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Environmental study of radioactive caesium in Greek lake fish after the chernobyl accident
J. Environ.
Radiouctivify, Copyright
Vol. 28 No. 3, pp. 285-293, 1995 0 1995 Elsevier Science Limited
Printed ELSEVIER
in Ireland. All rights reserved 0265-931X/95 $9.50 + 0.00
0265-931X(94)00047-6
Environmental Study of Radioactive Caesium in Greek Lake Fish After the Chernobyl Accident
P. Kritidis
and H. Florou
Inst. of Nuclear Technology - Radiation Protection, N.C.S.R. ‘Demokritos’, Athens, Greece (Received 2 1 June 1994; accepted 19 November 1994)
ABSTRACT The radiological status of radiocaesium in the Greek environment until 1986 has been characterized by the impact of world-wide fallout. During 1986, the Chernobyl nuclear accident resulted in an average deposition of total caesium (‘34Cs + 137Cs) of M 9 kBq mP2 in Greece, while regional of averages ranged within 3-45 kBq mP2. The radioactive contamination the lake ecosystems is potentially a radiologically important consequence of the accident. The effects of ‘37Cs and 134Cs introduced into a number of major Greek lake ecosystems has been evaluated in the present work by determination of their concentrations in various lake fish species during the years 1986, 1988 and 1989. Although the representative and predominant species typically doffer from lake to lake, while the local deposition of caesium varies signtficantly, the bioaccumulation of caesium by the examined species seems to depend rather on the fish species than on the local environmental parameters. The time-dependence of the fish contamination has been used to evaluate the contribution of lake fish consumption to the total ingestion dose of the population.
INTRODUCTION The environmental behaviour of the radionuclides after an accidental contamination is of interest because it largely determines their potential radiological impact to the environment and man. All freshwater ecosystems, except subterranean water bodies, have been exposed to world-wide 285
286
P. Kritidis, H. Florou
fallout due to nuclear tests and, in certain cases, to pollution from local sources. The large accidental releases to the atmosphere can be also a significant source of contamination for vast areas. This was the case of the Chernobyl nuclear accident, which resulted in considerable radioactive pollution of the Greek environment, mainly during the period 2-7 May 1986. Unlike the regions close to Chernobyl, the delayed impact of the accident on the Greek environment is related mainly to the two caesium isotopes 134Csand 137Cs. The variable precipitation conditions resulted in considerable variations of the regional averages of total caesium deposition - within ~3-45 kBq rnp2 - in the 132,000 km2 of Greek territory (Kritidis et al., 1990). Following the contamination event, elevated levels of radiocaesium were observed in abiotic and biotic components of the Greek lakes (Florou, 1987; Florou et al., 1989~). Our measurements during 1986 have shown that the bioaccumulation of caesium by lake freshwater fish is more than one order of magnitude higher than this observed in marine species (Florou et al., 19893). To follow the radiological and ecological time-behaviour of this contamination, samples of freshwater lake fish have been collected and measured during the years 1986, 1988 and 1989.
SAMPLING
AND MEASUREMENTS
The sampling network (Fig. 1) includes tive major Greek lakes (all natural except Aliakmon) located in the Northern part of the country (where the higher values of caesium deposition have been observed), as well as two lakes in Western Greece, which are characterized by lower values of caesium deposition (Kritidis & Papanicolaou, 1987). The fish species are representative and predominant for each lake, although the same species from different lakes were collected whenever this was possible. After identification of the collected species, the fish flesh parts were homogenized and measured in the wet state (typically for 70 000 s) by a PCbased high resolution gamma-spectrometry system with a HpGe detector of 20% relative efficiency with respect to a 3 x 3’ Nal detector. The results have been treated as follows: 1. The data have been corrected for the decay of 137Cs and 134Cs, in order to refer to the same sampling date for each of the three years. In fact, the sampling dates differ less than 7 days for each period (June 1986, May 1988, April 1989), therefore this procedure corrects for the radioactive decay of the two isotopes during the period between sampling and measurement.
Environmental study of radioactive caesium in Greek lake fish
I
\\
287
--
BULGARIA
,
’
,J,’ i
;,
,!
.
TOTAL CAESIUM DEPOSITION, kBq m” I pg@gj
es 5_,. Xl-20
:
‘“;Z
SAMPLING LOCATIONS 0 H El
GREECE MediterraneanSea
i.1
Aliakmon Vegoritis M. Prespa
El
Doirani
H
loanninon
c1
Trichonis
q Vistonis
Fig. 1. Sampling locations. Total caesium deposition after the Chernobyl accident.
