Migration of 238U and 226Ra Radionuclides in Technogenic Permafrost Taiga Landscapes of Southern Yakutia, Russia

This article describes the features and migration patterns of natural long-lived heavy radionuclides ²³⁸U and ²²⁶Ra in the major components of the environment including rocks, river waters, soils, and vegetation of permafrost taiga landscapes of Southern Yakutia, which helped us to understand the scale and levels of their radioactive contamination. Different methods have been used in this study to determine the content of ²³⁸U and ²²⁶Ra in various samples, including gamma-ray spectrometry, X-ray spectroscopy, laser excited luminescence, and emanation method. It was determined that the main source of radioactive pollution of soil and vegetation cover, as well as surface waters in these technogenic landscapes, are the dumps of radioactive rock that were formed here as the result of geological exploration carried out in this area during the last third of the 20th century. The rocks studied were initially characterized by a coarse, mainly stony gravelly composition and contrasting radiation parameters, where the gamma radiation exposure rate varied between 1.71 and 16.7 µSv/h, and the contents of ²³⁸U and ²²⁶Ra were within the range 126–1620 mg/kg and 428–5508 × 10⁻⁷ mg/kg, respectively, and the ²²⁶Ra: ²³⁸U ratio was 1.0. This ratio shifted later on from the equilibrium state towards the excess of either ²³⁸U or ²²⁶Ra, due to the processes of air, water, and biogenic migration. Two types of ²³⁸U and ²²⁶Ra radionuclides migration were observed in studied soils, namely aerotechnogenic and hydrotechnogenic, each of which results in a different intraprofile radionuclide distribution and different levels of radioactive contamination. In this study, we also identified plants capable of selective accumulation of certain radionuclides, including Siberian mountain ash (Sorbus sibiricus), which selectively absorbs ²²⁶Ra, and terrestrial green and aquatic mosses, which accumulate significant amounts of ²³⁸U.