## 3 Results and Discussion
### Vertical Distribution of Anthropogenic Radionuclides  
Figures 7 and 8 present the calculated isotopic ratios of Cs-137 horizon 10-20 cm/Cs-137 horizon 0-10 cm and Sr-90 horizon 10-20 cm/Sr-90 horizon 0-10 cm. In the lowland tundra zone located southwest of the Nes River basin, there is a marked vertical migration of Cs-137 observed along the 0-20 cm profile with an isotopic ratio greater than 1 for the two layers. In certain lowland areas, the activity of Cs-137 in the 10-20 cm layer surpasses the activity in the 0-10 cm layer by more than twice. A comparable pattern of migration was reported by [50] for a peat section in the Salym area of lowland Khanty-Mansiysk Autonomous Okrug in Russia. Higher Cs-137 activities are observed at a depth of 10-15 cm compared to the overlying layers. Figure 7: Features of Cs-137 migration based on isotope ratio. A low level of vertical migration of radionuclides (isotope ratio for two layers \(<1\)) is observed in the hill, slope, and plakor zones. This may be due to a more intensive runoff process along the sampling profile, resulting in an increase in radionuclide activity towards the lower part of the slope. Similarly, the distribution of Sr-90 in the areas of tundra lowland and slope bears resemblance to the radionuclide Cs-137. A stronger vertical migration trend is observed for Cs-137 compared to Sr-90. Several other studies [51] have demonstrated a similar migration pattern of anthropogenic radionuclides through variations in relief structure. Although Sr-90 has higher mobility in soils, this study's results indicate that its behavior and that of Cs-137 are quite similar in many ways. This may be due to the fact that postsedimentation mobility of radionuclides in soils decreases in arctic conditions, which are characterized by long periods of low temperatures, slow decomposition of organic matter, and minimal bioturbation [23]. Permafrost is a crucial factor hindering deep water infiltration in peat deposits and often results in waterlogging of the active peat layer, which is situated above mineral soils and is seasonally thawed. According to [52], the Nes River basin is situated in the cryolithozem with a discontinuous distribution of permafrost. The seasonally thawed layer's average depth up to 20 km from the Nes River mouth is 0.4 m.  
No peculiarities of Am-241, Pu-238, and Pu-239+240 distribution in two layers in different landforms are observed (Figure 9). Most likely, it is connected to the low content of these radionuclides in samples. The impact of relief on the migration of elements, including radionuclides, has been studied for a long time. For instance, [PERSON] [53] observed a substantial build-up of Cs-137 in lowland areas, while hilly areas are characterized by the transit character of the lateral migration of radionuclide. Additionally, the study highlights the intensified migration of radionuclides in the presence of high soil moisture and water stagnation in depressions. These phenomena are observable in the tundra lowland area southwest of Nes's village. Radionuclides Cs-137 and Sr-90 have significantly accumulated here, and their migration along the vertical profile is the most prevalent. The examination of increased activity in upper soil layers down the slope was also investigated in [54]. The incline with a gradient of 10 degrees resulted in a rise in Cs-137 and Am-241 radionuclide activity levels by 54% and 29%, respectively. In landscape-geochemical systems of the \"plakor-slope-lowland\" kind, cyclic variation of radionuclide markers Cs-137 and Sr-90 in soils was identified based on research [55]. The variability structure of both markers was similar but not identical, attributed to the differing chemical properties and migration peculiarities of these radionuclides. Earlier,  
Figure 8: Features of Sr-90 migration based on isotope ratio. it was noted that the Nes River basin exhibits similar migration properties for Cs-137 and Sr-90 radionuclides, although a more intensive migration is observed for Cs-137.