Assessment of Environmental Radionuclides and Controlling Factors in Volcanic Soils of Andean Patagonia

Natural radionuclides (40K, 210Pb, 226Ra, 232Th, and 238U) were evaluated for the first time on volcanic ash soils of the Argentine Patagonian Andes. The study was carried out along a topoedaphoclimatic gradient, encompassing soils from Xeric Mollisols to Udic Andisols, and different land uses. Median mass-specific activities of the lithogenic radionuclides 40K, 210Pb, 226Ra, 232Th, and 238U were 375, 8, 17, 19, and 29 (Bq kg−1), respectively, all falling within global natural background levels, yet distinct spatial and vertical patterns emerged. Radionuclide activities increased with sand content and decreased with organic matter, highlighting the role of the parent material and texture. In dry-site Mollisols, 40K and 210Pb increased with depth, while in humid-site Udands, activities declined with depth, suggesting leaching and surface accumulation by allophane–organic matter complexes. The 238U/226Ra activity ratio showed disequilibrium, indicating young, developing soil profiles. In Xerolls, where native forest was replaced by afforestation and rangeland use, erosion-driven degradation was evident. The distribution of radionuclides along the slopes was closely linked to the topographic position and slope gradient. These results underscore the sensitivity of radionuclide patterns to parent material, soil-forming processes and land use and provide a valuable reference for environmental monitoring in volcanic landscapes.