maars that formed in April 1977 on the Alaska Peninsula (8). Based on 14C radiometric dating of charcoal nested in pyroclastic maar deposits, the crater formed 400 +/- 100 years before the present (9). Following the end of explosive volcanism the newly formed crater filled with groundwater, thereby forming present day Lake Nyos (Fig. 4).
Table 1. Physical Parameters of Lake Nyos
Surface Area: 1.49-km2
Maximum Diameter: 1925-m
Maximum Depth: 208-m
Volume of H2O: 1.70 x108-m3
Climate: Wet (May - October), Dry (November - April)
Lake level Fluxuations: 1 - 2 m drop in dry season SCIENTIFIC INVESTIGATION OF LAKE NYOS
Initial reports from local authorities suggested that the tragedy was the result of a volcanic eruption. Investigators arriving at the lake several days after the event were puzzled by the lack of evidence for hot, caustic gases that normally accompany volcanic eruptions; plants on the lake perimeter were uncharred and apparently unharmed (Fig. 4). An important piece of the puzzle was revealed when researchers associated with the U.S. Foreign Disaster Assistance (USFDA) discovered that waters of Lake Nyos were highly stratified (Fig. 5) and contained a basal layer enriched in CO2(g) , i.e. a homogeneous mixture of H2O and CO2(g). Other evidence that supported explosive venting of trapped CO2(g) and weighed against a volcanic eruption hypothesis include- undisturbed lake-bottom sediments, cool lake water temperatures, and the absence in water samples of the gases SO2, CO, H2S. The presence of these gases are characteristic of volcanic activity.
Researchers hypothesized that on 26 August bottom-rich CO2(g) waters were disturbed, perhaps by a submarine landslide, driving the basal CO2(g)-rich waters toward the surface. As the gas-enriched waters approached the surface where the hydrostatic pressure was less, CO2(g) solubility decreased resulting in exsolving of gas, which erupted explosively from the lake surface (Fig. 4) . The process is analogous to shaking a bottle of soda. The dissolved CO2(g) is freed from the liquid medium and collects at the top of the bottle above the liquid surface. Upon removing the cap the gases vent violently. At Lake Nyos the volume of gas vented was sufficient to cause a 1-meter drop in lake level.
CO2(g) BUDGET AT LAKE NYOS
The research team estimated that the volume of CO2(g) released was about 1 x 109-m3 which translates into roughly 1.95 x 1012 grams of CO2(g). The concentration of CO2(g) varies as a function of depth; one liter of water at 40-m holds 2 to 3 liters of CO2(g) , at 190-m up to 11 liters of CO2(g). Preliminary models (advanced, educated guesses based on a diverse set of data) indicate an exit velocity of the gas equal to approximately 80 m/s. The velocity of the gas as it moved downslope is undetermined. For our purposes we will assume that victims situated 10-km from Lake Nyos died about 32 minutes after the eruption.
Since August 1986 an international science team has monitored water chemistry and gas content of Lake Nyos. In a paper published in 1994 they concluded that daily recharge of CO2(g) from hot springs on the lake floor is on the order of 6.07 x 106 mol/day (10). Diffusion of CO2(g) through the lake surface removes only about 1.7 x 108 moles annually. Hence, recharge is substantially greater than discharge. The CO2(g) is rapidly collecting at basal water levels (below about 50-m) of Lake Nyos.
The United Nations has provided funds to form an international team of scientists to 1) study this unique phenomenon, and 2) develop strategy for mitigating hazards at Lake Nyos. Because of your expertise as a chemist you have been selected to join the team.
Your team's first job is to quantify the CO2(g) budget at the lake. To assist you in this a series of questions have been posed in the following sections. As you answer these questions, you will better understand the nature of the hazard and the timeframe allowed for you to develop a strategy to mitigate hazards at Lake Nyos.
BACK OF THE ENVELOPE CALCULATIONS:
In July 1945 the first atom bomb was detonated near Alamogordo, New Mexico. Enrico Fermi, the brilliant physicist, was sitting in an open bunker some miles from ground-zero. Upon feeling the first shock waves, he released a fistful of torn fragments of paper above his head. As the shockwaves rolled over the bunker, the paper silently fluttered down and away from the growing mushroom cloud, landing about 2.5 yards behind him (11).
Fermi used the horizontal displacement of the paper to quickly approximate the energy released by the bomb. His estimate of an explosion equivalent to ten thousand tons of TNT was later confirmed using data collected by fancier analytical equipment that recorded the shock wave's velocity and pressure.
Fermi's efforts are referred to as a "back of the envelope" calculation. These types of calculations simplify an otherwise complex and difficult problem. The goal of these type of calculations is to arrive at a reasonable estimate. For example, calculating stream velocity using a bit of bark floating downstream provides a useful indication of flow velocity, but ignores the fact that flow velocity varies considerably through the water column. That is, water moving along the stream bottom may have a different velocity than water moving at the surface. "Back of the envelope" calculations are commonly used during the early stages of many scientific investigations to get a handle on expected (predicted) results.
In this exercise some of your calculations will be "back of the envelope" calculations. Like Enrico Fermi observing the detonation of the first atomic bomb, you will make assumptions to simplify an otherwise complex process so as to better understand the processes involved.
Good luck!
