Vertical distribution of natural radionuclides in soils

¹ Natural Radioactivity Group, Universidad de Extremadura Avd. Elvas s/n, 06071 Badajoz, Spain
² Laboratorio de Radiaciones Ionizantes, Facultad de Ciencias Universidad de Salamanca - 37008, Salamanca Spain

^* E-mail: fvt@unex.es

Abstract

Low-level alpha spectrometry techniques using semiconductor detectors (PIPS) and liquid scintillation counters (LKB Quantulus 1220™) were used in order to determine the activity concentration of ²³⁸U_, ²³²Th, ²³⁴U, ²³⁰Th, ²²⁶Ra, and ²¹⁰Pb in soil samples. The soils were collected from an old disused uranium mine located in southwest Spain. The soils were selected with different levels of influence from the installation, in such a way that they had different levels of radioactive contamination. The vertical profiles in the soils (down to 40 cm depth) were studied in order to evaluate the vertical distribution of the natural radionuclides. The possible contamination of subsurface waters depends strongly on vertical migration, and the transfer to plants (herbs, shrubs, and trees) also will depend on the distribution of the radionuclides in the root zone. The study of the activity ratios between radionuclides belonging to the same series allowed us to assess the differing behaviour of the radionuclides involved. The vertical profiles for these radionuclides were different at each sampling point, showing the local impact of the installation. However, the profiles per point were similar for the long-lived radionuclides of the ²³⁸TJ series (²³⁸U, ²³⁴U, ²³⁰Th, and ²²⁶Ra). Also, a major disequilibrium was observed between ²¹⁰Pb and ²²⁶Ra in the surface layer, due to ²²²Rn emanation and subsequent surface deposition of ²¹⁰Pb.