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Article

Assessment of the Impact of Anthropogenic Drainage of Raised Peat-Bog on Changing the Physicochemical Parameters and Migration of Atmospheric Fallout Radioisotopes in Russia’s Subarctic Zone (Subarctic Zone of Russia)

by
Evgeny Yakovlev
*,
Alexander Orlov
,
Alina Kudryavtseva
,
Sergey Zykov
and
Ivan Zubov
N. Laverov Federal Centre for Integrated Arctic Research of Ural Branch of the Russian Academy of Sciences (FECIAR UrB RAS), 109 Severnoj Dviny Emb., 163000 Arkhangelsk, Russia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(9), 5778; https://doi.org/10.3390/app13095778
Submission received: 20 March 2023 / Revised: 4 May 2023 / Accepted: 5 May 2023 / Published: 8 May 2023
(This article belongs to the Section Earth Sciences)
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Abstract

:
This paper reports on the research results of the radioactivity levels and upward dispersion of radionuclides of atmospheric fallout 210Pb, 137Cs, 241Am and 234,238U as well as key physicochemical parameters in a peat deposit subjected to drainage in 1969–1971. It was found that drainage of the peat bog led the natural moisture content to shrink in the peat. Active aeration in the peat deposit, accompanied by the growth of Eh, raises the strength of oxidative transformation processes of the organic part of the peat, leading to higher levels of peat degradation and major variations in the elemental content. Changes in these parameters affect the behaviour of radionuclides in the peat section. Thus, in contrast to the sites with a natural hydrological regime, within the dried peat deposit a geochemical barrier of sorption type is not formed, capable of retaining a significant proportion of 137Cs. In this connection, there is an acceleration of 137Cs migration. In addition, changes in the hydrological regime led to the predominance of vascular plants with a more developed root system than that of sphagnum, which also contributed to a more intense transfer of 137Cs by the root system up the section. The findings of 210Pb dating of the dried peat bog showed a significantly lower peat accumulation rate compared to the natural bog massif (0.15 ± 0.02 vs. 0.48 ± 0.08 cm/year). A factor examination of the dataset demonstrated that the leading role in the distribution of radionuclides belongs to redox and acid-base conditions, which have changed significantly after draining the peat deposit.

1. Introduction

Marshlands are complex auto-regulating and dynamically changing systems. Their basic constituents (characteristic of plants and animals, germs, water environment, peat bog, etc.) are deeply interconnected, and changes in any one of them bring about a considerable transformation of the whole system. Disturbance of the natural state of bog ecosystems may occur both in the course of economic–industrial development of these territories (for example, during drainage) and due to global climatic processes, such as global warming, which will certainly be accompanied by changes in the hydrological regime and physicochemical parameters of peat deposits and may cause ecological risks [1,2,3,4,5,6,7,8,9,10]. For example, peat deposits act as a difficult oxidation reduction, sulphide and sorption geochemical barrier [11]. Changes in redox and acid-base conditions can cause the mobilisation and migration of toxicants and radionuclides [12,13,14,15] both through their transition from reduced to oxidised forms and due to changes in the binding capacity of the peat organomineral matrix. The greatest danger among pollutants in terms of chemical mobilisation from the peat deposit is represented by man-made radionuclides, which have a high toxicity for biota and humans and long periods of decay [16,17,18]. As the main biocenoses in Northern Europe, the upland peatlands have become significant concentrators of man-made radionuclides emissions into the atmosphere because of nuclear weapons testing and radiation emergencies [19,20,21,22,23]. At present, a large spectrum of long-lived artificial radionuclides has been detected in the peatland of Northern Europe, which mainly comes from two sources: the 1963–1964 atmospheric test maximum and the Chernobyl nuclear power plant accident [24]. Radionuclides deposited in the northern peatlands on the one hand cause concern in terms of increased dose loads on the biota of peat-bog ecosystems, and on the other hand, require caution in the economic use of peat for fertilisers or fuel [25]. The drainage of wetlands for construction or agriculture disrupts the integrity of peatlands, their natural water and chemical regimes, triggering the destruction processes of the organic matter that retains radionuclides [26,27]. However, in organic matter there is a large gap in the assessment of activity levels and the migration of radionuclides in peatlands subjected to economic development. Few studies address the issues of changes in radionuclide content in connection with the development of peat deposits [28], while the impact of drainage processes on the behaviour of radioisotopes is poorly covered. To address this gap, we conducted research on the example of a small area of the Ilassky peat massif (Arkhangelsk oblast), which was drained with ditches in the later part of the 1960s and the first part of the 1970s to reduce watering of the area during the construction of the highway. As an object for comparison, we used a site with a natural hydrological regime within the same Ilassky massif, the initial (background) physicochemical and radiation parameters of which we studied earlier [29]. Given the single genetic type and the insignificant remoteness of the natural and drained sites from each other, we can assume that the initial concentrations of atmospheric radionuclides arriving at the marsh surface were the same, while differences in the hydrology significantly affected the further redistribution of isotopes. The proof of this working hypothesis is the subject of our paper. In this study, we set out to assess how draining a peat deposit affects the migration of radionuclides as a function of differences in physicochemical parameters. Among the radionuclides of the atmospheric fallout, we considered man-made 137Cs and 241Am and natural 210Pb, 234U and 238U. Among the physicochemical characteristics of peat, we studied the concentration of ash and organic matter (opposite parameter), carbonates, water soluble salt, moisture content and bulk density. The unstable parameters considered were the oxidation level, pH of salt and aqueous extracts. The elemental composition of peat core was also studied (C, H, O and N).

