Radioactive isotopes in soils of Rio de Janeiro state: reference values, spatial distribution and correlation with environmental factors

In the Rio de Janeiro State is located most of the Brazilian facilities for nuclear power generation and in the north region of the State is located a facility for concentration of heavy metals containing in monazite sand, which contain thorium and uranium associated. In addition, phosphate fertilizers, which contain natural radionuclides, are intensively used for agricultural purposes. A regional scale study was carried out in the state of Rio de Janeiro aimed to determine the levels of ⁴⁰K, ²²⁶Ra, ²²⁸Ra and ¹³⁷Cs in surface soils, their geographic distribution, quality reference values (QRV) and the external dose which the population is potentially exposed. With this aim, 263 samples of surface soil were collected in the overall State, representing all soil and geology types, and analyzed by gamma spectrometry. The ⁴⁰K, ²²⁶Ra and ²²⁸Ra concentrations were within the world average values for soils. The activity concentration varied from 12 to 1029 Bq kg^-1 to ⁴⁰K (geometric mean 111 Bq kg^-1), from 3.5 to 100 Bq kg^-1 for ²²⁶Ra (geometric mean 30 Bq kg^-1) and from 5.4 to 314 Bq kg^-1 for ²²⁸Ra (geometric mean 67 Bq kg^-1). Higher concentrations were found in soils with igneous and metamorphic origin (Leptosol and Cambisol), while the lowest concentration were observed to the Podzol, of sedimentary origin. The lowest concentrations of ²²⁶Ra were observed in soils occurring on a rock with a high grade of metamorphism (granulite). The QRV values were calculated, according to Brazilian law in the 75^th and 90^th percentiles. However, due to the large dispersion of concentration data, when values of VRQs for each soil class are compared, significant differences among them are found. The results put in the view the restriction of using the 75^th percentile and a unique QRV by radionuclide to all soil classes. Therefore, values of QRVs are suggested based on the 90^th percentile and for each individual soil class. The activity concentrations of ¹³⁷Cs are consistent with the low values expected for the southern hemisphere; the values ranged from <0.2 to 4.23 Bq kg^-1, with 86% of the values below the minimum detectable activity. The QRV for ¹³⁷Cs was calculated as 1.44 Bq kg^-1 (75^th percentile) and 1.78 Bq kg^-1 (90^th percentile), considering the year of 2015. The highest activity concentrations of ¹³⁷Cs were found in soils containing high levels of organic matter and clay. No influence of the rainfall index was observed on the ¹³⁷Cs concentration in soil. The maps showed the almost homogeneous occurrence of ⁴⁰K, while part of the Valley of Medium Paraiba do Sul River presented the highest concentration of ²²⁶Ra and ²²⁸Ra. In accordance with radium characteristics, soils containing high levels of clay and organic matter showed the highest concentrations of radium isotopes. However, also in the acidic soils high radium concentrations were found, which emphasizes the need for further investigation to explain such behavior. The external dose rate due to the studied natural radionuclides in the surface soil varied from 5 and 217 n Gy h^-1, mean and median values of respectively 67 and 62 nGy h^-1. In a general way, the levels of external exposure assessed indicate the State as an area of normal background radiation. (author)