Prediction of areas presenting a high radon exhalation potential: A new methodology based on the properties of geological formations and soils

¹ Institut de Protection et de Sûreté Nucléaire, DPRE/SERGD/LEIRPA, BP. 6, 92265 Fontenay-aux-Roses, France
² Laboratoire de Recherches Appliquées Atmosphère Hydrosphère, Université de Bretagne Occidentale, UFR Sciences et Techniques, 6 avenue Le Gorgeu, BP. 809, 29285 Brest cedex, France

Abstract

A research program carried out since 1997 produced a methodology for predicting areas with a strong potential for radon exhalation at the soil surface. This methodology is based on a quantification of the Rn exhalation rate, from a precise characterization of the main local geological and pedological parameters that control the radon source and its transport to the soil/atmosphere interface. It combines a cross mapping analysis of parameters used in a Geographic Information System with a model of the vertical transport of Rn by diffusion trough the soil. This code (TRACHGEO) calculates the radon flux density at the surface as a function of the properties of the rock and the soil. This approach is validated in 4 typical areas with different geological contexts, starting from in situ measurements of radon fluxes and of radon concentrations in dwellings. A lithogeochemical classification of the geological formations as a function of their U contents and their confrontation to Rn level measurements demonstrate the primordial influence of the U content of the basement on Rn exhalation. This study leads to an initial map of the exhalation potential by assigning a potential class to each lithogeochemistry. Nevertheless, in situ radon measurements reveal a high spatial variability on uraniferous lithologies. Tests made by the TRACHGEO tool show the need to take account of spatial heterogeneity of soils (in addition of geochemistry) to improve the mapping resolution. The TRACHGEO forecasts explain the variability of the Rn exhalation on a larger scale.