The results of long-term simultaneous measurements of radon exhalation rate, radon concentrations in soil gas and groundwater in the fault zone

Highlights: • Radon exhalation rate from soil surface, as well as radon concentrations in soil gas and ground water are measured in a fault zone at the Mount Beshtau (North Caucasus region). • The strong seasonal variations of radon exhalation and soil gas radon concentration with abnormal high values in summer and extremely low in winter are observed. While the groundwater radon concentrations varied during the year in a relatively small range without seasonal fluctuations. • Clear correlation between radon soil levels and air temperature were revealed. • Seasonal variations of the radon levels in fault zone are caused by the inversion of the direction of convective air flow in the fractures. The regular monthly radon measurements were carried out in the fault zone on the Western slope of the Beshtau magmatic massif (North Caucasus). The radon exhalation rate from the soil surface, as well as radon concentrations in soil gas at a depth of 0.5 m and in groundwater discharged at a spring located nearby have simultaneously been measured. High seasonal fluctuations in radon exhalation and radon concentration in soil gas, characterized by highs in summer and lows in winter, were registered. In summer, the radon exhalation reached 23.8 Bq m⁻²s⁻¹, and the radon concentration in the soil gas reached 166 kBq m⁻³. In winter, both the radon exhalation and the radon concentration in the soil dropped to 0.025 Bq m⁻²s⁻¹ and −3, respectively. The concentration of radon in ground water varied over the year in a relatively narrow range (100–210 Bq l⁻¹), and there were no seasonal fluctuations. A sharp increase in soil radon and radon exhalation in spring and a fall in autumn are timed to the moments when the temperature of the atmospheric air becomes, respectively, higher and lower than the temperature of the rock massif. Both the soil radon concentration and the radon exhalation show a close correlation with the temperature of atmospheric air, but in the first case the relationship is linear, and in the second – exponential. The obtained data confirm the assumption that the seasonal radon variations are caused by atmospheric air circulation in the shallow area of the fault due to the temperature difference between the atmosphere and the rock massif.