2. Material and Methods

2.1. Study Area and Sample Collection

The ISO-1 peat column sampling (64°19′16″ N, 40°41′01″ E) was conducted within the large Ilassky marshland, which is situated ~20 km to the south of Arkhangelsk city. The samples were taken in the eastern part of the Ilassky marshland, which was subjected to drainage in 1969–1972 by open drainage to remove excess moisture from the road-bed during highway construction [30]. The sampling location and photo of the drained peat-bog area are shown in Figure 1.
In morphological terms, an ISO-1 site is an area of convex oligotrophic bog in a state of effective drainage. The vegetation cover includes a moss-lichen layer, represented by Pleurozium schreberi green mosses, Politrichum commune, Dicranum bergeri and D. Polysetum, with a slight admixture of sphagnum mosses and Cladonia lichens; a herb shrub layer, composed of Vaccinium uliginosum, V. myrtillus and Ledum palustre; and a tree layer, consisting of Pinus sylvestris up to 7 m high.
The ISO-1 peat core is 1.5 m thick. The peat is homogeneous in its botanical content and consists of Sphagnum Moss as well as a few admixtures of shrubs, cotton-grass and pine wood throughout the peat profile. The degree of decomposition (R) varies between 7–35% and increases with depth. It should be noted that a slight increase in the R in the subsurface layer (0–10 cm), caused, apparently, by activation of the microbiological degradation of peat organic matter in an area of increased aeration. The basement rocks are a moraine of medium-textured structure, as the formation of the peat deposit began immediately after glacier retreat [31]. The groundwater level was between −9 and −42 cm throughout the study period.
The unbroken peat core up to 50 cm was removed by a PVC pipe. Following transportation to the laboratory, this peat core was divided into 2 cm layers, excluding the top 0–4 cm layer, and readied for next analysis steps. The peat samples were dried and reduced to a fraction of 0.5–2 mm by a series of sequential grinding in a rotary mill and sieving through a sieve with holes of an appropriate diameter. The obtained samples were used to assess the physicochemical parameters: active and exchange acidity of peat, water soluble salts, organic matter and ash content and C, H, N-elemental analysis. To determine the specific activity of 137Cs, 210Pb and 234,235,238U, the peat samples were additionally ground and sieved to a fraction of 0.1 mm. To estimate the botanical composition, the level of degradation and the natural saturation of humidity, the peat samples without sample preparation (with an undisturbed structure and natural moisture) were used. A summary of procedures for determining the radionuclide activity and physical and chemical values is given on the following pages.

2.2. Study of Physical, Chemical and Physicochemical Characteristics of Peat

The degree decomposition of peat was assessed by microscopy in transmitted light according to the area taken up by the degraded fraction in the aqueous preparation of peat on a slide. We analysed the species composition of vegetation according to [32,33]. The plant determination was made in five repetitions for each investigated layer (0–10, 10–20 cm, etc.).
The natural moisture content was determined by the gravimeters method based on sample weight loss while drying at 105 °C in a desiccator [29].
The bulk density of the peat samples was evaluated using the gravimeter method for the dried samples with a fraction size between 0.5 and 2 mm. An assessment of the bulk density was performed by a measurement of the samples volume with a calibrated cylinder followed by weight measurements in five parallel determinations [29].
The active and exchange acidity of peat samples were assessed by direct potentiometry in aqueous and chloralkali peat suspension, respectively. In the obtained aqueous suspension, we additionally evaluated the content of water-soluble salts by the conductometric method, as explained by Yakovlev et al. [29].
The ash content, concentrations of organic matter and CO32− were determined by the gravimeters method, by combustion and sequential burning of the peat layer in a muffle furnace at temperatures of 525 °C and 900 °C to a stable weight [29].
The C, H and N content was estimated by the EuroEA 3000 CHN analyser (Eurovector, S.p.A.). Content of O was defined as the difference between the total mass and total content of the other elements (C, H and N). Based on the data obtained, the oxidation level of the peat was estimated as described in [34].

2.3. In Situ Determination of ORP and PH of Peat Core

The evaluation of ORP (oxidation reduction potential) of the peat core layers was made by direct potentiometry with the original probe measuring devices developed by the authors. The measurement of ORP was carried out directly in the natural peat massif, with no peat samples extracted, excluding the effects of air. In addition to measuring ORP, the measurement of the temperature of each layer of the peat core was also recorded. Field pH determinations were also performed by the potentiometric method in peat water squeezed from the peat horizons (0–10, 10–20 cm, etc.). The methodology of measuring ORP and pH and converting the resulting ORP data in Eh4 (to normal circumstances—t = 25 °C with pH = 4.0) are presented in more detail in [29,35].

2.4. Gamma Spectrometry Measurements

The radionuclides 241Am and 137Cs were detected with a gamma-ray analyser CANBERRA Packard (USA) with a GX2018 coaxial Ge(Li) crystal-based semiconductor detector and Genie-2000 program. The resolving power of the gamma-ray spectrometer at 1.33 MeV (60Co) energy equals 1.75 keV and its comparative effective efficiency is 22.4%.
Detector calibration by energies and the estimation of registration efficiency was performed using standard samples with a set of radionuclides 241Am, 109Cd, 88Y, 137Cs and 152Eu. A flat vessel of 0.1 L volume was selected as the size of the geometry of the test due to the small mass for peat samples. The sample was measured for a period not less than 18,000 s.
The total activity of the 137Cs nuclide was measured by using the 661.66 gamma energy line having a quantum efficiency of 89.90%, which belongs to its 137mBa radiation nuclide. The radionuclide 241Am activity was detected with the 59.54 keV line with a gamma-quantum emission of 35.9 [36].
The minimum measured energies at exposition t = 18,000 s for the 0.1 litre flat vessel geometry for the GX2018 detector amounted to 0.3 Bq of 241Am and 0.1 Bq of 137Cs.

