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Distribution coefficients and concentration factors of 226Ra and 228Th in the Greek marine environment
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Distribution coefficients and concentration factors of 226Ra and 228Th in the Greek marine environment G. Trabidou, H. Florou, P. Kritidis, Ch. Chaloulou, Ch. Lykomitrou Institute of Nuclear Technology and Radiation Protection, NCSR Demokritos, 15310 Aghia Paraskevi, Athens, Greece
The levels of 226 Ra and 228 Th in sea water, sediment and biota were measured in some selected areas in the sublittolar zone of Ikaria Island, Loutraki in continental Greece and Milos Island. The concentrations of 226 Ra and 228 Th were found to be significantly elevated in seawater, sediments and biota in Ikaria Island. The results obtained were used for the evaluation of distribution coefficients and concentration factors by applying a linear and a non-linear regression analysis. The high distribution coefficients estimated for 226 Ra and 228 Th indicate that a large proportion of each radionuclide considered remains in the solid phase. In general, the estimated values of the concentration factors of 226 Ra and 228 Th in Algae and P. oceanica seem to follow the linear model of analysis in Ikaria and Loutraki, where the concentrations in sea water were high. The respective concentration factors from Milos were found to satisfy the non-linear model of analysis. Concentration factors in certain fish species were found to satisfy the non-linear model of analysis for the three areas studied.
1. Introduction 1.1. A brief state of the art As natural ecosystems are functional units composed by different parts of biotic and abiotic integrated compartments, concentrations of environmental components and transfer factors among them are parameters for the evaluation of the major pathways of radionuclide distribution and behavior. Radionuclide transfer between two different environmental components, of which one is considered as the source, is used to study the selective accumulation, magnification and/or bioaccumulation through the various levels of the environmental chain [1]. Concentration, biotransfer, biomagnification and bioaccumulation factors are important for the organisms because they reflect the response of different taxa to the varying lithospheric RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07143-8
© 2005 Elsevier Ltd. All rights reserved.
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Fig. 1. Conceptual model of radionuclide kinetics.
composition of their habitats. Besides, these factors are the major parameters determining the role of non-familiar materials present or introduced that affect the ecosystem sustenance. Concentration and bioaccumulation factors can be used as a tool for studying the radionuclide kinetics through the various levels of ecosystem organization (Fig. 1). Furthermore, the bioaccumulation factor is the main parameter considered for the selection of an organism as an indicator for environmental quality assessment, in terms of a specific radionuclide. Considering an ecosystem as a conceptual model of interactions among various components, the use of transport and/or transfer factors results in better understanding of its inherent structure and processes. In addition, prediction of radiation dose to plants, animals and man from radioactive materials present in or released to the environment can support countermeasures for protection of man and non-humans. In the present study, the distribution coefficients (Kd ) for the abiotic elements (sediments) and concentration factors (CF) in marine organisms for 226 Ra and 228 Th are studied in three areas of relatively high natural radioactivity, in Greece. Radium-226 (T1/2 = 1622 y) is one of the most biologically significant radionuclides. In terms of toxicity, it is considered as a bone seeker by replacing its analog Ca and being retained for a long time on bones and teeth. It is distributed in the environment by forming soluble salts and shows a reasonable biological mobility [2]. Thorium-228 (T1/2 = 1.91 y) is, relative to 226 Ra, unavailable for biological uptake with low mobility and low adsorption by organisms, because of its strong adsorption and adhesion onto inorganic material. It shows a tendency to form mostly insoluble compounds, which do not seem to have metabolic significance for organisms [2]. However, it is retained tenaciously by bones, following the Ca distribution. It is noteworthy that bone-deposited 228 Th has been cited to result in greater carcinogenic effects than 226 Ra [2,3]. The aim of this study is: (a) to obtain and present some important results on the levels of natural radioactivity in three selected coastal areas in Greece, (b) to select the appropriate model between linear and non-linear analysis [4] in order to apply this for CF estimations. 1.2. The investigated areas The natural radiation regime has been evaluated comparatively in three selected coastal areas with characteristic features. The areas considered present elevated levels of natural
Distribution coefficients and concentration factors of 226 Ra and 228 Th in the Greek marine environment 1169
Fig. 2. The areas investigated.
radioactivity, which are attributed to the local geological background. Furthermore, these areas are characterized by the presence of geothermal springs and vents [5–7]. The springs and vents provide a continuous flow of continental water into the sea. The investigated areas are shown in Fig. 2 and are described as follows: (i) The island of Ikaria (37◦ 59 N, 22◦ 58 E), with an area of 267 km2 , is located in the Eastern Aegean Sea in Greece. A mountainous area dominates this island. The island is divided into two equal parts, which are geologically distinct: (a) the eastern part that consists of largely metamorphosed sedimentary formations and (b) the western part mainly consisting of granite formations [8]. In the littoral zone around the island, there are several geothermal springs and in the sublittoral zone some springs emerge under the strata through the bottom to the seawater layer above. (ii) The island of Milos (36◦ 42 N, 24◦ 27 E), with an area of 150 km2 , is part of the Hellenic volcanic arc, which is located in the South Aegean Sea in Greece. The arc is parallel to the subduction zone of the lithospheric plates of the Eastern Mediterranean [9]. The island is characterized by the presence of geothermal vents, which are used experimentally for energy production by the Public Power Corporation. The underground hydrothermal fluids emitted from the vents reportedly have a direct and an indirect influence on the abiotic material and organisms of the coastal areas of Milos [10,11].
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Fig. 3. Maps of the sampling stations – S – and the results obtained by car-borne scintillometer.
