A radioactive gas rising from mountain springs in the Czech Republic has helped scientists reveal how groundwater moves through the hidden fracture networks of crystalline rock. A new study, based on 16 years of monitoring and underground tracer experiments at the St. Vojtěch Springs in the Giant Mountains, presents a mathematical model that links radon concentration to groundwater speed, fracture geometry and the distribution of uranium-bearing minerals. The approach could transform the way researchers identify concealed flow paths in hard-rock aquifers—systems that supply water in many mountainous and rural regions but are notoriously difficult to map. It may also improve assessments of radon exposure, groundwater contamination and the movement of fluids through fractured bedrock.
The focus of the research is radon-222, or ²²²Rn, a naturally occurring radioactive gas generated in the decay chain of uranium-238. Uranium ultimately produces radium-226, whose decay releases radon through a process known as alpha recoil. Because radon is chemically inert and has a half-life of approximately 3.8 days, it can enter groundwater from mineral surfaces and then travel with the moving water before decaying. Its short lifetime makes it unusually sensitive to changes in underground conditions. A sudden drop in radon concentration can indicate that groundwater has left a uranium-rich fracture zone, mixed with another water source or spent enough time in transit for radioactive decay to take effect. In this sense, radon acts as a natural stopwatch for subterranean water.
The gas does not emerge equally from every part of a rock. In crystalline bedrock, such as granite and orthogneiss, the bulk material is usually too impermeable for water to circulate freely. Instead, groundwater is concentrated in fractures, faults and narrow zones of crushed or altered rock. Radium and uranium located close to fracture surfaces are especially important because radon atoms produced within micrometres of a mineral–water interface have a greater chance of escaping into the water. The fraction of generated radon that becomes mobile is called the emanation coefficient. It depends on mineral composition, grain size, porosity, fracture surfaces and the degree to which minerals have been altered. Once in the water, radon is transported by advection, the bulk movement of groundwater, as well as molecular diffusion and hydrodynamic dispersion. Fast flow through wide fractures is dominated by advection, while slow and tortuous flow allows diffusion and mixing to become more influential.
St. Vojtěch Springs provide an unusually revealing natural laboratory because uranium-bearing material lies directly within the spring system. The springs sit between 954 and 981 metres above sea level in Horní Malá Úpa, in the northern Czech Republic’s Giant Mountains. The local bedrock includes metamorphosed rocks, quartzite, metabasite and carbonate bodies, together with orthogneiss formed when ancient granitoids were transformed during the Variscan mountain-building episode. A major tectonic zone developed near the contact between orthogneiss and dolomite, creating fractures that could conduct groundwater. The geological boundary also hosted uranium mineralization. A low-permeability clay-filled fault acts as a barrier, forcing water around it and helping determine where the springs emerge. Eleven individual springs have been identified, with radon activities ranging from 185 to 5,469 becquerels per litre and flow rates that differ substantially from one spring to another.
The site’s radioactive history was first recognized in 1951, when investigators from the Jáchymov Mines Exploration Group identified a cluster of springs with elevated radon activity. Exploration trenches and a 25-metre shaft were later excavated, following uranium mineralization to a depth of about six metres. Historical records indicate that approximately 15.9 kilograms of uranium were recovered at an average grade of 0.15 per cent, although much of the original geological documentation has disappeared. The area was rediscovered in 2008 during a systematic survey of radon-rich springs in the Eastern Giant Mountains. Subsequent work combined geological mapping, geophysical surveys, gamma spectrometry, emanometry and shallow borehole logging. The spring water itself is only weakly mineralized, with total dissolved solids below 100 milligrams per litre, and its calcium–sodium–bicarbonate chemistry is typical of shallow mountain groundwater. Temperatures between 5.2 and 9.6 degrees Celsius show that the water circulates relatively near the surface.
To determine how quickly the groundwater travels, the researchers injected tracers into the subsurface and tracked their arrival at the springs. They then compared those flow-path results with long-term measurements of discharge and radon activity. At one spring, the average groundwater velocity was approximately 0.44 metres per hour. The observations were fitted with an exponential model describing the progressive accumulation of radon as water moves through an “activation zone”—a stretch of fracture containing parent radionuclides. Incoming rainwater generally contains little radon. As it infiltrates, it contacts fracture walls enriched in radium-226 and begins accumulating newly generated radon. The activity rises toward a saturated value determined by the balance between radon production and radioactive decay. At the calibrated spring, the saturated activity was estimated at 5,160 becquerels per litre, while the activation zone extended for about 44 metres.
The model explains why radon concentrations can rise or fall as the flow rate changes. When groundwater moves rapidly, it spends less time in contact with radioactive fracture surfaces, so it may leave the activation zone before reaching the maximum possible activity. Slower water has more time to accumulate radon and may approach saturation. Yet the relationship is not simply a matter of contact time. Higher discharge can also change which fractures carry the water, alter mixing between shallow and deeper flow components and modify the effective volume through which the water travels. The model therefore treats radon concentration as the outcome of transport and decay rather than as a direct measure of uranium abundance. In mathematical terms, radon approaches saturation exponentially along the activation path, while its activity decreases after the water enters a “decay zone” lacking a continuing supply of parent isotopes.
Rock analyses help explain the unusually strong radon signal. Mineralized samples contained uranium concentrations from 102 to 1,151 parts per million, with an average of about 760 parts per million in five samples. Their radon emanation coefficients ranged from 25 to 43 per cent, averaging 33.7 per cent—meaning that a substantial fraction of the radon produced within the accessible mineral grains could escape into surrounding water or air. The principal uranium-bearing phase was identified as a hydrated uranium titanate known as uranium leucoxene, distributed through the rock and along fracture walls. It formed during alteration of dark mica, alongside hematite and anatase. The measurements also revealed strong disequilibrium between uranium and radium in some materials, evidence that groundwater and hydrothermal fluids have leached uranium or concentrated radium in particular zones. Such disequilibrium matters because radium, rather than the total uranium content alone, controls the immediate supply of radon.
By extending the equations to include activation and decay zones of different lengths, the researchers created a tool that can estimate hidden properties of fractured aquifers from routine monitoring. Where tracer tests are available, repeated measurements of flow, radon activity and travel time can be used to estimate the physical lengths of both zones. Where tracer experiments are impractical, the same framework can instead estimate effective pore volumes—the amount of connected space available to flowing water—although this does not necessarily reveal the actual geometric distance travelled. The distinction is important in crystalline rock, where a few connected fractures may carry most of the water while the surrounding rock contributes little to flow. The authors argue that the model is most robust when radon–flow relationships are observed across a wide range of hydrological conditions, including rainfall-driven changes.
The findings do not mean that every radon-rich spring poses the same health risk, nor that radon concentration alone can identify the complete structure of an aquifer. Radon in groundwater can escape into indoor air when water is used for bathing, washing or other household activities, and inhalation is the principal route by which radon is associated with lung-cancer risk. However, the study’s central contribution is geological: it shows that a radioactive decay product can serve as a high-resolution tracer of water movement through otherwise inaccessible rock. The St. Vojtěch record demonstrates the value of combining long-term observations with targeted tracer tests, especially in fractured environments where conventional groundwater models often oversimplify flow. By turning changes in radon activity into estimates of underground travel distance and storage, the new model offers a way to make the hidden architecture of hard-rock aquifers visible—one radioactive atom at a time.
