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Accumulation and distribution of long-living radionuclides in the forest ecosystems of the Kyshtym accident zone
the Science of the Total Environment ~n~ t i n r ~ , ~ i ~ t l t
ELSEVIER
~
IB I l l a m m i ~ l pw l ~ M m
The Science of the Total Environment 157 (1994) 147-151
Accumulation and distribution of long-living radionuclides in the forest ecosystems of the Kyshtym accident zone Ye. N. Karavaeva*, N.V. Kulikov, I.V. Molchanova, V.N. Pozolotina, P.I. Yushkov Institute of Plant and Animal Ecology, The Ural Dit~ion of the Russian Academy of Sciences, 8 March Str., Ekaterinburg,
620219, Russia
Abstract The concentration of Strontium-90 and Cesium-137 in soil and woody plants was studied in the Sverdlovsk region of the zone contaminated as a result of the Kyshtym accident in the southern Urals in 1957. In the forest ecosystems under study, the total concentration of strontium-90 varied from 1.5 to 63.4 kBq/m 2, while that of cesium-137 did not exceed 13 kBq/m 2. Soil contamination resulted in an increase in the concentration of radionuclides in woody plants. Thus, the concentration of strontium-90 in different parts of birch samples was higher than in the controls by a factor of 5-16, while for pine, it was higher by a factor of 2-4. Keywords: Kyshtym accident; Environmental contamination; Strontium-90; Cesium-137
1. Introduction One of the most serious nuclear accidents took place at Kyshtym in the southern Urals in 1957. About 74 PBq of radioactive substances were released into the atmosphere, which resulted in the contamination of a vast area. At present the main contaminant in this area is strontium-90. Some results of long-term research in this area, such as the levels of contamination, geochemical migration of radionuclides and their transfer along food-chains, etc., were discussed in a series of publications [1,2]. In this paper, we present the results of a study on the concentrations of stron-
* Corresponding author. Elsevier Science BV.
SSDI 0 0 4 8 - 9 6 9 7 ( 9 4 ) 0 4 2 7 6 - S
tium-90 and cesium-137 in some components of the forest biogeocenoses within the contaminated zone carried out 34 years after the accident.
2. Experimental Samples of the soils and woody plants were taken in the suburbs of Kamensk-Uralskyi (the Sverdlovsk region) and near the Tygysh lake along the centreline of the contaminated zone at a distance of 90-106 km from the epicentre of the accident (Fig. 1). The area is dominated by westerly winds. The average annual amount of precipitation is 350-450 mm, peak precipitation being observed in summer. Soils were sampled in a birch kolka (60 to 80 year-old trees) and in a p i n e / b i r c h kolka (25 to
148
Y.N. Karavaeva et aL / Sci. Total Environ. 157 (1994) 147-151
cover were disturbed by decontamination works carried out during the first years after the accident. The control plot was chosen in a birch/pine forest (40 to 50 year-old trees) near the village of Rassokha outside the contaminated zone. The vegetation cover there is poor, being represented by forest herbs on brown forest soil. The methods of soil sampling were carefully designed to avoid cross-contamination between the soil layers. After digging a pit, the layers of soil (5 cm thick) were cut horizontally to a depth of 30 cm taking into account the area of the soil layers. Aliquots of 0.5 kg were taken from each sample. Then these aliquots were dried and sieved (1-mm mesh sieve) before analysis. Several birch and pine trees were collected to prepare samples of bark, stem, branches, leaves, and needles. The samples were ashed at 500°C. The strontium-90 content of the samples was estimated radiochemically by its daughter, yttrium-90. The concentrations of cesium-137 were measured using a AM-A-02qbI multichannel 3'analyser with a scintillating NaI (T1) detector. The strontium-90 and cesium-137 determination were duplicates.
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I.rOm?, , / ,
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la
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/,1" D0 '
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Fig. 1. Map of region in southern urals in which the 1957
3. Results and discussion
accident occurred. Plot 1, virgin birch kolka; Plot 2, recultivated p i n e / b i r c h kolka; Plot 3, control, b i r c h / p i n e forest.
