Spatial distribution of natural and artificial radionuclides at the catchment scale (South Central Pyrenees)

Natural and artificial gamma-emitting radionuclides (238U, 226Ra, 232Th, 210Pb, 40K, and 137Cs) were measured in the soils of a small catchment in the Central Pyrenees, Spain. The study was carried out in a mountainous area that was representative of the Tertiary Flysch landscapes in the Southern Pyrenees. Bulk soil cores (n = 77) were collected at the intersections of a 200 × 200-m grid established in the Arnás River Catchment. Mean radioisotope activities (Bq kg−1) were 40 (238U), 27 (226Ra), 35 (232Th), 74 (210Pb), 48 (210Pbex), 590 (40K), and 31 (137Cs). The 210Pb and 137Cs exhibited the greatest variability, whereas 226Ra and 40K showed the least spatial variation. The relationships between basic soil properties and radionuclide activities indicate that only the radionuclides, 210Pbex and 137Cs, that are fixed to the fine fraction of the soil are directly correlated with the organic matter content, whereas the natural radionuclides are inversely correlated with the carbonate content.GIS and geospatial interpolations revealed patterns in the spatial concentrations of radionuclides and indicated important differences in their distributions showing the different behaviour of natural and fallout-derived radionuclides. The radionuclide spatial patterns were strongly correlated with physiographic features such as gradient, orientation, and vegetation cover of the slopes. Within the catchment, the least vegetated and steepest slopes had the lowest radionuclide activities, which suggest that physical processes such as erosion are primary factors in the mobilization of radionuclides in association with soil particles. The results provide insights into the main factors that have affected the spatial distribution of radionuclides in the soils of the catchment, which improves our knowledge of the behaviour of radionuclides in the environment and can lead to a better comprehension of the factors that affect their mobility within ecosystems.