2.5. Determination of 210Pb and U Isotopes

To determine 210Pb, a 5 g peat sample was decomposed with HNO3 under heating and H2O2. After decomposition, the sample was leached with hydrochloric acid (1:2) at boiling for one hour. Leachate solution was cleaned by hot HCl (1:4) passing through the filter. The residue was then leached along with the filter with HCl and H2O2 in a 1:2 ratio for 30 min. Afterwards, the filter solution was filtered again, and hot HCl (1:4) was passed through the filter as described above. HClO4 was poured over the total filtrate and vaporised to moist salts, then diluted in concentrated hydrochloric acid by boiling and the mixture was decolorised using ascorbic acid. The isotopes 210Po and 210Bi were extracted from the liquid through electro-chemical precipitation on a metal disk that had been simmering for two hours. The obtained sample was tested after 10 h of preparation because it was necessary for the coprecipitated short-lived decay products (218Po, 214Po, 216Po, 212Po; 210Bi and 210Bi) to decay. Activity measurements were performed using an Abelia alpha–beta radiometer [37].
To determine 234U and 238U, a 5 gram sample of peat was sprayed into a muffle kiln at about 600 °C. After being ashed, a 1 cm3 tracer of the radioisotope indicator RIK-232 containing 232U was included in the sample. The sample was then degraded by heating with strong hydrochloric and hydrofluoric acids in a ratio of 1 to 4. Radio-chemical purification of uranium from co-precipitated thorium and radium isotopes was carried out by extraction of 7 M HNO3 tributylphosphate [38]. Next, the cleaned U radionuclides were precipitated on metal disks by electrolysis for thirty minutes at 2A. The samples were measured on semiconductor alpha spectrometers Progress and Multrad-AS (NPP Doza, Russia).

2.6. 210Pb Chronology of the Peat Core

To perform the dating, we used the CF model [39,40,41,42]. For the specified dating model, we used the method of estimation provided by Sanchez-Cabeza and Ruiz-Fernandez [43].
The basic hypothesis of the CF model is the permanent flux of 210Pbex to the peat core area (fi = f(i) = k). The equation used: C i . t = 0 = f r i или f = C i , t = 0 r i [43].
To perform dating procedures for the ISO-1 peat profile, total 210Pb (210Pbtot) activity concentrations varying in depth zi profile were obtained. The equilibrium fraction of lead-210 (210Pbsup) was then calculated as the average (±SD, 1σ) activity of the 15 lower layers where the levels of 210Pbtot reached a steady state 210Pbsup = 21.77 ± 1.90 Bq/kg. Subtracting the supported lead activity 210Pbsup from the total 210Pbtot activity layer by layer, the unsupported portion of the radioisotope (210Pbuns) was determined. Only the 210Pbuns values were used for all subsequent dating steps. To verify the dating data, we used an independent 241Am chronometer characterised by insignificant migration along the peat section [44,45,46]. Peaks of 241Am, as well as a number of other man-made isotopes in natural sediments, correspond to the dates of intense radioactive fallout [47].

