Geophysical and geochemical tools for radon mapping at a regional scale.

G. Ielsch, D. Thiéblemont, J. Perrin, G. Tymen EOS, Transactions, American Geophysical Union, 1999 Fall Meeting, vol. 80, n°46, november 16, 1999/Supplement.

Radon (222Rn ) is the main source of natural radiation exposure for man, that has been recognized as a carcinogenic gas. Thus, a radon risk may exist when radon accumulates in enclosed environments, like dwellings. However, implementation of simple remedies can reduce the indoor radon concentrations. Several national or international organisations have already addressed themselves to the problem of high radon levels in specific areas. Indeed, identification of radon-prone areas permits to concentrate monitoring and remediation efforts. Therefore, a new method is being studied by the French Institute for Protection and Nuclear Safety (irsn) and the French Geological Survey (BRGM), in order to determine the higher risk areas in France. Radon levels can show important variations on a regional or local scale, which can be attributed to various geologic and pedologic parameters. The approach proposed is based on the study of the different factors related to the radon source term (radon emanation), and to the radon migration from its source up to the surface of the soil (radon exhalation). Firstly, the radon source is characterized by the different lithologies encountered and by their geochemistry (e.g., uranium content), and also by the radium content of the different soil types. Aerial radiometric surveys and measurements of the total gamma radioactivity by means of a mobile laboratory, are also used to characterize the spatial variability of the source term. Several parameters are taken into account for the transport phenomena: first and foremost, the fracturing and permeability of the subsoil. Indeed, radon generally migrates upwards through a network of fractures and, depending on the degree of fracturing, can cause local concentration anomalies in the atmosphere. Furthermore, the morphological characteristics of the soil, such as thickness, texture, porosity and humidity are studied in situ. The distribution of the different soil types is also analyzed. Moreover, these parameters are used for testing a code, which calculates the radon concentrations in the soil and the radon flux density at the soil surface. The quantification of these parameters is associated with different in situ radon measurements in the soil, at the soil surface and also in the atmosphere. The whole data set is incorporated into a Geographical Information System (G.I.S.) for its spatial analysis. The advantages of combining these different geochemical and geophysical tools to quantify and map the 222Rn level variations on a regional scale are exemplified by data from Brittany (western France). The studied area is geologically old and tectonically non-active; it is characterized by lithologies and soil fabrics yielding very different radon source potentials. Our method allowed to range the radon levels that could be expected on a specific area as a function of the geochemistry of its rock and soil association. Further studies will consist on comparing these results to indoor radon concentrations.