Factors controlling the air ventilation of a limestone cave revealed by ²²²Rn and ²²⁰Rn tracers

Abstract

We monitored ²²²Rn (half life: 3.82 d) and ²²⁰Rn (half life: 56 s) activities every hour in a limestone cave, Seongryu Cave, Korea, from May to July 2010. The activities of both nuclides in this cave were highest at night and lowest during the day owing to active ventilation with outside air during the day. As the outside air temperature increased from May to July, ²²²Rn activity increased by 2–3 times. A simple mixing model of hourly ²²²Rn variation revealed that the ventilation with outside air decreased by a factor of 3–4 from May to July. The daily average of ²²²Rn activities correlated well with the daily average temperature of the outside air, indicating that the ventilation with outside air is controlled mainly by outside-air density, rather than by the diurnal amplitude of the outside air temperature. Although the trend of the diurnal variation of ²²⁰Rn was similar to that of ²²²Rn, its increase from May to July was only 65%. Since the short-lived ²²⁰Rn does not have any stacking effect over different days and the diffusive flux is negligible, ²²⁰Rn activities indicated increases in the advective pore-air input rates of cave rocks from May to July, which are consistent with the modeling result of hourly ²²²Rn variations. Thus, our study suggests that the dual Rn tracers provide key information on the micro- and macro air-ventilation processes of caves.