Radiation monitoring of lake ecosystems within the Chernobyl
accident exclusion zone
NAZAROV Alexander1, GUDKOV Dmitri2,
2
State Specialized Scientific and Production Enterprise “Chernobyl Radioecological Centre” of the Ministry of Ukraine
on the Emergency and Affairs of Population Protection Against the Consequences of the Chernobyl Catastrophe,
Chernobyl, Ukraine, 07270. [email protected]
1
Department of Freshwater Radioecology, Institute of Hydrobiology of the National Academy of Sciences of Ukraine,
Kiev, Ukraine, 04210. [email protected]
Abstract: Lake ecosystems are efficient
“collectors” for the wide range of radionuclides, which
deposited in abiotic and biotic components after intake
by aquatic environments. For the time being, ecosystems
of lakes of the Chernobyl nuclear power plant exclusion
zone continue to suffer from heavy radioactive
contamination, dictating, thus, the need to carry out
further research works within the framework of
comprehensive regional radioecological monitoring.
Particular features of radionuclide accumulation by
hydrobionts substantially depend on formation of
hydrochemical composition of water, which, in its turn,
is a complex process, depending on functioning of
aquatic ecosystems and natural conditions of a water
catchment basin. Hydrochemical regimes in reservoirs
are determined by external factors and internal water
body processes. The results of 90Sr, 137Cs, 238Pu, 239+240Pu
and 241Am content in water, bottom sediment and
hydrobionts of different trophic levels of water objects
within the Chernobyl exclusion zone in 1990-2008 have
been analysed.
Keywords: Chernobyl exclusion zone, Lake
ecosystems,
Radiation
monitoring,
Radioactive
contamination.
1. Introduction
The territories of the Chernobyl exclusion zone are
characterised
by significant
heterogeneity of
radionuclide contamination, which is significantly
reflected by the radioactive substance contents in aquatic
ecosystem components. Primarily this is due to the
composition and the dynamics of radionuclide emissions
into the environment as a result of accident in 1986, as
well as to the subsequent processes of radioactive
substances transformation and biogeochemical migration
in the soils of catchment basin and bottom sediments of
reservoirs. Relatively low contents of radioactive
substances are found in the river ecosystems. Due to
high water change rate the river bottom sediments have
undergone decontamination processes (especially during
floods and periods of high water) and over the years that
passed since the accident have ceased to play the
essential role as a secondary source of water
contamination. The main sources of radionuclides in
rivers are currently the washout from the catchment
basin, the inflow from more contaminated water bodies,
as well as the groundwater. On the other hand, the closed
reservoirs, and in particular the lakes in the inner
exclusion zone, have considerably higher levels of
radioactive contamination caused by limited water
change and by relatively high concentration of
radionuclides deposited in the bottom sediments.
Therefore, for the majority of standing reservoirs the
level of radionuclide content is determined mainly by the
rates of mobile radionuclide forms exchange between
bottom sediment and water, as well as by the external
washout from the catchment basin.
Our studies were conducted: (1) to identify
dynamics of radionuclide distribution in components of
freshwater biocenose of lakes within the Chernobyl NPP
exclusion zone; (2) to study dynamic profiles of
radioactive contamination levels in hydrobionts of
different ecological groups and trophic levels; (3) to
assess the major factors, which determine distribution of
radionuclides in the freshwater ecosystems; (4) to study
the seasonal dynamics of radionuclides content in
macrophites and the role of main aquatic plant
associations in processes of radionuclides distribution in
biotic component of biohydrocenose; (5) to assess a
possibility to use hydrobionts of different trophic levels
as biological indicators of radioactive contamination of
water.
