Plutonium isotopes in the Qinghai-Tibet Plateau: Sources, distribution, and their environmental behaviors.

Due to the high radiotoxicity in high concentrations, plutonium isotopes have drawn high attentions in the consideration of radiation risk, their sources, level, environmental behaviors, including deposition, retention and migration behaviors. However, such research in the Qinghai-Tibet Plateau is still missing, where is deemed as an environmental sensitive area. 239,240Pu in surface soil collected from the Qinghai-Tibet Plateau were determined for the first time in this work. The concentrations of 239,240Pu are in the range of 0.0176-1.95 Bq/kg, falling into the reported ranges in the background areas from the similar latitude belt. The 240Pu/239Pu atomic ratio range was measured to be 0.146-0.225, which is similar with the global fallout values. Both indicate that the global fallout is the major source of plutonium in this region, and the low plutonium level will not cause any radiation risk so far. Based on the statistical analysis of the possible parameters (organic content, moisture content, average annual precipitation, altitudes, topography and human activity), the large variations of 239,240Pu concentrations were mainly attributed to the retention process related factors including soil organic content and human activity disturbances. While, the deposition related factors including the average annual precipitation, altitudes, topography made insignificant influence on the spatial distribution of 239,240Pu concentrations due to the low 239,240Pu concentrations in atmosphere, less wet deposition amount and insignificant re-suspended amount. The highest 239,240Pu concentrations of 0.805-1.95 Bq/kg were mainly due to the good retention condition in the sampling sites with higher soil organic content and less human activity disturbances.