Part I: CO2(g) Budget at Lake Nyos, Cameroon
The data table below is based on information found in the handout and also on calculations that you will have to perform. Complete as much of the table below as possible before moving on to Part II.
Part II: Complete the data table of the Daily and Annual CO2(g) Budget at Lake Nyos, Cameroon in Part I by answering the questions below. Use the information found in the handout CO2(g) and in Figure 5. Write neatly in pen and provide all your work with units on one or two sheets of paper.
a) Calculate the volume of CO2(g) erupted in August 1986 in km3;
b) i. Convert grams of CO2(g) to kg then to metric tonnes;
ii. Calculate the number of moles of CO2(g) erupted (concept of molar mass)
c) i. Explain why plants in the area were unharmed by the gas;
ii. Explain why the CO2(g) didn't simply rise harmlessly into the atmosphere. (Hint: Find the molar mass of CO2(g) and that of air (~76% N2, 22% O2, 1% Ar -the remaining 1% is a smorgasbourd of gases).
d) Calculate the downslope velocity of the CO2(g) gas following the eruption;
e) Determine how many grams of CO2(g) enter the lake each day;
f) What percent of recharged CO2(g) is vented annually through the lake surface?
g) i. Determine the moles of CO2(g) in Lake Nyos as of August 1997;
ii. Determine how much time will elapse before the bottom waters of Lake Nyos reach the dangerous CO2(g) levels of August 1986. (Be sure to remember to factor in CO2(g) that harmlessly diffuses, i.e. discharges, from the lake surface). Report the answer in units of days and years.
Part III. Given the data at hand it seems clear that Lake Nyos will produce another limnic eruption in the future. Your team's job is to propose a minimum of two- one principal and one alternative- methods for mitigating hazards associated with CO2(g) recharge at Lake Nyos. Each proposal must be accompanied by a sketch or two and if necessary a series of relevant chemical equations.
The report describing the principal and alternative proposals for mitigating hazards associated with limnic eruptions at Lake Nyos is to be two typewritten, single spaced pages. At this stage any proposal is preliminary and will likely have some shortcomings. Point out in sufficient detail potential problems (weaknesses) with each proposal type, note them, and provide suggestions on how to resolve these problems. You should submit a minimum of one principal and alternative proposal.
If references are used in your final report, please cite them. In the event the reference is a website provide the full address, webpage title, and author.
Evans, W.C. et al "Gas buildup in Lake Nyos, Cameroon: The recharge process and its consequences". Applied Geochemistry , 8, 207 (1993).
Giggenbach, W.F. "Water and gas chemistry of Lake Nyos and its bearing on the eruptive process". Journal of Volcanology and Geothermal Research , 42 , 337 (1990).
Kling, G.W. et al "The 1986 Lake Nyos disaster in Cameroon, West Africa". Science , 236 , 169 (1987).
Sano, Y., et al "Helium isotope evidence for magmatic gases in Lake Nyos, Cameroon". Geophysical Research Letters, 14, 1039 (1987).
Figures:
Figure 1.(gif) Map of Africa pinpointing the location of Lake Nyos in Cameroon. Other locales of interest include Lake Monoun, which experienced a similar but smaller limnic eruption in 1984, and Yaounde, the capital city of Cameroon.
Figure 2.(tiff)Figure 2.(gif) Cartoon of CO2(g)-rich cloud from Lake Nyos sweeping downslope over local villages situated among the fertile soils of river channels.
Figure 3.(tiff)Figure 3.(gif) Model of maar formation (12). Maar formation occurs as basaltic magma (~1100 o C) contacts cool groundwater. The groundwater flashes spontaneously into steam excavating a crater through a series of explosive bursts. Excavated materials are entrained in wet, turbulent, hot pyroclastic surges and redeposited outside the rapidly forming crater. Pyroclastic surges move outward at hurricane velocities- 100's of km/hr. See inset for associated phenomena, e.g. slumping of dense, wet material back into the freshly excavated crater.
Figure 4.(gif) Photo of Lake Nyos taken immediately after the limnic eruption of 26 August 1986. The rust color reflects the presence of iron-rich sediments in suspension following the event. Note that the vegetation along the lake perimeter appears unharmed.
Figure 5.(tiff)Figure 5.(gif) Model of CO2(g) exsolving from basaltic magma and migrating along fractures before being trapped in the dense, basal waters of Lake Nyos. Note that the abundance of CO2 gas (bubbles) decreases upward through the lake's water column.
Figure 6.(gif) Temperature inversion (cooler waters overlying warmer waters) at Lake Nyos resulting from effusion of warm groundwaters through the lake bottom. It should be noted that under normal conditions water temperature decreases with depth.
Arizona Western College is a community college located in Yuma, Arizona. Yuma is situated on the Colorado River in southwestern Arizona, 20-km north of Mexico. Northern Arizona University maintains a satellite campus at Yuma. Degrees offered on the AWC-NAU joint campus include associate, baccalaureate, and masters degrees in a variety of areas. The AWC annual student population comprises about 3000 full-time students and 3500 part-time students including in decreasing order of abundance- Hispanics, Caucasians, Blacks, Native Americans, and Asian students. Students are commonly bilingual. For many English is the second language and Spanish the first. A significant majority of AWC students are the first in their families to attend college.