A linear regression analysis of the relation Ck(6) versus Dk was performed, where Ck(6) is the average activity concentration (Bq kg-‘) of total caesium (134Cs + 137Cs) in the species of the lake ‘k’ at the 1986 reference sampling time and Dk - the regional average of the total caesium deposition in the administrative district of the lake ‘k’ during May 1986. To enhance the precision of this analysis, the deposition values used are averages - where possible of the results of our survey (Kritidis & Papanicolaou, 1987) and an independent study (Antonopoulos-Domis, Klouvas & Tervisidis, 1987), which covers the region of Northern Greece. In order to concentrate on the ecological behaviour of caesium, the concentrations C(8) and C(9) (years 1988 and 1989) have been further corrected for the radioactive decay of each caesium isotope during the period (19861989). The resulting values are CC(8) and CC(9), whereas CC(6) = C(6). Exponential fitting of the relations CCik(t), CC,(t) and CCi(t) has been performed, where CCik(t) is the value of the activity concentration and total caesium in the species ‘i’ from lake ‘k’, CC,(t) is the average value of CCi,(t) for all the species
P. Kritidis. H. Florou
288
from lake ‘k’, CCi(t) is the average value of CCik(t) for the species ‘i’ wherever it has been sampled and t is the time (a) after the nuclear accident. In order to overcome the poor statistics of certain very low 1989 values (relative errors higher than 50%), these calculations have been paralleled with calculation of the ratios Rik(8/6) = CCik(S)/ CCik(6), &(8/6) = CCk(S)/ CCk(6) and Ri(8/6) = CCi(8)/ CCi(6), which provide information of better statistical quality on the initial slope of the relations CC(t).
RESULTS AND DISCUSSION The average concentration of total caesium in the fish flesh of species from each lake during June 1986, Ck(6) is shown in Fig. 2 versus &, the regional deposition of total caesium in the lake district. The correlation coefficient equals 0.98 and the regression relation (zero y-intercept type) is: Ck(6) (Bq kg-‘) = 0.021 Dk (Bq mP2).
(1)
Note that the time (6) refers to the first week of June 1986, i.e. roughly one month after the period of the principal pollution. The relation (1) is not expected to depend on the type of caesium isotope. This is demonstrated by the coefficients of the similar relations, calculated on the basis of ‘37Cs and ‘34Cs alone and the activity ratio 2:l of the two isotopes at the moment of deposition. These values are 0.020 and 0.021, respectively. The good correlation observed is an indicator for the representativity of both the local deposition data and the fish sampling. On the other hand, this is also a first indication of the reduced influence of local abiotic parameters (such as the exchange rate of the lake water, the caesium dilution and sedimentation) on the relation C(6)/D.
0
10
20
30
40
50
Regional deposition of total caesium, kBqlm2 Fig. 2. The concentrations of total caesium in fish flesh (lake averages) versus the regional deposition of total caesium.
during June 1986
Environmental study of radioactive caesium in Greek lake fish
289
The ‘ecological half-lives’ T, of caesium in fish flesh (derived from the exponential fits of the decay-corrected values) and the ratios of total caesium concentrations in fish flesh (1988/1986) - R(8/6) - are given in Table 1. The term ‘ecological half-life’ is introduced to reflect the presence of various processes affecting the time-dependence of caesium concentrations in fish, such as the time-behaviour of the source term (e.g. delayed input due to weathering processes in the region of the lake), the caesium dynamics in the lake water (dilution, sedimentation, re-suspension) and the bioaccumulation of caesium by fish. As already mentioned, the poor statistics of certain 1989 results is reflected in the precision of the T, values for these results. If we consider in parallel the statistically better quality parameter R(8/6), we can note that: 1. There is no evidence of significant correlation between R(8/6) (or T,) and the lake location (which means also ‘the local caesium deposiTABLE 1
Ecological Half-Lives and Ratios 1988/86 of the Total Caesium Concentrations Fish Species Species Barbus albanicus
(Strosidi) Barbus perspensis
in Various
Lake
Cs Deposition (total), kBq nV2
Ecological half-life, a
Average
Ratio 1988/86
Ioanninon Trichonis M. Prespa
4 2.5 30.5
0.46 0.44 0.71
