Surface ground contamination and soil vertical distribution of 137Cs around two underground nuclear explosion sites in the Asian Arctic, Russia

Journal of Environmental Radioactivity 92 (2007) 123e143 www.elsevier.com/locate/jenvrad

Surface ground contamination and soil vertical distribution of 137Cs around two underground nuclear explosion sites in the Asian Arctic, Russia Valery Ramzaev a, Arkady Mishine a, Vladislav Golikov a, Justin Emrys Brown b,*, Per Strand b a

St.-Petersburg Institute of Radiation Hygiene, Mira street 8, 197101, St.-Petersburg, Russia Norwegian Radiation Protection Authority, Department for Emergency Preparedness and Environmental Radioactivity, Grini naeringspark 13, P.O. Box 55, N-1332 Østera˚s, Norway

b

Received 31 March 2006; received in revised form 28 August 2006; accepted 10 October 2006 Available online 5 December 2006

Abstract Vertical distributions of 137Cs have been determined in vegetation-soil cores obtained from 30 different locations around two underground nuclear explosion sites e ‘‘Crystal’’ (event year e 1974) and ‘‘Kraton-3’’ (event year e 1978) in the Republic of Sakha (Yakutia), Russia. In 2001e2002, background levels of 137Cs surface contamination densities on control forest plots varied from 0.73 to 0.97 kBq m2 with an average of 0.84  0.10 kBq m2 and a median of 0.82 kBq m2. 137Cs ground contamination densities at the ‘‘Crystal’’ site ranged from 1.3 to 64 kBq m2; the activity gradually decreased with distance from the borehole. For ‘‘Kraton-3’’, residual surface contamination density of radiocaesium varied drastically from 1.7 to 6900 kBq m2; maximal 137Cs depositions were found at a ‘‘decontaminated’’ plot. At all forest plots, radiocaesium activity decreased throughout the whole vertical soil profile. Vertical distributions of 137Cs in soil for the majority of the plots sampled (n ¼ 18) can be described using a simple exponential function. Despite the fact that more than 20 years have passed since the main fallout events, more than 80% of the total deposited activity was found in the first 5 cm of the vegetation-soil cores from most of the forested landscapes. The low

* Corresponding author. Tel.: þ47 67 16 2663; fax: þ47 67 14 7407. E-mail addresses: [email protected] (V. Ramzaev), [email protected] (J.E. Brown). 0265-931X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2006.10.001

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annual temperatures, clay-rich soil type with neutral pH, and presence of thick lichen-moss carpet are the factors which may hinder 137Cs transport down the soil profile. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Underground nuclear explosion; ‘‘Crystal’’; ‘‘Kraton-3’’; Radiocaesium; Soil; The Arctic

1. Introduction It is well known that the long-term behavior of 137Cs in the environment depends on a number of abiotic and biotic factors (Izrael et al., 1970; Moiseev and Ramzaev, 1975; Avery, 1996; Anspaugh et al., 2002; Whicker and Pinder, 2002). For terrestrial ecosystems, abiotic factors can include physico-chemical forms of fallout; amount of precipitation; physico-chemical properties of soil; soil and water erosion; temperature regime; and topography of a site. Important biotic factors are also numerous and can include, inter alia, the variability of interception surfaces; activity of soil fauna; interaction between mycorrhyzae and soil, etc. Finally, human activity, including application of countermeasures, can also govern movement of radionuclides in terrestrial ecosystems especially managed environments. The 137Cs soil vertical distribution, formed many years after a contamination event, can be regarded as an integrating parameter characterizing the resultant vector of prolonged interaction of the aforementioned factors. In turn, vertical distribution of radiocaesium in soil profiles may have a significant influence upon soil-to-plant transfer of the radionuclide, and, therefore, on internal dose formation (Ehlken and Kirchner, 2002; Ramzaev et al., 2006a). External doses are also strongly dependent on the depth of 137Cs penetration into the ground (Beck, 1966; Golikov et al., 1993; Jacob et al., 1994; Likhtarev et al., 2002; Ramzaev et al., 2006b). Systematic collection of experimental data regarding depth distributions and inventories of 137 Cs and other man-made radionuclides started in the beginning of the 1950s following the initial deposition of radioactivity from atmospheric nuclear weapons testing, i.e. global fallout (Beck and Bennet, 2002; UNSCEAR, 2000). A simple exponential model describing the radionuclides’ vertical distributions and the resulting dose rates in air was developed on the basis of these early observations (Beck, 1966). After cessation of atmospheric nuclear tests in 1963, a drastic decrease in the intensity of fresh fallout was observed. Nevertheless, problems associated with fallout-derived 137Cs persisted, often in Arctic areas with their increased radioecological vulnerability (Moiseev and Ramzaev, 1975; Hanson, 1982; Ramzaev et al., 1993; Strand et al., 2002). The results from several radioactivity surveys of surface soils (Burtcev and Kolodeznikova, 1997; Miretsky et al., 1997; Gedeonov et al., 2002) carried out in the Republic of Sakha (Yakutia) in the 1990s showed that in the Asian part of the Arctic, there were at least two sites where soil activity concentrations and inventories of 137Cs were two to three orders of magnitude higher than those expected from global fallout. Atmospheric releases from the ‘‘peaceful’’ underground nuclear explosions (PUNE), code named ‘‘Crystal’’ and ‘‘Kraton-3’’ were the sources of ground contamination by 137Cs and other radioactive products. The PUNE ‘‘Kraton-3’’ was conducted on August 25 (local time) 1978 for seismic sounding of the Earth’s crust. The special device was detonated at a depth of 577 m, and the energy output was equivalent to about 22 kt of TNT (Myasnikov et al., 2000). Due to technological mistakes and the fact that a borehole mouth was not sufficiently sealed, the event resulted in an accidental release of radioactivity into the atmosphere. The cloud of radioactive debris was

