Soil radon dynamics in the Amer fault zone: An example of very high seasonal variations

Soil radon levels of the Amer fault zone have been measured for a 4 year-period with the aim of checking seasonal fluctuations obtained in previous studies and to understand radon origin and dynamics. In this manuscript additional results are presented: updated continuous and integrated soil radon measurements, radionuclide content of soil materials and a detailed analysis of an urban profile by means of the electrical resistivity imaging technique and punctual soil radon, thoron and CO₂ measurements. Integrated and continuous measurements present a wide range of values, [0.2-151.6] kBq m^-3 for radon, [4.5-39.6] kBq m^-3 for thoron and [4.0-71.2] g m^-2 day^-1 for CO₂. The highest soil radon levels in the vicinity of the Amer fault (>40 kBq m^-3) are found close to the fractured areas and present very important fluctuations repeated every year, with values in summer much higher than in winter, confirming previous studies. The highest radon values, up to 150 kBq m^-3, do not have a local origin because the mean value of radium concentration in this soil (19 ± 5 Bq kg^-1) could not explain these values. Then soil radon migration through the fractures, influenced by atmospheric parameters, is assumed to account for such a high seasonal fluctuation. As main conclusion, in fractured areas, seasonal variations of soil radon concentration can be very important even in places where average soil radon concentration and radium content are not especially high. In these cases the migration capability of the soil is given not by intrinsic permeability but by the fracture structure. Potential risk estimation based on soil radon concentration and intrinsic permeability must be complemented with geological information in fractured systems.