2. Methodology
Our researches were carried out during 1997-2008
in Azbuchin Lake, Yanovsky (Pripyatsky) Crawl,
cooling pond of the Chernobyl nuclear power plant, the
lakes of the left-bank flood plain of the Pripyat River Glubokoye Lake and Dalekoye-1 Lake (Fig. 1). The
radionuclide content in biological tissues was measured
for 28 higher aquatic plant species, 6 species of bivalve
molluscs and gastropod snails as well as 18 species of
fish. The tendency of the aquatic organisms to
accumulate radionuclides, traditionally expressed as the
concentration factor (CF), which is determined by
calculating the ratio of the specific activity of
radionuclides in tissue to the average annual content (for
molluscs and fish) or to the average content in the
environment water during the vegetation period (for
higher aquatic plants).
Fig. 1 Map of the major water bodies within the
Chernobyl exclusion zone
The 137Cs content was measured by g-spectrometry
complex: PGT IGC-25 detector (France), “Nokia LP
4900 B” analyser (“Nokia”, Finland), low-volt feeding
source - crate NIM BIN, amplifier NU-8210
(“Elektronicus Merokeszulekek Gyara”, Hungary) and
100 mm thickness leaden protection. The 90Sr content
was
measured
on
low-background
NRR-610
b-radiometer (“Tesla”, Czech). Minimal detectable
activity was 0.04 Bq under 1000 s sample exposition.
238
Pu and 239+240Pu content in electrolytic samples was
determined by a-spectrometric tract by NUC-8192
impulse analyser (“Elektronicus Merokeszulekek Gyara”,
Hungary). The 241Am content was measured by x-rayspectrometric line including x-ray detector EG&G Ortec
LOAX-51370/20 CFG-SU-GMX (“EG&G Ortec”,
USA) and analyser “Nokia LP 4900 B”. The results of
the radionuclide content measurements in hydrobionts
are expressed in Bq kg-1 of wet weight at natural
humidity and the mistake of estimated radionuclide
concentration fell within 15-20%.
3. Results
3.1 Water and bottom sediments
The highest radionuclide activity in water among
the studied objects was found in the Azbuchin Lake (Tab.
1). During 1997-2008, the average content of 90Sr and
137
Cs in water of lake reached up to 190 and up to
23 Bq l-1 respectively. The radionuclide contamination
density values found in the lake bottom sediments for
90
Sr, 137Cs, 238+239+240Pu and 241Am averaged at 6.70,
11.50, 0.24 and 0.22 TBq km-2 respectively, with the
maximum values of 33.30, 14.40, 1.10 and
0.29 TBq km-2. The 90Sr and 137Cs content in the water
of Glubokoye Lake come to 74-160 and 3.5-14 Bq l-1
respectively. The average values of contamination
density in the bottom sediments by 90Sr, 137Cs,
238+239+240
Pu and 241Am in 1998 were 2.6, 5.6, 0.07 and
0.06 TBq km-2, with the maximum values being 10.0,
13.7, 0.22 and 0.23 TBq km-2 respectively. In
Dalekoye-1 Lake the average content of 90Sr and 137Cs in
the research period reached 45 and 2.4 Bq l-1
respectively. The maximum value of radionuclide
contamination density in the bottom sediments by 90Sr in
1999 was 18.9, by 137Cs - 15.2, by 238+239+240Pu - 0.6 and
by 241Am - 0.4 TBq km-2. The average values were,
accordingly, 4.0, 3.1, 0.08 and 0.08 TBq km-2.