3. Results and Discussion

3.1. The Physical and Chemical Parameters of Peat Core

Active acidity levels in researched peat core range from 3.50 to 4.38; meanwhile the exchange acidity was in the angrier pH range and amounts from 2.56 to 3.48 (Figure 2). Based on the information received, the investigated peat can be referred to the highly acidic type as the pH is below 4.5 units. The pH of peat is determined by the existence with available acids, such as acetic acid, formic acid, oxalic acid and various types of acids [48]. Vertically, the active and exchange acidity are observed to decrease insignificantly with depth.
There are increased pH values in the near-surface layer of peat 0–6 cm associated with leaching low-molecular-mass organic acids, which can affect the mobility of radionuclides. On the other hand, the washout of low-molecular-mass acids deep into the peat core, where reducing conditions prevail, may contribute to the processes of acetoclastic and autotrophic methano genesis, during which H+, acetate and formate ions are actively consumed [49,50]. The content of carbonates in the studied peat core is in trace amounts and varies between 0.05 and 0.32%. The curve of the vertical range of carbonate concentrations demonstrates its gradual decrease, which is limited to a depth of 38–40 cm, which practically coincides with the minimum of the bog water level for the studied period. The underlying part of the peat profile (40–50 cm), on the contrary, is characterized by a slight increase in the carbonate fraction.
The content of ash value for the peat core is between 0.69 and 6.21%. In the vertical distribution, the maximum content of ash values is observed in the 6–8 cm horizon, perhaps because of washout of ash contents out of the top peat layers and its accumulation in there.
Previously, for the natural site (ISNO-1 [29]), the maximum ash content was fixed at a depth of 19–21 cm, whereas at the current dried site there is a significant shift of the horizon enriched in ash components upward along the section, which is probably associated with compaction of the peat structure and decomposition of its organic part due to drainage. Anthropogenic impacts manifested in the history can also cause a sharp increase in the ash index [51]. Below the depth of 6–8 cm, there is a trend of a non-monotonic reduction in the ash content in the transition on the bottom levels of structures, linked to the atmospheric type of swamp feeding. A lot of the research ash from peat is 98–99% aluminium, oxides of silicon, calcium, iron, sodium, magnesium, sulphur and phosphorus. [49]. The organic matter is 94–99%, which allows us to categorise the peat massif as a low-ash type with a low level of degradation [52].
The water soluble salts concentration varies between 0.52 and 2.73 mg/g. The highest concentration of salts is in the top peat horizon (0–4 cm). Under the horizon, an almost monotonic decrease in this indicator is observed, which is caused by the atmospheric mechanism of the deposit feeding as well as by the activity of the rhizosphere of the peat formers.
The obtained values of the physicochemical parameters (Figure 2) identify the researched peat as an acidic phase of oligotrophic peat soils [53].
Based on the data of the experiment, the studied section of the peat core profile consists primarily of sphagnum and sphagnum-furry types of peat. At the same time, the natural site was characterised by the prevalence of peat sphagnum species. The trends of changes in structure-sensitive peat parameters, such as the level of decomposition® and bulk density (Table 1), for the horizons of the ISO-1 peat core differ in principle from the core of the same peat-bog with a natural hydrological regime, characterised in detail in the work [29]. Their change along the depth of the layer (0–50 cm), which includes the acrotelm and the upper part of the catotelm, and, as a consequence, the most exposed to the influence of swamp water level fluctuations and sensitive to drainage, is not monotonous. The most significant increase in the degree of degradation and bulk density is observed in the 0–10 cm layer (deposit layer formed since the beginning of drainage works in 1971–1975) (Table 1) and is apparently caused by the activation of the microbiological decomposition organic matter of the peat in the zone of increased aeration. In general, dewatering is largely accompanied by vertical shrinkage of the major part of the peat deposit; in particular, due to a significant decline in proportion of moisture in the peat core, leading to the compaction of its structure. Therefore, the values of bulk density 0.185 ÷ 0.410 g/cm3 are recorded for the dried area, which significantly exceeds this parameter that we previously recorded for the natural section of this bog (0.075 ÷ 0.109 g/cm3) [29] and other upland peat lands of the Pribelomorsky type in undisturbed conditions [34,54]. The same patterns are observed for the natural density of the deposit; one of the key parameters used to assess the dynamics is the accumulation of the peat deposit.
As noted above, drainage is expected to be accompanied by a decline in the natural moisture proportion. The moisture content of the drained area does not have clearly expressed dynamics on the depth of occurrence and is between 6.3 ÷ 9.4 g/g and is significantly lower than the values established earlier for this bog site with a natural hydrological regime (15 ÷ 24 g/g) [29]. Such a change in this parameter in combination with the fluctuation of the bog water level leads to large fluctuations in the aeration and redox regime of the deposit. On the one hand, a decrease in the bog water level contributes to oxidative processes, but prolonged drainage may contribute to the compaction of the peat structure, which, apparently, may be the reason for a decrease in the diffusion of atmospheric oxygen into deeper layers of the peat core.
The trend of the Eh4 and rH parameters (Table 1) by the depth of the analysed peat column indicates the existence of an expressed redox regime, in that the moderate-oxidizing conditions are slowly changed by reducing ones when moving from the surface to the depth of the peat core. The average indicators (for the spring–autumn period of 2021) of the redox potential for the 0–50 cm layer, recorded within the site with stable drainage, are within 195 ÷ 521 mV, which is slightly higher than we noted earlier (109 ÷ 428 mV) [29] for the site with a natural hydrological regime. This is caused by a wider interval of bog water level fluctuations, which was −42 ÷ 0 cm during the sampling period. It is also necessary to note the greater seasonal variability of the Eh4 parameter for the drained site compared to the natural site, which is associated with greater temporal instability of the swamp water level.
The proportion of major components in the peat organic matter was 0.3 ÷ 1.0 (N); 31.0 ÷ 35.0 (C); 43.4 ÷ 47.3 (H); 16.6 ÷ 23.6 (O) atomic %. (Figure 3, Table 2).
The obtained values slightly differ from the results previously obtained for this bog area with a natural hydrological regime [29], which is probably associated with the intensification of the oxidative and microbial processes of biogeotransformation of organic matter of peat. These processes result, in particular, in a reduction in the N content, growing in the O composition and the oxidation level parameter (ω), as well as a decrease in the H/C ratio. The level of oxidation of organic matter (ω) for the site with sustainable drainage is in the range of −0.39 ÷ 0.09 (−0.66 ÷ −0.29) for the natural site [29] and varies nonlinearly with the depth of the profile, probably due to the course of differently directed oxidation-reduction processes, the upright transport of organic matter in the peat core and also the individual characteristics of the original chemical content of the peat-forming plants. However, the most pronounced growth of the oxidation level (ω) relative to the natural site is characteristic of the layer of 0–40 cm, which is limited to the fluctuation level of groundwater (0–42 cm).
The H/C ratio varies in the interval 1.3 ÷ 1.5, indicating a distinct prevalence of aliphatic constructions in the organic fraction of the core (H/C > 1) [55], and varies insignificantly along the depth of the studied profile and changes insignificantly along the depth of the studied profile. It should be noted that the H/C ratio recorded for the peat of the drainage area is less than that for the area with the original hydrological regime of 1.3 ÷ 2.0 [29], which indicates a decrease in the content of aliphatic (primarily carbohydrate) compounds in the organic matter of the peat depth of the studied profile. Thus, we can say that the aliphatic component of the peat undergoes biodegradation in the course of the deposit drainage, while the aromatic part is more thermodynamically stable. The C/N fractional index is 33 ÷ 114, which is common of oligotrophic peatland formed mainly by sphagnum mosses [56] and indicates a strong depletion of nitrogen during the drainage processes.