Maximum strontium-90 concentrations were observed for the 0- to 5-cm thick intact top soil layer in the birch kolka, which exceeded the control concentrations by a factor of ~ 7 (Table 1). The radionuclide content decreased with depth, being undetectable at a depth of 20-25 cm with the methods used.
35-year-old trees) located at a distance of 0.5 km from each other and 0.5 km from the shore of the Tygysh lake. The birch kolka stands on virgin grey forest soil, while the pine/birch kolka has grown up in an area where the soil and the vegetation
Table 1 9°Sr and 137Cs ( B q - k g - 1 dry wt.) in the soils of the investigated forest stands Depth (cm)
0-5 5-10 10-15 15-20 20-25 25-30
Plot 1 (Virgin) 90Sr 646 + 235 212 + 40 40+4 6+ 2 BDL BDL
BDL, below detection limit.
137Cs
Plot 2 (Recultivated) 90Sr
314 _____100 19 5- 1 95:1 8 5- 1 8_ 1 15 5:9
12 + 16 + 7+ 7+ 11 + 27 +
1 6 1 5 6 16
137Cs
Plot 3 (Control) 90Sr
137Cs
75 + 12 + 21-i45 + 11 + 20 +
90 + 15 10 5- 2 4+ 1 BDL BDL BDL
150 + 3 23 + 2 5 5- 1 7+ 1 5 5- 1 BDL
42 4 13 35 6 1
Y.N. Karavaeva et al. / Sci. Total Environ. 157 (1994) 147-151
A different pattern of strontium-90 concentration distribution was observed for the soils in the pine/birch kolka. The highest concentration was recorded at a depth of 25-30 cm. In the 0- to 5-cm top layer, it was lower than in the controls by a factor of seven. This decrease in strontium-90 concentration and its reversal in the soil profile may be due to partial removal of the contaminated top layer and its burial at a certain depth during the decontamination works carried out after the accident. This is reflected in the morphology of the soil, in which the humus layer was buried at a depth of 20-30 cm. The cesium-137 concentrations of the soil samples from the three locations are very much similar. Maximum values were obtained for the humus-saturated 0- to 5-cm top layer (Table 1). As the concentration of strontium-90 in different soil layers varies, the contamination of these layers is also different. Thus, it is 34 k B q / m 2 for the intact soils (location 1) and only 5 k B q / m 2 for the soils disturbed by the decontamination works (Table 2). These values exceed those for the controls by a factor of 20 and 3, respectively. For cesium-137, the total contamination of the soils amounts to 13.6 and 15.2 k B q / m 2, the control value being 6.5 k B q / m 2. According to the data obtained for the central Urals in the 1970s, global fallout here was 2.6 k B q / m 2 for strontium-90 and 8.9 k B q / m 2 for cesium-137 [3]. In collaboration with our colleagues from the International Union of Radioecologists and using our data obtained earlier, we estimated the contribution of various radioactive contamination sources to the total contamination [4]. It appears that the contribution of the Kyshtym accident to the activity of the samples collected was 70-95% for strontium-90 and 8% for cesium-137. A considerable contribution to the level of contaminaTable 2 Total soil contamination (kBq. m-2) in the investigated forest stands Location
90Sr
137Cs
Plot 1 (virgin) Plot 2 (recultivated) Plot 3 (control)
34.3 5.4 1.7
13.6 15.2 6.5
149
