Long-term investigations of radiocaesium activity concentrations in carp in North Croatia after the Chernobyl accident.
ABSTRACT Long-term investigations of radiocaesium activity concentrations in carp in the Republic of Croatia are presented. The radiocaesium levels in carp decreased exponentially and the effective ecological half-life of (137)Cs was estimated to be about 1 year during 1987-2002 and 5 years during 1993-2005. The observed (134)Cs:(137)Cs activity ratio in carp was found to be similar to the ratio observed in other environmental samples. The concentration factor for carp (wet weight) was estimated to be 128+/-74 Lkg(-1), which is in reasonable agreement with model prediction based on K(+) concentrations in water. Estimated annual effective dose received by adult members of the Croatian population due to consumption of carp contaminated with (134)Cs and (137)Cs are small: per capita dose from this source during 1987-2005 was estimated to be 0.5+/-0.2 microSv. Due to minor freshwater fish consumption in Croatia and low radiocaesium activity concentrations in carp, it can be concluded that carp consumption was not a critical pathway for the transfer of radiocaesium from fallout to humans after the Chernobyl accident.
Long-term investigations of radiocaesium activity concentrations in carps in
north Croatia after the Chernobyl accident
Zdenko FRANIĆ and Gordana MAROVIĆ
Institute for Medical Research and Occupational Health, Ksaverska cesta 2
PO Box 291, HR-10000 Zagreb, Croatia
Long-term investigations of radiocaesium activity concentrations in carps in the
Republic of Croatia are presented. The radiocaesium levels in carps decreased exponentially
and the effective ecological half-life of 137Cs in carps was estimated to be about 1 year for
1987-2002 period and 5 years for 1993-2005 period.
The observed 134Cs:137Cs activity ratio in carps has been found to be similar to the ratio
that has been observed in other environmental samples.
Concentration factor for carps (wet weight) was roughly estimated to be 128 ± 74 Lkg-1,
which is in reasonable agreement with model prediction based on K+ concentrations in water.
Estimated annual effective doses received by 134Cs and 137Cs intake due to consumption
of carps for an adult member of Croatian population are small, per caput dose for the 1987 –
2005 estimated to be 0.5 ± 0.2 µSv.
Due to minor freshwater fish consumption in Croatia and low radiocaesium activity
concentrations in carps, it can be concluded that carps consumption was not a critical pathway
for the transfer of radiocaesium from fallout to humans after the Chernobyl accident.
Key words: carp; concentration factor; 137Cs; 134Cs; Chernobyl accident; dose
Nuclear tests conducted in the atmosphere and releases of radioactive material from
nuclear facilities are the main causes of man-made radioactive contamination of the
environment. Once released to the atmosphere, long-range atmospheric transport processes
can cause widespread distribution of such radioactive matter, although it may, as in the case of
the Chernobyl accident, originate from a single point.
The resulting fallout, consisting of short and long-lived radionuclides, eventually
impacts on humans, either directly or indirectly, by entering the food chain through plants and
animals. In both cases radiation exposure represents a potential health hazard to the
population through the direct irradiation and consumption of contaminated foodstuffs.
Among man-made radionuclides, those of caesium and strontium, particularly 137Cs and
90Sr, are regarded as a particular hazard to organisms. This results from the unique
combinations of relatively long half-lives (30.14 and 29.12 years, respectively) and the
chemical and metabolic properties of these radionuclides, which closely resemble potassium
and calcium, respectively.
Investigations of the distribution and fate of natural, nuclear weapons-produced and
reactor-released radionuclides in foodstuffs, including freshwater fish, have been conducted as
a part of an extended and ongoing monitoring programme of radioactive contamination of the
environment in Croatia (Popović, 1966-1978; Bauman et al., 1979 – 1992; Kovač et al., 1993
– 1998; Marović et al., 1999 - 2006). Regular measurements of radioactive contamination of
freshwater fish started one year after the Chernobyl accident, in 1987.
Among the freshwater fish studied, special attention has been given to the common carp
(Cyprinus carpio). As a widespread and rapidly breeding freshwater fish, carp species are
fished and farmed across the world. In Croatia, carp represent the majority of the total
freshwater fish catch and consumption. During the period 1991 – 2005, the total catch of carp
in Croatia was about 55,000 tonnes or 3,800 ± 1,550 tonnes annually (Central Bureau of
Statistics, 2000 – 2006). As virtually all this catch is consumed in Croatia, it implies an annual
consumption of about 0.9 kg of carp per person. Unfortunately, data for carp catch and
consumption for the 1987 – 1990 period are unavailable for the Republic of Croatia.