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dispersed downwind in a northeasterly direction (Figs. 1 and 2), and contamination on the ground at a distance of about 31 km occurred by dry and wet deposition (Antonov and Dekhtarenko, 1999; Myasnikov et al., 2000; Logachev, 2001). The maximum width of the trace was about 1.5 km. The plume passed over the temporary settlement (Fig. 2), where participants of the experiment were staying. Despite an urgent evacuation precipitated by extremely high gamma-dose rates in air, humans received up to 300 mSv of acute external accidental irradiation (Logachev, 2001). In 1979, it became clear that the larch-tree forest located at the head of the trace (area about 1 km2) had been destroyed (Burtcev and Kolodeznikova, 1997; Miretsky et al., 1997). In 1981, the technical area around the borehole and a part of the contaminated forest were cleaned up and re-cultivated (Myasnikov et al., 2000). Contaminated topsoil (down to a depth of approximately 20 cm and in selected places 1 m) and vegetation were removed using a bulldozer. The wastes were buried in a pit and a trench, excavated at a distance of about 10e15 m to the west of the borehole mouth. The pits and the borehole mouth were covered with ‘‘clean’’ excavated soil and surrounded by a banking made of soil to protect the site from thaw and rain waters. In 2001e2002, lethal damage to w100% of adult larches was observed for an area covering circa 1.2 km2 (Fig. 2). A detailed description of the present status of the injured forest ecosystem can be found in Ramzaev et al. (2004). The PUNE ‘‘Crystal’’ was conducted on October 2, 1974, with the purpose of constructing a reservoir dam for the diamond fields ‘‘Udachnaya’’. The depth of detonation was about 98 m with the power equivalence of the device being estimated to be 1.7 kt of TNT (Myasnikov et al., 2000). As a result of the ‘‘Crystal’’ explosion, a ground mound with a height of 14 m and

Fig. 1. Map of the Russian Federation, showing sampling locations. The territory of the Republic of Sakha (Yakutia) is marked with gray color, and the main area of the study is indicated as the black quadrant. The black asterisks indicate locations of the ‘‘Kraton-3’’ and ‘‘Crystal’’ explosion sites. The arrows starting from the asterisks show the positions of the trajectories of the radioactive clouds ‘‘Kraton-3’’ and ‘‘Crystal’’. Major rivers are also marked.

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Fig. 2. Maps of the ‘‘Kraton-3’’ site (the upper frame) and its near area (lower frame), showing locations of the sampled plots.

a diameter of 180 m was formed. In accordance with the planned technological conditions, up to 4% of radioactive products of the explosion could be vented to the atmosphere (Myasnikov et al., 2000). The length of the primary radioactive plume (on the day of explosion) was estimated to be about 10e12 km (azimuth 70 ), and its maximal width reached 300e400 m. Because of the significant radioactive contamination of the environment and because of economical reasons, the created ground mound has not been used for the purposes originally planned for. To minimize possible access of the local population to radioactively contaminated objects, the ground mound and the central crater were covered with barren rocks from the nearest diamond field ‘‘Udachnaya’’ in 1992. At present, the ground surface above the site of the explosion is covered with an artificial rock mound (sarcophagus) about 150 m in diameter and 7e20 m in height (Fig. 3). The negative effects to the environment observable at the ‘‘Kraton-3’’ site were registered at the ‘‘Crystal’’ site as well, but the area associated with zones of environmental damage were significantly smaller (Burtcev and Kolodeznikova, 1997; IRH, 2002). Lethal damage to adult larches is observed for an area covering circa 5.5 ha of forest surrounding the mound in all directions. The explosion itself and the huge sarcophagus have also changed the hydrological status of the site, leading to water-logging on a local scale.

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Fig. 3. Locations of the sampled plots (encircled diamonds) in the affected forest of the ‘‘Crystal’’ site.

As both sites represented potential long-term sources of contamination and a health hazard for the local human populations, a complex study was initiated with the main aim of estimating levels of human exposure from the ‘‘Crystal’’ and ‘‘Kraton-3’’ PUNEs (IRH, 2002; Ramzaev et al., 2004; Ramzaev and Go¨ksu, 2006). Four expeditions were conducted to the sites and the nearby settlements in 2001e2002. This paper deals with some of the materials obtained from two field expeditions and the subsequent laboratory analyses with respect to 137Cs behavior in the affected and control areas within Yakutia. The work is focused on a comparative study of vertical distributions of 137Cs in the ground many years after contamination events from three different sources e fallout from nuclear explosions in the atmosphere, local fallout from the ‘‘Crystal’’ explosion, and local fallout from the ‘‘Kraton-3’’ explosion. The ranges and, to some extent, the current spatial distributions of 137Cs contamination originating from these three sources were also evaluated. 2. Materials and methods 2.1. Study area The area of interest administratively belongs to the Mirny district of the Republic of Sakha (Yakutia) in the Russian Federation (Fig. 1). The main study area is located at the eastern part (Viliy’s plateau) of the Middle-Siberia Plateau. The underlying geology is characterized by sedimentary flat carbonate rocks and the average altitude of the plateau table is approximately 300 m above sea level. The area is dominated by sod-carbonate slightly alkaline soil of pH 7.05e8.45 (Burtcev and Kolodeznikova, 1997; Petrovsky and Koroleva, 2002; Chevychelov and Sobakin, 2004). There are several rivers of varying sizes and numerous streams. These separate the Viliy’s Plateau into gradually sloping low mountains and hills with an average relative altitude of about 200e400 m. The ‘‘Crystal’’ site (latitude 66.5 N, longitude 112.5 E) is located on the left bank of the UlakhanBysyttaah Brook (a left tributary of the Daldyn River), some 7.5 km from the town of Udachny. The ‘‘Kraton-3’’ site (65.9 N, 112.3 E) is located on the right bank of the Markha River, approximately 40 km east of the nearest settlement of Aikhal. The area under study belongs to the cool temperature zone (permafrost area). The duration of the period of snowfall is around 8 months (from the end of September to the end of May). According to the data of the nearest weather station, Schelogonci, located about 100 km east of both explosion sites (Fig. 1), the

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average annual temperature is circa 12  C. The total amount of annual precipitation is low, 250e 400 mm, of which approximately 80% falls during the warm springesummer season (Petrovsky and Koroleva, 2002). The depth of the topsoil layer, which melts during summertime, ranges from 10 to 50 cm at the flat tops and on the upper terraces, down to 1e2 m at well-drained mountain slopes and at river floodplains. Despite the low precipitation, the soils have, generally, a high moisture content. For our sites, the mass proportion of water in soil ranged from 17 to 48% with an average of 31  7% (1 standard deviation, median ¼ 30%; n ¼ 30). This phenomenon is explained by constant thawing of the permafrost during summertime. The ‘‘Crystal’’ and ‘‘Kraton-3’’ sites are located within the North-East Siberian Taiga (boreal forest) with a high predominance (up to 96%) of larch (Larix gmelinii) (Petrovsky and Koroleva, 2002; Ramzaev et al., 2004). Generally, sparse growth of trees is observed. A lichens-moss carpet covers the major part of the ground surface. The vegetation of the floodplains and wetter, low land areas consists of numerous species of grasses, frequently of Carex spp. Other vegetation includes shrubs and bushes, mostly of Salix spp. and Betula spp.

