Insights on the distribution and environmental implications of the radio-isotope 235U in surface soils and glaciers of the Tibetan Plateau

Atom ratio between U-235 and U-238 is often used as an indicator of U contamination as the isotopic signature of products generated by the nuclear and military industry significantly vary from the natural isotopic ratio of U. In this study, surface soils and glaciers samples were collected in the Tibetan Plateau (TP) and its surrounding areas and analyzed for U isotopic composition. Results show that the U-235/U-238 atom ratios in the surface soils of the TP ranges from 0.007122 to 0.007615, with an average value of 0.007378 +/- 0.00011; while in the snow/ice dust from high-altitude glaciers it ranges from 0.007254 to 0.007687, with an average value of 0.007345 +/- 0.000128. These ratios are slightly higher than the typical crustal value, indicating that the TP was affected by an anthropogenic input of U-235, especially in its northeast and southwest sectors. The variability of our results suggests that the spatial distribution of this contamination is not uniform, pointing to differences in the potential sources and transmission paths of radioactive particles. Combining the knowledge of past tests and activities conducted in the geographic areas around the TP with the knowledge of prevailing winds, we hypothesize that the observed U-235 contamination in the TP surface soils and glaciers may have originated mainly from the previous nuclear related activities in surrounding areas (e.g., north Gobi Desert and South Asia). In addition, the horizontal and vertical wind field around the Tibetan Plateau, as well as the atmospheric aerosol optical thickness data also demonstrated the possible transport paths of the radionuclides, that is, originated from in northern Gobi desert and South Asia and reached the TP crossing the Himalayas.