High seasonal variations of the radon exhalation from soil surface in the fault zones (Baikal and North Caucasus regions)

Abstract

The seasonal variations of radon exhalation rate from soil surface were studied in two seismically active regions of the Russian Federation - the Baikal rift and the North Caucasus. In each region, monthly measurements of the radon exhalation have been carried out at two relatively proximal sites, one of which was located within the active fault zone and the other outside of the fault zone. The Open Charcoal Chamber Method was used. Very high radon exhalation rate values were found in the fault zones at both regions. At the Baikal rift, the radon exhalation reached 1.4 Bq m^-2 s^-1, and at the Caucasian region in some periods it even achieved 24 Bq m^-2 s^-1, which is an extremely high value. The same pattern of seasonal variations of radon levels with abnormal high radon exhalation rate values in summer and extremely low in winter were observed in both the Baikal and Caucasus regions. Clear correlation between radon exhalation and air temperature were also revealed. The obtained data and simulation results indicate that seasonal fluctuations in the radon exhalation rate are caused by the inversion of the direction of convective air flow in the fractured zones of the rock massif. In summer, the convective air flow is directed from the rock massif to the atmosphere and in winter, vice versa, from the atmosphere to the rock massif. This phenomenon is similar to the well-known "chimney effect", i. e. in winter there is a direct draft in the system of fractures, and in summer - the reverse one. Thus, the detected radon anomalies are due to near-surface convective air circulation in permeable zones of the mountain ranges and most probably are not associated with deep crustal or mantle degassing. Seasonal thermally induced radon anomalies should be taken into account both in the radon risk mapping and in the application of radon as a tracer of natural processes in various fields of geology and geophysics.

Keywords: Faults; Radon anomaly; Radon exhalation rate; Soil radon measuring; Thermally induced convection.