The beautiful, lush environment they call home is both a volatile and unpredictable neighbour. Mount Nyiragongo is an active volcano that can stir at any time and Lake Kivu could potentially release destructive gases into the atmosphere. In 2021, geophysicists sponsored by UNESCO’s International Geoscience Programme set out to study this fascinating environment, in order to assess the myriad threats simmering beneath the surface.
Mount Nyiragongo is an active stratovolcano in the Virunga Mountain range, one of the most consistently active volcanic ranges in the world. So far, its activity has essentially been effusive, which means that eruptions produce lava flows, rather than explosions.
At 3,470 m tall, Mount Nyiragongo towers over the cities of Goma in the Democratic Republic of Congo, which has a population of almost 750,000, and Gisenyi in Rwanda with over 80,000 inhabitants. Goma city centre lies 15 km south of the volcano’s eastern side.
Previous eruptions of Mount Nyiragongo have resulted in disaster. An eruption in 2002 rendered around 120,000 people homeless and caused the tragic deaths of approximately 250 individuals due to the effects of asphyxiation, burns and the explosion of a gas station triggered by lava flowing at high speed on account of its low viscosity.
In May 2021, the volcano erupted once again. Lava flowed as far as the Buheme district on the northern fringes of Goma, destroying more than 3,500 homes, seven schools, the city’s main reservoir and four health centres. An estimated 31 people died. According to official reports from the Ministry of Communications, 232,433 people had to be evacuated from the region but other estimates placed the number of people who had to flee their homes at 450,000. The eruption was followed by intense tremors which damaged infrastructure, buildings and homes. As of August 2022, the population had not received any compensation.
In 2022, project leader Catherine Meriaux submitted her team’s findings to UNESCO on the interactions they had observed between Mount Nyiragongo, Lake Kivu and the health of local communities in the aftermath of the 2021 eruption. Their study suggested, in particular, that magma would likely flow towards the city of Goma and Lake Kivu during the next eruption, based on their analysis of the stresses driving apart the two tectonic plates, combined with stresses related to topography. This is consistent with the orientation and direction of the magma flow during the last two eruptions.
Dr Meriaux is based at the East African Institute for Fundamental Research hosted by the University of Rwanda, an institute which operates under the auspices of UNESCO. Her team includes experts from ISTerre at the French Institute for Research for Development, the University of Antwerp in Belgium and from the Rwanda Environment Management Authority (REMA).
Toxic gasses from the volcano are damaging the population’s health
The 2021 eruption released tons of sulphur dioxide and carbon dioxide, both of which are acidic gases. The emissions of sulphur dioxide have not ceased since. Since May 2002, Mount Nyiragongo’s permanent lava lake has been emitting up to 5,357 tons of sulphur dioxide daily into the atmosphere.
Most of the 43 people surveyed by the project team in August 2022 mentioned that the 2021 eruption had affected their health, the most frequent complaints being eye irritations and coughing. This is not surprising, as volcanic gases cause respiratory illnesses. However, it is the acid rain from the eruption in the form of sulphuric acid that is most harmful for the population because it can severely damage vegetation, reduce crop yields and milk production in cattle and contaminate waterways with heavy metals, as well as with acidic chloride and corrosive fluoride.
The project’s second phase got under way in April 2023. The team plans to survey health providers in the region by the end of this year before examining the health of the population on a broader scale. Whereas short-term health conditions related to a volcanic eruption are well known, the effect on health of the long-term passive degassing of volcanoes such as Mount Nyiragongo is less well understood. Nevertheless, some recent studies suggest that these long-term effects should not be underestimated.
In particular, the volcanic environment seems to be causing a chronic health problem: podoconiosis. This incurable and debilitating disease causes an inflammation of the lower limbs. It is thought to be caused by the long-term exposure of bare feet to volcanic soils, as cases have also been reported in other highland tropical and subtropical countries, but there is no solid scientific evidence yet to support this hypothesis. A previous community-based study found that podoconiosis was prevalent in Western Rwanda.
Other factors could also be responsible for podoconiosis, such as a micro-organism in the soil. That is why the team has now invited the Centre for Biodiversity at the University of Rwanda, which operates under the auspices of UNESCO, and ISTerre to participate in analysing soil samples later this year to gain insights into what drives the disease.
Could Lake Kivu erupt?
Lake Kivu represents a greater potential danger than Mount Nyiragongo. The lake sits on the border of the Democratic Republic of Congo and Rwanda, atop a rift valley that is responsible for the heightened volcanic activity in the area. The lake reaches depths of more than 450 m. It contains 300 km3 (2.6 gigatonnes) of dissolved carbon dioxide, corresponding to about 5% of global annual greenhouse-gas emissions. It also contains 60 km3 of methane mixed with toxic hydrogen sulfide.
