Accumulation of natural and artificial radionuclides in cryoconite holes on Alpine glacial environment

Abstract

Over the last century, alpine glaciers have melted rapidly. According to current climate models, it is predicted that 80% of these glaciers will likely disappear between the 2060s and the 2080s. The storage of natural and anthropogenic contaminants in these ice masses, which might be released with water, creates a potential threat for communities, especially those closely related to glacierised regions, and the surrounding glacier habitats. In recent years, cryoconite – a mineral-organic debris that accumulates on the glacier surface - has been the subject of interest due to its ability to accumulate specific substances, surpassing levels found in other terrestrial ecosystems (e.g., proglacial, sediments, soil, lichens, mosses). This phenomenon is attributed to a combination of natural and anthropogenic factors, yet still not well-studied. The majority of artificial radionuclides released into the environment can be attributed to nuclear reactor accidents like Chernobyl (1986) and the stratospheric global fallout. Our main objective is to comprehensively understand the accumulation of natural (210Pb) and artificial (137Cs, 238,239,240Pu) radioisotopes in cryoconite and identify the different sources of contamination based on isotopic and mass ratios of subject radionuclides. To achieve these objectives, we analysed activity concentrations, their relation with the global and local signals, and their variability between glaciers. Samples were collected from eight glaciers in the European Alps, including the glaciers Blanc, Gries, Mandrone, Pastrze, Preda Rossa, Tsanteleina, Ventina, and Zebrù. The highest values of 210Pb were found in cryoconite from the Ventina and the Zebrù Glaciers (more than 11,000 Bq kg−1). The lowest values of 210Pb in individual samples (<100 Bq kg−1) were found in the Gries and Mandrone Glaciers. Sediment from the Ventina Glacier had the highest 137Cs activity concentration (up to 12,000 Bq kg−1), while the one from Gries Glacier exhibited the lowest values (4.00 Bq kg−1). The highest activity concentrations of 238Pu (up to 5.0 Bq kg−1) and 239+240Pu (up to 103.0 Bq kg−1) were observed also on the Ventina Glacier. The atomic ratio 240Pu/239Pu and activity ratio 238Pu/239+240Pu showed that the plutonium-related radioactivity from different Alpine glaciers is mostly compatible with global radioactive fallout. On average, more than 88.0% to 97.7% of the Pu found in cryoconite samples are from global fallout. In contrast, the major contribution of 137Cs is identified as the Chernobyl accident. Our observations found a positive correlation between the activity concentrations of studied isotopes and organic matter content. These results confirm the ability of cryoconite to accumulate radioactivity and show that multiple regional and global sources influence the radioactive signature of Alpine cryoconite. Also, activities in cryoconite are significantly higher than those in the published matrices usually used for the environmental monitoring of radioactivity. Moreover, due to its organic matter content (and the positive correlation of the latter to the amount of the studied radionuclides), cryoconite effectively captures (mostly from atmospheric deposition) and collects the impurities present in meltwater.