Determination of plutonium in cryoconite on glacier surfaces in the northeast Tibetan Plateau: Implications for...

Plutonium isotopes have drawn public attention because of their high radiotoxicity and risk for internal radiation. Cryoconite, dark sediments on the surface of glaciers, is rich in anthropogenic radionuclides. Therefore, glaciers are regarded as not only a temporary sink for radioactive pollutants in the past decades, but also a secondary source during their melting. However, study on activity concentration and source of Pu isotopes in cryoconite materials in Chinese glaciers have not been studies thus far. The present study determined the ^239+240Pu activity concentration and ²⁴⁰Pu/²³⁹Pu atom ratio in cryoconite and other environmental samples collected in August-one ice cap, northeast Tibetan Plateau. The results showed that the ^239+240Pu activity concentration in cryoconite is 2-3 orders of magnitude higher than the background value, suggesting the exceptional capacity of cryoconite to accumulate Pu isotopes. The high ^239+240Pu level in cryoconite in the study area was significantly correlated with organic matter and slope, indicating their dominant influence. The average values of the ²⁴⁰Pu/²³⁹Pu atom ratio for proglacial sediments (0.175) and grassland soils (0.180) suggested that the global fallout is the dominant source of Pu isotopes pollution. In contrast, the measured ²⁴⁰Pu/²³⁹Pu atom ratios in the cryoconite were distinctly lower at 0.064-0.199, with an average of 0.157, indicating that the close-in fallout Pu isotopes from Chinese nuclear test sites is another source. In addition, although the relatively lower activity concentrations of ^239+240Pu in proglacial sediments implies that most Pu isotopes are retained in this glacier instead of being redistributed along with cryoconite by meltwater, the potential health and ecotoxicological risks to the proglacial environment and downstream areas cannot be ignored. These results are important for understanding the fate of Pu isotopes in the cryosphere and can be applied as baseline data for radioactivity evaluation in the future.