Tab. 1 Average annual specific activity of 90Sr and 137Cs in
water of lakes within the Chernobyl exclusion zone in 19972008, Bq l-1
Yanovsky Dalekoye
Azbuchin
Glubokoye
Year
Crawl
Lake
Lake
Lake
Sr
Cs
Sr
Cs
Sr
Cs
Sr
Cs
1997
30
5.2 45 4.5
85 12.7 100 13.2
1998
35
4.3 50 3.4 120 17.2 120 14.0
1999
38
3.7 45 2.8 190 22.8 120 13.6
2000
49
2.8 48 1.7 133 13.0 103 7.8
2001
26
2.7 35 2.6 110 9.9
79
7.1
2002
23
3.2 29 2.0
52
5.6
74
7.2
2003
22
2.4 40 2.3
49
8.7 102 6.8
2004
19
2.8 55 2.2
56
6.7 135 6.2
2005
17
2.3 58 2.1
74
6.8 140 6.3
2006
16
2.2 –*
–
61
6.3 160 3.6
2007
19
2.1
–
–
47
5.2 110 3.5
2008
11
3.2
–
–
37
4.8
95
3.7
The average specific activity values for 90Sr and
Cs in water of Yanovsky Crawl for the period 19972008 were 25 and 2.9 Bq l-1 respectively. The
radionuclide contamination of the bottom sediments of
reservoir is extremely heterogeneous, which is obviously
caused by the non-uniform character of the nuclear fallout and by the absence of wind-induced turbulence in
deep water. The average content of 90Sr, 137Cs,
238+239+240
Pu and 241Am in bottom sediments was,
respectively, 16.3, 14.8, 0.4 and 0.3 TBq km-2. At the
same time, within the bounds of silt sediment deposition,
some sites with abnormally high density of
contamination by 90Sr, 137Cs and 238+239+240Pu (307.1,
251.6 and 5.3 TBq km-2 respectively, which is 20 times
higher than the average values in the crawl), were found.
The cooling pond of the Chernobyl NPP has
undergone the highest radionuclide contamination in
comparison with other reservoirs of exclusion zone. In
the course of time, after the cessation of radioactive
emissions into the atmosphere and due to disintegration
137
1990s specific activity of 137Cs in tissues of higher
aquatic plants from Krasnensky flood plain was much
higher than specific activity of 90Sr in them, whereas in
the late 1990-s these values became comparable, and at
present specific activity of 90Sr is much higher than that
of 137Cs.
a
Bq/kg w .w .
900
700
Sr-90
R2 = 0,8653
Cs-137
R2 = 0,8565
1998
800
1995
of short-lived isotopes, 90Sr and 137Cs have become the
main radioactive contaminants of the cooling pond water.
During 1998–2001, the specific activity of 90Sr in the
cooling pond water was found to be within the range of
1.7-1.9, with the range being 2.7-3.1 Bq l-1 for 137Cs. The
heterogeneity of the bottom sediment contamination in
the cooling pond is currently determined by the nature of
the silt accumulation processes. The height of silt layers
at the depth of over 11 m (for up to 35 per cent of the
bottom area) reaches up to 100 cm., with the density of
contamination by 137Cs at 18.5-133.2 TBq km-2. The
bottom at the depth of 3-11 m consists of primary soils,
which are covered, with a 1-6 cm layer of silt, with the
contamination density by 137Cs in the range of 1.55.9 TBq km-2.
600
500
400
300
200
100
Year
2007
2006
2005
2004
2003
2002
2001
2000
1999
1996
1994
1993
1990
0
b
Bq/kg w .w .
20000
18000
16000
Sr-90
R2 = 0,778
14000
Cs-137
R2 = 0,0044
12000
10000
8000
6000
4000
2000
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
0
1993
3.2 Higher aquatic plants
The radionuclide contents in higher aquatic plants
(macrophytes) in the studied water bodies were largely
determined by the nature of radionuclide contamination
of the water objects and nearby territories, as well as by
the hydrochemical regime in the reservoirs. The latter
affects the forms of radionuclides in the reservoirs, thus
affecting the level of their bioavailability to the
hydrobionts.
The patterns of 90Sr and 137Cs accumulation have
been shown to be species-specific. Among the species
with relatively high 137Cs content are the helophytes (airwater plants) of genus Carex, Phragmites australis,
Glyceria maxima, Typha angustifolia, as well as strictly
water plant species Myriophyllum spicatum and
Stratiotes aloides. The low values of 137Cs activity in all
reservoirs were found in the representatives of family
Nymphaeaceae - Nuphar lutea and Nymphaea candida
as well as Hydrocharis morsus-ranae. Relatively high
content of 90Sr was shown by the species of genus
Potamogeton. Obviously this is related to this plant’s
tendency to accumulate large quantities of calcium
(which is not washed off during standard sampling) on
its surface during photosynthesis. At the same time,
calcium carbonate that is removed from the plant could
contain 7-20 times more radioactive strontium than the
plant tissue [1]. Thus, Potamogeton species makes a
good prospective radioecological monitoring object as a
specific accumulator of 90Sr.