3.2. Variations in the Radioisotopes Content in the ISO-1 Peat Profile

The 137Cs concentrations in the analysed ISO-1 peat core sampled in the dried section of the Ilassky bog ranges from 2.6 to 142.6 Bq/kg (Figure 4). The maximum activity (142.6 Bq/kg) is concentrated in the upper horizon 0–4 cm. In the underlying horizon of 4–6 cm, the activity of 137Cs falls sharply and makes 68.8 Bq/kg with the further smooth decrease down the section without expressed peaks. The 137Cs distribution in the ISO-1 profile corresponds to the typical distribution pattern in upland peatlands, which is explained by the high mobility of radio-caesium in peat waters and its absorption by both sphagnum mosses, which are not demanding for mineral nutrition, and by more demanding vascular plants [22,47,57]. However, this distribution is characteristic, as a rule, for natural bogs without anthropogenic impact on peat deposits [24]. At the same time, in addition to the 137Cs maximum concentration in the upper part of the section, a similar maximum of 137Cs activity at 19–21 cm was detected in the natural section within the studied bog massif (core ISNO-1). Studies of the physical and chemical characteristics of the natural peat core have revealed the presence of a complicated geochemical hurdle of redox, sulphide and sorption sort in the 19–21 cm horizon, conditions leading to the containment of 137Cs [29]. This is probably due to a disturbance of the natural hydrological regime of the dried ISO-1 core, in which the formation of a pronounced geochemical barrier does not occur and there are no factors limiting the migration of 137Cs up through the section. This is especially noticeable in the differences in the 137Cs activities in the upper horizons of the natural ISNO-1 and dried ISO-1 peat core (45.6 Bq/kg in the ISNO-1 profile and 142.6 Bq/kg in the ISO-1 profile, respectively).
With close total stocks of 137Cs in both peat deposits, changes in the hydrological regime lead to significant differences in the vertical distribution of 137Cs. From this, we can conclude that drainage of the peatland in the conditions of the Arkhangelsk oblast leads to the mobilisation of 137Cs and accelerates its migration up the section.
The 210Pb concentration in the ISO-1 peat column sampled at a site with a disturbed hydrological regime ranges from 6.3 to 567.9 Bq/kg. The maximum activity of 210Pb (567.9 Bq/kg) falls on a horizon of 4–6 cm. In the uppermost horizon of 0–4 cm, the activity of 210Pb is 500.8 Bq/kg. The concentration of 210Pb maximums in the upper horizons is a natural process confirming the entry of 210Pb into the peatland with atmospheric precipitation [58,59,60]. Below the 4–6 cm horizon, 210Pb activity decreases exponentially to a depth of 20–22 cm (from 567.9 to 23.9 Bq/kg). From the horizons 22–24 and down to the lowest layer, the activity of 210Pb ceases to change and varies slightly from 19 to 23 Bq/kg, indicating no excess atmospheric lead in the lower layers. The vertical distribution of 210Pb in the ISO-1 profile also shows significant differences from the site with a natural hydrological regime of ISNO-1 [29]. Therefore, the natural deposit of ISNO-1 is characterised by the active leaching of 210Pb from the overlying horizons and concentrating on the geochemical barrier at the range of 19 to 21 cm. At the same time, the concentration of 210Pb at the range of 19 to 21 cm is close to the values of the upper horizon (310.8 Bq/kg in the 0–3 cm horizon and 243.9 Bq/kg in the 19–21 cm horizon, respectively), indicating the intensive downward migration of 210Pb under conditions of the natural hydrological regime. The differences in 210Pb activities between the top horizons of cores from the dried ISO-1 and natural ISO-1 sites (567.9 Bq/kg vs. 310.8 Bq/kg) with comparable reserves are probably associated with this process; that is, redistribution of 210Pb across the section is observed. What is also significant is the difference in the depths in which the presence of excess lead is recorded. In the natural deposit of ISNO-1, excessive lead was fixed at a depth of 35–37 cm, while in the drained area atmospheric lead was not detected below the horizon 22–24. This indicates significantly lower rates of peat mass accumulation within the dried section of ISO-1, associated with compaction of the peat. The accumulation of the ISO-1 peat column will be discussed in more detail in the section “210Pb dating”.
The U concentration in the dried ISO-1 peat core fluctuates between 0.20 and 2.37 mg/kg, for a mean concentration of 0.75 mg/kg, which is the norm for peatlands of the damp areas of the European North (subarctic zone) of Russia [61]. The range of variation and mean value of uranium concentration in the ISO-1 profile is comparable to the natural site of ISNO-1 (0.10–3.06 mg/kg, average 0.83 mg/kg). In a vertical distribution of uranium in the ISO-1 profile, an interval of 6–18 cm with a maximum concentration of U is clearly distinguished, which is probably due to the behaviour of past U deposits with atmospheric deposition to the top of the peatland. The uranium isotope ratio of 234U/238U varies between 0.88 and 1.82, which is generally the typical range for surface waters, whose main source of nutrition is atmospheric precipitation [62].
In addition to the man-made isotope 137Cs, the trans-uranic radionuclide 241Am was found in the dried peat deposit of ISO-1. Significantly, 241Am was detected only in the depth interval of 8–20 cm with an activity variation from 1.0 to 2.1 Bq/kg. In other horizons, the activity of 241Am was below the detection limit. At the same time, ISNO-1 was not reliably detected on any horizon at the natural site. Given the proximity of the ISO-1 and ISNO-1 sites and the apparently uniform atmospheric flux of 241Am, we can assume that the possibility of detecting 241Am in the dried section of ISO-1 is related to the compaction of the deposit and the “concentration effect”. An evaluation of the influence of changes in the physical and chemical parameters of the dried peat column and radionuclide activity is presented in the “The statistical analysis” section.
Thus, variations in the hydrological regime of a peat deposit resulting from drainage contribute to important variations in both the physicochemical parameters and the vertical distribution of a selected number of radioisotopes. However, in spite of the fact that the physicochemical regime of the deposit is an important criterion of radioisotope distribution and migration, in this study we practically did not consider the influence of bog water movement and the biogenic component on migration processes. For example, in [63] the predominant role of biogenic migration of radionuclides for forests and associated landscapes is noted. At the same time, researchers [54] have fixed considerable modifications in species diversity of the flora, structure and space distribution of plant cover during drainage processes that, in our opinion, undoubtedly will reflect on migration features of radionuclides in the deposit and demands additional attention. This is particularly important because changes in the structure and chemical content of peat deposits as a result of bog drainage are not reversible and are not completely recompensated by rewetting [54], as it limits the possibilities for restoring drained oligotrophic peatlands to initial condition.