2.2. Sampling and measurements Soil samples were collected within the affected forest ecosystems (including the bulldozed area at the ‘‘Kraton-3’’ site) (Figs. 2 and 3), as well as at five nearby control forested plots located at sites that did not lie on sectors affected by the main plume trajectories. One more control forest site was sampled in the Enerdek area (64.3 N, 116.9 E). The vegetation at the control plots had a normal superficial appearance. Besides the forested areas, five ‘‘background’’ grassland plots were sampled. Two of them were located at the Markha River floodplain near the ‘‘Kraton-3’’ site and the others at distant areas of Aikhal (65.9 N, 111.6 E), Udachny (66.5 N, 112.3 E) and Enerdek. With respect to fallout contamination, these grass plots cannot be considered as a perfect control, because of the possible influence of water erosion and spatial redistribution of the radioactivity with time. The positions of all 30 sampled sites (plots) with respect to the source of contamination, together with the type of ecosystem are summarized in Table 1. Between three to five cylindrical vegetation-soil cores, each with a ground surface of 20 cm2, were taken at a plot to determine the activities per unit mass (Bq kg1) and per unit area (Bq m2). A special dismountable steel sampler was used. The construction of the sampler made it possible to obtain intact vegetation-soil carpet in one core. The overall depth of such cores ranged from 10 to 20 cm (Table 1), depending on the presence of pebbles and big stones in underlying soil. The cores taken were cut on site into slices about 2 or 5 cm thick. On one occasion (plot K-1), the 20 cm cores were sectioned into upper and lower parts with thickness of 10 cm each. Plant roots and stones were retained within soil samples for analyses. The same layers of different samples at a site were mixed and double bagged in plastic to hold soil moisture. A total of 124 such sub-samples were collected from all 30 plots in Yakutia in 2001e 2002. The cores were taken according to two geometrical patterns. In a ‘‘triangle’’ configuration (22 plots), the individual samples were taken in the corners of an isosceles triangle with the sides of approximately 3 m. In an ‘‘envelope’’ configuration (8 plots), the samples were taken in the corners and in the center of a 10  10 m quadrant. The large quadrants were needed for collection of a variety of vegetation species. In order to assess possible small-scale variability of 137Cs horizontal distribution in the localities of the 10  10 m plots, additional sampling of topsoil layer (thickness of circa 5 cm) in one block with covering vegetation was carried out within the affected areas at the ‘‘Kraton-3’’ site at plots K-2, K-5, K-11, K-12 and at plot K-16 (control). Five randomly distributed samples were taken in each plot using the soil sampler. Each sample was analyzed individually. Average ground 137Cs deposition (Bq cm2) together with its standard deviation and coefficient of variance [CV ¼ (standard deviation/average)  100%] were calculated for a plot. Wet weight of soil was determined within 2 weeks after sampling. The samples were air dried, well mixed, additionally dried at 60  C for 12e24 h, and re-weighed. The ‘‘in-sampler’’ density (g cm3)

Table 1 Sampling locations, depth of sampling, 137Cs current ground depositions (Cs-137) and plots sampled in Yakutia in 2001e2002

137

Cs depth distribution parameters (Bl, Bz and F ) for vegetation-soil cores from the Cs-137 (kBq m2)

Bl (cm1)

Bz (m2 kg1)

F (%)

0 12 13 14

20 12 15 15

2960 79.2 487 6890

0.22 0.53 0.79 0.05

0.021 0.042 0.063 0.008

ND 93 97 28

Kraton-3, affected forested area K-5 210 m K-6 215 m K-7 230 m K-8 240 m 245 m K-9a K-10a 290 m K-11 310 m K-12 340 m K-13 1200 m K-14 2470 m

12 14 10 359 357 350 25 30 40 60

12 15 15 15 10 15 12 12 12 12

896 464 919 140 25.2 1.72 39.6 13.6 285 181

0.65 0.74 0.71 0.66 0.67 0.87 0.76 0.89 0.86 0.94

0.064 0.068 0.077 0.052 0.072 0.176 0.179 0.123 0.090 0.099

94 99 86 97 93 96 97 98 98 99

Kraton-3, control forested area 450 m K-15a K-16 1200 m K-17a 1380 m

345 180 200

15 12 20

0.82 0.82 0.97

0.72 0.91 0.70

0.099 0.147 0.143

89 97 96

Kraton-3, floodplain K-18 K-19a

160 m 260 m

240 280

20 15

1.23 0.36

NC 0.41

NC 0.034

22 78

Crystal, affected forested area C-1 90 m C-2 95 m C-3 100 m C-4 135 m 180 m C-5a

40 40 40 40 40

15 15 15 15 15

64.3 15.3 13.4 8.72 1.32

0.36 0.61 0.40 0.50 0.87

0.050 0.057 0.039 0.059 0.137

66 95 82 84 95

Distance from borehole

Kraton-3, bulldozed area K-1a 0 K-2 115 m K-3 120 m K-4 135 m

Azimuth (degree)

V. Ramzaev et al. / J. Environ. Radioactivity 92 (2007) 123e143

Depth (cm)

Code of plot

(continued on next page) 129

130

Distance from borehole

Crystal, control forested area C-6a 900 m C-7a 1000 m

Azimuth (degree)

Depth (cm)

Cs-137 (kBq m2)

Bl (cm1)

Bz (m2 kg1)

F (%)

320 180

15 18

0.73 0.74

0.56 0.54

0.065 0.065

88 88

Udachny, lawn P-1

15 kmb

260b

18

1.32

0.35

0.055

66

Aikhal, lawn A-1a

39 kmc

280c

18

0.90

0.55

0.072

82

Enerdek, control forested area E-1 210 kmc

130c

12

0.95

0.45

0.079

67

Enerdek, floodplain E-2 210 kmc

130c

18

1.19

NC

NC

9

Note: Bl ¼ empirical coefficient for Eq. (1); Bz ¼ empirical coefficient for Eq. (2); F ¼ fraction of radioactivity in the first 5 cm of soil; ND ¼ no data; NC ¼ not calculated. a The number of layers with measured 137Cs concentrations is equal to two. b Distance and direction from the ‘‘Crystal’’ site. c Distance and direction from the ‘‘Kraton-3’’ site e the nearest source of possible local radioactive contamination.

