Insight into radionuclide 235U deposition in multiple-environmental media in Muztagh Ata Glacier Basin of the eastern Pamirs

This study concerns the ²³⁵U/²³⁸U ratios in environmental samples collected in the Pamir region (Central Asia). Cryoconite (a supra-glacial sediment), soil and river water were sampled in the Muztagh Ata Glacier Basin, a secondary basin belonging to Gaizi River watershed. The aim of the research is to assess the impact of anthropic nuclear activities in such a remote area, being the ²³⁵U/²³⁸U ratio highly sensitive to anthropogenic disturbances. Results indicate that the ²³⁵U/²³⁸U atomic ratio in the region ranges from 0.007256 to 0.007263, with an average of 0.007259±0.000002. Such values are slightly higher than the natural isotopic reference (0.007256), suggesting a modest enrichment of ²³⁵U. This is related to the input of uranium with an anthropogenically modified isotopic signature. The ²³⁵U/²³⁸U ratios are higher in cryoconite compared to that of surface soil/sediment and river water, reflecting differences in the release, transmission and retention of ²³⁵U across various environmental media in the Pamir region. The variability of the isotopic ratio was also explored in relation to some key geographic parameters and compared with data from a wider geographic context. The altitude distribution of the ²³⁵U/²³⁸U ratio indicates that the deposition of ²³⁵U in the Muztagh Ata area primarily results from upper atmospheric deposition sources of the global fallout of radionuclides, which were probably released by historical human nuclear activities of global range, but without clear evidence of local uranium contamination. A conceptual model for interpretation of the ²³⁵U/²³⁸U ratios and profile distribution in eastern Pamir is provided. Comparative analysis of the ²³⁵U/²³⁸U ratios in our samples and in samples from other locations in the northern hemisphere shows that the level in the environment of the Muztagh Glacier area is cleaner, with subtle enriched uranium and not contaminated by depleted uranium. Therefore, this work is of importance in providing a complete view on the migration, dispersion, and removal of radionuclide ²³⁵U in the atmosphere and cryospheric glacier watershed of the remote Pamir region, thus offering new insights into the distribution and behavior of radionuclides in the Pamir.