0.45 0.77
0.094 0.107 0.162
Trichonis
2.5
0.45
0.45
0.391
Doirani Vistonis Ioanninon Vegoritis
16.5 8 4 28
0.83 0.60 0.47 I.21
0.63
0.156 0.152 0.124 0.322
Aliakmon Doirani Vegoritis Doirani Ioanninon Trichonis Trichonis
42 16.5 28 16.5 4 2.5 2.5
0.50
0.51
Vegoritis M. Prespa Ioanninon
28 30.5 4
o-43 0.48 ‘0.47
(Mpriana) Caracius caracius gibelio
(Petaluda) Cyprinus carpio
(Cyprinos, Grivadi) Esox lucius
1.27
(Tourna) Perka fluviatilis
(Perki) Rurilus rut&s doiranensis
(Tsironi) Rut&s rubilio
(Dromitsa) Scardinius acarnanicus
0.52 0.53 0.47 0.50 0.51 0.57
0.50 0.50 0.57
0.037 0,041 0.047 0.027 0.135 0.127 0.267
(Tsiroukla) Salmo gairdneri
(Pestrofa) Tinca tinca
(Glini) All species
0.46 0.47 0.60
0.038 0.029 0.101
290
P. Kritidis. H. Florou
tion’). It is noticeable, that two pairs of extremely different R(8/6) values, E. lucius - O-32, S. gairdneri - 0.038 (Vegoritis lake) and C. caprio - O-16, R. r. doiranensis - 0.027 (Doirani), have been observed in the same lakes. 2. In contrast, the ecological half-lives T, and the ratios R(8/6) appear to depend definitely on the fish species. This is demonstrated by Table 1, where the ratios R(8/6) appear fairly grouped according to the fish species. We have to conclude that the retention/elimination rate of caesium in the species is a major factor influencing the time behaviour of the fish contamination, while the abiotic environmental parameters are of lesser importance in this case, although they can play, generally, an important role as well (Whicker & Schultz, 1982; Whicker et al., 1972). One must note that the biological parameters include not only the metabolism rates, but also the habits of the species (taking into account the non-homogeneous distribution of caesium in the lake water during the initial period of the impact). An important external factor influencing the bioaccumulation of caesium is the life stage of the organisms during the pollution event: the season of the event could be of importance.
RADIOLOGICAL
IMPACT
The effective half-lives T,K of 137Csand ‘34Cs are related to the ecological half-life of caesium T, as follows: Tefi = T,T,(T,
(2)
+ TJ’
where T, refers to all ecological processes affecting the concentrations of caesium in fish, except the radioactive decay and TP is the physical half-life (due to the radioactive decay). The integrated concentration IC of a radionuclide with radiological half-life Teg and initial concentration C, is given by IC = C,, T&ln2)-I According to (1) C can be expressed the local deposition D as
statistically -
C, [Bq kg-‘] = O-021D [Bq rnp2]
as a function of (4)
Combining (3) and (4) we can express the integrated concentration of caesium in certain fish species per unit of local caesium deposition as a function of its radiological half-life time: IC [Bq kg-la] = 0.021 (ln2))’ Tee [a]D [Bq me21
(5)
Environmental study of radioactive caesium in Greek lake fish
The committed effective dose equivalent H due to the consumption lake fish at annual rate of A4 [kg a-‘] is H=ICMK
291
of (6)
where K [Sv Bq-‘1 is the ingestion considered.
conversion
factor for the isotope
K(‘34Cs) = 1.9 x lo-’ Sv Bq-’ and K(137Cs) = 1.3 x lo-’ Sv Bq-’ (FAO et al., 1992).
The derived effective half-lives, the integrated concentrations in fish flesh per unit deposition and the committed effective dose equivalent estimates for the critical group are given in Table 2, separately for ‘37Cs and ‘34Cs. For species collected in more than one lake, the specimens from the lake with the maximum deposition value has been used. The dose estimations for specific fish species correspond to a rather extreme case of stable feeding preference for this species. The estimation in the last row (=3OO@v total) is based on the average effective life-time of the species and the maximum regional deposition observed (Lake Vegoritis). This approach should correct for the possible missing of representative fish species collected from the Lake Vegoritis. The maximum value which could be considered as ‘critical group dose’ is close to 400 ,uSv (E. lucius in Vegoritis). These values can be compared with the 1600 $v estimate for the ingestion-related critical group dose in Greece (Kritidis & Papanicolaou, 1987). The last value does not include a specific component related to lake fish consumption as long as its ingestion term is based on the average diet of the Greek and the value of maximum caesium contamination.