V. Ramzaev et al. / J. Environ. Radioactivity 92 (2007) 123e143

Table 1 (continued ) Code of plot

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was determined for all soil sub-samples as the ratio of weight after drying to fresh soil volume inside the sampler. 137 Cs was determined in all 149 sub-samples by direct gamma-spectroscopy using high-purity Ge detectors and multi-channel analysers. The detectors were calibrated with standards made up to the same geometry as the samples. The standards from the Russian Federal Center for Measuring, Instrument Testing and Certification (St.-Petersburg) had mass densities of 0.2 g cm3 (sawdust), 1.0 g cm3 (epoxy resin) and 1.6 g cm3 (quartz sand). The necessary correction for the mass density of a sample was implemented. In most cases the overall measurements’ uncertainty was approximately within 15%. Minimal detected activity (MDA) of 137Cs varied from 1.4 to 1.8 Bq kg1 depending on the type of detector, mass of sample and its matrix, and the duration of measurement. The activity data were decay corrected to the date of sampling.

3. Results 3.1. Summary of data on the measured

137

Cs activity concentrations

A summary of measured activity concentrations for 149 sub-samples of vegetation-soil cores from all 30 sampled plots is given in Table 2. As expected, the lowest 137Cs activity concentrations were detected for the set of samples collected at control (‘‘background’’) areas. The maximal activity concentrations for the subsamples from the control areas did not exceed 100 Bq kg1; for 16 sub-samples (w30% to the total) the contents of 137Cs appeared to be below MDA (1.4e1.8 Bq kg1). All these sub-samples were taken from soil layers located below a depth of 6e15 cm. The samples collected in the affected forest at the ‘‘Crystal’’ site exhibited significantly elevated activities e the measured concentrations were generally more than an order of magnitude higher than those for background plots. The samples from the affected areas at the ‘‘Kraton-3’’ site had the highest, quite significant, levels of radiocaesium contamination. The mean and maximal 137Cs activity concentrations for the ‘‘Kraton-3’’ samples were about 1000 times higher than those registered at the background plots. 3.2. 137

137

Cs activity concentrations vs. depth

From the analyses of the subsequent layers in vegetation-soil cores, two different types of Cs depth distribution can be distinguished.

Table 2 Summary of the activity concentrations of 137Cs in sub-samples of vegetation-soil cores (depth of sampling varied from 5 to 20 cm) sampled in Yakutia in 2001e2002 (Bq kg1, dry weight) Sites, areas

Total (n) Below MDA (n) Mean  Std (Bq kg1) Median (Bq kg1) Range (Bq kg1)

Control areas 55 Crystal, affected forest 15 Kraton-3, affected areas 79

16 2 4

17  21 317  580 10 400  20 200

7.9 61 597

(<1.4)e86 (<1.4)e2130 (<1.5)e86 700

Note: n ¼ number of sub-samples. Data below minimal detected activity (MDA) were excluded from calculations of mean, standard deviation (Std) and median.

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For the majority of plots (n ¼ 27), except plot K-1 from the decontaminated area near the ‘‘Kraton-3’’ borehole and two plots from the Markha River floodplain (points K-18 and E2), maximum activity concentrations were registered within the upper layer of the core. As a rule, 137Cs concentrations sharply decreased with depth. Several examples for control areas, and for the ‘‘Kraton-3’’ and ‘‘Crystal’’ affected areas are given in Figs. 4e6. It should be noted that the complete pattern of the vertical activity distributions within sampled cores can be reproduced only for the heavily contaminated plots. As one may see in Figs. 4e6 [panel (a)], vertical distributions of the activity concentrations observed with majority of the experimental data can be very closely fit to a simple exponential function: AðLÞ ¼ A0 expð  Bl LÞ;

ð1Þ

where A(L) is the activity concentration (Bq kg1) at the depth L (cm), A0 and Bl are empirical coefficients having dimension of Bq kg1 and cm1, respectively. Coefficient Bl determines the gradient of the activity concentration within a core. The larger the absolute value of Bl, the steeper the change in activity concentrations with depth. The values of Bl for individual plots are given in Table 1. The regression coefficients (R2) were calculated for those plots (n ¼ 18) where the number of layers with detectable activity concentrations was not less than three. Regression coefficients for the majority of plots are rather high, ranging from 0.82 to 0.99 (average ¼ 0.96  0.05, median ¼ 0.98, n ¼ 18). The activity concentrations were also plotted against mass depth expressed in term of g cm2 (Figs. 4e6, panel (b)). The mass depth is defined as a mass of the material in a vertical core of soil divided by the core area. The presentation of data in this way has a more robust physical basis (Mattsson, 1975), and it may be useful for comparing activity distributions for 1000 U-1 K-16

(b) U-1 K-16

100 DL

10

Activity (Bq kg–1)

Activity (Bq kg–1)

100

1

0.1

0.01

0.001

1000

(a)

DL

10

1

0.1

0.01

0

5

10

Depth (cm)

15

20

0.001

0

5

10

15

Dry mass depth (g cm–2)

Fig. 4. Depth distribution of 137Cs (Bq kg1, dry weight) at a ‘‘background’’ grassland (U-1) and a control forest (K-16) plots. Horizontal bar indicates detection limit (DL) for the radiocaesium quantitative determination. Also shown are exponential functions (Eqs. (1) and (2)) fitted to the experimental points with the measured activities. For abbreviation of the plots, see Table 1.