tion of cesium-137 (58-74%) comes from unknown sources. One such source may be Lake Karachay, which is an open storage area for radioactive wastes. In 1967-1970, a severe drought led to the evaporation of water from this lake, which resulted in a patchy contamination of the region by the dust from the lake bed spread by winds over considerable distances [4]. As is known in forest ecosystems, the distribution of radionuclides in the soil and vegetation cover stabilizes at a certain level in about 10-12 years [3,5-7]. Further redistribution of radionuclides in the biocenosis is associated mainly with their migration deeper into the soil and their accumulation by plants, predominantly through the root system. An increased strontium-90 content of the soils in the contaminated zone leads to a higher concentration of this radionuclide in the local woody plants (Table 3). Thus, as compared with the control, the concentration of strontium-90 in the birch from the first location near the Tygysh lake (intact top layer) is higher by a factor of 2-30 depending on the part of the tree. For the birch and pine from the second, decontaminated location, the factor is 2-6. For both species, minimum strontium-90 concentrations were observed in the tissue. Twigs and bark showed a greater extent of contamination. This has been noted for other species of plants also [5,7,8], which is due to the fact that strontium-90 penetrates into a plant through its peripheral layers of tissue, the diffusion towards the core being slow. No clearly marked gradient was observed for strontium-90 concentrations in the bark along the trunk height. The woody species under study differ considerably in the absorption of strontium-90. Birch accumulates 1.5-5 times more of this radionuclide than pine, which is supported by the data from other authors [3,7,9]. Birch features increased absorption of Ca, being a typical calciphyl species. It is easy to discriminate between Sr and Ca in its surface part [9]. The above differences between these species may also depend on a different arrangement of their root systems in the soil profile. Birch has a 1.5-2 times larger mass of active, fine 'sucking'
150 Table 3 9°Sr and
Y.N. Karavaeva et al. / Sci. Total Environ. 157 (1994) 147-151
137Cs(Bq'kg-1
dry wt.) in trees
Location
Species, part of tree
Plot 1 (Virgin)
Betula verrucosa Ehrh.
Plot 2 (Recultivated)
Leaves Small branches Bark base Bark middle Bark top Wood Betula verrucosa Ehrh. Leaves Small branches Bark base Bark middle Bark top Wood
9oSr
137Cs
269 + 24 594 + 23 659 + 35 427 + 19 936 + 88 95 + 5
4.7 + 1.2 4.6 + 0.8 11.2 + 1.4 7.2 + 1.8 3.5 + 1.0 0.4 + 0.1
106 + 10 178 + 10 165 -t- 8 196 + 14 139 + 13 13 + 1
11.1 + 3.5 + 3.3 + 3.5 + 3.7 + 2.1 +
1.9 1.7 1.5 1.0 1.2 1.1
Pinus silvestris L.
Plot 3 (Control)
Needles Small branches Bark base Bark middle Bark top Wood Betula verrucosa Ehrh. Leaves Small branches Bark base Bark middle Bark top Wood
65 + 14 58 + 17 70 + 2 30 + 18 52 + 1.0 10 + 1
5.1 + 1.0 3.0 + 1.3 16.8 + 2.5 6.2 + 1.5 13.5 + 2.2 1.6 + 0.2
51 + 6 36 + 3 28 + 2 31 + 4 34 -t- 2 7+ 1
10.5 + 2.7 5.7 + 2.0 17.0 + 4.0 6.0 -t- 1.8 6.7 + 1.1 0.7 + 0.2
20 + 12 16 + 3 14 + 1 17 + 3 28 + 1 3+1
8.5 + 1.1 11.0 + 1.0 24.1 + 3.1 7.5 + 1.6 3.2 + 1.4 1.2 + 0.2
Pinus silvestris L.