3.3. 210Pb Dating

The behaviour of 210Pb over the analysed peat deposit has an almost exponential character with a slight fluctuation of activity in the upper part. The R-square determination coefficient for the entire data series ln (210Pbuns) is 0.96, which is acceptable for the CF dating model (CRS) used by us and indicates an insignificant effect of migration on 210Pb [64].
The dating data for the ISO-1 peat column are presented in Figure 5. Based on the fact that below the horizon 22–24 cm excess lead is not detected, the model is limited to a layer of 20 cm, the date of formation of which corresponds to 1896. The age of the horizons below 22 cm is more than 150 years old and beyond the 210Pb dating method.
A comparison of data on the distribution of the independent marker of man-made 241Am indicates the existence of an activity peak in the 12–14 cm horizon, which according to the CF dating model (CRS) corresponds to the age of 1963 (Figure 6), the period of maximum atmospheric 241Am arrival to the earth’s surface [65,66,67].
The consistency of these data indicates the correctness of the chosen model and the adequacy of the dating results in relation to the ISO-1 peat core.
During the dating of the ISO-1 profile, the linear rate of peat accumulation s (SAR) was calculated (Figure 7).
Thus, the linear accumulation rate varies from 0.05 ± 0.03 to 0.24 ± 0.02 cm/year, the mean value is 0.15 ± 0.02 cm/year. At the same time, for the previously studied ISNO-1 core sampled at the site with a natural hydrological regime, s values varied in a wider range (from 0.09 ± 0.02 to 1.3 ± 0.05 cm/year), and the average accumulation rate was more than three times higher than 0.48 ± 0.08 cm/year. Obviously, this is due to changes in the hydrological regime at the ISO-1 profile sampling site due to drainage, in which there is significantly less accumulation of peat biomass and compaction of the peat.
The calculated meaning of the atmospheric flux of 210Pb to the peatland surface was 77 ± 4 Bq/m2·yr, which is comparable to the above data from the [64,68] and our earlier data for the European subarctic (zone) of Russia for an ISNO core, a value was 69.13 ± 10 Bq/m2·year [29].

3.4. The Statistical Analysis

The correlation matrix on measured parameters in the peat profile ISO-1 is displayed in Figure 8. A significant positive correlation (r ˃ 0.50) is observed for the sampling depth, pH of the water extract, organic matter, C and H content, indicating a tendency for these parameters to increase with increasing depth.
At the same time, high negative relations (r < −0.70) are characteristic for the depth and activity of radionuclides 137Cs, 210Pb, U, ash, water-soluble salts, Eh and rH, which in turn indicates a decrease in these parameters with depth. The isotope 137Cs has a very high positive correlation with 210Pb (r = 0.91), which confirms its single input source. Additionally, 137Cs is positively correlated at the high correlation level (r ˃ 0.70) with carbonates, ash components, pH of salt extract and water-soluble salt content. 137Cs has a noticeable positive relationship (r ˃ 0.50) with the parameters Eh and rH. 137Cs has a high negative bond (r = −0.72) with organic matter. 210Pb is characterised by a high positive correlation with ash components (r = 0.83) and the water-soluble salts concentration (r = 0.79). A noticeable positive relationship (r ˃ 0.50) is observed for the 210Pb-carbonate and 210Pb–pH salt-extract pairs. 210Pb has a high negative bond with organic matter (r = −0.83). The uranium isotope ratio of 234U/238U is virtually unrelated to the other parameters studied except for the presence of a moderate correlation with N (r = 0.32).
Uranium at the level of high positive correlation (r = 0.75) correlates only with ash components, with the correlation of water-soluble salts, O content, oxidation degree, Eh and rH values being noticeable (r from 0.5 to 0.7). For the uranium content, there is an inverse high correlation with organics (r = −0.75); a marked inverse relationship is observed with the pH of water extract and H content (r = −0.64 and r = −0.53, correspondingly). The relationships of other physicochemical parameters and components of the elemental composition are shown in Figure 3. As shown in the correlation matrix, there is a rather complex structure of the connections between the involved parameters, but these general trends indicate that the distribution of 137Cs, 210Pb and U is controlled mainly by the content of carbonates, ash, pH of salt extract, water soluble salts, Eh and rH. Those identified relationships are in line with the data of a factor analysis, as shown in Table 2. It should be noted that a couples correlation analysis does not always give clear and correct results for complex natural systems, where the dependent variable is simultaneously determined by the influence of several characteristics.
To identify the nature of general relationships between the studied parameters of peat and the accumulation of radioisotopes in the ISO-1 peat core, a factor analysis was carried out. A summary of the factor analysis for the ISO-1 core is shown in Table 2 and Figure 9. Numerically, a data set has been divided by five factors (group) explaining 93.98% of the total dispersion. The first factor with a variance of 42.46% on high loadings isolated the characteristics of the elemental content of organic matter—O, H/C, O/C and the oxidation level (ω), and physicochemical parameters—Eh and rH. This is logical and confirms the existing idea that the oxidation-reduction mode of the peatland is one of the key factors affecting the course of chemical and biochemical processes of the biogeotransformation (diagenesis) of organic matter of plant residues in peat [5,15,69].
For the second factor with a variance of 27.49% of the general amount by positive loadings radionuclides 137Cs, 210P and U, the ash content, water-soluble salts and physicochemical parameters Eh and rH are isolated. The association between radionuclides and ash is explained by their main influx with the atmospheric fallout. The detected influence of the Eh (rH) parameter on the radionuclide distribution is due to the following: redox and acid-base conditions are the determining factors for the existence of different forms of U and 210Pb in the deposit, whose solubility is unequal, and thus affect their mobility.
You can also identify the indirect influence of Eh on the distribution and mobility of 137Cs, 210Pb and U through the influence of redox conditions on the state of the organic and mineral component of peat, involved in the procedures of physical and chemical adsorption of these pollutants [70,71]. In particular, Husson [15] found that Pb binding increases with an increasing pH and decreasing Eh, which is explained by its interaction with organic matter, as well as Mn compounds and sulphur sulphides.
The third factor accounts for 11.58% of the dispersion with high factor loadings on 137Cs, 210Pb and CO32− carbonate content, water soluble salts, salt extract pH and N. The isolation of 137Cs and 210Pb in this factor indicates the influence of additional mechanisms explaining the migration of these radionuclides in the peat core of the dried section of ISO-1. Apparently, most of the 137Cs in peat and soils [72] is bound reversibly by the ion-exchange mechanism, as evidenced by the presence of a strong relationship with the exchangeable acidity index pHsalt. In addition, a small part of 137Cs is in a dissolved state in the form of compounds with humus and low molecular weight organic acids. In this case, there is a dynamic equilibrium in its distribution among the solid and liquid phases of the peat. 210Pb is present in peat mostly in the associated state with mineral and organic parts, and the share of mobile lead due to the presence of its compounds with fulvic acids and low molecular weight organic acids is small, which determines its low mobility and low rate of migration in the peat profile.
The fourth factor with a variance of 7.19% by positive loading isolates the carbonates CO32− and elemental parameters N and H.
The fifth factor accounts for 5.26% of the dispersion with a positive load on uranium isotope ratio 234U/238U.
At the same time, several parameters, such as the pH of the aqueous extract, organic matter, elemental components of organic matter C and C/N, but also the natural moisture content and bulk density, are not included in any factor.

