Radon-222 as a tracer of water–air dynamics in the unsaturated zone of a geological carbonate formation: Example of an underground quarry (Oligocene Aquitain limestone, France)

This article investigates the isotope Radon-222 (hereafter referred to as radon) evolution in an underground quarry atmosphere, during three hydrological cycles (January 2002 to December 2004). The unsaturated zone has a complex chemical and dynamic operation which varies in time and space. The aim is to use radon as a natural tracer of water–air dynamics. Several parameters have been measured: the outside and inside temperature, the volumetric water content of porous rock, and the radon concentration. A simple diffusion model can explain the natural radon signal. The diffusion coefficient is a very weighty parameter in the calculated predictive signal of radon. Nevertheless, radon migration is more complex than the sole diffusion in the unsaturated zone. Three dynamic processes are at work during the hydrologic cycles: (i) a piston effect and water degassing, (ii) a diffusion process and (iii) a ventilation process. Several periods in the radon time series are individualized by the expression of one of these major processes.

► Radon-222 (with a surface origin) is a natural tracer in the vadose zone.
► Effective diffusion coefficient is important in the modelling (porosity about 41%).
► Olesen's model, using porous media properties, yields the best modelling result.
► Three different processes control radon dynamics during the hydrologic cycle.
► Radon exemplifies the complex transfers in a natural environment.