Radon in the soil

During the radioactive decay of uranium-238 in the earth, radium is formed, which in turn decays to radon. Part of the radon is released into the pores of the soil and rocks. The more uranium the soil contains, the more radon is present.

Radon in the soil

Together with other soil gases, radon gets from the soil to the earth's surface through flows and diffusion and is released into the atmosphere.

Weather conditions influence radon concentration in the soil

Up to a depth of under one metre, the radon concentration in the soil varies considerably, depending on the weather conditions:

• For example, as a result of rain, snow, or frost, the pores of the soil and rocks are filled with more water or, respectively, freeze. Thereby, it is more difficult for radon-containing air to escape from the soil. The air remains there, and as a result, the radon concentration in the top soil layers increases.
• The radon concentration in the soil also increases with rising air pressure: The atmospheric pressure additionally presses air from the atmosphere into the pores of soils and rocks and, as a result, it is more difficult for the radon-containing air to leave the soil and it remains there. When air pressure falls, a greater amount of radon is released.

Only in deeper soil layers is the radon concentration stable. The more permeable the soil is to gas, the greater is the influence of weather conditions – and the deeper a stable radon concentration can be found.

Radium – during whose decay radon is formed in the soil – has a long half-life of approximately 1,600 years. On account of this long half-life, the radon concentration in the soil air is stable, even in the long run. If the radon concentration at a site is known, new measurements are only reasonable when greater interventions in the underground have taken place.

Soil conditions influence the propagation of radon

The transport of radon from the depths to the surface is determined by the gas permeability of the soil and locally occurring flow paths. The more cracks and gaps exist in the underground, the more easily radon propagates. At some places, the radon concentration in the soil air can be clearly above the levels that are typical of the region – for example

• at fissures:

  Fissures are geological faults in the soil that provide pathways for water. Radon dissolved in water, which is formed during the decay of uranium, can deposit at the edges of fissures, where it releases radon during its radioactive decay.

• at subsidences:

  At subsidences, the rock is generally loosened and thus more permeable for radon-containing soil air.

• at the boundary between two types of rock:
  At the boundary of two different types of rock, more uranium may have been deposited than at other places. During its decay, radon is formed.

If required, developers and architects can get information e.g. about the radon situation at a building site through a subsoil expertise.

Groundwater transports radon

Radon can also dissolve in the groundwater, with which it is then transported in the geological underground.

Where does radon occur in the soil in Germany?

In Germany, the concentration of radon in the soil varies, since regionally, different amounts of uranium and radium-226, during whose decay radon is formed, occur in Germany. The same applies to the permeability of the soil.

The Federal Office for Radiation Protection (BfS) has created maps depicting the regional distribution of radon in the soil. No information about single buildings or plots can be derived from the forecast maps.