The initiative, coordinated by Marco Dentz (IDAEA-CSIC), combines cave characterisation, experiments and digital models to understand water flow and contaminant transport in karst systems worldwide
Profile of the Markov Spodmol cave, Slovenia. | Tanguy Racine, Karst
In the current context of climate change, intense storms, droughts and floods are becoming increasingly frequent. These extreme events not only affect the Earth’s surface but also alter underground aquifers, on which millions of people depend for drinking water. However, despite their essential role in the hydrological system, little is still known about how water circulates underground.
The European Karst project, funded by a prestigious Synergy Grant from the European Research Council (ERC) and coordinated by Marco Dentz, from the Institute of Environmental Assessment and Water Research (IDAEA-CSIC), tackles this major challenge: modelling and characterising underground cave systems worldwide in order to predict water flow and contaminant transport. The project began in 2023 and, after three years of work, has already achieved significant progress, including the construction of the world’s largest database of cave networks. With three more years ahead, the research team aims to refine its models to anticipate the movement of water and contaminants and thus assess the real impact of floods or spills.
Understanding underground water and contaminant flow
Karst systems are limestone caves formed by the dissolution of karstic rocks (limestone, gypsum, dolomite), giving rise to highly branched and extensive cave networks. They are estimated to cover around 10% of the Earth’s surface and are therefore fundamental to global hydrology. Due to the properties of limestone, water is not stored but flows rapidly through the cavities. As a result, these systems are highly sensitive to changes in water volume. For example, during heavy rainfall, flooding can easily occur, while during drought periods they may empty completely.
To understand how water flows through these underground systems, classical fluid mechanics laws have traditionally been applied. However, these models do not adequately capture their real complexity.
“A cave is not a smooth, perfectly symmetrical pipe. Its walls are rough, fractured and contain cavities where water forms eddies or can be temporarily stored,” explains Marco Dentz, researcher at IDAEA and coordinator of Karst.
The first step of the project was therefore to understand what happens inside a karst cave conduit. To study these complex geometries, the team carried out LIDAR (laser-based) scans of 16 caves across Europe to obtain highly detailed 3D digital models of cave interiors.
Plan view of a LiDAR point cloud obtained in the Markov Spodmol cave, Slovenia, coloured by elevation. | Tanguy Racine
From these scans, researchers developed numerical simulations of water flow and transport. They also created physical replicas using 3D printing, some more than two metres long, faithfully reproducing the real cave structure. These replicas are being used to perform laboratory-scale flow and transport experiments, where water circulation is observed under controlled conditions and results are compared with mathematical simulations. The aim is to establish the physical laws governing the movement of water and solutes, dissolved substances such as minerals or contaminants, in real karst conduits.
The world’s largest cave database
One of the major achievements of the Karst project so far has been the compilation of information on karst systems worldwide, in collaboration with caving clubs and explorers, to characterise their topology: how cave conduits connect, their degree of branching and their dimensions.
This effort is building the most comprehensive global database of karst cave networks. It enables the identification of common patterns using attributes such as diameter, structure or linearity to classify network types and generate synthetic models with realistic properties.
To date, the database includes 172 cave systems, 15 of them in Spain. They are classified into four main morphological categories: branched caves, labyrinthine caves, anastomotic caves (where conduits repeatedly split and rejoin) and sponge-like caves. Morphology depends on geological context, the type of rock, whether the cave is coastal or mountainous, and whether it is hypogenic (formed by water rising from depth) or epigenic (formed by water infiltrating from the surface). Structural analysis helps decipher the origin of cave systems and facilitates numerical simulation and, therefore, prediction of water flow through caves.
Thanks to deep learning models, the research team is also reconstructing unknown sections of cave networks that are inaccessible to humans.
Cave passage with an elliptical profile in the Hölloch cave system, Switzerland. | Tanguy Racine
Assessing and tracing pollution
The high speed at which water flows through karst systems makes these aquifers particularly vulnerable. During heavy rainfall events, they can quickly become saturated and cause flooding. Likewise, if a contaminant spill occurs, it can travel within hours or days to a drinking water well.
One of the best-known cases is the Walkerton Tragedy, which occurred in 2000 in Canada. After heavy rainfall, the bacterium Escherichia coli from agricultural manure contaminated several drinking water wells. Because the protection system underestimated how quickly the contaminant could move through the karst aquifer, more than 2,300 people fell ill and seven died.
Avoiding similar situations requires precise understanding of underground water flow. In this regard, the Karst project is also developing forensic hydrogeology tools, a discipline that uses numerical models of subsurface flow to reconstruct the origin, pathway and impact of a contaminant. A well-known example is the real case portrayed in the film ‘Erin Brockovich’, in which it was demonstrated how highly toxic hexavalent chromium migrated from an industrial plant to groundwater wells supplying the town of Hinkley, California, causing cancer, reproductive disorders and other serious illnesses among residents.
The Karst project shows how fundamental research in fluid physics and mathematical modelling can become a key and practical tool to address water resource challenges in the current climate change scenario.
Alicia S. Arroyo
Communication and Outreach | IDAEA
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