V. Ramzaev et al. / J. Environ. Radioactivity 92 (2007) 123e143 100000

100000

100

10

1

0.1

C-2

10000

1000

(b)

K-5

Activity (Bq kg–1)

10000

Activity (Bq kg–1)

(a)

K-5 C-2

133

1000

100

10

1

0

5

10

0.1

15

0

5

10

15

Dry mass depth (g cm–2)

Depth (cm)

Fig. 5. Depth distribution of 137Cs (Bq kg1, dry weight) at the plots K-5 and C-2 located in the affected forest at the ‘‘Kraton-3’’ and the ‘‘Crystal’’ sites, respectively. Also shown are exponential functions (Eqs. (1) and (2)) fitted to the experimental points. For abbreviation of the plots, see Table 1.

sites that differ significantly from each other in terms of their soil bulk density (usually dried soil). For our plots, density of the dried material inside a core as a whole varied considerably from 0.58 g cm3 to 1.28 g cm3 with an average  standard deviation of 0.92  0.17 g cm3 and median of 0.92 g cm3 (n ¼ 30). The wet density ranged from 1.03 to 1.61 g cm3 with an average of 1.32  0.17 g cm3 and a median of 1.35 g cm3 (n ¼ 30). 100000

100000

(a)

10000

Activity (Bq kg–1)

Activity (Bq kg–1)

10000 K-4 K-2 K-1

1000

100

10

1

(b)

K-4 K-2 K-1

1000

100

10

0

5

10

Depth (cm)

15

20

1

0

5

10

15

20

Dry mass depth (g cm–2)

Fig. 6. Depth distribution of 137Cs (Bq kg1, dry weight) at three plots from the decontaminated area at the ‘‘Kraton-3’’ site. Also shown are exponential functions (Eqs. (1) and (2)) fitted to the experimental points. For abbreviation of the plots, see Table 1.

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Figs. 4e6 [panel (b)] demonstrate that in most cases, the distribution of experimental points can be also reasonably well (R2 ¼ 0.94  0.06, n ¼ 18) described, again, with a simple exponential function: AðZÞ ¼ A0 expð  Bz ZÞ;

ð2Þ

where A(Z ) is activity concentration (Bq kg1) at the mass depth Z (kg m2), A0 and Bz are empirical coefficients having dimension of Bq kg1 and m2 kg1, respectively. For those plots where only two subsequent upper layers demonstrated measurable activities (those are marked by the letter ‘‘a’’ in Table 1, 10 plots totally), any simple depth distribution, such as polynomial, exponential or power function, could be applied with equal validity. For the sake of compatibility, exponential distributions were also applied here. On average, the highest values of Bz and Bl were calculated for the affected forested areas at the ‘‘Kraton-3’’ site and for the control forests (Table 1), which indicated that here, the activity concentrations decreased most sharply with depth. From the vertical distribution patterns of activity observed at the forested areas, one may reasonably assume that for these plots, insignificant amounts of 137Cs have migrated deeper than 20e25 cm. In order to check this hypothesis experimentally, we dug a pit of 40 cm depth on the heavily contaminated forested plot K-5. A soil sample was taken from the layer within a depth of about 20e30 cm. The measured 137Cs activity concentration for this sample was found to be 15 Bq kg1. This value constitutes only 0.02% of the activity concentration of 66 900 Bq kg1 in the first 2 cm of the soil obtained with core sampling in this plot. The other type of 137Cs activity vertical distribution was observed near the ‘‘Kraton-3’’ borehole (plot K-1) and on two floodplain plots (K-18, E-2). For these locations, maximal activity concentrations were found below a depth of 10 cm (Figs. 6 and 7). To give analytical approximations of the experimental data we used a third order polynomial function for floodplain plots and a simple exponential Eq. (1) for the plot K-1. 12

8

6

4

2

0

(b)

E-2 K-18

10

Activity (Bq kg–1)

Activity (Bq kg–1)

10

12

(a)

E-2 K-18

8

6

4

2

0

5

10

Depth (cm)

15

20

0

0

5

10

15

20

Dry mass depth (g cm–2)

Fig. 7. Depth distribution of 137Cs (Bq kg1, dry weight) at two plots in the floodplain of the Markha River. Also shown are polynomial (third order) functions fitted to the experimental points. For abbreviation of the plots, see Table 1.

V. Ramzaev et al. / J. Environ. Radioactivity 92 (2007) 123e143

3.3.

137

135

Cs current ground deposition at the plots sampled

Current inventory of 137Cs in soil (Table 1) was calculated as the radioactivity of each soil layer divided by the core area and summed over sampling depth. For those plots, where a part of deeply positioned sub-samples did not reveal activities above MDA, expected activities for such sub-samples were calculated assuming exponential vertical distribution of the activity concentrations in the cores (Fig. 4). Bl values presented in Table 1 were used. At the control plots located in forests, contamination densities of 137Cs varied within a comparatively narrow range, from 0.74 to 0.97 kBq m2 with an average of 0.84  0.10 kBq m2 (1 standard deviation) (n ¼ 6) and a median of 0.82 kBq m2. For the grasslands (n ¼ 5), the range of the individual values was much wider, from 0.36 to 1.32 kBq m2. The average and the median of 137Cs contamination density for the grass plots were calculated to be 1.00  0.39 kBq m2 and 1.19 kBq m2, respectively. 137 Cs ground contamination densities of the affected forest at the ‘‘Crystal’’ site were generally higher (average ¼ 21  25 kBq m2; median ¼ 13 kBq m2; n ¼ 5) than levels at the background areas. The maximum contamination density was found at the plot C-1 located near the foot of the artificial rock mound at a distance of about 90 m from ground zero (Fig. 3). The contamination gradually decreased with distance, and at the boundary area between affected and normal forests (point C-5), the 137Cs contamination density was only 1.32 kBq m2. 137 Cs ground contamination density of the affected areas at the ‘‘Kraton-3’’ site varied drastically from 1.7 to about 6900 kBq m2 with an average of 955  1870 kBq m2 (n ¼ 14) and a median of 233 kBq m2. The pattern of radiocaesium contamination at the ‘‘Kraton-3’’ site was much more complex than the one observed at the ‘‘Crystal’’ site. The maximum level of the contamination density was found within the bulldozed area (plot K-4) at a distance of about 135 m from the ‘‘Kraton-3’’ borehole. Inside the affected forest, maximal ground contamination densities (464e919 kBq m2) were detected on the axis of the radioactive trace characterized by the maximum primary damage of the ecosystem (Ramzaev et al., 2004). If one moves transversely the affected area starting from the axis of the trace, e.g. points K-7 / K-8 / K9 / K-10 (Fig. 2), the 137Cs surface activity declines quickly (Table 1). Elevated levels of 137 Cs ground contamination were found at distances of 1.2 and 2.5 km from the borehole. The fraction (F ) of 137Cs activity in the first 5 cm of vegetation-soil cover was also calculated (Table 1) for all sites, except plot K-1, where the first soil layer had a thickness of 10 cm. For the overwhelming majority of mechanically undisturbed plots (24 cases of 26), the fraction exceeded 65%. Low values of F were calculated for two plots (K-18 and E-2) located on the Markha River floodplain. This was consistent with the vertical distribution patterns of the specific activity concentrations observed at these plots (Fig. 7). The inventories of 137Cs for the plots sampled were estimated with some deviation originating from measuring uncertainties and uneven spatial distribution of the activity within a plot. To assess a scale for this resultant deviation, five samples per plot were analyzed individually at five selected plots from the ‘‘Kraton-3’’ site (see Section 2). The coefficient of variance (CV) was used as a measure of the total deviation. The subsequent CV values were obtained: K-2 e 108%, K-5 e 20%, K-11 e 49%, K-12 e 42% and K-16 (control forest) e 15%. Taking into account that CV is a relative unit, the overall relative error of radiocaesium activity measurement for an individual sample for this set did not exceed 10%. Therefore, the deviation observed at plots K-2, K-11, and K-12 reflects mostly real, and sometimes quite prominent, spatial variability of the deposit within 10  10 m plots. For example, on plot K-2 from the bulldozed