During the period of studies since 1990 till 2007,
dynamics of the main radionuclide concentrations in
tissues of the higher aquatic plants from river
ecosystems of the exclusion zone indicated a decrease of
specific activity of 90Sr and 137Cs (Fig. 2a). So far as
concerns closed of low flowage water bodies, the most
representative retrievals obtained in the period of 19932007 have shown that since the late 1990-s the higher
aquatic plants related to different ecological groups is
indicating a frank tendency to 90Sr content increase in
tissues (Fig. 2b). The specific activity of 137Cs in higher
aquatic plants of the lakes under study either decreases
or remains practically constant. Hence, in the middle
Year
Fig. 2 Dynamics of radionuclide content in higher aquatic
plants of the Chernobyl Exclusion Zone: a - reed-mace
(Typha angustifolia L.) from the Pripyat River; b - watersoldier (Stratiotes aloides L.) from Glubokoye Lake
It is suggested that 90Sr specific activity in tissues of
macrophytes and fish of Krasnensky flood plain
increases due to dynamics of radionuclide
transformation in soils of water catchments area and
bottom sediments of water bodies. Due to construction a
complex of flood control dams, which preventing
radioactive substance washing off the soils of
contaminated areas not only, but changed the
hydrological regime of flood plain flows during floods,
this became the reason for intensification of waterlogging and swamping of dam-locked areas. This
confirms the increase of a role of water-marsh floristic
complex in structure of vegetative cover of Krasnensky
flood-lands [2]. As a result of swamping, at the
background of general tendencies of 90Sr mobile form
increase in the soils of water catchment areas and bottom
sediments of the exclusion zone water bodies; in
swamped soils of Krasnensky flood plain fulvic and
humic acid concentration increases, that decreases pH-
250000
90
Sr
200000
150000
100000
50000
0
Lymnaea
stagnalis
Viviparus
viviparus
Planorbarius
corneus
Stagnicola
palustris
Unio pictorum
Lymnaea
stagnalis
Planorbarius
corneus
Viviparus
viviparus
Stagnicola
palustris
Unio pictorum
Radix ovata
3.3 Mollusks
Freshwater molluscs are often considered as
bioindicators of radionuclide contamination of water
objects. These invertebrates accumulate practically all
the radionuclides found in water and, due to their high
biomass, molluscs play an important role in
bioaccumulation
processes
and
radionuclide
redistribution in aquatic ecosystems. The 137Cs and 90Sr
content in molluscs of reservoirs of the Chernobyl
exclusion zone are shown in Fig. 3.
determined for Lymnaea stagnalis - 0.1 and 0.2 Bq kg-1
respectively in Dalekoye-1 Lake, 2.7 and 6.4 in
Glubokoye Lake. The highest content was determined
for Stagnicolia palustris from Glubokoye Lake - 14 and
36 Bq kg-1 respectively. The highest activity among
gastropods was shown in Planorbarius corneus - 1 and
2 Bq kg-1 respectively from Dalekoye-1 Lake; 25 and
53 Bq kg-1 in Glubokoye Lake. Dreissena polymorpha
from the cooling pond of the Chernobyl NPP showed
238
Pu and 239+240Pu contents of 3 and 6 Bq kg-1
respectively.
Activity, Bq/kg
value in water and intensifies a denuding of watersoluble forms of 90Sr forming soluble complexes with
fulvic acids. Hence, an increase of concentrations of
mobile forms of the radionuclide and their inclusion to
biotic turnover of aquatic ecosystems are observed. This
also confirms the increase of 90Sr specific activity,
observed in recent years in Krasnensky flood plain lakes
at the background of stabilization of this index for 137Cs.