Common carp prefer large bodies of water with slow or standing water and soft,
vegetated sediments. Since they can survive in stagnant and even polluted waters, which most
other fish do not inhabit, carp represent good bioindicator species for freshwater radioecology.
2. Materials and methods
Carp samples were obtained once a year, usually in late spring or early summer, from
commercial fish markets in the cities of Zagreb (45° 50' N, 16° 00' E) and Osijek (45° 30' N,
18° 40' E). Samples of river waters were also collected in the same cities, (the Sava river in
Zagreb and the Drava river in Osijek), usually in late spring. A map showing the locations
(cities of Zagreb and Osijek) of water and fish sampling sites is shown in Figure 1.
FIGURE 1 ABOUT HERE
From each site, 2 to 4 individual carp were obtained with masses of approximately 2 kg
per individual. Therefore, they were all “large” (fresh weight more than 0.1 kg) according to
the classification by Smith et al. (2002). The fish were cut into small pieces in order to obtain
composite samples. Fish samples were dried in an oven and then ashed in a muffle furnace at
450 oC for 24 h. The 137Cs activity concentrations in carp are reported on a wet weight basis.
Samples of river waters were collected in the cities of Zagreb (the Sava river) and
Osijek (the Drava river), usually in late spring. Total 137Cs activity concentrations, i.e.
dissolved and solid forms, were determined in water samples. Prior to analysis, samples of 50
L were evaporated to 1 L volume in order to concentrate the 137Cs content. It was not possible
to collect water samples from the local fishponds in which carp were farmed and it is assumed
that water from adjacent rivers is a good representation of surface waters in the study areas.
Fallout samples were collected monthly in the city of Zagreb at the Institute for Medical
Research and Occupational Health (45° 50' 7.3" N, 15° 58' 58.7" E). Funnels, which were
used for rainwater collection, had a 1 m2 collection area. Precipitation was measured by a
Hellman pluviometer. As in the case of river water samples, rainwater was evaporated to a
volume of 1 L in order to concentrate the 137Cs activity concentration prior to analysis.
(FWHM 1.87 keV at 1.33 MeV 60Co and relative efficiency of 15.4% at 1.33 MeV) and
ORTEC HPGe detector (FWHM 1.75 keV at 1.33 MeV 60Co and relative efficiency of 21% at
Gamma-ray spectrometry systems based on a low-level ORTEC Ge(Li) detector
1.33 MeV) coupled to a computerized data acquisition system were used to determine
radiocaesium and 40K levels in the samples from their gamma-ray spectra. Ash from the fish
samples was packed in cylindrical plastic containers of appropriate volume, which were
placed directly on the detector. River water and fallout samples were measured in Marinelli
beakers. Counting times depended on sample activities, ranging from 10,000 to 250,000
seconds, typically being 80,000 s.
Quality assurance and intercalibration measurements were performed through
participation in International Atomic Energy Agency (IAEA) and World Health Organization
(WHO) international intercalibration programmes, which include regular checks on blank and
background samples as well as quality control measurements.
Radiocaesium activity concentrations in fish samples in this paper were reported as
averages of two sampling locations, which assumes that the water bodies in both locations had
similar characteristics. The relative error between both locations never exceeded 25 %.
3. Results and discussion
137Cs activity concentrations in carp
Fallout from highly radioactive atmospheric plumes originating from the damaged
Chernobyl nuclear reactor was spread and transported all over Europe. Fortunately, due to the
prevailing meteorological conditions at the time after the accident, Croatia was only partially
affected by the edge of one of the plumes (United Nations Scientific Committee on the Effects
of Atomic Radiation, 1988), as indicated in Figure 2.
FIGURE 2 ABOUT HERE
The highest observed 137Cs activity concentrations in Chernobyl fallout were recorded
in May 1986, resulting in a surface deposition of 6200 Bq m-2 (Bauman et al., 1978 – 1991;
Franić, 1992a). The highest 137Cs activity concentration recorded in carps was 19.5 ± 1.5 Bq
kg-1 in 1987, while in 2004 the value was only 0.07 ± 0.01 Bq kg-1. The latter value is
comparable with the 137Cs activity concentration found in the late 1990s for trout (Bauman et
al., 1978 - 1991) although no data for 137Cs activity concentrations in trout for the 1980s and
early 1990s are available.
3.2. Effective ecological half-life of 137Cs in carp
To study the time course the 137Cs activity concentrations and to assess the effective
ecological half-life of 137Cs in carp, measurement data have been graphed as a function of
time. The distribution shows a relatively rapid decrease during the first six years after the
Chernobyl accident. After approximately six years, this decrease was significantly less rapid.
