Quantification of the radon exhalation rate at the surface of the soil : a new predictive tool based on the physical and chemical properties of geological formations and soils.

Ielsch., G., Ferry, C., Tymen, G. and Robé, M.-C. Third European Symposium on Protection against Radon at home and at work - Liège, 10-11 May 2001

The main difficulty encountered in management of risks related to radon in the home throughout France is due to the great variability of exposure to radon. In preparing risk management programs, a predictive methodology needs to be developed to define areas in which the density of buildings with high radon levels is likely to be highest. Identification of priority areas would then help to target actions taken by the government, and particularly screening measurements in existing buildings. A research program carried out since 1997 produced a methodology for predicting areas with a strong potential for exhalation of radon at the soil surface. This methodology is based on a quantification of the radon flux density at the surface, starting 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. The proposed methodology is innovative due to the combination of a cross mapping analysis of parameters used in a Geographic Information System with a simplified model of the vertical transport of radon by diffusion in the soil pores. This code is called TRACHGEO and calculates the radon flux density at the surface as a function of the chemical and physical properties of the rock and the soil. The main objective was to validate this methodology in 4 typical areas with different geological contexts, starting from in situ measurements of the radon exhalation rate at the soil surface and the radon volume activity in the soil, the atmosphere and dwellings. Our approach is innovative firstly due to the creation of a global lithogeochemical classification of geological formations as a function of their uranium contents. This is a first essential analysis step, demonstrating the primordial influence of the U content of the basement on the exhalation of radon. This study leads to an initial zoning of the exhalation potential by assigning a potential class to each defined lithogeochemistry. Furthermore, our radon flux density measurements reveal a high spatial variability on uraniferous lithologies. Therefore the first zoning of the exhalation potential determined from the lithogeochemical study sometimes overestimates this potential. Tests made by the TRACHGEO tool show the need to take account of spatial heterogeneity of pedological characteristics (in addition of geochemistry) to improve the mapping resolution. The second zoning deduced from TRACHGEO forecasts explains the variability of the radon exhalation on a larger scale. The vast majority of our forecasts are in good agreement with observations in the field. However, they can significantly underestimate exhalation in some cases, particularly in faulted areas, suggesting that exhalation variations may be caused by structural anomalies. The exhalation potential amplification factor induced by fracturing remains to be studied in order to improve the methodology developed so far which ignores this phenomenon.