Needles Small branches Bark base Bark middle Bark top Wood
roots in the 0- to 30-cm top layer of soil comp a r e d with p i n e [5,10]. T h e r e m o v a l of r a d i o n u c l i d e s f r o m soil by plants d e p e n d s greatly o n the d i s t r i b u t i o n of r a d i o n u clides in the soil profile. T h e r e m o v a l is greatest from the 20- to 30-cm deep layer [9,10]. A s a result of d e c o n t a m i n a t i o n in the s e c o n d s a m p l i n g location, the most c o n t a m i n a t e d h u m u s layer was b u r i e d just at this depth. This m a y b e o n e of the r e a s o n s why the r e m o v a l of r a d i o n u c l i d e s by the surface parts of the plants in the first a n d second s a m p l i n g sites differs by a b o u t a factor of three, w h e r e a s the total c o n t a m i n a t i o n of the soils dif-
fers by a b o u t a factor of six. Besides, we suppose that a part of the radioactivity is in the d e e p e r soil layers. A s regards cesium-137, its c o n c e n t r a t i o n was f o u n d to be similar in the h o m o l o g o u s organs of the birch a n d p i n e from different s a m p l i n g sites ( T a b l e 3), t h o u g h with a n o t i c e a b l e g r a d i e n t in the b a r k at various heights a n d with a m a x i m u m at the base. Thus, 34 years after the Kyshtym accident, the soils in the c o n t a m i n a t e d area show different levels of total c o n t a m i n a t i o n . This difference as regards the c o n c e n t r a t i o n of s t r o n t i u m - 9 0 in the
Y.N. Karavaeva et al. / Sci. Total Environ. 157 (1994) 147-151
soil and the vegetation cover may be attributed to the decontamination works performed after the accident. The concentration of strontium-90 in various parts of the birch and pine in the contaminated zone exceeds the background level by a factor of 2-30 and is dependent not only on the radionuclide content of the soil but also on the distribution of the radionuclides in the soil profile and specific differences between plant species. The maximum cesium-137 content of the soil in the contaminated zone was found to be 15 k B q / m 2, a considerable contribution to it coming from unknown sources in the area under study. References [11 A.I. Burnazyan (Ed.), Results of Studies and Experience in Elimination of the Consequences of Accidental Contamination of a Territory with the Products of Uranium Fission. Energoatomizdat, Moscow, 1990, p. 143. [2] B.V. Nikipelov, E.I. Mikerin, G.M. Romanov, D.A. Spirin and Yu. B. Khlopina, The radiation accident in the Southern Urals in 1957 and the cleanup measures implemented. Proceedings of an International Symposium 'Recovery Operations in the Event of a Nuclear Accident or Radiological Emergency', IAEA, Vienna, 1990, pp. 373-403.
151
[3] R.M. Alexakhin and M.A. Naryshkin, Migration of radionuclides in forest biogeocenoses. Science, Moscow, (1977) 144. [4] A. Aarkrog, H. Dahlgaard, M. Frissel, L. Foulquier, N.V. Kulikov, I.V. Molchanova, C. Myttenaere, S.P. Niellsen, G.G. Polikarpov and P.I. Yushkov, Sources of anthropogenic radionuclides in the southern Urals. J. Environ. Radioact., 15 (1992) 69-80. [5] F.A. Tikhomirov, The Effect of Ionizing Radiation on Ecologycal Systems. Atomizdat, Moscow, 1972, p. 174. [6] C.F. Jordan, J.R. Kline and D.S. Sasscer, A simple model of strontium and manganese dynamics in a tropical rain forest. Health Phys., 24 (1973) 477-482. [7] A.A. Molchanov, E.A. Fiodorov, R.M. Alexakhin, M.M. Alexakhina, N.N. Mishenkov, M.A. Naryshkin, L.N. Tiumenev, V.A. Uhanova and V.P. Julanov, Some regularities of distribution of fission products deposited in composition of global fallout in forest vegetation. Russian J. For., 6 (1968) 18-23. [8] R. Scott Russell (Ed.), Radioactivity and Human Diet, Pergamon, London, 1966, p. 376. [9] R.M. Alexakhin, M.A. Naryshkin and M.A. Bocharova, On the problem concerning the peculiar features and quantitative prognostication of the cumulative accumulation of Sr-90 in arboreal plans. Rep. Acad. Sci., 193(5) (1970) 1192-1194. [10] F.M. Tikhomirov, N.I. Sanzharova and A.A. Shpazhnikow, Migration of anthropogenic radionuclides in the soil-plant system in forest biogeocenoses. Problems of forest radioecology. Gydrometeo, Moscow, (1979) 83-96.