The same pattern has been observed for other fish species (Smith et al., 2000a; Hanslik et. al.,
2005) as well as various other foodstuffs (Schwaiger et al., 2004).
A first order kinetic equation was used to parameterise time changes in the 137Cs activity
concentrations in carp. Therefore, for the periods of 1987-1992 and 1993-2005 the measured
data were fitted to the following exponential function:
Acarp(t) = Acarp(0) e – kt
Acarp(t) is time-dependent activity concentration of 137Cs in carp (Bqkg-1 wet
initial activity concentration of 137Cs in carp (Bqkg-1 wet weight) and
ln(2)/k=T1/2,eff effective (observed) ecological half-life of 137Cs in carp (years).
In the immediate period after the Chernobyl accident, i.e. 1987 –1992 the observed
effective ecological half-life of 137Cs in carp was about 1.05 years (r = 0.92, P(t) < 0.01 for 4
degrees of freedom). However, the effective ecological half-life of 137Cs in carp increased to
5.05 years for the 1993 –2005 period (r = 0.82 with P(t) < 0.001 for 11 degrees of freedom).
The observed and modeled 137Cs activity concentrations in carp are shown in Figure 3.
FIGURE 3 ABOUT HERE
In order to obtain the standard deviation of T1/2,eff, Monte Carlo simulations were
performed. To ensure conservative estimates, as well as to simplify calculations, a uniform
distribution has been assumed over the A ± 2σ value of 137Cs activity concentrations in carp
for respective years, although a normal distribution would perhaps be more realistic. For each
year a random value was generated over the interval [A - 2σ, A + 2σ] and a ln(2)/k value
estimated by fitting equation (1) to the stochastically generated data. This process was
repeated 100 times and 100 values for ln(2)/k = T1/2,eff were thus obtained. From this data set
the mean values and standard deviations for T1/2,eff were calculated to be 1.05 ± 0.06 years for
1987 – 1992 period and 5.05 ± 0.32 years for 1993 – 2005 period.
These results are in good agreement with literature data. Hanslik et al. (2005) reported
the observed 137Cs half-lives for freshwater fish species (both piscivorous and herbivorous) in
the Vltava River basin (Bohemia) to be about 1 year during 1986 – 1992 and 5.1 years during
1994 – 2002. The effective ecological half-life for freshwater fish in the post Chernobyl
period in Nordic countries was discussed in the Nordic nuclear safety research report NKS-
123 (Nielsen and Andersson, 2006). During 1988-2002 the observed ecological half-life of
137Cs in perch in Finnish lakes varied from 3 to 9 years while half-lives in trout and in Arctic
char varied from 1.4 to 4.7 years during 1988 – 2004. Trout in Norwegian lakes experienced a
more rapid reduction in 137Cs burdens compared with Finnish lakes, the highest observed
ecological half-life, determined for the time period 1989 – 2000, being 6.3 years. Smith et al.
(2000a) reported effective biological half-life of 137C in young fish, water and terrestrial
vegetation during the first five years after the Chernobyl accident to be 1 – 4 years. However,
after that period, the effective biological half-life of 137C increased to 6 – 30 years.
The effective ecological half life results from biological elimination of 137Cs from fish
as well as radioactive decay. In order to estimate the ‘true’ ecological half-life the effective
constants k should be corrected for radioactive decay. Therefore constant k from the equation
(1) can be written as:
k = λ + kR
where ln(2)/λ = 30.14 y is the physical half-life of 137Cs and ln(2)/kR = T1/2,e is ecological half-
life for 137Cs.
From equation (2) the ‘true’ ecological half-lives for 137Cs in carp T1/2,e were found to
be 1.09 ± 0.06 and 6.06 ± 0.47 years, which are slightly higher than the effective ecological
half-lives for the respective ‘fast’ and ‘slow’ periods.
137Cs : 134Cs activity ratio in carp
The presence of 134Cs (half-life of 2.06 years) in the Croatian environment was
detected for the first time in May 1986. The estimated amount of caesium released after the
reactor explosion at Chernobyl was 3.7 × 1016 Bq of 137Cs (13% of total reactor inventory)
and 1.9 × 1016 Bq of 134Cs (10% of total reactor inventory (International Atomic Energy
Agency, 1986). Thus, the initial value for the 134Cs:137Cs activity ratio in May 1986 was 0.51.
This activity ratio was not altered during the passage of the radioactive plume from Chernobyl
to other geographical areas in Europe.