CONCLUSIONS 1. The initial bioaccumulation of radiocaesium in lake fish is strongly correlated with the local caesium deposition, with a correlation coefficient of 0.98. Local abiotic parameters do not seem to affect this relation, therefore the estimated ecological half-lives of caesium do not depend on the initial deposition during 1986. Considering the elimination rate of caesium, we can assume, based on the ecological half-lives, that the biological parameter (fish species) plays the main role, while the environmental parameters are of minor importance in this case. 2. Based on the average radiological half-lives of ‘37Cs and ‘34Cs and the maxima of caesium deposition observed, it is estimated that the
0.49 0.49 0.46 0.45 0.44 0.44 0.59
0.50
13’CS 1.22 0.75 0.62 0.56 0.40 0.40 0.38 0.38 0.37 0.37 0.46
0.41
‘34cs 0.78 0.56 0.48 0.45
Notes: 1. The activity ratio ‘37Cs:‘34Cs = 2:l is assumed for May 1986. 2. The consumption rate of fish is supposed to be 30 kg aa’.
0.50 0.50 0.47 0.46 0.45 0.45 0.60
R. r. doiranensis R. rubilio T. tinca S. gairdneri B. albanicus C. c. gibelio All species
1.27 0.77 0.63 0.57
0.51
lucius g. perspensis carpio acarnanicus
P. fluviatilis
E. B. C. S.
Doses
0.015 0.015 0.014 0.014 0.013 0.013 0.018
0.015
13’cs 0.037 0.023 0.019 0.017 0.012 0.012 0.012 0.011 0.011 0.011 0.014
0.012
‘34cs 0.024 0.017 0.015 0.014
of Greek
28 4 4 30.5 4 2.5 42
42
13’cs + lJ4cs 28 30.5 16.5 2.5
Maximum deposition kBq mm2
Due to Consumption
Ratio of the integrated concentration to deposition, m2 a kg-t
TABLE 2 and Committed Critical Group Chernobyl Accident
Effective half-life, a
per Unit Deposition
Ecological half-life, a
Concentrations
Species
Integrated
109 16 14 109 13 9 195
166
“‘Cs 270 181 81 12
65 10 10 65 8 6 110
97
‘34cs 125 97 46 6
Critical comm. eff dose equivalent pSv (see notes)
Lake Fish After the
a $ E
-;
9. 2. %
P
Environmental study of radioactive caesium in Greek lake fish
293
committed effective dose equivalent to the critical group due to the consumption of lake fish does not exceed 300 ,uSV, which represents about 20% of the ingestion-related critical group dose in Greece.
ACKNOWLEDGMENT We thank Dr Ch. Daulas from the National Center for Marine Research/ Dep. of Inland Water, for his kind help in fish identification.
REFERENCES Antonopoulos-Domis, M., Klouvas, A. & Tervisidis, F. (1987). Deposition of long-lived radionuclides ‘34Cs and 137Csin Macedonia and Thrace soils after the Chernobyl nuclear accident. Proc. Nation. Conf., ‘The Impact of the Chernobyl Accident on Greece’, NCSR ‘Demokritos’, Athens, 19-20 November 1987, pp. 77-82. FAO, IAEA, ILO, NEA, PAHO, WHO (1992). Basic Safety Standards, IAEA, Vienna, 302 pp. Florou, H. (ed.). (1987). The impact of the Chernobyl nuclear accident to the Greek marine environment. Technical Report. N.C.M.R./68, February 1987, 36 pp. (In Greek.) Florou, H., Kritidis, P. & Probonas, M. (1989a). Radiocaesium in marine and lake fish after the Chernobyl accident. Proc. Int. Symposium on Radiation Protection Selected Topics, Boris Kidric Inst. of Nuclear Sciences, Dubrovnik, October 2-6, 1989 pp. 376380. Florou, J., Kritidis, P. & Synetos, S. (19896). A comparative study of radiocaesium in marine and lake fish. Proc. Nat. Symposium on Environmental Science and Technology. Mytilini, September 1989, pp. 19-23. Kritidis, P., Florou, H. & Papanicolaou, E. (1990). Delayed and late impact of the Chernobyl accident on the Greek envrionment. Radiation Protection Dosimetry,
30(3),
87-190.
Kritidis, P. & Papanicolaou, E. (1987). Deposition of caesium and contamination of certain products: a correlation study. Regional IRPA Congress, Rome, October 1987. Whicker, F. W., Nelson, W. C. & Callegos, A. F. (1972). Fallout and 90Sr in trout from mountain lakes in Colorado. Health Phys., 23(4), 519-530. Whicker, F. W. & Schultiz, V. (1982). Radioecology: Nuclear energy and the environment. CRC Press, Inc., Florida, Vol. I, 212 pp.
