To conduct the radiological investigations in the Kostomuksha Nature Reserve, the research team collected samples of plants (moss and lichens), as well as leaves, branches and the trunk of a birch tree, making a total of seven samples. Samples of bottom sediments (99 samples) were taken from Kamennoye Lake with the help of a motor boat and a bottom scoop. A soil pit was also extracted at the nature reserve and five samples of different genetic horizons were collected from it. Collected samples of plants, soils and bottom sediments were placed in plastic bags and marked, and appropriate sample details were recorded in the field log. After delivery to the laboratory, the samples were dried in a drying box at 105 °С to constant weight, and the bottom sediments were studied to determine their grain composition.

Radiological investigations of the environmental components within the limits of the Kostomuksha Nature Reserve and adjacent territories were carried out using a variety of radiometric instruments with different functional purposes, including a mobile gamma spectrometer RS-700 (Radiation Solution, Canada), laboratory gamma spectrometer Progress-Gamma (NPP Doza, Russia), and automatic radon radiometer RRA-01М-03 with sampling extension POU-04 (NTM-Zashchita, Russia).

Terrain radioactivity surveys were carried out in situ using a high-precision mobile scintillation gamma spectrometer system RS-700 (2017). The RS-701 system is built with an advanced digital spectrometer (ADS) that has a high definition capability (1024 channels) and can be used for real-time measurements of total radioactivity in cps, as well as separate measurement of total concentrations of uranium (ppm), thorium (ppm) and potassium (% wt.). The system has an integrated GPS receiver for precision geo-referencing of each measurement. The system’s RAD Assist software allows the measurement data to be processed in the field.

Measurements were made at a height of 1 metre above ground level from a vehicle. In total, the profile of gamma spectrometer survey of the nature reserve territory includes over 4500 measurement points for each of the monitored parameters.

Laboratory tests of radioactive elements in the samples of plants, soils and bottom sediments were carried out with the help of a stationary gamma spectrometer Progress-Gamma. Energy calibration of the gamma spectrometer to ensure parameter security was carried out after each measurement using a proprietary calibration source OISN-137-1 (¹³⁷Cs+⁴⁰K). A measuring cell (Marinelli beaker) was used for exposure of the loads.

Studies of the volumetric activity of radon in natural waters, soil air and ambient air were conducted in situ using an automatic radon radiometer RRA-01М-03 with sampling extension POU-04 and included examination of 19 radon samples in total.

The results of measuring gamma activity on the nature reserve territory from north-west to south-east demonstrate lack of uniformity and significant variations in the distribution of total radioactivity. Total gamma activity of the terrain varies from 0.006 mcSv/hr to 0.071 mcSv/hr at an average of 0.04 mcSv/hr. The differentiation of total radioactivity and a large spread of measurement values in the nature reserve is a consequence of the complex geology with diversity of magmatic rocks and presence of faults and dikes. At the same time, maximum values of total radioactivity in the reserve are approximately 0.1 mcSv/hr lower than the average value of natural background gamma radiation (Smyslov 1974), which meets the prescribed radiation safety standards. Concentrations of naturally occurring radioactive elements U, Th and K in the nature reserve are also determined by the variety of rocks in the area. The content of naturally occurring uranium varies in the range of 0.5 to 9.9 ppm, sometimes exceeding the bulk earth values for the earth crust (~4 ppm) by a factor of two. Thorium was also found to have higher than normal (13 ppm) bulk earth concentrations reaching 30 ppm. These high concentrations of U and Th correspond to their natural concentrations typical of alaskite granites, as well as of nephelinic and agpaitic syenites that occur widely across the study area (Smyslov 1979). Percentage abundance of potassium in the earth crust is at 2.4% (Smyslov 1974). The highest concentrations of potassium (up to 4.8 %) that were double its bulk earth value were observed in the acid rocks of the nature reserve.

Presence of rocks within the limits of nature reserve and adjacent territories that feature high concentrations of radioactive elements requires an investigation of their decay products in different media; their primary decay product is the radon gas, which may pose a significant hazard by adding to the background radiation dose. Moreover, several radon sources were found around the town of Kostomushka and within the limits of the nature reserve (Kolomytsev and Shiltsova 1998). One of these sources, to the south-west of the town, is properly organised and used for balneotherapy.