As the half-life of 137Cs (30.14 years) is about 15 times longer than that of 134Cs, the
134Cs:137Cs activity ratio R(t) should decrease, in a predictable way, due to differential
radioactive decay, according to the following relationship:
107 . 3
T1 and T2 are the physical half-lives for 137Cs and 134Cs, respectively.
is time elapsed after the Chernobyl accident and
In 1987 the observed 134Cs:137Cs activity ratio in carp was 0.41 ± 0.06, compared with
the theoretical prediction of 0.38. In 1988 that ratio was 0.27 ± 0.04 (Figure 4).
FIGURE 4 ABOUT HERE
Similar ratios, decreasing according to equation (3), have been found in most other
environmental samples (Bauman et al., 1987 - 1992; Kovač et al., 1993 - 1998; Marović et
al., 1999 – 2004; Franić et al. 2006) the only exception being mushrooms (Franić et. al,
1992b). In mushrooms, excess 137Cs from pre-Chernobyl fallout, affected the 134Cs:137Cs
concentration ratios. As 134Cs decayed and migrated to deeper layers within soils, the
observed 134Cs:137Cs concentration ratios approached the values theoretically predicted.
In 1990 and afterwards, contamination of carp by 134Cs originating from the Chernobyl
accident was detectable only at a very low level.
3.4 Concentration factors
Radionuclides introduced into natural waters undergo various biological processes and
may enter complicated food-webs. Therefore, a variety of mathematical models have been
developed to predict the behavior and fate of radionuclides in fresh water ecosystems, as
described by IAEA (2000) and Monte et al. (2003).
Since one of the routes of human exposure to artificial radionuclides from the
freshwater environment is through ingestion of contaminated organisms, it is interesting to
investigate the bioaccumulation of 137Cs in carp. The level of radioactive contamination of
aquatic biota by specific radionuclides is usually defined in terms of a concentration factor
(CF) which is defined as the ratio of activity concentration in a biological material to the
activity concentration in the ambient water:
is the concentration factor of radionuclide m for a specific organism (L kg-1)
is the activity concentration of radionuclide m in that organism (Bq kg-1 wet
s the activity concentration of radionuclide m in water (Bq L-1).
Use of concentration factors assumes equilibrium between organisms and water, which
may not be the case in the real environment due to complex inputs of radionuclides and the
variability in parameters influencing activity concentrations in abiotic compartments.
Concentration factors do not have any mechanistic basis, simply representing the ratios of
radionuclide activity concentrations between biological material and ambient water.
Nevertheless, if the concentration factor of a specific radionuclide is known for a given
organism, the level of the radionuclide activity concentration in this organism can be
predicted, based on its activity concentration in the ambient water.
Since carp were obtained in fish markets in the cities of Zagreb and Osijek, without
knowing the exact locations of the fish farms from which they came, CRs were calculated
using 137Cs activity concentrations observed in the rivers Sava and Drava. The calculated
value of CF for 137Cs in North Croatian carp is 128 ± 73 L kg-1. If the data for Zagreb and
Osijek are analyzed separately, respective CFs are 129 ± 64 and 125 ± 82 L kg-1. The small
deviations in these CF estimates is expected given the assumptions made. It should be noted
that non-predatory fish, like carp, are expected to have lower CFs for radiocaesium than
predatory fish (Kryshev, 1995). Nevertheless, these CFs correspond with other reported CF
values for 134Cs and 137Cs, which fall in the range 3 × 101 – 3 ×103 (IAEA, 1994). Similar
ranges of CFs have also been observed by other authors. Strand et al. (1999) reported CFs for
selected fresh water fish (pike, perch and roach) in artificial reservoirs in the southern Urals,
(Russia) in the range of 240 –1400 L kg-1. Smith et al. (2000b, 2002) have reported fish-water
CFs for perch, rudd, roach, gudgeon, ruffe, bream and goldfish in the range 102 – 104 L kg-1 in
10 lakes in Russia, Belarus and Ukraine. Hanslik et al. (2005) have reported somewhat greater
fish-water CFs for the fish sampled in Orlik Reservoir (the Vltava river basin), in the range of
1000 – 2000 L kg-1.