Radiouctivify, Copyright
Vol. 28 No. 3, pp. 285-293, 1995 0 1995 Elsevier Science Limited
Printed ELSEVIER
in Ireland. All rights reserved 0265-931X/95 $9.50 + 0.00
0265-931X(94)00047-6
Environmental Study of Radioactive Caesium in Greek Lake Fish After the Chernobyl Accident
P. Kritidis
and H. Florou
Inst. of Nuclear Technology - Radiation Protection, N.C.S.R. ‘Demokritos’, Athens, Greece (Received 2 1 June 1994; accepted 19 November 1994)
ABSTRACT The radiological status of radiocaesium in the Greek environment until 1986 has been characterized by the impact of world-wide fallout. During 1986, the Chernobyl nuclear accident resulted in an average deposition of total caesium (‘34Cs + 137Cs) of M 9 kBq mP2 in Greece, while regional of averages ranged within 3-45 kBq mP2. The radioactive contamination the lake ecosystems is potentially a radiologically important consequence of the accident. The effects of ‘37Cs and 134Cs introduced into a number of major Greek lake ecosystems has been evaluated in the present work by determination of their concentrations in various lake fish species during the years 1986, 1988 and 1989. Although the representative and predominant species typically doffer from lake to lake, while the local deposition of caesium varies signtficantly, the bioaccumulation of caesium by the examined species seems to depend rather on the fish species than on the local environmental parameters. The time-dependence of the fish contamination has been used to evaluate the contribution of lake fish consumption to the total ingestion dose of the population.
INTRODUCTION The environmental behaviour of the radionuclides after an accidental contamination is of interest because it largely determines their potential radiological impact to the environment and man. All freshwater ecosystems, except subterranean water bodies, have been exposed to world-wide 285
286
P. Kritidis, H. Florou
fallout due to nuclear tests and, in certain cases, to pollution from local sources. The large accidental releases to the atmosphere can be also a significant source of contamination for vast areas. This was the case of the Chernobyl nuclear accident, which resulted in considerable radioactive pollution of the Greek environment, mainly during the period 2-7 May 1986. Unlike the regions close to Chernobyl, the delayed impact of the accident on the Greek environment is related mainly to the two caesium isotopes 134Csand 137Cs. The variable precipitation conditions resulted in considerable variations of the regional averages of total caesium deposition - within ~3-45 kBq rnp2 - in the 132,000 km2 of Greek territory (Kritidis et al., 1990). Following the contamination event, elevated levels of radiocaesium were observed in abiotic and biotic components of the Greek lakes (Florou, 1987; Florou et al., 1989~). Our measurements during 1986 have shown that the bioaccumulation of caesium by lake freshwater fish is more than one order of magnitude higher than this observed in marine species (Florou et al., 19893). To follow the radiological and ecological time-behaviour of this contamination, samples of freshwater lake fish have been collected and measured during the years 1986, 1988 and 1989.
SAMPLING
AND MEASUREMENTS
The sampling network (Fig. 1) includes tive major Greek lakes (all natural except Aliakmon) located in the Northern part of the country (where the higher values of caesium deposition have been observed), as well as two lakes in Western Greece, which are characterized by lower values of caesium deposition (Kritidis & Papanicolaou, 1987). The fish species are representative and predominant for each lake, although the same species from different lakes were collected whenever this was possible. After identification of the collected species, the fish flesh parts were homogenized and measured in the wet state (typically for 70 000 s) by a PCbased high resolution gamma-spectrometry system with a HpGe detector of 20% relative efficiency with respect to a 3 x 3’ Nal detector. The results have been treated as follows: 1. The data have been corrected for the decay of 137Cs and 134Cs, in order to refer to the same sampling date for each of the three years. In fact, the sampling dates differ less than 7 days for each period (June 1986, May 1988, April 1989), therefore this procedure corrects for the radioactive decay of the two isotopes during the period between sampling and measurement.
Environmental study of radioactive caesium in Greek lake fish
I
\\
287
--
BULGARIA
,
’
,J,’ i
;,
,!
.
TOTAL CAESIUM DEPOSITION, kBq m” I pg@gj
es 5_,. Xl-20
:
‘“;Z
SAMPLING LOCATIONS 0 H El
GREECE MediterraneanSea
i.1
Aliakmon Vegoritis M. Prespa
El
Doirani
H
loanninon
c1
Trichonis
q Vistonis
Fig. 1. Sampling locations. Total caesium deposition after the Chernobyl accident.