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area, about a 10-fold difference between maximum and minimum radiocaesium activities was registered. 4. Discussion 4.1.

137

Cs ground deposition at control plots

The present anthropogenic background contamination in the study area of Yakutia arises from the integration of radioactive inputs from three sources: (a) global fallout due to nuclear weapons tests and explosions; (b) the local fallout from the nuclear explosions carried out at the Novaya Zemlya nuclear test site (NZTS); and (c) Chernobyl debris. A rough estimation of the intensity of 137Cs global fallout within the study area can be made on the basis of 90Sr latitude-dependent global pattern, which is systematized in the latest preChernobyl UNSCEAR (1982) report. Taking into account radioactive decay of 137Cs (T½ ¼ 30.0 years), the deduced value of 1.79 kBq m2 (in 1980) decreased to 1.09 kBq m2 in 2001e2002 (the time of our sampling). According to UNSCEAR (2000), the integrated global 90Sr (and, therefore, 137Cs) deposition in Northern Hemisphere increased very little after 1980 (up till 1999) and the value of 1.1 kBq m2 for 137Cs remains still valid for the 60e70 N latitude band in the beginning of the new millennium. Systematic long-term (1965e1984) measurements of 137Cs cumulated ground deposit in the Russian Arctic (territory of the Extreme North in Russian literature) demonstrated a broad range of the average annual values from 703 Bq m2 in Chukotka (eastern Russian Arctic) in 1981 up to 2923 Bq m2 in the Komi Republic (western Russian Arctic) in 1973 (Ramzaev et al., 1993). The variability observed among radiocaesium deposition densities, to some extent, was dependent on distance from NZTS. Furthermore, the observed variability of radiocaesium global deposition in the Russian Extreme North was closely connected to precipitation (Ramzaev et al., 1993). Apparent relationships between intensity of global fallout and precipitation were also reported for Canada (Blagoeva and Zikovsky, 1995), Sweden (Isaksson et al., 2000), Norway (Bergan, 2002), and Iceland (Palsson et al., 2002). For Yakutia with its comparatively low precipitation, the measured values of 137Cs cumulated ground deposition declined from 1665 Bq m2 (45  7 mCi km2) in 1969 to 740 Bq m2 (20  5 mCi km2) in 1984 (Ramzaev et al., 1993). The last value gives w500 Bq m2 at our sampling time due to radioactive decay of 137 Cs. All points considered, a range of 0.5e1.1 kBq m2 should represent a reasonable estimate of the present residual from 137Cs global fallout in the area of the ‘‘Kraton-3’’ and ‘‘Crystal’’ PUNEs. This range corresponds well to variability observed among the actual results of 137 Cs surface contamination density measurements for our control forest plots (0.73e 0.97 kBq m2) and for the ‘‘background’’ grasslands (0.36e1.32 kBq m2). Besides measurement errors, this variability may reflect initial spatial irregularities of the fallout within the region under study as well as the following redistribution of the deposited activity due to weathering processes. The most recent additional radioactive contamination occurred in the Yakutia territory with Chernobyl debris in 1986. The official estimates of Izrael et al. (2000) give the intensity of the Chernobyl 137Cs deposition for the Middle Siberia region of approximately 0.02e 0.04 kBq m2 (0.5e1.0 mCi km2) only. Latter values are in good agreement with a possible contribution of Chernobyl releases to total 137Cs deposit (850 Bq m2) of less than 48 Bq m2 that have been derived by Bossew et al. (2000) for North-East Siberia (Chukotka) in 1994.

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The present (as of 1999) total cumulated 137Cs ground deposition from nuclear tests and the Chernobyl accident for Western Yakutia (the eastern part of Middle-Siberia plateau) is officially estimated to be about 1.48e1.85 kBq m2 (40e50 mCi km2) (Izrael et al., 2000). Mean measured cumulative deposition of 137Cs of 0.84 kBq m2 and the individual values for six control forested plots around the ‘‘Kraton-3’’ and ‘‘Crystal’’ sites are certainly below the values deduced in the afore-cited study of Izrael and co-authors. Furthermore, the range of radiocaesium deposit at the control plots from the present study is close to those recently reported from other areas in Yakutia for total 137Cs deposit: 0.270e1.19 kBq m2 (Stepanov et al., 2004), 0.35e 1.11 kBq m2 (Sobakin, 2004). Therefore, from the comparison presented, one may state that the control forest plots around the ‘‘Kraton-3’’ and ‘‘Crystal’’ sites, which have been selected on the basis of normal outward appearance of vegetation and direction from the boreholes, have 137Cs contamination originating from sources different from the ‘‘Kraton-3’’ and ‘‘Crystal’’ explosions. The fallout (global þ local) from nuclear weapon tests seems to have been the dominating source. 4.2.