The tendency of the root contamination of plant
tissues by 90Sr was also observed for terrestrial plants in
the exclusion zone [3, 4]. Currently, some authors [5]
suggest that 90Sr mobility in soils of the exclusion zone
is maximal, and this will last during the nearest decade.
Thereafter, the rate of radionuclide decomposition will
exceed the rate of its mobilization.
40000
Sr
137
10700
2200
Dalekoye-1 Lake
26900
2900
Azbuchin Lake
30000
Cs
0
10000
52500
7600
Glubokoye Lake
20000
73700
40000
60000
Activity, Bq/kg
20000
80000
0
100000
Fig. 3 Average specific activity of radionuclides in
freshwater mollusks of Chernobyl exclusion zone on 19972008, Bq kg-1
The highest CF for both 90Sr and 137Cs were found
in bivalve mollusc Dreissena polymorpha and Unio
pictorum, which are the most active filter molluscs. The
highest CF for 90Sr was noted in Dreissena polymorpha in excess of 1100, while for 137Cs the highest CF (about
500) was found in the tissues of Unio pictorum.
Considerably lower CF was determined for the
gastropod species Lymnaea stagnalis, Planorbarius
corneus, Viviparus viviparous and Radix ovata (Fig. 4).
The average contents of transuranic elements 238Pu
239+240
and
Pu in mollusc tissues in Glubokoye Lake and
Dalekoye-1 Lake were as follows: the lowest value was
Radix ovata
430
Cs
90
1400
Activity, Bq/kg
Cooling pond
Yanovsky Crawl
137
873
Fig. 4 Average values and diapasons of specific activity of
90
Sr and 137Cs in mollusks of Glubokoye Lake
Average values of radionuclide CF for gastropod
snails and bivalve molluscs of the exclusion zone located
in the descending series in the following way: pond snail
- 241Am (4120) > 238Pu (590) > 239+240Pu (450) > 90Sr
(412) > 137Cs (310); zebra mussel - 241Am (29250) >
238
Pu (9200) > 239+240Pu (8730) > 90Sr (1100) > 137Cs
(350).
3.4 Fish
The fish species that are found at the upper levels of
the food webs may also constitute a part of human diet
a
Bq/kg
35000
30000
2
Cs-137
R = 0,0114
Sr-90
R = 0,8733
2
25000
20000
15000
10000
5000
2007
2006
2005
2004
2003
2002
2001
0
Year
Bq/kg
b
35000
30000
Cs-137
R 2 = 0,1418
Sr-90
R 2 = 0,9582
25000
20000
15000
10000
5000
2007
2006
2005
2004
2003
2002
2001
2000
0
1999
and, therefore, are of a particular interest in
radioecological research of water ecosystems.
The specific activity of 90Sr in fish of the lakes was
registered in a range 660-139500 Bq kg-1, with highest
values in Azbuchin Lake, and 137Cs - 810-27000 Bq kg-1,
with highest values in Dalekoye-1 Lake. In comparison
with river ecosystems within the exclusion zone and in
Kiev reservoir (on the area joining to the exclusion zone)
the specific activity of 90Sr and 137Cs was registered,
accordingly, in a range 1-99 and 3-530 Bq kg-1, with
maximal middle indexes for both radionuclides in fish
of the Pripyat River. The highest values of specific
activity of 137Cs in all reservoirs are determined for
predatory fish. Average values of specific activity of
238
Pu, 239+240Pu and 241Am in fish of Glubokoye Lake
were, accordingly, 0.4, 0.8 and 6.2 Bq/kg.
The analysis of radionuclide distribution on organs
and tissues of fish in lakes of the exclusion zone showed
that more 90% of 90Sr is concentrated in a skeleton and
scale, up to 3.7% - in muscular tissues and up to 3.5% in gills. The highest value of specific activity of 90Sr is
registered in scale, the least one - in skin, internal organs
and muscles. Basic part of 137Cs (more 60%) is
concentrated in muscular tissues, about 30% - in bones
and scale, and about 10% is on the other organs and
tissues.