A linear regression analysis of the relation Ck(6) versus Dk was performed, where Ck(6) is the average activity concentration (Bq kg-‘) of total caesium (134Cs + 137Cs) in the species of the lake ‘k’ at the 1986 reference sampling time and Dk - the regional average of the total caesium deposition in the administrative district of the lake ‘k’ during May 1986. To enhance the precision of this analysis, the deposition values used are averages - where possible of the results of our survey (Kritidis & Papanicolaou, 1987) and an independent study (Antonopoulos-Domis, Klouvas & Tervisidis, 1987), which covers the region of Northern Greece. In order to concentrate on the ecological behaviour of caesium, the concentrations C(8) and C(9) (years 1988 and 1989) have been further corrected for the radioactive decay of each caesium isotope during the period (19861989). The resulting values are CC(8) and CC(9), whereas CC(6) = C(6). Exponential fitting of the relations CCik(t), CC,(t) and CCi(t) has been performed, where CCik(t) is the value of the activity concentration and total caesium in the species ‘i’ from lake ‘k’, CC,(t) is the average value of CCi,(t) for all the species
P. Kritidis. H. Florou
288
from lake ‘k’, CCi(t) is the average value of CCik(t) for the species ‘i’ wherever it has been sampled and t is the time (a) after the nuclear accident. In order to overcome the poor statistics of certain very low 1989 values (relative errors higher than 50%), these calculations have been paralleled with calculation of the ratios Rik(8/6) = CCik(S)/ CCik(6), &(8/6) = CCk(S)/ CCk(6) and Ri(8/6) = CCi(8)/ CCi(6), which provide information of better statistical quality on the initial slope of the relations CC(t).
RESULTS AND DISCUSSION The average concentration of total caesium in the fish flesh of species from each lake during June 1986, Ck(6) is shown in Fig. 2 versus &, the regional deposition of total caesium in the lake district. The correlation coefficient equals 0.98 and the regression relation (zero y-intercept type) is: Ck(6) (Bq kg-‘) = 0.021 Dk (Bq mP2).
(1)
Note that the time (6) refers to the first week of June 1986, i.e. roughly one month after the period of the principal pollution. The relation (1) is not expected to depend on the type of caesium isotope. This is demonstrated by the coefficients of the similar relations, calculated on the basis of ‘37Cs and ‘34Cs alone and the activity ratio 2:l of the two isotopes at the moment of deposition. These values are 0.020 and 0.021, respectively. The good correlation observed is an indicator for the representativity of both the local deposition data and the fish sampling. On the other hand, this is also a first indication of the reduced influence of local abiotic parameters (such as the exchange rate of the lake water, the caesium dilution and sedimentation) on the relation C(6)/D.
0
10
20
30
40
50
Regional deposition of total caesium, kBqlm2 Fig. 2. The concentrations of total caesium in fish flesh (lake averages) versus the regional deposition of total caesium.
during June 1986
Environmental study of radioactive caesium in Greek lake fish
289
The ‘ecological half-lives’ T, of caesium in fish flesh (derived from the exponential fits of the decay-corrected values) and the ratios of total caesium concentrations in fish flesh (1988/1986) - R(8/6) - are given in Table 1. The term ‘ecological half-life’ is introduced to reflect the presence of various processes affecting the time-dependence of caesium concentrations in fish, such as the time-behaviour of the source term (e.g. delayed input due to weathering processes in the region of the lake), the caesium dynamics in the lake water (dilution, sedimentation, re-suspension) and the bioaccumulation of caesium by fish. As already mentioned, the poor statistics of certain 1989 results is reflected in the precision of the T, values for these results. If we consider in parallel the statistically better quality parameter R(8/6), we can note that: 1. There is no evidence of significant correlation between R(8/6) (or T,) and the lake location (which means also ‘the local caesium deposiTABLE 1
Ecological Half-Lives and Ratios 1988/86 of the Total Caesium Concentrations Fish Species Species Barbus albanicus
(Strosidi) Barbus perspensis
in Various
Lake
Cs Deposition (total), kBq nV2
Ecological half-life, a
Average
Ratio 1988/86
Ioanninon Trichonis M. Prespa
4 2.5 30.5
0.46 0.44 0.71