ELSEVIER
~
IB I l l a m m i ~ l pw l ~ M m
The Science of the Total Environment 157 (1994) 147-151
Accumulation and distribution of long-living radionuclides in the forest ecosystems of the Kyshtym accident zone Ye. N. Karavaeva*, N.V. Kulikov, I.V. Molchanova, V.N. Pozolotina, P.I. Yushkov Institute of Plant and Animal Ecology, The Ural Dit~ion of the Russian Academy of Sciences, 8 March Str., Ekaterinburg,
620219, Russia
Abstract The concentration of Strontium-90 and Cesium-137 in soil and woody plants was studied in the Sverdlovsk region of the zone contaminated as a result of the Kyshtym accident in the southern Urals in 1957. In the forest ecosystems under study, the total concentration of strontium-90 varied from 1.5 to 63.4 kBq/m 2, while that of cesium-137 did not exceed 13 kBq/m 2. Soil contamination resulted in an increase in the concentration of radionuclides in woody plants. Thus, the concentration of strontium-90 in different parts of birch samples was higher than in the controls by a factor of 5-16, while for pine, it was higher by a factor of 2-4. Keywords: Kyshtym accident; Environmental contamination; Strontium-90; Cesium-137
1. Introduction One of the most serious nuclear accidents took place at Kyshtym in the southern Urals in 1957. About 74 PBq of radioactive substances were released into the atmosphere, which resulted in the contamination of a vast area. At present the main contaminant in this area is strontium-90. Some results of long-term research in this area, such as the levels of contamination, geochemical migration of radionuclides and their transfer along food-chains, etc., were discussed in a series of publications [1,2]. In this paper, we present the results of a study on the concentrations of stron-
* Corresponding author. Elsevier Science BV.
SSDI 0 0 4 8 - 9 6 9 7 ( 9 4 ) 0 4 2 7 6 - S
tium-90 and cesium-137 in some components of the forest biogeocenoses within the contaminated zone carried out 34 years after the accident.
2. Experimental Samples of the soils and woody plants were taken in the suburbs of Kamensk-Uralskyi (the Sverdlovsk region) and near the Tygysh lake along the centreline of the contaminated zone at a distance of 90-106 km from the epicentre of the accident (Fig. 1). The area is dominated by westerly winds. The average annual amount of precipitation is 350-450 mm, peak precipitation being observed in summer. Soils were sampled in a birch kolka (60 to 80 year-old trees) and in a p i n e / b i r c h kolka (25 to
148
Y.N. Karavaeva et aL / Sci. Total Environ. 157 (1994) 147-151
cover were disturbed by decontamination works carried out during the first years after the accident. The control plot was chosen in a birch/pine forest (40 to 50 year-old trees) near the village of Rassokha outside the contaminated zone. The vegetation cover there is poor, being represented by forest herbs on brown forest soil. The methods of soil sampling were carefully designed to avoid cross-contamination between the soil layers. After digging a pit, the layers of soil (5 cm thick) were cut horizontally to a depth of 30 cm taking into account the area of the soil layers. Aliquots of 0.5 kg were taken from each sample. Then these aliquots were dried and sieved (1-mm mesh sieve) before analysis. Several birch and pine trees were collected to prepare samples of bark, stem, branches, leaves, and needles. The samples were ashed at 500°C. The strontium-90 content of the samples was estimated radiochemically by its daughter, yttrium-90. The concentrations of cesium-137 were measured using a AM-A-02qbI multichannel 3'analyser with a scintillating NaI (T1) detector. The strontium-90 and cesium-137 determination were duplicates.
'\ '- ~
rk "L,
.lliltl~K- IIWIL.~K1y I:|Ly
/ \
'\
56 °
Z~9}
/
Ile~tta,' s i ;a
0
I.rOm?, , / ,
$
la
/, j I~ 30"
/,1" D0 '
tS
\\
~ ki
-,
i
Fig. 1. Map of region in southern urals in which the 1957
3. Results and discussion
accident occurred. Plot 1, virgin birch kolka; Plot 2, recultivated p i n e / b i r c h kolka; Plot 3, control, b i r c h / p i n e forest.
