Radon

Radon can move from the minerals where it is formed towards the pores and fractures of rocks. That is why measurements of its concentration can provide information about the circulation of gases and fluids in the subsurface. Researchers study these variations to understand the permeability of geological materials and changes in subterranean transport. However, the signal also depends on humidity, pressure, temperature and other factors, so it does not allow identifying a single fracture in isolation.

Radon has several radioactive isotopes, but their half-lives and decay chains are different. Radon-222, belonging to the uranium-238 series, has a half-life of about 3.8 days. Other isotopes, such as radon-220, known as thoron, last much less. These differences determine how far they can move before decaying and in what contexts they can be detected. Therefore, identifying the isotope is important when interpreting geological and environmental measurements.

Radon present in the ground can move through pores, cracks and spaces between building materials. Pressure differences between the ground and the inside of a building can favour its entry through joints, conduits or cracks in foundations. The ease of movement depends on the permeability of the ground, construction characteristics and ventilation. This process explains why two nearby buildings can have very different concentrations, even when built on the same type of ground.

Radon can accumulate in groundwater when it comes into contact with minerals and materials that release it. When that water reaches rivers, lakes or coastal areas, the radon concentration can change and serve as an indication of groundwater inputs. Researchers use these variations to study water exchanges that are not always visible on the surface. The method requires comparing the concentrations with other indicators and considering radioactive decay, mixing and gas loss to the atmosphere.

Radon can be used as a natural tracer to investigate gas exchange between soil, water and atmosphere. Its concentration and distribution help to study processes such as soil ventilation, groundwater circulation and certain exchanges between oceans and air. As it is radioactive, its temporal changes also provide information about the transport and renewal of water or air masses. Interpretation requires considering environmental conditions and the decay of the isotope itself.

Radon cannot be identified by the naked eye, so it is measured by instruments that detect its radioactivity or that of its decay products. Some devices record alpha particles, while others use electronic detectors or air sample analysis methods. There are equipment for continuous measurements and others designed to obtain averages over long periods. The choice depends on the purpose of the study, the necessary sensitivity and sampling conditions. Results must be interpreted by knowing the limitations of each technique.

Radon is part of radioactive decay chains that include uranium and thorium isotopes. Its mobility and short half-life can provide information about recent geological processes, although it is not usually the main clock for determining the age of ancient rocks. In specialised research, the imbalances between isotopes in a radioactive chain help study the transport of elements and the evolution of natural systems. Interpretation requires knowing the production, migration and decay rates of the radionuclides involved.