0.45 0.77
0.094 0.107 0.162
Trichonis
2.5
0.45
0.45
0.391
Doirani Vistonis Ioanninon Vegoritis
16.5 8 4 28
0.83 0.60 0.47 I.21
0.63
0.156 0.152 0.124 0.322
Aliakmon Doirani Vegoritis Doirani Ioanninon Trichonis Trichonis
42 16.5 28 16.5 4 2.5 2.5
0.50
0.51
Vegoritis M. Prespa Ioanninon
28 30.5 4
o-43 0.48 ‘0.47
(Mpriana) Caracius caracius gibelio
(Petaluda) Cyprinus carpio
(Cyprinos, Grivadi) Esox lucius
1.27
(Tourna) Perka fluviatilis
(Perki) Rurilus rut&s doiranensis
(Tsironi) Rut&s rubilio
(Dromitsa) Scardinius acarnanicus
0.52 0.53 0.47 0.50 0.51 0.57
0.50 0.50 0.57
0.037 0,041 0.047 0.027 0.135 0.127 0.267
(Tsiroukla) Salmo gairdneri
(Pestrofa) Tinca tinca
(Glini) All species
0.46 0.47 0.60
0.038 0.029 0.101
290
P. Kritidis. H. Florou
tion’). It is noticeable, that two pairs of extremely different R(8/6) values, E. lucius - O-32, S. gairdneri - 0.038 (Vegoritis lake) and C. caprio - O-16, R. r. doiranensis - 0.027 (Doirani), have been observed in the same lakes. 2. In contrast, the ecological half-lives T, and the ratios R(8/6) appear to depend definitely on the fish species. This is demonstrated by Table 1, where the ratios R(8/6) appear fairly grouped according to the fish species. We have to conclude that the retention/elimination rate of caesium in the species is a major factor influencing the time behaviour of the fish contamination, while the abiotic environmental parameters are of lesser importance in this case, although they can play, generally, an important role as well (Whicker & Schultz, 1982; Whicker et al., 1972). One must note that the biological parameters include not only the metabolism rates, but also the habits of the species (taking into account the non-homogeneous distribution of caesium in the lake water during the initial period of the impact). An important external factor influencing the bioaccumulation of caesium is the life stage of the organisms during the pollution event: the season of the event could be of importance.
RADIOLOGICAL
IMPACT
The effective half-lives T,K of 137Csand ‘34Cs are related to the ecological half-life of caesium T, as follows: Tefi = T,T,(T,
(2)
+ TJ’
where T, refers to all ecological processes affecting the concentrations of caesium in fish, except the radioactive decay and TP is the physical half-life (due to the radioactive decay). The integrated concentration IC of a radionuclide with radiological half-life Teg and initial concentration C, is given by IC = C,, T&ln2)-I According to (1) C can be expressed the local deposition D as
statistically -
C, [Bq kg-‘] = O-021D [Bq rnp2]
as a function of (4)
Combining (3) and (4) we can express the integrated concentration of caesium in certain fish species per unit of local caesium deposition as a function of its radiological half-life time: IC [Bq kg-la] = 0.021 (ln2))’ Tee [a]D [Bq me21
(5)
Environmental study of radioactive caesium in Greek lake fish
The committed effective dose equivalent H due to the consumption lake fish at annual rate of A4 [kg a-‘] is H=ICMK
291
of (6)
where K [Sv Bq-‘1 is the ingestion considered.
conversion
factor for the isotope
K(‘34Cs) = 1.9 x lo-’ Sv Bq-’ and K(137Cs) = 1.3 x lo-’ Sv Bq-’ (FAO et al., 1992).
The derived effective half-lives, the integrated concentrations in fish flesh per unit deposition and the committed effective dose equivalent estimates for the critical group are given in Table 2, separately for ‘37Cs and ‘34Cs. For species collected in more than one lake, the specimens from the lake with the maximum deposition value has been used. The dose estimations for specific fish species correspond to a rather extreme case of stable feeding preference for this species. The estimation in the last row (=3OO@v total) is based on the average effective life-time of the species and the maximum regional deposition observed (Lake Vegoritis). This approach should correct for the possible missing of representative fish species collected from the Lake Vegoritis. The maximum value which could be considered as ‘critical group dose’ is close to 400 ,uSv (E. lucius in Vegoritis). These values can be compared with the 1600 $v estimate for the ingestion-related critical group dose in Greece (Kritidis & Papanicolaou, 1987). The last value does not include a specific component related to lake fish consumption as long as its ingestion term is based on the average diet of the Greek and the value of maximum caesium contamination.