137

Cs ground contamination at the affected areas

Soil samples from the affected forested areas around the PUNEs exhibited elevated levels of Cs contamination, both in terms of Bq kg1 and kBq m2. For the ‘‘Crystal’’ site, maximal registered ground deposition was about 80 times the background deposition, while for the affected forest at the ‘‘Kraton-3’’ site, the contamination levels were 1000 times higher than those at the control areas. However, the highest activity concentrations and respective surface activity levels (up to some 8000 times the background) were detected within the decontaminated area at the ‘‘Kraton-3’’ site. These observations are in agreement with the results of the 1993 radioecological survey, which revealed about 20 times difference between the two explosion sites with respect to maximal 137Cs ground contamination level (Burtcev and Kolodeznikova, 1997). The highest residual caesium contamination densities up to 16 240 kBq m2 were also reported for soils from the bulldozed area of the ‘‘Kraton-3’’ site (Burtcev and Kolodeznikova, 1997). Taken together, these and our findings indicate that the cleanup procedures implemented on the technical area and in the contaminated forest around the ‘‘Kraton-3’’ borehole in 1981 were not completely successful. Because of a pioneering character of such work carried out under the extreme climatic conditions, and because of the technologically difficult soil structure (existence of underlying permafrost table giving permanent input of water from thawing ice to the surface; clayey type soil; presence of large amount of stones), the low efficiency of mechanical decontamination might have been expected. The central and the most contaminated area of the ‘‘Crystal’’ site is presently covered with a thick rock mound, which may promote the existence of a constant frozen state of the surface soil and, therefore, ‘‘fixing’’ the activity. 137Cs concentrations in the soil samples (depth 0e 5 cm), which were collected within this area in 1991 shortly before the mound creation, ranged from 85 to 19 340 Bq kg1 (Gedeonov et al., 2002). The last value is about 9% of the maximum activity concentration (217 000 Bq kg1) reported by the same authors for the ‘‘Kraton-3’’ site. Updated inventories of 137Cs in surface soils (top 10e20 cm) in the areas affected by ‘‘Kraton-3’’ are comparable with those in soils of two other heavily 137Cs-contaminated areas in the Russian Federation e the Bryansk Region and the Techa River basin in the Urals. In 2001, the residual Chernobyl-borne contamination densities in some settlement areas from the western part of the Bryansk Region were as high as 1000e4000 kBq m2 (Ramzaev et al., 2006b). In the Techa River floodplain, residual ground contamination densities by the 137Cs, which

137

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had been discharged from the Mayak Production Association plant, reached 4810 and 5490 kBq m2 for the areas of Muslyumovo and Bradokalmak settlements, respectively, in 1998 (Shutov et al., 2002). It should be emphasised, however, that the area with elevated levels of 137Cs contamination at the ‘‘Kraton-3’’ site is much smaller than those in the Bryansk Region and in the Urals. 4.3. Depth distribution of

137

Cs in the ground

Present radionuclide vertical distribution pattern observed in the soil cores, that were collected from background plots in Yakutia, is a result of downward percolation, lateral migration and physical decay of 137Cs originating from a number of events that occurred throughout an approximately 40-year period ending in 1986 (the Chernobyl accident). Taking into account that the Chernobyl radiocaesium hardly contributes more than 10% to the total cumulated inventory in the study area, the average age of the background fallout may be roughly estimated to be 35e40 years old. After the ‘‘Crystal’’ and ‘‘Kraton-3’’ explosions, the radioactive debris was deposited onto the ground surface as a result of a single-pulse process. Therefore, for the plots from affected areas at the ‘‘Kraton-3’’ and ‘‘Crystal’’ sites, the ‘‘age’’ of 137Cs deposition is around 24 and 27 years, respectively. Despite the fact that the 137Cs depositions occurred many years before our sampling, a very surficial position of the contaminant within the soil profile was registered for the absolute majority of the mechanically undisturbed plots, irrespective of site and source of contamination. This is documented by (a) graphical presentation of the activity concentration plotted against depth, which demonstrates 137Cs maximum value in the first layer of a core (Figs. 4 and 5); (b) a large proportion of 137Cs total inventory that is retained in the first 5 cm of soil (F parameter); and (c) exponential vertical distributions of 137Cs concentrations with large values of Bl and Bz (Eqs. (1) and (2)). Depth distributions of ‘‘aged’’ 137Cs that tend to decline exponentially with depth in undisturbed soils have been observed in several countries (Blagoeva and Zikovsky, 1995; Dubasov, 1997; Isaksson and Erlandsson, 1995; Whicker and Pinder, 2002). Although, the individual reported Bl values varied widely from 0.14 to 0.64 cm1, on average, these were lower than the values calculated for the majority of our soils (Table 1). Our findings that indicate a very limited vertical migration of radiocaesium in most of the Arctic soils studied are in good agreement with the results and estimation given by Chevychelov and Sobakin (2004) with respect to 137Cs mobility at the ‘‘Kraton-3’’ site in the beginning of the new millennium. The authors found 91e97% of the total radiocaesium inventories in the top 4 cm of the vegetation-soil carpets. It is worth mentioning, however, that frequently, the depth distributions of ‘‘aged’’ radiocaesium in undisturbed soils demonstrate significant deviations from the afore-cited observations and from the exponential model conservatively adopted by UNSCEAR for global and Chernobyl fallout (UNSCEAR, 1988, 2000). Thus, the maximum 137Cs activity density and concentrations at the depth of several cm (up to 20 cm) were found many years after the deposition in some types of undisturbed soil in the European part of Russia (Roed et. al., 1996), Germany (Schimmack et al., 1998; Kirchner, 1998), Sweden (Rose´n et al., 1999) and Italy (Jia et al., 1999). The registration of a 137Cs peak at some depth usually indicates enhanced mobility of the radionuclide within the soil profile. Of the 30 plots sampled in the present study, this type of radiocaesium distribution was found only for two floodplain plots and for a plot