The results of in situ measurements of radon activity within the limits of the nature reserve and adjacent territories in the ambient air, soil air and natural waters are provided in Table 1.

Recorded concentrations of radon in the ambient air were remarkably low, not exceeding 21 Bq/m³, except in the area around the radon spring near Shchuchya Lambine in the north-east of the reserve, where volumetric activity of radon was recorded at 539 Bq/m³. In contrast, the ambient air around the Kostomushka healing radon water spring is characterised by low activity of ²²²Rn (< 20), which is explained by the fact that the spring is enclosed, and there is no direct emission of radon into the air. The values of radon activity in the air of the nature reserve and adjacent territories away from radon springs generally match the average yearly activity of ²²²Rn typical of the north-western regions, which equals 21 Bq/m^{3 1}.

Radon activity in the soil air varies from below 20 Bq/kg to 629 Bq/kg. The wide range of values is explained by presence of radium in the bottomset beds and permeability of the medium. The measured values generally remain within the range typical of the rocks in this area (Korshunov et al. 2012). No radon was found in the soil air of the swampy area in the north-east of the reserve, because water-flooded peat bogs make a perfect screen that blocks migration of radon ²²²Rn from underlying rocks to the surface.

Concentrations of radon in the water of two adjacent radon springs located near Shchuchya Lambine in the north-east of the reserve are 250 Bq/l and 27.5 Bq/l, respectively. These springs are found in the wetland; they are low-yield springs draining into the lambine on top of the moss cover. Radon activity values of 1300 Bq/l for the first spring and 250 Bq/l for the second spring were recorded in 1998 (Kolomytsev and Shiltsova 1998). The difference in radon concentrations in the water is most likely explained by changes in the dynamic state of the springs that occurred over more than a decade after the initial observations. Radon activity in the water of the organised Kostomushka healing radon water spring is 478.8 Bq/l. Surface waters of the nature reserve and adjacent territories, as well as municipal water in the town of Kostomushka, show very low concentrations of radon, below 2.8 Bq/l, while health standards allow a radon concentration of 60 Bq/l in drinking water.²

Data on soil radioactivity measured at different genetic horizons is provided in Table 2.

Podzolic soil is characterised by a rich content of radiocesium in horizon А_оА₁ (2–5), with concentrations reaching 142 Bq/kg. Horizons А₂В (5–10) and Bfe (10–25) are rich in radium and potassium, which is explained by the presence in these horizons of parent rock represented by small pebbles. In general, natural radioactive elements in genetic horizons of podzolic soils are within the range of prescribed bulk earth values (Smyslov 1974).

The content of radionuclides in woody plants of the reserve was found to be lower than, for example, on the Kola Peninsula (Melnik and Kiseyev 2006).

The sphagnum samples collected in the reserve showed a high activity of ¹³⁷Cs isotope among the other studied plant samples (431 Bq/kg). This value is much higher than in the moss samples collected in the Arkhangelsk Region, with a mean radioactivity of ¹³⁷Cs around 40 Bq/kg (Kiselev and Druzhinin 2009), which means that a more in-depth study of the activity of radionuclides in the moss of the nature reserve must be conducted.

Since radionuclides are chiefly accumulated by bottom sediments that absorb over 80% of the radioactivity (Trapeznikov 2007), examination of the bottom sediments appears to be a key objective of radioecological investigations. Bottom sediments in lakes constitute a vital component of the lake ecosystem, carrying exhaustive information about the history of water bodies (Subetto 2009) and accumulating harmful elements.

Some 46% of bottom sediment samples from Kamennoye Lake contained anthropogenic isotope ¹³⁴Cs, whose activity varied from 1 to 12 Bq per kg of dry weight. Mean activity of ¹³⁴Cs is 4.2 Bq/kg. Irregular distribution of the isotope and its presence only in pelite samples in small quantities (below 12 Bq/kg), plus its fairly short half-life (~2 years), may indicate that it is no longer being supplied to the bottom sediments at this time, while the measured values of its activity are residual values from past fallout and the probability of ¹³⁴Cs disappearing altogether over the next several years is very high.

Specific activity of ¹³⁷Cs in bottom sediments of the lake varies from 100 to 873 Bq/kg (Fig. 2).

