## 2 Materials and Methods
### Methods  
Peat samples were taken on a 20 \(\times\) 20 cm plot in 0-10 cm and 10-20 cm layers. Sample preparation, radiochemical preparation, and measurements were carried out at the Environmental Radiology Laboratory of the Federal Research Centre for Integrated Arctic Research in Arkhangelsk. The selected samples were brought to dryness in a BINDER E28 desiccator (BINDER GmbH, Tuttlingen, Germany) at 105 \({}^{\circ}\)C. After drying, the soil and peat samples were asked at a temperature not exceeding 400 \({}^{\circ}\)C to avoid loss of radionuclides.  
The levels of radionuclides Cs-137 and Am-241 were assessed using a low-background semiconductor gamma spectrometer, ORTEC (Atlanta, GA, USA), equipped with a coaxial detector GEM40. The detector was constructed using extremely pure germanium (HPGe) and SpectraLine software (version: 1.5.5396, [[https://www.lsrn.ru/](https://www.lsrn.ru/)]([https://www.lsrn.ru/](https://www.lsrn.ru/)), 2 January 2021). A resolution of 1.75 keV was reached along the 1.33 MeV (Co-60) line, and the relative efficiency was 43%. Plastic beakers of varying volumes were selected to determine the activity. The duration of measurement was selected between 2 to 5 h based on the detector loading. Additionally, it was ensured that uncertainties of the photopeak areas at 59.54 keV and 661.66 keV did not exceed 5%. The activity of radionuclide Cs-137 was calculated using the gamma line 661.66 with a quantum yield of 89.90%. The activity of radionuclide Am-241 was calculated using gamma line 59.54 keV with a quantum yield of 35.90%.  
The activity of the radionuclide Sr-90 was assessed through its decay product, Y-90. The activity of Sr-90 was calculated from the measurement of Y-90 separated by radiochemical methods using the following formula:  
\[A_{sr90}=\frac{N_{\mathrm{s}}-N_{b}}{Eff\times M\times ChLos\times K_{abs}} \tag{1}\]  
where \(A_{sr90}\)--Sr-90 activity in the sample, Bq\(\times\)kg\({}^{-1}\); \(N\)--sample count rate on the radiometer, count per second; \(N\)--background count rate on the radiometer, count per second; \(Eff\)--beta-radiation energy efficiency indicator of the radiometer Y-90 (E = 2260 keV), %;  
Figure 2: Digital elevation model of the Nes River basin and shallow bogs in different types of elementary landscapes.  
\(M\)--mass of counting sample, kg; _Chlos_--radiochemical yield of Y-90, %; _Kabs_--beta absorption coefficient, %. After radiochemical preparation, Sr-90 was determined using an alpha-beta radiometer RKS-01 Abelia (NTC Amplitude, Moscow, Russia) and a 10-channel alpha-beta radiometer LB 770 (Berthold Technologies GmbH & Co., KG, Bad Wildbad, Germany).  
Plutonium isotope determination was performed following its radiochemical preparation, which included transferring radionuclides from the sample to the solution, extracting plutonium isotopes and separating them from the matrix and interfering alpha-emitting radionuclides, and electrolytic preparation of the counting sample.  
Radiochemical purification involves extracting and separating plutonium isotopes from interfering alpha-emitters and macrocomponents, followed by electrolytic precipitation. A solution consisting of 30% tributyl phosphate (TBP) in toluene is used as the extractant. Chromatographic purification is carried out using anionic AB-17-8.  
Following the radiochemical preparation, plutonium isotope activity was determined with an alpha-radiation energy spectrometer SEA-13P1 (NTC Aspect, Moscow, Russia).  
The pollution densities of anthropogenic radionuclides were determined by accounting for both the sampling area and the total mass of the sample.  
The main measured unit in this paper is the pollution density expressed in Ci\(\times\)km\({}^{-2}\) and Bq\(\times\)m\({}^{-2}\) units. We have chosen to use the unit Ci\(\times\)km\({}^{-2}\) for ease of comparison with the existing data on pollution in Russia.