By the main factors, determining quantitative
content of radionuclides in fish and their subsequent
dose-forming significance in lakes of the exclusion zone,
are the level and composition of radioactive
contamination of lakes and adjacent territories, intensity
of water exchange rate, and also correlation in water of
basic cations - chemical analogues and, accordingly,
discriminators of 90Sr and 137Cs. In the processes of
radionuclide concentration by the fish of lakes within the
exclusion zone the primary value of relation of the sum
of Ca2+ and Mg2+ to the sum of Na+ and K+ above
correlation of radionuclides in water and levels of
contamination of adjacent territories is determined.
In Glubokoye Lake the steady tendency of increase
of specific activity of 90Sr in fish’s tissue of different
ecological groups is registered. Due to this the specific
activity of 90Sr in pray fish considerably exceeded such
index for 137Cs, and the radiation dose rate due to
incorporated radionuclides grew on comparison with the
end of 1990-s more than in 6 times. It is possible to
suppose, that the main reason of such dynamics is
building of complex of water-protection dams on the
left-bank flood lands of the Pripyat River, entailing the
change of the hydrological mode, and also strengthening
of processes of overwetting and swamping of the
dammed territories. As a result on a background of
general tendencies of increase of mobile forms of 90Sr in
soils of catchment basin and bottom sediments of water
bodies within the exclusion zone, there is the increase of
specific activity of radionuclides in water of the lakes
located within dammed territory and intensity of their
concentration by biota.
Year
Fig. 5 Dynamics of radionuclide content in predatory (a)
and prey fish (b) from Glubokoye Lake
Specific activity of radionulcides in fish of lakes
within the exclusion zone during researched period in all
cases manyfold exceeded permissible levels, to the
norms accordant accepted in Ukraine for fish products
[6] - on the average in the hundred one times for 90Sr and
in the ten one times for 137Cs.
3.4 Radionuclide distribution on the main
components of lake ecosystems
The studies of radionuclide distributions in basic
biotic and abiotic components of lake ecosystems,
located at dam-fenced site of Krasnenskaya flood plain,
indicated depositing of the main quantity of
radionuclides
to
bottom
sediments.
Hence,
concentrations of radionuclides equalled as follows: by
90
Sr - 89 and 95% for Glubokoye and Dalekoye-1 lakes,
respectively; by 137Cs - 99%; by transuranic 238+239+240Pu
and 241Am elements - almost 100% of total content in
ecosystems (Tab. 1, 2). The difference in 90Sr
contribution to bottom sediments of the lakes and
contents of other radionuclides is stipulated by elevated
migration activity of 90Sr compared with 137Cs and
transuranic elements. This is the determining reason for
Macrobentos
Fish
100%
As suggested, such differences in 90Sr distribution
in ecosystems of lakes is, firstly, related to high biomass
of the higher aquatic vegetation, intensively
overgrowing Glubokoye Lake (covering about a half of
water plane). It is known that as growths of macrophytes
increase and become denser, the oxygen regime becomes
worse, significant quantities of organic substances and
biogenic elements are accumulate, and pH of the water
medium decreases. As pH decreases, radionuclides are
desorbed more actively and transit into the dissolved
state, 90Sr, first of all. This is also confirmed by lower
average pH in Glubokoye lake (8.2) compared with
Dalekoye-1 lake (8.8) and the ratio between specific
activity of 90Sr and 137Cs in water equal, on average, 12.5
in Glubokoye lake and 7.3 in Dalekoye-1 lake,
respectively. Contamination densities of bottom
sediments by different radionuclides are also typical: the
а
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Sr-90
Tab. 3 Total content and ratio of radionuclides in the main
components of Glubokoye Lake ecosystem
137
238+239+240
90
Cs
Pu, 241Am
Sr
Component
MBq
MBq
MBq
%
%
%
Bottom
444000
962000
25900
sediments
89.02
98.64
99.80
50900
6200
10
Water
10.21
0.64
0.04
800
2471
Seston
–*
0.16
0.25
3035
4598
42
Biota
0.61
0.47
0.16
* - no measurements were performed
Gastropod snails
Macrophytes
Am-241
The content of transuranic elements in biotic
components of ecosystems was minimal 0.07 and 0.16%,
respectively; for 137Cs it equals 0.14 and 0.47%, and for
90
Sr - 0.25 and 0.61%, respectively. Maximal values
were determined for Glubokoye Lake, where dissolved
90
Sr concentration in water was also higher.
