Significance of seasonal outdoor releases of thoron from airflow through a point source during natural ventilation of a mine-complex in thorium-rich bedrock

Concentrations of naturally occurring radioactive gas can be high in caves and mines, and it has been shown that releases of radon (222Rn) to the outside environment through ventilation can be large. We assess airflow and associated activity concentrations of thoron (220Rn) and progeny (TnP) being released through a drainage pipe from an old mine-complex situated in thorium-rich bedrock. Outdoor thoron concentrations in this area have been thought to arise solely from thoron exhalation from the ground. However, thoron concentrations in outwards airflow in the drainage pipe range from 25 000 Bq m−3 to 42 000 Bq m−3 and discharges can in summer be as high as 1 GBq d−1. The drainage pipe can be considered as a point source adding to exhalation from the soil and deposits of waste rock in the area. Statistical analyses including meteorological data suggest that outdoor temperature is the main factor affecting this airflow and that ventilation of these mines resembles chimney ventilation, but other weather variables may also contribute. During summer with warm outdoor temperatures, colder and denser air within the mines escapes through the drainage pipe, which is located on low ground. In winter, when outdoor temperatures are lower than those within the mines, the direction of airflow in the drainage pipe is inwards into the mines, while air escapes upwards and outwards through larger mine openings located on higher ground. Statistical outliers indicate Venturi effects by strong winds and syringe effects by internal water level.