Radon (Rn-222) level variations on a regional scale: influence of the basement trace element (U, Th) geochemistry on radon exhalation rates

The approach proposed in this study provides insight into the influence of the basement geochemistry on the spatial distribution of radon (Rn-222) lev els both at the soil/atmosphere interface and in the atmosphere. We combine different types of in situ radon measurements and a geochemical classifica tion of the lithologies, based on 1/50 000 geological maps, and on their tr ace element (U, Th) contents. The advantages of this approach are validated by a survey of astable basement area of Hercynian age, located in South Br ittany (western France) and characterized by metamorphic rocks and granitoi ds displaying a wide range of uranium contents. The radon source-term of th e lithologies, their uranium content, is most likely to be the primary para meter which controls the radon concentrations in the outdoor environment. I ndeed, the highest radon levels (greater than or equal to 100 Bq m(-3) in t he atmosphere, greater than or equal to 100 mBq m(-2) s(-1) at the surface of the soil) are mostly observed on lithologies whose mean uranium content can exceed 8 ppm and which correspond to peraluminous leucogranites or meta granitoids derived from uraniferous granitoids. (C) 2000 Elsevier Science L td. All rights reserved.