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Bukavu is equally in danger
- Although not threatened by volcanoes as Goma is, Bukavu is equally in danger from a potential limnic eruption from Lake Kivu, in which vast quantities of dissolved carbon dioxide and methane could explode from the lake and threaten the lives of the 2 million people who live near the lake.
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Sep 23, 2021 · The picturesque lake, nestled between the DRC and Rwanda, has the potential to explosively release these gases in a rare phenomenon known as a limnic eruption.
Oct 16, 2020 · The high levels of carbon dioxide and methane in the deep waters of Lake Kivu could cause a devastating limnic eruption if disturbed (Credit: Richard Gray) But Lake Kivu is more than...
- Overview
- How Lake Kivu became a ticking time bomb
- Inside Rwanda’s efforts to defuse Lake Kivu—and fuel the country
- How current degassing methods may increase the risk of an eruption
- Balancing safety and scaling
The bottom of Lake Kivu, one of the African Great Lakes, contains combustible carbon dioxide and methane that threatens countless communities.
Lake Kivu is framed by imposing cliffs, nestled within a verdant valley straddling Rwanda and the Democratic Republic of Congo. On the lake, fishermen float out in small boats, singing to time their paddle strokes as they catch the day’s meal.
Under the surface, that tranquility vanishes.
Lake Kivu is a geological anomaly, a multi-layered lake whose depths are saturated with trapped carbon dioxide and methane. Only two other such lakes—Lake Nyos and Lake Monoun—share these characteristics, and both have erupted in the past 50 years, spewing a lethal cloud of gas that suffocated any humans and animals in its path. When Lake Nyos erupted in 1986, it asphyxiated nearly 2,000 people and wiped out four villages in Cameroon. Folklore in the area speaks of “the bad lake” and its evil spirits that emerged to kill in an instant. Concerningly, Lake Kivu is 50 times as long as Lake Nyos and more than twice as deep. Millions live on its shoreline.
Lake Kivu sits along the East African Rift Valley, dotted with hot springs that feed carbon dioxide and methane into its depths.
“Kivu has a complicated vertical structure,” Sergei Katsev, a limnologist at University of Minnesota Duluth, explains. While “the top [200 feet] or so mix regularly,” the rest of the lake remains stratified. Nearly 72 cubic miles of dissolved carbon dioxide and 14 cubic miles of methane, laced with toxic hydrogen sulfide, remain trapped in the bottom of the lake. They sit beneath a “main density gradient” at 850 feet below the surface.
These gasses could explode above the surface. “When the lake reaches 100 percent saturation—and it is currently somewhere over 60 percent—it will erupt spontaneously,” says Philip Morkel, an engineer and founder of Hydragas Energy, which is seeking funding for a project to extract methane from the lake for electricity. “It’s like a boiling pot of water. It looks quiet—until it starts to bubble.”
The lake could also erupt if its layers are sufficiently disturbed, for instance by “an earthquake or a large lava intrusion,” Katsev says. Beyond the rift zone directly beneath the lake, there are two active volcanoes within 15 miles.
Lake Kivu’s eruption would be catastrophic. “[The lake] would release the equivalent of 2-6 gigatonnes of carbon into the atmosphere in a day,” Morkel says. For reference, current global carbon dioxide emissions are approximately 38 gigatonnes each year, in total. “That erupted gas would hang over the lake in a foggy cloud for days to weeks.”
For those around the lake at the time of eruption, this would be fatal, according to Morkel: “The gas would be extremely toxic. If anyone were in that cloud, it would take a minute to kill them.”
Faced with this potential catastrophe, the Rwandan government authorized KivuWatt to extract methane from the lake and convert it into energy.
“It works relatively simply,” Martin Schmid, a researcher at the Swiss Institute for Water and Environmental Research, says. “You take up the water from a certain depth” below the gradient barrier, where the gas-rich water sits, and “at the surface, you separate the water” from the CO2 and methane. “Then, the degassed water is put back into the lake.”
However, some experts believe that current efforts to remove gas from the lake may trigger an eruption—and a local extinction event.
“It’s a compromise of safety versus commercial exploitation in the long term,” says Katsev. “If you return the water deep in the lake, you dilute your resource zone for future years. However, if you dump it higher up,” as KivuWatt is currently doing, “the water generates a plume as it sinks downward through the density layer, causing the water to mix vertically. The risk of limnic eruption is linked to this vertical movement.”
However, Schmid believes the current method is safe: “We know the degassing process changes the stratification of the lake. This was predicted. We don’t think this is really a problem—but predictions are never completely correct.”
“From my point of view both methods are feasible. It’s a political decision, whether you want to take a higher risk, to extract more of the methane from the lake,” continues Schmid. The effort “is relatively small scale, so it doesn’t really affect the lake significantly.”
At current rates, “it would take centuries to remove a large fraction of the gas from the lake,” Schmid says. However, to reach that goal more quickly, KivuWatt is planning to scale up; "the next phase of the project," their website says, “will create a total capacity of over 100 MW.”
But Morkel warns that increasing capacity would also elevate the risk: “What these projects do at this scale is not too damaging, but if any more were added…they could reach the point of irreversible damage.”
Schmid agrees that before “upscaling to larger amounts of extraction, there needs to be an agreement on which method to use.”
He explains that, because each method disrupts the layers of the lake in different ways, the two could disturb each other if both are conducted at the same time. And, Katsev adds, “you cannot easily adjust where the pipe goes, because it is placed at a fixed distance from the surface.” This means that once capacity is built using a certain method, it may be costly or difficult to change in the future.
Oct 14, 2020 · Scientists have found what may be evidence of at least one previous limnic eruption at Kivu that likely occurred between 3,500 and 5,000 years ago, and possibly several more recent ones.
May 29, 2021 · Scientists say underground magma has been flowing toward Lake Kivu since the deadly eruption of the nearby Mount Nyiragongo volcano on Saturday. An eruption from under the lake could trigger the...
Scientists have found what may be evidence of at least one previous limnic eruption at Kivu that likely occurred between 3,500 and 5,000 years ago, and possibly several more recent ones. Sediment cores taken from the bottom of the lake have revealed features known as brown layers that are unlike the surrounding sediments.
Lake Kivu is a fresh water lake and, along with Cameroonian Lake Nyos and Lake Monoun, is one of three that are known to undergo limnic eruptions (where overturn of deepwater stratified layers releases dissolved carbon dioxide).