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from the ‘‘Kraton-3’’ decontaminated area. In the latter case, significant mechanical disturbance connected with the process of covering the decontaminated ground surfaces by a cleaner material is quite probable. As for two floodplain plots, such 137Cs vertical distribution in floodplain soils is believed to reflect the on-site erosion and accumulation (burial) processes. For plots K-1 and E-2, it seems quite probable that a significant proportion of activity of radiocaesium may be deposited deeper than the depth of our sampling (18e20 cm). Based on the above discussion, it can be concluded that in the absence of mechanical disturbance, the Yakutian soils posses a very strong fixation with respect to fallout 137Cs. Several factors, which may influence the fixation of radiocaesium in the vegetation-soil profiles, should be mentioned. First, the area under study is characterized by an extremely low average annual temperature. The existence of permafrost certainly suppresses hydraulic conductivity of soil and bioturbation processes. Blagoeva and Zikovsky (1995) from analyses of soil cores, that were collected in a number of geographical areas in Canada (45e68 N) in 1989e1990, found that in the north, 137 Cs deposited from global fallout remained much closer to the surface than in the south. The authors explained this difference by lower annual temperature which hinders the vertical migration of Cs. Schuller et al. (2002) also suggest that the long freezing period and the long lasting snow cover are the factors that explained smaller values of the diffusion coefficient and the convection velocity of radiocaesium in soil under a polar climate in comparison to these transport parameters determined in temperate zones. These observations and hypotheses are in line with the data of Alexeev et al. (1993), who studied the soil cores obtained from a permafrost area at the northern Yakutia (70e71 N, 127 E) in 1992. The authors could easily measure 137Cs activity (4e55 Bq kg1) only in the first 5 cm of soil. The activity concentrations of the radionuclide in the deeper layers were below detection limit of 2.0e0.5 Bq kg1. Second, the soils collected in Yakutia demonstrate pH values that are very close to neutral. It is well known fact that caesium migrates faster under acidic conditions, while increasing soil pH from 3.5 to 7.0 may decline the wash-off of Cs from a substrate (Izrael et al., 1970; Moiseev and Ramzaev, 1975). Third, the carbonate soils analyzed are rich with clay (Chevychelov and Sobakin, 2004). Many studies have demonstrated that clay minerals bind caesium very strongly (Moiseev and Ramzaev, 1975; Avery, 1996). Even in organic layers of forest soil, 137Cs is not only present in the fraction ‘‘bound to organic matter’’, but mainly in the clay mineral fractions (Bunzl et al., 1998). Finally, the presence of a thick lichen-moss cover on the soil surface may prevent a significant proportion of the initially deposited radiocaesium from percolation into deeper layers of the vegetation-soil carpet. The ability of terricolous lichens and mosses to absorb and retain substantial amount of the fallout-derived Cs has been demonstrated in different Arctic and sub-arctic areas for many years (Mattsson, 1975; Hanson, 1982; Ramzaev et al., 1993; Nifontova, 2005). For the ‘‘Kraton-3’’ forest, the actual fraction of 137Cs total ground deposition present in intact, living lichens-moss cover, 24 years after the accident, was estimated to range from 20% (Ramzaev et al., 2005) to 53% (Ushnitsky and Sobakin, 2004). In the case of possible climatic changes (the global warming) or as a result of a forest fire in future, existing 137Cs fixation within the vegetation-soil profiles may be altered. This will lead to some increase in the rate of downward percolation of the radionuclide and to an enhancement in the lateral mobility of the contaminant. The last aspect provides a basis for a radiological concern (Kiselev et al., 2004) because a proportion of the presently well-fixed radiocaesium (and other artificial radionuclides) can be transported from the unpopulated terrestrial areas via water bodies (DaldyneMarkhaeViliyeLena river ecosystem) to the populated areas of Yakutia.

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5. Conclusion Two expeditions that were conducted in 2001e2002 to the sites of the underground nuclear explosion ‘‘Crystal’’ and ‘‘Kraton-3’’ in Yakutia (Russia) confirmed the existence of heavily contaminated Arctic terrestrial environs in the locations of both explosions. The radioactive contamination of the environment at the ‘‘Crystal’’ site occurred due to a planned release of the products of the explosion into the atmosphere, while accidental venting to atmosphere was the reason for the surface contamination by man-made radionuclides at the ‘‘Kraton-3’’ site. The expeditions also found significant damage within the terrestrial ecosystems. The affected ecosystems were additionally disturbed by mechanical treatment of the ground surface near the boreholes during decontamination efforts in remote phases after contamination. Background levels of 137Cs surface contamination densities on control forest plots varied from 0.73 to 0.97 kBq m2 with an average of 0.84  0.10 kBq m2 and a median of 0.82 kBq m2. Both the median and the average were somewhat lower than the current value of 1.1 kBq m2 expected from the 137Cs global fallout (UNSCEAR, 1988) for the 60e70 N latitude band. This difference may be attributed to a comparatively low precipitation in the area under study. 137 Cs ground contamination of the affected forest around the ‘‘Crystal’’ site was about an order of magnitude higher than levels determined at background plots. The maximum contamination density (64 kBq m2) was found near the foot of an artificial rock mound at a distance of about 90 m from ground zero. 137Cs contamination gradually decreased with distance from the borehole, and at the boundary area between affected and normal forests the contamination density was only 1.32 kBq m2. 137Cs ground contamination density at the affected areas around ‘‘Kraton-3’’ varied drastically from 1.7 to 6900 kBq m2; the maximal contamination level was found within bulldozed area at a distance of about 135 m from the ‘‘Kraton-3’’ borehole. Considerable ground contamination densities, 285 and 181 kBq m2, were detected at the distance of 1.2 and 2.5 km, respectively, in a northeasterly direction from the borehole. At all forest plots, radiocaesium activity decreased throughout the whole vertical soil profile, while for two plots from the Markha River floodplain it increased in the first 15 cm of soil depth. Vertical distribution of 137Cs in the ground for the majority of the plots sampled can be rather well described as an exponential function of depth. The numerical parameters characterizing depth distribution of the contaminant are practically the same in the control forest and in the affected areas around the ‘‘Kraton-3’’ site; the cores from the ‘‘Crystal’’ site demonstrated a less prominent gradient of decreasing activity concentration with depth. Despite the fact that more than 20 years have passed since the major fallout events, at least 80% of the total deposited activity has been retained in the first 5 cm of vegetation-soil carpet in almost all forest plots. Comparing the results from the present study with the relevant data and estimations given by other authors, one may conclude, that in the Arctic areas under study, radiocaesium vertical migration in soil is more limited than that observed in temperate climatic zones. The low annual temperatures, clay-rich soil type with neutral pH, and presence of thick lichenmoss carpet hinder 137Cs transport down the soil profile.

Acknowledgement This work was supported by the Government of the Republic of Sakha (Yakutia) and the EC Copernicus project ‘‘EPIC’’ (Environmental Protection for Ionizing Contaminants in the

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Arctic). We would like to thank Mr. Ivan Burtcev from the former Radiation Mitigation Board of the Republic of Sakha (Yakutia) for his excellent coordination of the fieldwork during expeditions to the sites of explosions. The expeditions could not have been undertaken without the generous cooperation of the diamond company ‘‘ALROSA’’. We appreciate the invaluable local support by the staff of Udachny and Aikhal mining and enrichment combines in helping us to perform the field studies. The authors are grateful to two anonymous reviewers for their valuable suggestions and comments.

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