Assessment of natural radioactivity levels and their relationship with soil characteristics in undisturbed soils of the northeast of Buenos Aires province, Argentina

Abstract

Surface and depth profile concentrations (down to 50 cm) of 232Th chain, 226Ra, and 40K radionuclides were determined in undisturbed coastal and inland soils of La Plata city region, Argentina, through their gamma-ray activity using a high-purity Ge detector spectrometer. These results were compared with superficial activities determined in soils from the surroundings of the Centro Atómico Ezeiza (Ezeiza Atomic Center) located in Ezeiza, Buenos Aires Province, Argentina. The hyperfine and magnetic Fe phase’s properties of soil profiles were characterized by Mössbauer spectroscopy, magnetic hysteresis loops and AC magnetic susceptibility. No dependence of the activity of the 232Th natural chain on depth was found, whereas variations for 226Ra and 40K activities were observed. Positive correlations, determined by the Pearson correlation coefficients, were established between 40K, 226Ra and 232Th activity concentrations for the whole set of soil samples. The annual external equivalent dose for adults was similar for La Plata and Ezeiza regions, with average values of 0.08 0.01 mSv and 0.06 0.02 mSv, respectively. The thermal dependence of the AC magnetic susceptibility revealed the existence of magnetite and hematite. The Mössbauer spectra of all soils were made up of signals associated with a-Fe2O3, a paramagnetic relaxation component, and Fe3þ and Fe2þ doublets. In addition, the spectra of inland soils revealed the presence of Fe3O4. A negative correlation was found between the activity concentrations and the a-Fe2O3 and Fe3O4 relative fractions, whereas a positive correlation was found between the Fe3þ relative fraction and the 40K activity.