RENNES — Hell, the netherworld or even just “the bad place” — this is hardly good advertising for the underground realm, a space rich in microorganisms and vital to many of our basic needs. Still, despite its importance, the study of what exists deep below the ground is still in the early stages.
But now to advance this effort, a new research site under France‘s National Centre for Scientific Research (CNRS) has opened in Rennes, Brittany: an experimental hall dedicated to environmental sciences, with a particular focus on the microbiology of underground spaces.
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This laboratory boasts ceilings 30 feet high, yet the most fascinating discoveries lie beneath researchers’ feet. This unique European facility features boreholes that plunge more than 300 feet underground, explains Dimitri Lague, director of the Rennes Observatory.
The need to study soils and subsoils has become a global focus, most recently at last month’s COP16 conference on desertification and soil degradation, in Riyadh, Saudi Arabia. According to a United Nations report released the next day, droughts driven by climate change and unsustainable water and soil management could affect 75% of the global population by 2050.
Restoring soils unfit for cultivation — an important climate buffer that absorbs CO2 — could cost more than $310 billion annually, according to the report.
In Rennes, the Environmental Science Observatory focuses on topics such as the effects of soil artificialization on water flow and sediment transport, or the chemical pollution affecting subsoils and water tables, such as pesticide residues and PFAS, so-called “forever chemicals” with harmful effects on human health.
Warming beneath our feet
The scale of the research matches the complexity of the subject, involving 600 researchers and 140 doctoral students across 12 different research units.
Understanding what happens on the surface often requires going underground. CNRS researcher Maria Klepikova studies how human activities, like groundwater pumping, affect subsoil quality.
Deep bacteria are the second largest biomass on the planet after plants.
“Groundwater pumping affects underground temperatures,” she says. Data from sensors on fiber optics in an older borehole near the lab showed a 3 °C rise in underground temperatures, enough to impact bacterial behaviors. It’s easy to think of earthworms as being the life under our feet, but much further down thrive billions of bacteria.
“Deep bacteria are the second largest biomass on the planet after plants,” says Tanguy Le Borgne, physicist at the University of Rennes. Collaborating with biologist colleagues, Le Borgne examines how groundwater movement influences these microorganisms and their role in water quality. Some bacteria can effectively filter water under specific temperatures, or oxygen or iron levels in the soil.
“They play a role very similar to plants in the atmosphere,” says Julien Farasin, a research engineer at the lab.
Carbon storage
The good news is that these bacteria are extraordinarily resilient. They can survive up to 4 kilometers below the surface and some can endure temperatures as high as 120 °C. Some even “eat” rocks and capture CO2, and experiments in Iceland have already shown how bacteria in the soil store CO2 found in basalt rocks.
But bacteria need water to survive, which is why researchers are looking at how agricultural water pumping might disrupt these systems.
“The effects of water pumping on humid zones and on microorganisms is a real question,” Farasin says. “We could go from a system that is capturing carbon, to one that releases it.”
The effects of groundwater pollution on subsoil bacteria are being watched closely. But for the moment, scientists are working the hardest to create “chemical landscapes” of the underground, describing how chemical substances, such as nitrates, antibiotics and other forever chemicals, move through the groundwater.
“This chemical signature, obtained thanks to tracer injections, gives a sort of digital imprint of the chemical substances presented in the soil,” says Joris Heyman, a fluid mechanics specialist with the CNRS and a researcher at the Rennes lab. “It’s like a telescope aimed at the underground.”
A controversial solution
Because the water cycle takes about 30 years on average from the start to the end, the long-term effects of groundwater pollution could take decades to surface. For example, the green algae blooms plaguing Brittany’s coastline are tied to nitrate pollution from years past.
“Our work consists of constructing past events to model for the future,” Lague says — a succinct definition of the lab’s work, and a reminder to be more attentive in how we treat the environment today.
At the new CNRS experimental hall in Rennes, researchers are studying underground CO2 storage. While some industries have adopted the technique — particularly off the coast of Norway — the cost remains steep, at about 4 per ton of carbon. But interest is growing.
Globally, around 20 sites are currently equipped to store CO2.
France’s ministry of industry has put out a call for projects to scale up carbon capture and storage nationwide. Globally, around 20 sites are currently equipped to store CO2. Rennes researchers are working to optimize the process and reduce associated risks, such as pressure imbalances and potential CO2 leaks that could acidify groundwater. Storage efficiency can be more or less optimal depending on the conditions underground.
Despite heavy oversight, the method has critics. Environmental NGOs argue it lets polluters off the hook for reducing emissions. For now, the high cost limits storage to unavoidable industrial emissions, such as those from cement or steel production.
