Abstract. The main factors affecting the value of ²²²Rn activity concentration in groundwater are the emanation coefficient of reservoir rocks (K_em), the content of parent ²²⁶Ra in these rocks (q), changes in the volume and flow velocity as well as the mixing of various groundwater components in the circulation system. The highest values of ²²²Rn activity concentration are recorded in groundwaters flowing towards an intake through strongly cracked reservoir rocks undergoing weathering processes. Because of these facts, waters with hazardous radon concentration levels, i.e. containing more than 100 Bq dm^{−3 222}Rn, could be characterised in the way that follows. They are classified as radon waters, high-radon waters and extreme-radon waters. They belong to shallow circulation systems (at less than a few dozen metres below ground level) and are contemporary infiltration waters, i.e. their underground flow time ranges from several fortnights to a few decades. Because of this, these are usually poorly mineralised waters (often below 0.2–0.5 g dm⁻³). Their resources are renewable, but also vulnerable to contamination.

Waters of this type are usually drawn from private intakes, supplying water to one or at most a few households. Due to an increased risk of developing lung tumours, radon should be removed from such waters when still in the intake. To achieve this aim, appropriate legislation should be introduced in many countries.