RADON RISK MAPS: THE PORTUGUESE EXPERIENCE
Since then, several studies were carried out by the authors to recognize the geological constraints of radon distribution in soil air, water and indoor air. These studies show that the highest radon concentrations are usually found in association with Hercynian granites, in particular with porphyritic coarse-grained biotite varieties, as well as with metasediments of pre-Ordovician age of their contact metamorphism aureole. The most important radon sources, however, are U-enriched faults that in Central Portugal frequently crosscut both igneous and metamorphic lithologies. Their U content can be as high as a few thousand ppm; soil air radon concentrations above 30,000 pCi/l are common in these structures. Mineralized faults produce radon anomalies in soil air that spread laterally from a few meters to tens of meters, according to type of fault; the affected belt can reach a few hundred meters in the largest structures. The most productive faults correspond to the N30-40ºE and to the N55-65ºE systems.
Considering the typology of soils, very thin in most part of the country, and the secondary nature of the U-mineralization (usually occurring up to a maximum depth of 50 m), a specific methodology has been established for the purpose of radon risk mapping, at a very small-scale (1: 5 000 or less). This methodology combines geological information, detailed gamma-ray surveys, U contents of rocks, and soil-gas radon measurements; risk predictions are tested against direct measurements in dwellings. We present the first radon risk map officially adopted in Portugal by a local municipality for the purpose of land use planning, developed according to the principles referred above.