Pu238+239+240
Tab. 2 Total content and ratio of radionuclides in the main
components of Dalekoye-1 Lake ecosystem
90
137
238+239+240
Sr,
Cs,
Pu, 241Am,
Component
MBq
MBq
MBq
%
%
%
Bottom
37000
51800
1100
sediments
95.35
99.11
99.90
1650
236
0.27
Water
4.25
0.45
0.03
58
155
Seston
–*
0.15
0.30
73
0.81
96
Biota
0.25
0.14
0.07
* - no measurements were performed
average specific activities of transuranic elements in
both lakes were practically equal, whereas the ratio
between average concentrations of 137Cs and 90Sr in
Dalekoye-1 and Glubokoye lakes equalled 1.3 and 2.2,
respectively. Therefore, affecting hydrochemical regime
of water bodies, the intensity of closed lake overgrowth
by associations of higher aquatic plants may also change
the type of distribution of radionuclides in the
components of ecosystems. It is also obvious that a part
of radionuclides transit to water from biomass of dead
plants and return to the biotic turnover of the water body.
In zoobenthos associations of Dalekoye-1 lake the
overwhelming majority of 90Sr (99%) is concentrated in
bivalve molluscs. Total 90Sr concentration in these
molluscs equals about 89% of its content in the lake
biota. About 8.5% is accumulated in higher hydrophytes,
1.5% - in fishes, about 1% - in gastropods, and less than
1% - in representatives of “soft” zoobenthos
(Oligochaeta and Chironomidae). Similar situation is
observed for distribution of transuranic elements.
However, in this case, quantities accumulated in bivalve
molluscs are decreased (down to 70-80%), as well as
these accumulated in gastropods and fishes (to fractions
of percent), whereas the value of macrophytes increases
to 21-29% (Fig. 6).
Cs-137
higher concentration of dissolved 90Sr in waters of both
lakes (10.2 and 4.3%) compared with 137Cs (0.6 and
0.5%) and transuranic elements (0.04 and 0.03%) and,
vice versa, lower 137Cs concentration in seston (0.16 and
0.15%) compared with 137Cs (0.25 and 0.30%),
respectively.
Fig. 6 The distribution of radionuclides on the main groups
of hydrobionts in the biotic component of Dalekoye-1 Lake
ecosystem
In the bottom invertebrates the part of 137Cs
accumulated in bivalve molluscs is 25% of its total
amount or lower. This relates to high CF values for this
radionuclide in “soft” zoobenthos representatives, in
which, despite lower biomass compared with bivalve
molluscs (10-fold lower), 137Cs content gives about 65%
of its total quantity. About 85% of 137Cs present in the
lake biota is accumulated in high aquatic plants, 7 and
8% - in zoobenthos and fish, respectively, and less than
0.1% - in gastropods.
In Dalekoye-1 Lake biota higher aquatic plants
occupying less than 5% of the water plane dominate
exclusively in 137Cs distribution, whereas in Glubokoye
Lake comparative biomass of macrophytes is so high
that they accumulate about 90% of all radionuclides
present in the biotic component of the lake (Fig. 7). The
exception is 90Sr, for which the contribution of aquatic
plants is slightly more than 50%, and the rest of it is
mostly concentrated in gastropod and bivalve mollusc
valves.
