## 3 Results and Discussion
### Vertical Distribution of Anthropogenic Radionuclides
Figure 9: Features of Am-241 (**a**), Pu-238 (**b**), and Pu-239+240 (**c**) migration based on isotope ratio. It is evident that in assessing the flushing of radionuclides by surface runoff in undisturbed peat areas, it is crucial to be aware of the radionuclide concentration in the upper few millimeters of peat [55]. Furthermore, in order to identify the characteristics of vertical migration, it is essential to extract soil and peat cores from a depth of at least 50 cm owing to the varying degree of deep migration of anthropogenic radionuclides [24]. However, using radionuclide activity ratios in various layers (specifically 0-10 cm and 10-20 cm), this study has distinguished differences in radionuclide activity across important relief structures (plakor, slope, and lowland areas). The obtained outcomes are consistent with prior research, yet they also provide new insights into inadequately researched regions, such as the tundra areas in the subarctic zone of Russia. Estimation of Pollution Sources Based on Isotope Ratio Calculations and HYSPLIT Air Mass Trajectory Model  
To determine probable sources of radionuclide pollution, we performed calculations for isotopic ratios of Sr-90/Cs-137, Pu-238/Pu-239+240, Pu-239+240/Cs-137, and Am-241/Pu-239+240, and created linear regression plots depicting the correlations between these radionuclides (Figure 10). The R\({}^{2}\) values presented in Figure 10 are not relevant to this study. Table 1 illustrates the computed mean isotope ratio and slope of the linear regression model for the Nes River basin, together with a comparison to data gathered from S  
Figure 10: Linear regression plots to assess the relationships between (**a**) Cs-137 and Sr-90, (**b**) Pu-238 and Pu-239+240, (**c**) Pu-239+240 and Cs-137, and (**d**) Am-241 and Pu-239+240. Isotope ratio calculations reveal that none of the results obtained for the Kanin tundra match the characteristics of global fallout. The average values of Sr-90/Cs-137 and Am-241/Pu-239+240 ratios are similar to those of global fallout. However, because the regression slope significantly deviates from the mean value, it suggests that these results do not reflect a single source of pollution. Furthermore, the isotopic ratio of Am-241/Pu-239+240 in global fallout is continually rising as a result of the growing Am-241 activity owing to the decay of Pu-241. This hinders the usage of this parameter in evaluating the origin of pollution. It is worth mentioning the resemblance of the findings, both in terms of the average value parameter and regression slope, to those of [PERSON] et al. (2018); [PERSON] et al. (2018); [PERSON] et al. (2018). Based on the studies conducted, it can be inferred that the primary contributors to radionuclide pollution on Svalbard are the global atmospheric fallout and the Chernobyl accident. As a result, it is reasonable to assume that the same sources, as well as possible local tropospheric fallout from nuclear tests on the Novaya Zemlya archipelago, are responsible for the pollution observed in the Nes River basin. The presence of the Chernobyl fallout is verified through the Pu-238/Pu-239+240 isotopic ratio mean values (0.102) and regression slope (0.0524) in the study area, which differ marginally from the global fallout's value (0.025). According to ([PERSON] et al., 2018; [PERSON] et al., 2018), excessive Pu-238 content characterizes the Chernobyl fallout, and the Pu-238/Pu-239+240 isotopic ratio ranges from 0.19 to 0.55. This explains the excess Pu-238 content in the study area. Additional data for analyzing sources of radioactive pollution is obtained by calculating pollution trajectories using the widely used HYSPLIT model ([PERSON] et al., 2018). The methodology involves acquiring a set of air mass trajectories from specific sources based on meteorological parameters ([PERSON] et al., 2018). The chosen sources were thermonuclear explosions and nuclear fission-based tests executed on the Novaya Zemlya archipelago between 1955 and 1962 ([PERSON] et al., 2018).