## 3 Results and Discussion
### Vertical Distribution of Radionuclides in the Peat Profile  
The activity of \({}^{137}\)Cs in the studied ISNO-1 peat profile varies from 3.2 to 45.6 Bq\(\cdot\)kg\({}^{-1}\). (Figure 4). According to the vertical distribution graph, two pronounced peaks of \({}^{137}\)Cs activity are noted: the first peak with an activity of 45.5 Bq\(\cdot\)kg\({}^{-1}\) is at a depth of 19-21 cm, and the second peak with an activity of 45.6 Bq\(\cdot\)kg\({}^{-1}\) is at a depth of 3-5 cm. This distribution of \({}^{137}\)Cs is typical of raised bogs, where the maximum of \({}^{137}\)Cs activity is characteristic of the upper part of the peat profile [14; 26; 28]. This is partly due to the chemical affinity between Cs and K, which are actively transported together up the peat section by plant roots [5]. Another reason for the high mobility of \({}^{137}\)Cs in high-moor peatlands is indicated by the lack of suitable mineral particles for \({}^{137}\)Cs adsorption [12; 13; 24; 33]. However, in the considered case, in addition to the peak of \({}^{137}\)Cs activity in the upper part of the profile, comparable activity is observed at a depth of 19-21 cm (45.6 Bq\(\cdot\)kg\({}^{-1}\) in the upper part of the profile and 45.5 Bq\(\cdot\)kg\({}^{-1}\) at a depth of 19-21 cm, respectively). A similar pattern of changes in caesium activity and ash content with depth (Figure 3) is probably associated with the mechanisms of caesium input with atmospheric fallout since caesium-137 and ash enter peat with atmospheric fallout [21]. However, for the uppermost layers (up to 14 cm), there are different trends in \({}^{137}\)Cs and ash changes, which are associated with  
Figure 3: Change in the physicochemical parameters of the peat profile by depth. the high mobility and bioavailability of \({}^{137}\)Cs in an acidic environment for peat sphagnum vegetation. As can be seen from the \({}^{137}\)Cs distribution plot, the maximum depth of caesium uptake by vegetation is 14 cm for the studied profile, since below at a depth of 20 cm, one can observe a peak of activity that exceeds the maximum upper readings in value and is probably due to global fallout. Probably, for plots with a different leaching regime and a type of vegetation with a more powerful root system, for example, sedges, shrubs, and woody vegetation, a different distribution of \({}^{137}\)Cs will be observed. The activity of \({}^{210}\)Pb in the studied peat profile varies from 26.2 to 310.8 Bq kg\({}^{-1}\). The maximum activity of \({}^{210}\)Pb (310.8 Bq kg\({}^{-1}\)) falls on the uppermost horizon, which is a natural phenomenon given that the only way \({}^{210}\)Pb enters the peatland is atmospheric fallout. Below the horizon of 35-37 cm, the activity of \({}^{210}\)Pb ceases to change and amounts to \(\sim\)26 Bq kg\({}^{-1}\) in all underlying horizons. Obviously, below the horizon of 35-37 cm, there is practically no excess atmospheric lead, and the observed activity of \({}^{210}\)Pb in these horizons is maintained due to the decay of \({}^{226}\)Ra. However, the near exponential decrease in \({}^{210}\)Pb in the upper part of the peat profile is interrupted by the activity peak at the 19-21 cm horizon, which is 243.9 Bq kg\({}^{-1}\). Taking into account the average rate of formation of peat deposits in the Subarctic region of European Russia [63] and the relatively constant flux of lead from the atmosphere at a depth of 19-21 cm, the activity of \({}^{210}\)Pb should be at least 2-2.5 times less than on the surface of the peat section. The presence in the section of the horizon (19-21 cm) with \({}^{210}\)Pb activity close to the level of the current \({}^{210}\)Pb influx from the atmosphere indicates the downward migration of this radionuclide down the peat profile. It should be noted that the peak of \({}^{210}\)Pb activity in the 19-21 cm horizon also coincides with the peak of \({}^{137}\)Cs activity and the maximum ash content, which could be associated with global atmospheric fallout in 1963.