4. Conclusions

The studied physicochemical parameters of the dried peat deposit ISO-1 is significantly different from the previously recorded values characteristic of the site with a natural hydrological regime. The content of natural moisture in the peat is significantly reduced, which contributes to more active aeration and an increase in Eh. Increased peat degradation is natural, so a decrease in the relative N content of the peat is observed. There is also a decrease in the H/C ratio and an increase in the level of oxidation of organic matter. Changes in these parameters determine the behaviour of the studied radioisotopes in the peat core.
The concentration of 137Cs in the ISO-1 core gradually decreases down the section with maximum activity in the uppermost horizon. Such distribution is essentially different from the natural peat profile studied previously. Migration of 137Cs goes upwards through the section as it is not limited by the geochemical barrier. In addition, the significant proportion of vascular plants in the vegetation cover of the dried area contributes to the transfer of 137Cs by the root system up the section. The vertical distribution of 210Pb in the ISO-1 deposit is characterised by significant differences from the site with a natural hydrological regime. In the natural deposit, excess lead was recorded up to a depth of 35–37 cm, while at the drained site atmospheric lead was not detected below the 22–24 horizon. This indicates a significantly lower rate of peat mass accumulation at the dried area. The U content in the dried peat profile on average corresponds to the values typical of peat formations in the damp area of Russia’s subarctic zone. 241Am with an activity up to 2.1 Bq/kg was found in the dried peat deposit. The results of the dating of the peat deposit showed a significantly lower rate of accumulation of peat in the drained site compared with the natural site (0.15 ± 0.02 against 0.48 ± 0.08 cm/year). The consistency of the estimates of peat accumulation rates with the distribution of the independent marker 241Am indicates the correctness of the chosen model and the validity of the dating results as applied to the ISO-1 peat core. A statistical analysis of the data on physical, chemical and physicochemical parameters and radionuclides showed that the leading role in the distribution of isotopes in the dried bog massif belongs to redox and acid-base conditions.
The obtained results allow us to expand our views on the processes of atmospheric radionuclide migration in peatlands as a result of the impact of anthropogenic activity on the peat deposit.

Author Contributions

Conceptualization, E.Y.; methodology, E.Y., A.O., A.K. and S.Z.; software, E.Y.; validation, E.Y., A.O., A.K., S.Z. and I.Z.; formal analysis, E.Y.; investigation, S.Z.; resources, E.Y. and A.O.; data curation, E.Y.; writing—original draft preparation, E.Y., A.O., A.K., S.Z. and I.Z.; writing—review and editing, E.Y., A.O. and A.K.; visualization, E.Y.; supervision, E.Y.; project administration, E.Y. and A.O.; funding acquisition, E.Y. and A.O. All authors have read and agreed to the published version of the manuscript.

Funding

The study was supported by the Russian Science Foundation (project No 22-27-20085).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets presented in this study can be obtained upon request to the corresponding author.

