Limnic eruption

A limnic eruption (also called "explosive lake phenomenon") is a rare natural disaster, in which carbon dioxide erupts suddenly from the depths of a lake, suffocating wildlife, livestock and humans. Such an eruption can also cause tsunamis in the lake as CO2 rises to the surface, displacing water. Scientists believe that landslides, volcanic activity or certain explosions can trigger such an eruption. Some characteristics of limnic activity in lakes are:

• high CO concentrations₂ in the water;
• a cold lacustre background, indicating absence of direct volcanic interaction with the waters;
• layers of stratum with different levels of CO saturation₂;
• close to areas of volcanic activity.

Scientists have recently determined that limnic eruptions and volcanic eruptions, although indirectly related, are actually different types of natural disasters.

Cases in history

To date, this phenomenon has only been observed twice. The first time was in Lake Monoun in Cameroon in 1984, which caused the suffocation and death of 37 people living around the lake. The second case was a deadly eruption that occurred in neighboring Lake Nyos in 1986, which released more than 80 million cubic meters of CO2 and killed almost 1,800 people, also due to asphyxiation.

Due to the nature of the event, it is difficult to determine whether limnic eruptions have happened elsewhere. However, a third lake, Lake Kivu shared by Rwanda and the Democratic Republic of the Congo, contains large amounts of dissolved CO₂. Professor Robert Hecky of the University of Michigan took sample sediments from the Kivu, which showed that an event causes the extinction of living beings in the lake every approximately one thousand years, and causes nearby vegetation to sink to the bottom. lacustrine.

In 2016, Spanish scientific studies confirmed the existence of an artificial lake in Puebla de Guzmán (Huelva), which could contain 80,000 m³ of CO₂, a gas formed by the acidic waters produced by the ancient mining activity in the area, in contact with soil carbonates.

Causes

For a limnic eruption to occur, a lake must be saturated with a gas. In the two known cases, the largest component was CO₂, which came from volcanic gas emitted under the lake. Before a lake is saturated, it behaves like a soft drink, as the CO₂ dissolves in the water. In both soda and lake, gas dissolves much faster when it is under higher pressure. This is why bubbles in a bottle form only after the soda is opened; The pressure is released and the carbonate leaves solution. In the case of lakes, their bottom has a much higher pressure (the greater the depth, the greater the pressure). This means that enormous amounts of CO₂ can be dissolved in large, deep lakes. Additionally, CO₂ dissolves faster in cold water, such as that at the bottom of a lake. However, a difference of a few degrees of temperature is not enough to dissolve such quantities of gas; the role of pressure is more decisive for this.

Once the lake is saturated with CO₂, it reaches the critical, unstable point, after which a trigger is all that is needed for the eruption to break out. In the 1986 Lake Nyos case, landslides were the supposed trigger, but a volcanic eruption, earthquake, or even a storm may be the necessary trigger. In any case, the trigger causes part of the saturated water to rise to the surface of the lake, where the pressure is insufficient to keep the CO₂ in solution. Bubbles then begin to form, and the water increases its buoyancy in the lake, releasing even more CO₂ from the solution. This process forms a column of gas. At this point, the water at the bottom of this column rises by suction, losing its CO₂. This eruption pours CO₂ into the air, and displaces water to form a tsunami.

The following are reasons why this type of rash is so rare. First, there has to be a source of CO₂. Also, the temperature of the lake must be stable for the CO₂ to concentrate until saturation; If there is a variation in temperature between winter and summer, as in the Great Lakes region (North America), the differences generate currents that mix water from the bottom and surface of the lake, so the CO₂ rises to the surface and is released little by little. A lake must also be deep enough to have enough pressure to maintain a high amount of CO₂ in the solution of the deep strata. That is, only deep, stable, tropical and volcanic lakes, such as Lake Nyos, are prone to limnic eruptions.

Consequences

Once the eruption occurs, a large cloud of CO₂ forms over the lake and expands to the surrounding areas. Because CO₂ is denser than air, it tends to sink to the ground as it pushes breathable air up. As a result, living things that need oxygen suffocate once the CO₂ cloud reaches them. CO₂ can acidify human fluids, causing poisoning. As victims struggle to breathe, they end up causing more damage to themselves through greater inhalation of CO₂.

At Lake Nyos, the gas cloud descended from the lake to a nearby village where it settled, killing almost all of its inhabitants. In this eruption, some people died more than 25 km away from the lake. A change in the skin color of some corpses led scientists to think that the gas cloud might have contained a dissolved acid, such as hydrogen chloride. Thousands of wild animals and cattle also died from suffocation, but they were not accurately counted. On the other hand, the vegetation was not largely affected, except for that immediately adjacent to the lake. There the vegetation was devastated by a 5 meter high tsunami caused by the violent eruption.

Possible solution: Degas the lakes

Several efforts have aimed to find a solution to remove the gas from these lakes and prevent an explosion that could lead to another catastrophe. A team of French scientists began experimenting in Lake Monoun and Nyos in 1990, using siphons to degas the waters in a controlled manner. A tube is positioned vertically in the lake with its mouth above the surface of the water. The water saturated with CO₂ enters through the bottom of the tube and rises through it. The lower pressure at the surface allows the gas to exit the solution, forming only bubbles. The degassed water acts as a pump, drawing more water from the bottom of the tube, so the flow continues to sustain itself. This is the same process that leads to a natural eruption, but in this case it is controlled by the diameter of the tube.

Each tube has a limited pumping capacity, so several are required to degas a significant fraction of the lake's deep water and keep the lake safe. These waters are slightly acidic due to dissolved CO₂, which causes corrosion in the tubes and constant electronic maintenance. There is also a fear that the CO₂ from the tubes could remain on the surface of the lake, forming a thin layer of unbreathable air, causing problems for wildlife.

In January 2001, a single tube was installed in Lake Nyos. A second tube was installed in Lake Monoun in late 2002. These two tubes are believed to be sufficient to prevent an increase in CO2 levels by removing approximately the same amount of gas that would naturally enter. to the lake bed.

Potential danger of Lake Kivu

Lake Kivu is not only 2,000 times larger than Nyos, but it is also located in a more densely populated area, with more than 2 million people living on its banks. Fortunately, it has not reached high CO₂ saturation yet. If the water were to become more saturated, human and animal life would be put at serious risk, since it is located near a potential trigger: Mount Nyiragongo, an active volcano that erupted in January 2002. It is also in an area of constant seismic activity.

Degassing of the lake, similar to Lake Monoun and Nyos, has not yet been implemented, because the size of the lake and the volume of the gas mean very expensive operating costs, in the millions of dollars. No plan has been initiated to reduce the potential danger to Lake Kivu.

Since May 17, 2016, the KivuWatt power plant has been operating, which produces electricity by extracting methane gas from the waters of Lake Kivu, becoming one of the most effective, profitable and advanced methods to prevent a future limnic eruption. This is located in Kibuye, in the Karongi district, Western Province, 130 km from the capital of Rwanda, Kigali. It was built on an inland platform, where methane gas is extracted by pumping gas-saturated water 350 meters deep.. As the water and gas rise through the tubes, they separate due to the change in pressure. The methane is then sent through a pipeline to a second facility on shore, where it is transformed into electricity. The CO2 is reinjected into the lake, at a depth precisely calculated so as not to destabilize it.

This plant is projected to generate about 25MW in the first stage, approximately 30% of the country's total consumption. It is projected that in the future the production capacity could be expanded to about 100 MW.