Researchers from Brazil’s Nuclear Technology Development Center have developed a method to track groundwater movement and identify discharge points into surface water bodies using radon and thoron as natural tracers. The technology could help more quickly locate potential contamination pathways from former uranium mines and other mining operations.
After uranium mines are closed, environmental problems do not disappear. Waste rock dumps continue to interact with rainwater and air, generating acid mine drainage. This acidic water can leach uranium, heavy metals, and other contaminants from the rock, which may then enter surface water bodies. Detecting groundwater discharge points at the surface, however, is extremely challenging. Traditional approaches use artificial tracers, such as salt solutions, which are injected into wells and then tracked. Such studies are time-consuming and do not always yield reliable results in complex geological conditions.
The Brazilian scientists proposed using naturally occurring radioactive gases, specifically radon-222 and thoron (radon-220), for this purpose. As water moves underground, it becomes enriched with these gases through contact with rock formations. When a groundwater flow discharges into a stream, radon concentrations in the water spike sharply. By measuring these concentrations along the watercourse, groundwater inflow zones can be identified without drilling additional wells or using artificial tracers.
The researchers tested the method at a former uranium mine site in Minas Gerais state. Radon concentrations in groundwater were found to be roughly a thousand times higher than in surface water. This allowed them to detect two locations where groundwater was discharging into a stream adjacent to the waste dump.
Using two isotopes simultaneously made the observations more precise. Radon-222 has a half-life of nearly four days, making it suitable for tracking groundwater movement over relatively long distances. Thoron, by contrast, decays in just 56 seconds and pinpoints the actual discharge point at the surface. The combined use of the two natural tracers enabled the researchers to distinguish between local groundwater sources and more extensive flow systems.
In addition, the scientists estimated groundwater velocity using the “radon deficit” method, which compares radon content in fresh groundwater with that in water that has remained in a monitoring well for some time. Compared with conventional hydrogeological tests, surveying a single well took about one hour instead of the usual 6–8 hours.
The study also showed that such monitoring is best conducted across different seasons. During the dry period, groundwater inflows were clearly identifiable by radon concentration, whereas during the rainy season, concentrations dropped by nearly a factor of eight due to dilution by surface water, making discharge points less conspicuous.
In the researchers’ view, the proposed approach can be applied not only to former uranium mines but also to other mining sites. It helps identify groundwater flow paths and locate points through which contaminants can enter surface water bodies. This would enable more effective environmental monitoring and site remediation.
