Study of Abnormal Seasonal Variations in the Radon Exhalation Rate in a Fault Zone

Abstract

Strong seasonal variations in the radon exhalation rate from the soil surface were revealed during a two-year monitoring cycle in a fault zone on a slope of Mount Beshtau in the Caucasus Mineral Water Resort area, Russia. The variations are characterized by very high radon levels in summer and low levels in winter. In the warm season (from late April to early October), the radon exhalation rate values ranged from 4.5 to 23.8 Bq m–2 s–1. During the cold period (from November to March), the radon levels decreased to 0.04–0.17 Bq m–2 s–1. A strong correlation was found between the radon exhalation and atmospheric air temperature (the Pearson correlation coefficient is 0.8). The simulation results show that seasonal fluctuations in the radon exhalation rate are caused by an inversion of the convective radon transport direction in the fractured zones of the rock massif. In summer, the convective flow of radon is directed from the rock massif to atmosphere, whereas this flow in winter comes from the atmosphere to rock massif. The rate of convective transport varies from 5 to 28 m day–1 during the warm season and from –2 to –4 m day–1 during the cold one. Seasonal radon anomalies in fault zones are most probably caused by thermally induced convective air flows in the highly permeable rock zone due to the temperature difference between the rock air and atmospheric air. Seasonal radon anomalies should be taken into account both when mapping radon risk and when using radon as a tracer of gas transport in geochemical and geophysical research.