Another important factor which affects rates of uptake and elimination of radiocaesium
in fish, and therefore CFs, is the influence of potassium as well other chemical parameters
(IAEA, 1994; Poon and Au, 1999; Smith et. al., 2002; Smith et al., 2005). Specifically, fish-
water CFs are inversely proportional to potassium concentrations and expressions have been
derived to describe this relationship (Coughtrey and Thorne, 1983; IAEA, 1994; Smith et al.,
For non-predatory fish, like carp, it has been observed (Smith et al., 2005) that:
CF ≈ 61.3 / [K+]
CF is the fish-water CF for 137Cs in m3 kg-1 and
[K+] is the potassium concentration measured in µmol L-1.
from gamma spectrometric data for 40K, were in the range 170 - 250 µmol L-1. Similar
potassium concentrations, 6.7 mg L− 1 (≈ 170 µmol L-1) have been observed elsewhere (Neal
The potassium concentrations in the waters of the Drava and Sava rivers, estimated
and Robson, 2000). Therefore, using equation (5), CFs would fit into the range 245 – 370 L
kg-1 which is slightly higher than the observed CF values for carp. It should be noted,
however, that potassium concentrations in some Croatian lakes, as well as ponds, is
significantly higher than in the Sava and Drava rivers (Marović et al., 1999 – 2006) while
137Cs activity concentrations are quite similar. Consequently, the related CF values for carps
are likely to be lower than the ones predicted by equation (5) using the data from the Sava and
Due to the relatively high contribution of ingestion dose to total dose received by the
Croatian population after nuclear fallout (Lokobauer et al., 1998), a reliable knowledge of
ingestion dose is of particular importance. Data on activity concentrations of 134Cs and 137Cs
in carp allow us to estimate the doses incurred by consumption of this fish species. Dose
conversion factors, i.e. effective dose per unit intake via ingestion for adult members of the
public, are 1.3×10-8 SvBq-1 and 1.9×10-8 SvBq-1 for 137Cs and 134Cs respectively (IAEA,
1996). As the ratio of dose conversion factors for 137Cs and 134Cs is ≈ 0.7, it implies that
ingestion of 134Cs contributes about 30% more to the dose, compared with ingestion of the
same activity concentration of 137Cs.
In the Republic of Croatia, overall consumption of fish, including fresh water and sea
species, is relatively small, being between 7 and 8 kg per year per person (Central Bureau of
Statistics, 2000 – 2006). The collective effective dose incurred due to food consumption over
a specific time period depends on the activity concentration within the food and on the
quantity of that food which is consumed. The dose can be expressed as:
E is the effective dose in Sv,
C is the total annual per capita consumption of food,
D is the dose conversion factor for radionuclide m, i.e. effective dose per unit intake, which
converts the ingested activity to effective dose and
Am is the mean annual specific activity of radionuclide m in food (Bqkg-1).
Based on the statistical data for the consumption of carp during 1991 – 2005 (3800 ±
1550 tonnes annually) the same consumption rate was assumed for the period 1987 – 1990 as
well. The estimated collective effective dose due to 137Cs and 134Cs ingestion by carp
consumption for the Croatian population (4.5 ×106 inhabitants) during 1987-2005 is 2.2 ± 0.9
person-Sv. 1.5 ± 0.6 person-Sv can be attributed to 137Cs and 0.7 ± 0.3 person-Sv to 134Cs. The
majority of the overall dose was incurred during the 1987 (1.0 ± 0.4 person-Sv and 0.6 ± 0.2
person-Sv for 137Cs and 134Cs, respectively).
The annual effective collective doses are shown in Figure 5.
FIGURE 5 ABOUT HERE
It should be noted that the doses were not corrected to take into account inedible parts of
the fish such as bones and scales. Nevertheless, these are rather small doses, since for the
1987 – 2005 period per capita dose is 0.5 ± 0.2 µSv. However, the doses are small not only
because of low activity concentrations of radiocaesium in carp, but also due to the low
consumption of freshwater fish in Croatia. In summary, it can be argued that freshwater fish
consumption is not a critical pathway for human intake of radiocaesium from the
A relatively rapid decrease in 137Cs activity concentrations in carp was observed for the
immediate post-Chernobyl period (1987 – 1992), the effective ecological half-life of 137Cs in
carp within this period being approximately 1 year. However, approximately 6 years after the
accident, this rate of decrease slowed significantly and the effective ecological half-life of
137Cs in carp increased to 5 years, the same pattern being observed for some other fish and
The concentration factor (CF) for carp (wet weight) was estimated to be 128 ± 74 L kg-
1, a value which is in reasonable agreement with model predictions based on K+
concentrations in water as well as with CF values observed elsewhere.
The activity concentrations of 137Cs in carp became quite low within a few years after
the Chernobyl nuclear accident, while 134Cs activity concentrations were generally below
detection limit after 1990. Consequently, only very low doses have been received by the
general population as a result of freshwater fish consumption and it can be concluded that carp
consumption was not a critical pathway for the transfer of radiocaesium to humans after the
This study is a part of a research project Radioecology of the Adriatic Sea and
Coastal Areas, supported by the Ministry of Science, Education and Sports of the Republic of
The authors wish to thank Mrs. Ljerka Petroci for her excellent technical assistance and
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