Maximum specific activity of ¹³⁷Cs isotope (682 to 873 Bq/kg) was found in brown and light-brown silty pelites, and minimum activity (up to 14 Bq/kg) – in grey and brown fine sands. A distinctive feature of the distribution of anthropogenic isotopes ¹³⁴Cs and ¹³⁷Cs is the direct correlation between their presence and the grain composition of the soil. These radionuclides accumulate only in silty pelite sediments. It should be noted that tests of bottom sediment samples from Kamennoye Lake revealed the highest values of activity of anthropogenic ¹³⁷Cs (>800 Bq/kg) among all the studied lakes of Karelia and the Arkhangelsk Region. Mean values of specific activity of radiocesium for the lakes of Karelia and Arkhangelsk Region are within the range of 30 Bq/kg (Kiselev et al. 2017). Similarly high activities of ¹³⁷Cs were observed in the bottom sediments of lakes around the Kostomushka iron ore deposit located in close proximity to the nature reserve. These samples showed cesium activity values of up to 547 Bq/kg (Kiselev et al. 2017). When processing the body of data on the activity of cesium in the lakes of Karelia and the Arkhangelsk Region, we discovered that activity of ¹³⁷Cs varies depending on the longitude of the lake’s location. Concentration of ¹³⁷Cs in bottom sediments increases significantly to the west, reaching its maximum at a longitude of 30°E, at the boundary of the study area near the border with Finland (Kiselev et al. 2017). This pattern is quite likely explained by the nature of global fallout of anthropogenic radiocesium, specifically by the accident at the Chernobyl Nuclear Plant, since the density of soil pollution in Finland and Sweden after the accident was much higher than in Karelia (Bakunov 2013).

In the regional perspective, maximum values of ²²⁶Ra reaching 76 Bq/kg were found in the north-east of the lake, while in the other parts of the lake this isotope’s activity does not exceed 20 Bq/kg, on average. The distribution of the specific activity of ²³²Th varies from 1.2 to 45 Bq/kg, with a mean value of 16.2 Bq/kg. The activity of ⁴⁰K varies from 27.9 to 624 Bq/kg, with a mean value of 230.9 Bq/kg. Mean activities of natural radionuclides in the bottom sediments remain within the range of prescribed bulk earth values.

Measurement data acquired during the radioecological investigations in the Kostomushka State Nature Reserve bring us to the following highlights of the study.

Total radioactivity in the territory of the nature reserve is around 0.04 mcSv/hr, which is almost twice lower than mean global values for natural gamma background radiation. Concentrations of naturally occurring radionuclides in the nature reserve are governed by differences in rock composition. The measured concentrations that were in excess of the bulk earth values for U, Th and K are natural for acid magmatic rocks forming the terrain in the study area.

The ambient air of the study area shows increasingly low concentrations of radioactive radon gas that do not exceed 21 Bq/m³. The concentration of ²²²Rn in the ambient air increases rapidly to 539 Bq/m³ in the area where radon water springs up to the surface. At the organised source of healing radon water near the town of Kostomushka, where there is no direct emission of radon gas into the air, activity of ²²²Rn in the air remains below the limit of detection.

Concentration of ²²²Rn in the water of radon springs in the nature reserve was found to be much lower than during previous studies. This significant decrease in radon concentration in the water of radon springs is most likely explained by changes in ground water dynamics since the original observations. This speaks to the necessity of occasional monitoring of ²²²Rn activity in the springs.

Mean specific activity values for natural radionuclides in bottom sediments of Kamennoye Lake, which covers almost a quarter of the reserve’s surface area, are within the range of prescribed bulk earth values for such isotopes. The pattern of variation of specific activity of radionuclides in the lake’s bottom sediments is governed by the grain and lithological composition of the bottom sediments, as well as the regional features of global fallout.

Development of the Kostomushka iron ore deposit, which involves extraction of rocks characterised by a naturally high content of radioactive elements (granitic gneiss), as well as the operations of the mining and processing plant, do not exert any considerable impact on the radiological situation in the nature reserve. Data obtained during the study indicate that the overall radiological situation in the reserve is acceptable and meets the relevant radiation safety standards. On the other hand, high levels of radiocesium found in the moss and bottom sediments of the nature reserve require additional research to determine the wider pattern of distribution of anthropogenic radioactivity across adjacent territories and to study the processes of buildup and migration of radionuclides in aquatic organisms of Kamennoye Lake.