Baseline and Quality Reference Values for Natural Radionuclides in Soils of Rio de Janeiro State, Brazil

A first large-scale systematic survey of natural radioactivity contents of soils of the state of Rio de Janeiro is presented, focused on the establishment of Quality Reference Values (QRVs). Undisturbed soil samples were collected from 243 areas and analyzed by gamma spectrometry. The activity contents varied largely, ranging from 12.2 to 1,029 Bq kg(-1) for K-40 (geometric mean of 111.1 Bq kg(-1)), from 3.5 to 99.8 Bq kg(-1) for Ra-226 (geometric mean of 29.7 Bq kg(-1)), and from 5.4 to 314.5 Bq kg(-1) for Ra-228 (geometric mean of 67.1 Bq kg(-1)). The highest contents of radium isotopes were found in soils developed on igneous rocks (Leptosol), and the lowest in a soil of sedimentary origin (Podzol). Among the different soil types, the radioisotope contents differed substantially. Separate QRVs were calculated for each radionuclide by the 75th and 90th percentile approach, and the QRVs were estimated for each soil type. The results emphasized the restrictiveness of QRVs based on the 75th percentile or of a single overall QRV for all soils. Therefore, rather than estimating a separate QRV for each radionuclide for the State, we suggest the use of an upper threshold value, defined as the 90th percentile, and a specific QRV for each soil type area.