Time-dependent variations of groundwater radon: Insights from a twelve-year study in the Baikal region, East Siberia, Russia

Time-dependent variations of ²²²Rn concentration (Q) in groundwater have been monitored for twelve years (2012–2023) at eight sites of groundwater discharge within the Baikal region in East Siberia, Russia. The concentrations of radioactive gas at different sites vary from 30% to 60% of average values (Q_av). The sampled waters are of three groups with Q_av ≈ 15 Bq/l (I), ≈30 Bq/l (II₁), and ≈50 Bq/l (II₂). Cluster analysis shows closest linkage between the two subgroups of group II due to similarity in the discharge mechanisms. Fourier analysis of periodic ²²²Rn behavior reveals major cycles of 365, 180, 126, and 30 days correlated with variations of air temperature and pressure, as well as with the patterns of groundwater discharge. In addition, radon anomalies are related with seismicity. Earthquakes are reflected in the radon field as three distinct anomaly types, occurring either subsequent to or prior to the seismic event. The anomalies responding to earthquake nucleation can be considered as precursors and used in earthquake prediction. The revealed trends make basis for a model designed to predict Q variations in groundwater of the area to 80% average efficiency. The external and internal factors that affect the concentration of radioactive gas in groundwater are linked in a hierarchic system and are classified according to the degree and type of their influence.