Maximum strontium-90 concentrations were observed for the 0- to 5-cm thick intact top soil layer in the birch kolka, which exceeded the control concentrations by a factor of ~ 7 (Table 1). The radionuclide content decreased with depth, being undetectable at a depth of 20-25 cm with the methods used.
35-year-old trees) located at a distance of 0.5 km from each other and 0.5 km from the shore of the Tygysh lake. The birch kolka stands on virgin grey forest soil, while the pine/birch kolka has grown up in an area where the soil and the vegetation
Table 1 9°Sr and 137Cs ( B q - k g - 1 dry wt.) in the soils of the investigated forest stands Depth (cm)
0-5 5-10 10-15 15-20 20-25 25-30
Plot 1 (Virgin) 90Sr 646 + 235 212 + 40 40+4 6+ 2 BDL BDL
BDL, below detection limit.
137Cs
Plot 2 (Recultivated) 90Sr
314 _____100 19 5- 1 95:1 8 5- 1 8_ 1 15 5:9
12 + 16 + 7+ 7+ 11 + 27 +
1 6 1 5 6 16
137Cs
Plot 3 (Control) 90Sr
137Cs
75 + 12 + 21-i45 + 11 + 20 +
90 + 15 10 5- 2 4+ 1 BDL BDL BDL
150 + 3 23 + 2 5 5- 1 7+ 1 5 5- 1 BDL
42 4 13 35 6 1
Y.N. Karavaeva et al. / Sci. Total Environ. 157 (1994) 147-151
A different pattern of strontium-90 concentration distribution was observed for the soils in the pine/birch kolka. The highest concentration was recorded at a depth of 25-30 cm. In the 0- to 5-cm top layer, it was lower than in the controls by a factor of seven. This decrease in strontium-90 concentration and its reversal in the soil profile may be due to partial removal of the contaminated top layer and its burial at a certain depth during the decontamination works carried out after the accident. This is reflected in the morphology of the soil, in which the humus layer was buried at a depth of 20-30 cm. The cesium-137 concentrations of the soil samples from the three locations are very much similar. Maximum values were obtained for the humus-saturated 0- to 5-cm top layer (Table 1). As the concentration of strontium-90 in different soil layers varies, the contamination of these layers is also different. Thus, it is 34 k B q / m 2 for the intact soils (location 1) and only 5 k B q / m 2 for the soils disturbed by the decontamination works (Table 2). These values exceed those for the controls by a factor of 20 and 3, respectively. For cesium-137, the total contamination of the soils amounts to 13.6 and 15.2 k B q / m 2, the control value being 6.5 k B q / m 2. According to the data obtained for the central Urals in the 1970s, global fallout here was 2.6 k B q / m 2 for strontium-90 and 8.9 k B q / m 2 for cesium-137 [3]. In collaboration with our colleagues from the International Union of Radioecologists and using our data obtained earlier, we estimated the contribution of various radioactive contamination sources to the total contamination [4]. It appears that the contribution of the Kyshtym accident to the activity of the samples collected was 70-95% for strontium-90 and 8% for cesium-137. A considerable contribution to the level of contaminaTable 2 Total soil contamination (kBq. m-2) in the investigated forest stands Location
90Sr
137Cs
Plot 1 (virgin) Plot 2 (recultivated) Plot 3 (control)
34.3 5.4 1.7
13.6 15.2 6.5
149