Acknowledgments

The authors are grateful to everyone who showed interest in the authors’ research and rendered all possible assistance in the study of bog ecosystems.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 05778 g001 550
Figure 1. The sample location where the ISO-1 peat profile was collected (Arkhangelsk oblast).
Figure 1. The sample location where the ISO-1 peat profile was collected (Arkhangelsk oblast).
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Figure 2. Variation in the physicochemical characteristics of the ISO-1 peat core.
Figure 2. Variation in the physicochemical characteristics of the ISO-1 peat core.
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Figure 3. Variation in the elemental compositions of the ISO-1 peat core.
Figure 3. Variation in the elemental compositions of the ISO-1 peat core.
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Figure 4. Variation in the radioisotopes in the ISO-1 peat core by depth.
Figure 4. Variation in the radioisotopes in the ISO-1 peat core by depth.
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Figure 5. The plot of 210Pb chronology of the ISO-1 peat core under the CF (CRS) model.
Figure 5. The plot of 210Pb chronology of the ISO-1 peat core under the CF (CRS) model.
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Figure 6. The graphs of total activity concentration (a) and unsupported (b) 210Pb, 241Am (c) for the ISO-1 peat column.
Figure 6. The graphs of total activity concentration (a) and unsupported (b) 210Pb, 241Am (c) for the ISO-1 peat column.
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Figure 7. The plot of the linear rate of peat accumulation in the ISO-1 peat core by depth.
Figure 7. The plot of the linear rate of peat accumulation in the ISO-1 peat core by depth.
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Figure 8. Correlation matrix of the parameters studied for ISO-1 peat profile.
Figure 8. Correlation matrix of the parameters studied for ISO-1 peat profile.
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Figure 9. Plot of the results of the calculated factor load matrix for the characteristic group of the ISO-1 peat core.
Figure 9. Plot of the results of the calculated factor load matrix for the characteristic group of the ISO-1 peat core.
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Table
Table 1. Variation in the level of peat degradation, bulk density, moisture content, rH-index and ORP (Eh4) of peat by depth.
Table 1. Variation in the level of peat degradation, bulk density, moisture content, rH-index and ORP (Eh4) of peat by depth.
Horizon, cmLevel of Peat Degradation
R, %		Moisture Content, g/gBulk Density, g/cm3Eh4, mV *rH
0–10107.39 ± 0.370.320 ± 0.006510 ± 6025.3 ± 2.0
10–2076.55 ± 0.400.259 ± 0.005521 ± 2825.7 ± 0.9
20–3076.25 ± 0.440.185 ± 0.003409 ± 2921.8 ± 1.0
30–4079.39 ± 0.030.229 ± 0.005298 ± 5018.1 ± 1.7
40–50157.49 ± 0.350.410 ± 0.001195 ± 5114.6 ± 1.7
* Eh4 values are average for the 2021 summer low water period.
Table
Table 2. Factor loadings matrix for the set of results for the ISO-1 peat core.
Table 2. Factor loadings matrix for the set of results for the ISO-1 peat core.
ParameterFactor 1Factor 2Factor 3Factor 4Factor 5
Depth−0.55−0.62−0.45−0.29−0.06
137Cs0.100.440.880.100.03
210Pb0.010.640.710.020.14
234U/238U−0.09−0.030.110.110.94
U0.300.800.000.25−0.29
CO32−0.170.220.600.71−0.01
Ash0.030.900.340.230.04
pHwater−0.67−0.430.43−0.350.12
pHsalt−0.33−0.050.920.100.10
Water soluble salts0.380.500.680.29−0.15
Organic matter−0.03−0.90−0.34−0.23−0.04
N−0.390.110.500.560.28
C−0.96−0.10−0.020.110.16
H−0.39−0.720.000.51−0.09
O0.830.39−0.03−0.35−0.08
H/C0.88−0.310.020.19−0.25
C/N0.21−0.18−0.34−0.68−0.27
O/C0.880.31−0.02−0.29−0.12
Oxidation level0.750.48−0.03−0.41−0.06
Moisture content−0.08−0.200.04−0.950.03
Bulk density−0.870.110.24−0.09−0.30
Eh0.720.490.220.420.04
rH0.710.490.220.420.04
Variance9.776.322.661.651.21
Percentage of variance (%)42.4627.4911.587.195.26
Cumulative (%)42.4669.9581.5388.7193.98
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Yakovlev, E.; Orlov, A.; Kudryavtseva, A.; Zykov, S.; Zubov, I. Assessment of the Impact of Anthropogenic Drainage of Raised Peat-Bog on Changing the Physicochemical Parameters and Migration of Atmospheric Fallout Radioisotopes in Russia’s Subarctic Zone (Subarctic Zone of Russia). Appl. Sci. 2023, 13, 5778. https://doi.org/10.3390/app13095778

AMA Style

Yakovlev E, Orlov A, Kudryavtseva A, Zykov S, Zubov I. Assessment of the Impact of Anthropogenic Drainage of Raised Peat-Bog on Changing the Physicochemical Parameters and Migration of Atmospheric Fallout Radioisotopes in Russia’s Subarctic Zone (Subarctic Zone of Russia). Applied Sciences. 2023; 13(9):5778. https://doi.org/10.3390/app13095778

Chicago/Turabian Style

Yakovlev, Evgeny, Alexander Orlov, Alina Kudryavtseva, Sergey Zykov, and Ivan Zubov. 2023. "Assessment of the Impact of Anthropogenic Drainage of Raised Peat-Bog on Changing the Physicochemical Parameters and Migration of Atmospheric Fallout Radioisotopes in Russia’s Subarctic Zone (Subarctic Zone of Russia)" Applied Sciences 13, no. 9: 5778. https://doi.org/10.3390/app13095778

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