Macrobentos
Fish
100%
Gastropod snails
Macrophytes
б
90%
80%
70%
60%
50%
40%
30%
20%
10%
Am-241
Pu238+239+240
Cs-137
Sr-90
0%
Fig. 7 The distribution of radionuclides on the main groups
of hydrobionts in the biotic component of Glubokoye Lake
ecosystem
The main factors determining the contribution of
various ecological groups of hydrobionts to total content
of radionuclides in the biotic component of lake
ecosystems are CF and aquatic organism biomass.
Moreover, CF values may be so significant that
sometimes the biomass factor is displayed in the only
cases, when the difference in this parameter between
different groups of hydrobionts reaches two orders of
magnitude.
4. Conclusion
Radioactive contamination of the Chernobyl
accident exclusion zone has complex distribution
structure in different landscapes and characterised by
dynamic character of the state of radioactive substance
forms, which affect their migration and redistribution by
components of ecosystems. The basic questions of
radioactive safety of the restriction zone concern
radionuclides wash-off with surface drainage water to
river network, their export outside the exclusion zone
and affection of the water quality in the Dnieper River.
Self-purification of closed water bodies in the
Chernobyl exclusion zone is extremely slow process.
Therefore, ecosystems of the majority of lakes, dead
channels and crawls possess high level of radionuclide
contamination of all the components. The main quantity
of radionuclides in aquatic landscapes of the restriction
zone are deposited to bottom sediment of closed water
bodies, hence, the distribution of radioactive substances
in biotic and abiotic components of hydrobiocoenoses is
defined
by
biogeochemical
regularities
and
transformations of radioactive substances in bottom
sediments of water bodies and in soils of adjacent
territories.
The construction of a complex of flood retarding
dams and degradation of existing melioration systems at
the site of the left-bank flood plain of the Pripyat River
implied a change of hydrological regime and the
character of water object overgrowth. The absence of
flowage in water bodies, stagnation effects during spring
flooding and seasonal runoffs intensified waterlogging
and swamping of dam-fenced territories. As a result, at
the background of general tendencies of 90Sr mobile
forms increase in the soils of water catchment areas and
bottom sediments of the restriction zone water bodies
located at dam-fenced sites, as well as increasing
intensity of this radionuclide concentrating by higher
aquatic plants and, obviously, other autotrophic
organisms. For some species of macrophytes, 90Sr
concentration compared with the early 1990s has
increased by more than an order of magnitude and
exceeds the specific activity of 137Cs. Obviously the
specific activity of 90Sr will also increase at higher
trophic levels, however, at present, such dynamics was
not reliably detected. It is suggested that for
radioecological monitoring of aquatic ecosystems in the
restriction zone, higher aquatic plants possessing high
CF values of radionuclides are the most sensitive testobjects for recording increasing specific activity of
mobile forms of radionuclides in the water of test site
water bodies.
The results of tests performed confirm the tendency
to further deterioration of the radiation situation in
aquatic ecosystems of the restriction zone. Swamping of
contaminated territories leads to acceleration of 90Sr
deposited form mobilisation processes and their
migration and redistribution in closed aquatic landscapes.
Hence, an original “depot” of mobile forms of
radioactive substances is formed, which in high-flood
periods may become a source of increasing 90Sr drainage
to the Pripyat River and then outside the restriction zone.
In this connection, the necessity to implement hydraulic
engineering procedures preventing underflooding of
territories with high densities of radionuclide
contamination; optimisation and enhancement of
radioecological monitoring system, and further
development of investigations of radionuclide behaviour
in aquatic ecosystems of the exclusion zone - the
important components in the complex of measures
related to forecasting and minimisation of the Chernobyl
disaster consequences.
Acknowledgement
This study was financially supported by the
Ukraine Ministry for Emergency Situations and by the
National Academy of Sciences of Ukraine (Projects
0101U004987, 0102U004665 and 0102U003541).
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