CONCLUSIONS 1. The initial bioaccumulation of radiocaesium in lake fish is strongly correlated with the local caesium deposition, with a correlation coefficient of 0.98. Local abiotic parameters do not seem to affect this relation, therefore the estimated ecological half-lives of caesium do not depend on the initial deposition during 1986. Considering the elimination rate of caesium, we can assume, based on the ecological half-lives, that the biological parameter (fish species) plays the main role, while the environmental parameters are of minor importance in this case. 2. Based on the average radiological half-lives of ‘37Cs and ‘34Cs and the maxima of caesium deposition observed, it is estimated that the
0.49 0.49 0.46 0.45 0.44 0.44 0.59
0.50
13’CS 1.22 0.75 0.62 0.56 0.40 0.40 0.38 0.38 0.37 0.37 0.46
0.41
‘34cs 0.78 0.56 0.48 0.45
Notes: 1. The activity ratio ‘37Cs:‘34Cs = 2:l is assumed for May 1986. 2. The consumption rate of fish is supposed to be 30 kg aa’.
0.50 0.50 0.47 0.46 0.45 0.45 0.60
R. r. doiranensis R. rubilio T. tinca S. gairdneri B. albanicus C. c. gibelio All species
1.27 0.77 0.63 0.57
0.51
lucius g. perspensis carpio acarnanicus
P. fluviatilis
E. B. C. S.
Doses
0.015 0.015 0.014 0.014 0.013 0.013 0.018
0.015
13’cs 0.037 0.023 0.019 0.017 0.012 0.012 0.012 0.011 0.011 0.011 0.014
0.012
‘34cs 0.024 0.017 0.015 0.014
of Greek
28 4 4 30.5 4 2.5 42
42
13’cs + lJ4cs 28 30.5 16.5 2.5
Maximum deposition kBq mm2
Due to Consumption
Ratio of the integrated concentration to deposition, m2 a kg-t
TABLE 2 and Committed Critical Group Chernobyl Accident
Effective half-life, a
per Unit Deposition
Ecological half-life, a
Concentrations
Species
Integrated
109 16 14 109 13 9 195
166
“‘Cs 270 181 81 12
65 10 10 65 8 6 110
97
‘34cs 125 97 46 6
Critical comm. eff dose equivalent pSv (see notes)
Lake Fish After the
a $ E
-;
9. 2. %
P
Environmental study of radioactive caesium in Greek lake fish
293
committed effective dose equivalent to the critical group due to the consumption of lake fish does not exceed 300 ,uSV, which represents about 20% of the ingestion-related critical group dose in Greece.
ACKNOWLEDGMENT We thank Dr Ch. Daulas from the National Center for Marine Research/ Dep. of Inland Water, for his kind help in fish identification.
REFERENCES Antonopoulos-Domis, M., Klouvas, A. & Tervisidis, F. (1987). Deposition of long-lived radionuclides ‘34Cs and 137Csin Macedonia and Thrace soils after the Chernobyl nuclear accident. Proc. Nation. Conf., ‘The Impact of the Chernobyl Accident on Greece’, NCSR ‘Demokritos’, Athens, 19-20 November 1987, pp. 77-82. FAO, IAEA, ILO, NEA, PAHO, WHO (1992). Basic Safety Standards, IAEA, Vienna, 302 pp. Florou, H. (ed.). (1987). The impact of the Chernobyl nuclear accident to the Greek marine environment. Technical Report. N.C.M.R./68, February 1987, 36 pp. (In Greek.) Florou, H., Kritidis, P. & Probonas, M. (1989a). Radiocaesium in marine and lake fish after the Chernobyl accident. Proc. Int. Symposium on Radiation Protection Selected Topics, Boris Kidric Inst. of Nuclear Sciences, Dubrovnik, October 2-6, 1989 pp. 376380. Florou, J., Kritidis, P. & Synetos, S. (19896). A comparative study of radiocaesium in marine and lake fish. Proc. Nat. Symposium on Environmental Science and Technology. Mytilini, September 1989, pp. 19-23. Kritidis, P., Florou, H. & Papanicolaou, E. (1990). Delayed and late impact of the Chernobyl accident on the Greek envrionment. Radiation Protection Dosimetry,
30(3),
87-190.
Kritidis, P. & Papanicolaou, E. (1987). Deposition of caesium and contamination of certain products: a correlation study. Regional IRPA Congress, Rome, October 1987. Whicker, F. W., Nelson, W. C. & Callegos, A. F. (1972). Fallout and 90Sr in trout from mountain lakes in Colorado. Health Phys., 23(4), 519-530. Whicker, F. W. & Schultiz, V. (1982). Radioecology: Nuclear energy and the environment. CRC Press, Inc., Florida, Vol. I, 212 pp.