Natural radioactivity in soils of the state of Rio de Janeiro (Brazil): Radiological characterization and relationships to geological formation, soil types and soil properties

Located in the south-western part of Brazil, the state of Rio de Janeiro is geotectonically contained within a complex structural province that resulted in the amalgamation of the Western Gondwana Paleocontinent. To undertake an extensive radiological characterization of this complex geological province and investigate the influence of bedrock, soil type and soil chemical-physical characteristics on natural radionuclide levels in soils, 259 surface soil samples were collected that encompassed the main soil types and geological formations throughout the state. Gamma spectrometry analysis of the samples resulted in median values of 114 Bq.kg(-1) for K-40, 32 Bq.kg(-1) for Ra-226 and 74 Bq.kg(-1) for Ra-228. The median value for Ra-226 was similar to the world median value for soils, the K-40 value was well below the worldwide value, and that for Ra-228 exceeded the world median value. The intense weathering caused by the high rainfall rates and high temperatures may be responsible for the low levels of K-40 in the soils, of which the strongly acidic and clayey soils are markedly K-depleted. A soil from a high-grade metamorphic rock (granulite) presented the lowest Ra-226 (18 Bq.kg(-1)) content, whereas the highest levels for Ra-226 (92 Bq.kg(-1)) and Ra-228 (139 Bq.kg(-1)) were observed in a young soil enriched in primary minerals (Leptsol). A lowland soil (Gleysol) showed the highest median of K-40 (301 Bq.kg(-1)). Strongly acidic soils tended to present high amounts of Ra-226, and sandy soils tended to contain low levels of Ra-228. The external radiation dose indicates that the state has a background radiation level within the natural range.