The chemical composition of the lake poses the risk of a limnic eruption, also known as a lake overturn. ‘In a limnic eruption’, explains Dr Meriaux, ‘dissolved gasses such as carbon dioxide or methane suddenly separate from the water and rise to the surface from the depths of the lake, forming a gas cloud capable of suffocating wildlife, livestock, crops and humans'.
Limnic eruptions can be caused by differences in temperature – such as in the event of heightened volcanic activity – and by a change in gas concentration in the upper and lower layers of a lake. Methane and carbon dioxide levels in the lake can rise over time, building up pressure as they become saturated until, one day, this pressure becomes too much and spontaneous bubbling starts, followed by a gas eruption. Once the methane reaches the surface, the gas can ignite if there is a sufficient concentration of it, creating a fireball. A catastrophic scenario.
‘Scientists have found evidence to suggest that volcanic eruptions, landslides, earthquakes and cool rainwater can all trigger limnic eruptions’, explains Dr Meriaux, ‘but it is also possible for a lake to become unstable on its own. This happens when bacteria present in the sediments on the lake bottom decompose and ferment organic matter, raising the concentration of gases’.
She recalls how ‘in 1986, Lake Nyos in Cameroon overturned, suffocating over 1,700 people and 3,000 livestock. Lake Kivu is almost double the size of Lake Nyos and contains a thousand times more gas than Lake Nyos’, she says. ‘This means that an overturn of Lake Kivu could potentially do far more damage than an eruption of Mount Nyiragongo. Today the concentration of gases in Lake Kivu is well below the critical saturation level but the risk remains; there just needs to be a trigger’.
Logically, pumping water from the depths of the lake to extract natural gas should reduce the concentration of dissolved methane and, thus, keep pressure levels down. But could pumping also cause a limnic eruption?
In 2016, ContourGlobal, a UK-based energy company, began extracting methane from Lake Kivu under license to the Rwandan government. The US$200 million extraction project is currently in Phase 1, supplying 26 MW of electricity for the local grid through the company’s KivuWatt Power Station. in Phase 2, enough power for about 40,000 Rwandan households. This would almost double Rwanda’s existing grid capacity. According to the national , only 47% of Rwandans were connected to the grid in 2022.
ContourGlobal pumps water from the lake’s depths then separates the dissolved methane from the water and reinjects the lighter, degassed water back into the lake. Over time, the lake water is, thus, becoming less dense.
‘Gasses are dissolved in water at depth’, observes Dr Meriaux. ‘This makes the water denser at depth and, thus, stable. As long as the gas is dissolved, it won’t rise to the surface in gaseous form and lead to a cataclysmic explosion and fireball once the methane is exposed to the air and ignites’. But what if the water should become less stable? ‘That is one question we hope to answer through the second phase of our research project’, says Dr Meriaux.
Linking the dots between the volcano and the lake
Dr Meriaux’s team has worked with REMA to study phytoplankton in Lake Kivu, since phytoplankton seem to be very sensitive to environmental conditions, making them good indicators of change in the mixing layer of the lake.
The research team has noticed a clear change in the population of phytoplankton found in the first 60 m of the lake before and after the May 2021 eruption. Species of phytoplankton that had been abundant prior to the eruption were less so afterwards and species that had been less present before the eruption were more abundant afterwards.
The team plans to undertake further research to determine whether this change is a consequence of the eruption, atmospheric conditions or methane exploitation in Lake Kivu.
The scientists are also keen to know whether the heightened activity of Mount Nyiragongo could trigger a gas explosion in Lake Kivu. Previously, REMA had found evidence of lava having flowed through underground fractures beneath the city of Goma and Lake Kivu during the 2021 eruption. A day later, a tremor seems to have triggered part of a sandbar by the lake to collapse. That guests staying at a lakeshore hotel at the time should have reported seeing what looked like boiling waters offshore suggests that the collapse of this sandbar may have caused a small release of gases in that particular spot on the lake.
During the project’s second phase, the team will be working with REMA again. They will be using meteorological data to determine whether there is a correlation between the data they have collected on phytoplankton and the area’s atmospheric conditions. A freshly trained geophysicist from the East African Institute for Fundamental Research may join the team, as REMA does not currently have a geophysicist.Dr Meriaux’s team also plans to do some modelling. One aim will be to simulate the dispersion of the volcanic gas being released daily by the volcano. The team will also simulate a scenario whereby carbon dioxide and methane are dispersed in the event of a limnic eruption of Lake Kivu.
