Spatial distributions of natural radionuclides in soils of the state of Pernambuco, Brazil: Influence of bedrocks, soils types and climates

To investigate whether and in which extension the levels of natural radionuclides in soils vary with parent rock, soil type, and climatic condition factors, a large-scale survey was carried out to cover the semiarid and tropical wet climate regions of the territory of Pernambuco state (Brazil). The radionuclide concentrations were analyzed by gamma spectrometry. The median values of the activity concentrations of ²²⁶Ra (20), ²²⁸Ra (38) and ⁴⁰K (458) (Bq kg⁻¹) in the soils agreed with the values reported worldwide. The concentrations of ⁴⁰K in the soils from the semiarid region were higher than those in the soils from the tropical region, whereas the radium isotope levels were higher in soils from the tropical region. The less-developed soils from the semiarid region, such as Leptosols and Neosols, had the highest ⁴⁰K levels, unlike the more developed soils (Acrisols and Ferrosols) from the tropical wet area, which showed the lowest contents of ⁴⁰K and the highest content of radium isotopes. The low ⁴⁰K contents in well-developed soils may be attributed to the leaching of ⁴⁰K by the high rainfall rates of the tropical area. In contrast, the rainfall scarcity and high potential evaporation rates of the semiarid environment may be responsible for ⁴⁰K accumulation in soils. The highest Ra isotope concentrations in the well-drained soils from the tropical climate may be ascribed to the presence of heavy minerals and adsorption on Fe oxyhydroxides, which are very common in those soils. The more developed the soil the lower the K concentration, regardless of the climate. For Ra, the differences are more significant for well-drained soils, and in this case, the tropical climate, with its high rainfall rate, makes the difference. As a conclusion, climate and soil formation time showed high and contrasting influence on the ⁴⁰K and radium isotope contents in soils. Less developed soils from semiarid had the highest content of ⁴⁰K, whereas the more developed soils from tropical areas had the highest content of radium.