Role of the Structural Factor in the Distribution of High-Radon Groundwater in the Southwestern Flank of the South Baikal Rift Basin

Abstract

This article discusses the distribution patterns of high-radon groundwater at the southwestern shore of Lake Baikal. This region is a flank of the South Baikal rift basin, characterized by high geodynamic activity and complex fault patterns, without any special geochemical conditions with regard to the content of uranium in the rocks. Based on our observations and measurements, we consolidated the first massive database on radon volume activity (Q) in a variety of local water sources. In the Kultuk-Vydrino area, the Q values vary from zero to 81.1 Bq/l, according to the analysis of the water samples from 93 springs, lakes, small streams, wells, and drilled holes. The highest concentrations of radon are discovered in the groundwater samples. Such values are unevenly scattered across the study area. The chain of the maximum Q values trends northwestwards along the Lake Baikal shore. This distribution pattern of radon, as well as the locations of individual water sources with Q>15 Bq/l are predetermined by the structural factor. The paragenetic analysis of faults and joints in the Kultuk-Vydrino area shows that this factor includes both the structure and stages in the development of the regional largest Main Sayan fault zone (the southwestern flank of the South Baikal basin is a segment of this zone). The water sources with increased concentrations of radon are located in zones with a high density of the 2 nd order faults, especially on sites wherein the NW-striking faults cross the transverse faults that have experienced repeated activation. Temperature T is an additional factor influencing the degree of radioactivity in water. A relationship between T and Q is reverse. Water sources with Q>15 Bq/l associated with the 2 nd order fault zones may occur also due to a locally lower temperature of groundwater. Our study gives evidence that the southwestern coast of Lake Baikal is promising for finding high-radon water sources. Using such water in balneology can become a valuable contribution to the tourism and recreational potential of the region. This prospect is especially important for the town of Baikalsk that is now developing without its township-forming enterprise: the notorious pulp-and-paper mill has been completely shut down.