Assessment of the vertical distribution of natural radionuclides in a mineralized uranium area in south-west Spain

Low-level alpha spectrometry techniques using semiconductor detectors (PIPS) and liquid scintillation (LKB Quantulus 1220™) were used to determine the activity concentration of 238U, 234U, 230Th, 226Ra, 232Th, and 210Pb in soil samples. The soils were collected from an old disused uranium mine located in southwest Spain. The soils were sampled from areas with different levels of influence from the installation and hence had different levels of contamination. The vertical profiles of the soils (down to 40 cm depth) were studied in order to evaluate the vertical distribution of the natural radionuclides. To determine the origin of these natural radionuclides the Enrichment Factor was used. Also, study of the activity ratios between radionuclides belonging to the same radioactive series allowed us to assess the different types of behaviors of the radionuclides involved. The vertical profiles for the radionuclide members of the 238U series were different at each sampling point, depending on the level of influence of the installation. However, the profiles of each point were similar for the long-lived radionuclides of the 238U series (238U, 234U, 230Th, and 226Ra). Moreover, a major imbalance was observed between 210Pb and 226Ra in the surface layer, due to 222Rn exhalation and the subsequent surface deposition of 210Pb.