tion of cesium-137 (58-74%) comes from unknown sources. One such source may be Lake Karachay, which is an open storage area for radioactive wastes. In 1967-1970, a severe drought led to the evaporation of water from this lake, which resulted in a patchy contamination of the region by the dust from the lake bed spread by winds over considerable distances [4]. As is known in forest ecosystems, the distribution of radionuclides in the soil and vegetation cover stabilizes at a certain level in about 10-12 years [3,5-7]. Further redistribution of radionuclides in the biocenosis is associated mainly with their migration deeper into the soil and their accumulation by plants, predominantly through the root system. An increased strontium-90 content of the soils in the contaminated zone leads to a higher concentration of this radionuclide in the local woody plants (Table 3). Thus, as compared with the control, the concentration of strontium-90 in the birch from the first location near the Tygysh lake (intact top layer) is higher by a factor of 2-30 depending on the part of the tree. For the birch and pine from the second, decontaminated location, the factor is 2-6. For both species, minimum strontium-90 concentrations were observed in the tissue. Twigs and bark showed a greater extent of contamination. This has been noted for other species of plants also [5,7,8], which is due to the fact that strontium-90 penetrates into a plant through its peripheral layers of tissue, the diffusion towards the core being slow. No clearly marked gradient was observed for strontium-90 concentrations in the bark along the trunk height. The woody species under study differ considerably in the absorption of strontium-90. Birch accumulates 1.5-5 times more of this radionuclide than pine, which is supported by the data from other authors [3,7,9]. Birch features increased absorption of Ca, being a typical calciphyl species. It is easy to discriminate between Sr and Ca in its surface part [9]. The above differences between these species may also depend on a different arrangement of their root systems in the soil profile. Birch has a 1.5-2 times larger mass of active, fine 'sucking'
150 Table 3 9°Sr and
Y.N. Karavaeva et al. / Sci. Total Environ. 157 (1994) 147-151
137Cs(Bq'kg-1
dry wt.) in trees
Location
Species, part of tree
Plot 1 (Virgin)
Betula verrucosa Ehrh.
Plot 2 (Recultivated)
Leaves Small branches Bark base Bark middle Bark top Wood Betula verrucosa Ehrh. Leaves Small branches Bark base Bark middle Bark top Wood
9oSr
137Cs
269 + 24 594 + 23 659 + 35 427 + 19 936 + 88 95 + 5
4.7 + 1.2 4.6 + 0.8 11.2 + 1.4 7.2 + 1.8 3.5 + 1.0 0.4 + 0.1
106 + 10 178 + 10 165 -t- 8 196 + 14 139 + 13 13 + 1
11.1 + 3.5 + 3.3 + 3.5 + 3.7 + 2.1 +
1.9 1.7 1.5 1.0 1.2 1.1
Pinus silvestris L.
Plot 3 (Control)
Needles Small branches Bark base Bark middle Bark top Wood Betula verrucosa Ehrh. Leaves Small branches Bark base Bark middle Bark top Wood
65 + 14 58 + 17 70 + 2 30 + 18 52 + 1.0 10 + 1
5.1 + 1.0 3.0 + 1.3 16.8 + 2.5 6.2 + 1.5 13.5 + 2.2 1.6 + 0.2
51 + 6 36 + 3 28 + 2 31 + 4 34 -t- 2 7+ 1
10.5 + 2.7 5.7 + 2.0 17.0 + 4.0 6.0 -t- 1.8 6.7 + 1.1 0.7 + 0.2
20 + 12 16 + 3 14 + 1 17 + 3 28 + 1 3+1
8.5 + 1.1 11.0 + 1.0 24.1 + 3.1 7.5 + 1.6 3.2 + 1.4 1.2 + 0.2
Pinus silvestris L.
