Radon concentration in groundwater sources of the Baikal region (East Siberia, Russia)

In this study, we conduct a systematic study of groundwater radioactivity in the territories around Lake Baikal in Siberia, including Pribaikalie and Transbaikalia. The database includes more than 200 measurements of Rn-222 from monitored water sources in the study region. We classify the groundwater sources in the Baikal region into seven groups (I-VII) that differ in radon concentration in space and time. The spatial distribution of water sources belonging to certain groups depend on geochemical and structural factors. In Transbaikalia, intrusive rocks with elevated contents of uranium are abundant. In Pribaikalie, many water sources are classified into Groups I and II (Q < 100 Bq/L). Group I includes groundwater with background concentrations, and Group II includes groundwater with anomalous concentrations of radon. Water sources belonging to elevated radon concentration Groups III to VI are considered as anomalies. Faulting is one of the main causes of radon anomalies as groundwater in the fault zones is enriched with Rn-222 due to the increased permeability of rocks and rock crushing resulting from displacements of the fault walls. Monitoring of water sources in Pribaikalie shows that Rn-222 concentrations vary also in time mainly within the range of values corresponding to a particular group. The oscillatory pattern of the temporal variations is clearly revealed by radon measurements conducted every two weeks and depends mainly on meteorological factors. Seasonal variations in air temperature and atmospheric pressure lead to changes in groundwater temperature, dilution of groundwater due to precipitation, freezing the top layer of soil, and other phenomena. As a result, the concentration regularly increases in winter and decreases in summer. In this study, seismic activity has not been revealed as significant factor of impact on the concentration of radon in groundwater, since strong earthquakes did not occur during the monitoring period.