Needles Small branches Bark base Bark middle Bark top Wood
roots in the 0- to 30-cm top layer of soil comp a r e d with p i n e [5,10]. T h e r e m o v a l of r a d i o n u c l i d e s f r o m soil by plants d e p e n d s greatly o n the d i s t r i b u t i o n of r a d i o n u clides in the soil profile. T h e r e m o v a l is greatest from the 20- to 30-cm deep layer [9,10]. A s a result of d e c o n t a m i n a t i o n in the s e c o n d s a m p l i n g location, the most c o n t a m i n a t e d h u m u s layer was b u r i e d just at this depth. This m a y b e o n e of the r e a s o n s why the r e m o v a l of r a d i o n u c l i d e s by the surface parts of the plants in the first a n d second s a m p l i n g sites differs by a b o u t a factor of three, w h e r e a s the total c o n t a m i n a t i o n of the soils dif-
fers by a b o u t a factor of six. Besides, we suppose that a part of the radioactivity is in the d e e p e r soil layers. A s regards cesium-137, its c o n c e n t r a t i o n was f o u n d to be similar in the h o m o l o g o u s organs of the birch a n d p i n e from different s a m p l i n g sites ( T a b l e 3), t h o u g h with a n o t i c e a b l e g r a d i e n t in the b a r k at various heights a n d with a m a x i m u m at the base. Thus, 34 years after the Kyshtym accident, the soils in the c o n t a m i n a t e d area show different levels of total c o n t a m i n a t i o n . This difference as regards the c o n c e n t r a t i o n of s t r o n t i u m - 9 0 in the
Y.N. Karavaeva et al. / Sci. Total Environ. 157 (1994) 147-151
soil and the vegetation cover may be attributed to the decontamination works performed after the accident. The concentration of strontium-90 in various parts of the birch and pine in the contaminated zone exceeds the background level by a factor of 2-30 and is dependent not only on the radionuclide content of the soil but also on the distribution of the radionuclides in the soil profile and specific differences between plant species. The maximum cesium-137 content of the soil in the contaminated zone was found to be 15 k B q / m 2, a considerable contribution to it coming from unknown sources in the area under study. References [11 A.I. Burnazyan (Ed.), Results of Studies and Experience in Elimination of the Consequences of Accidental Contamination of a Territory with the Products of Uranium Fission. Energoatomizdat, Moscow, 1990, p. 143. [2] B.V. Nikipelov, E.I. Mikerin, G.M. Romanov, D.A. Spirin and Yu. B. Khlopina, The radiation accident in the Southern Urals in 1957 and the cleanup measures implemented. Proceedings of an International Symposium 'Recovery Operations in the Event of a Nuclear Accident or Radiological Emergency', IAEA, Vienna, 1990, pp. 373-403.
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[3] R.M. Alexakhin and M.A. Naryshkin, Migration of radionuclides in forest biogeocenoses. Science, Moscow, (1977) 144. [4] A. Aarkrog, H. Dahlgaard, M. Frissel, L. Foulquier, N.V. Kulikov, I.V. Molchanova, C. Myttenaere, S.P. Niellsen, G.G. Polikarpov and P.I. Yushkov, Sources of anthropogenic radionuclides in the southern Urals. J. Environ. Radioact., 15 (1992) 69-80. [5] F.A. Tikhomirov, The Effect of Ionizing Radiation on Ecologycal Systems. Atomizdat, Moscow, 1972, p. 174. [6] C.F. Jordan, J.R. Kline and D.S. Sasscer, A simple model of strontium and manganese dynamics in a tropical rain forest. Health Phys., 24 (1973) 477-482. [7] A.A. Molchanov, E.A. Fiodorov, R.M. Alexakhin, M.M. Alexakhina, N.N. Mishenkov, M.A. Naryshkin, L.N. Tiumenev, V.A. Uhanova and V.P. Julanov, Some regularities of distribution of fission products deposited in composition of global fallout in forest vegetation. Russian J. For., 6 (1968) 18-23. [8] R. Scott Russell (Ed.), Radioactivity and Human Diet, Pergamon, London, 1966, p. 376. [9] R.M. Alexakhin, M.A. Naryshkin and M.A. Bocharova, On the problem concerning the peculiar features and quantitative prognostication of the cumulative accumulation of Sr-90 in arboreal plans. Rep. Acad. Sci., 193(5) (1970) 1192-1194. [10] F.M. Tikhomirov, N.I. Sanzharova and A.A. Shpazhnikow, Migration of anthropogenic radionuclides in the soil-plant system in forest biogeocenoses. Problems of forest radioecology. Gydrometeo, Moscow, (1979) 83-96.