(iii) Loutraki (37◦ 36 N, 26◦ 17 E) is located in Korinthiakos gulf in Central Greece. It occupies the western part of the Hellenic volcanic arc. Several geothermal springs are located in the littoral and sub-littoral zone and in the part of the area considered here [7]. The gamma-radiometry mapping of the three investigated areas was used as a guide for the selection of sampling stations, as shown in Fig. 3.
2. Materials and methods The methodologies used for both the in-situ and laboratory measurements are described in detail in a number of our published papers [5,12]. A car-borne total gamma-scintillometer
Distribution coefficients and concentration factors of 226 Ra and 228 Th in the Greek marine environment 1171
with a 2 × 2 NaI (Tl) cylindrical detector (sensitivity 1 cpm per 3.5 × 10−3 µR h−1 for 226 Ra at 1 m above ground) was used for in-situ gamma-radiometry in the investigated areas. Marine abiotic material and biota were sampled in a five-year period under warm and cold weather conditions (Fig. 3). The samples were physically treated and measured by gamma-spectroscopy in the laboratory, whereas seawater samples were treated radiochemically [13]. For gamma spectroscopy, two high-resolution systems with HpGe detectors of 20% relative efficiency were used. The statistical error (1 σ ) of the measurements did not exceed 18%. 2.1. Distribution coefficient Based on the radionuclide levels in seawater and sediment, the distribution coefficients can be evaluated as follows: Kd = Ysed /Xsw ,
(1)
where Kd is distribution coefficient, Ysed is concentration of radionuclide in sediment (Bq kg−1 ), and Xsw is concentration of radionuclide in seawater (Bq L−1 ). The calculation of Kd was performed on the assumption that concentrations are at dynamic equilibrium. 2.2. Concentration factor To reveal biological pathways for radionuclide transfer to biota, the concentration factor CF can be evaluated by considering a dual-compartment uptake system, consisting of a donor and receiving compartment, as follows: CF = Yorg /Xdonor
(2)
where CF is concentration factor, Yorg is concentration of radionuclide in organism (Bq kg−1 wet weight), and Xdonor is concentration of radionuclide in the donor compartment (Bq L−1 or Bq kg−1 ). The donor compartment Xdonor (i.e., seawater, sediment, or fish diet) provides the radionuclide to the receiving compartment Yorg (i.e., fish, algae, angiosperm). The calculation of CF is generally performed on the basis of three assumptions [1,14,15]: – The donor and receiving compartment must be at a dynamic equilibrium. – The radionuclide concentrations in the receiving compartment are linearly correlated with the radionuclide concentrations in the donor compartment. – Only one radionuclide is available for biological uptake by the receiving compartment – competitive or synergetic actions are not taken into account. In the case that these assumptions are not satisfied, the evaluation of CF by this linear relation may lead to underestimations or overestimations, as shown in Fig. 4 [14,15]. Therefore, in non-linear equilibrium conditions due to weathering processes, re-suspension, bioturbation and seasonal variation of spring current, which may not be covered by a reasonable time series sampling, a non-linear relation for CF may be used, as follows [4]: b Yorg = (CF)Xdonor ,
(3)
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Fig. 4. The underestimation and the overestimation of a linear model [4]. Y for radionuclide in receiving compartment, X for radionuclide in donor compartment.
where CF is concentration factor, Yorg is concentration in organism (Bq kg−1 wet weight), Xdonor is concentration in donor (Bq L−1 or Bq kg−1 ) and b is exponential term. The exponential term b is given by regression analysis. If b is statistically different from 1 at the 95% significance level, the relation (3) is applied instead of (2), thus allowing for a more accurate evaluation of CF.
3. Results and discussion 3.1. Gamma radiometry The results of the dose rates of gamma-radiometry measurements in Ikaria, Loutraki and Milos are shown in Fig. 3. The dose rates in Ikaria are in the range of 0.05–0.21 µGy h−1 . The highest values were measured in the vicinity of geothermal springs, with an average of 0.14 µGy h−1 . The dose rates in Loutraki areas are in the range of 0.01–0.04 µGy h−1 . The dose rates in Milos range at 0.05–0.20 µGy h−1 . The derived annual dose rate equivalent in Ikaria is 307–1226 µSv y−1 , in Loutraki 60–167 µSv y−1 and in Milos is 307–1226 µSv y−1 . One can note that the geothermal springs in Loutraki do not affect the background radiation of the coastal environment. Considering Greece, a mean value of 0.08 µGy h−1 for dose rates and a consequent annual dose rate equivalent of 490 µSv y−1 have been reported for other regions [10]. 3.2. Concentrations of natural gamma emitters in abiotic material and biota The results of gamma-spectroscopy measurements of 226 Ra and 228 Th in seawater, sediment, algae and fish are given in Tables 1, 2, 3, respectively. These results show that elevated concentrations of 226 Ra and 228 Th are detected in sea water and sediments in the coastal areas of Ikaria in comparison to the respective values from Loutraki and Milos (Table 1). The elevated concentrations in the abiotic environmental materials of Ikaria are reflected in the concentrations of 226 Ra and 228 Th in the examined species of algae compared to those from Milos and Loutraki (Table 2). The concentrations of 226 Ra in P. oceanica are found elevated, whereas those of 228 Th are found in the same levels for Milos and Ikaria. Considering the pelagic fish species Boops boops and Trachurus trachurus, higher concentrations have been found in flesh tissue in Boops boops from Loutraki and Milos (Table 3).
Distribution coefficients and concentration factors of 226 Ra and 228 Th in the Greek marine environment 1173 Table 1 Activity concentrations of 226 Ra and 228 Th in seawater (Bq L−1 ) and sediments (Bq kg−1 ) in Ikaria, Milos and Loutraki
Ikaria MV ± SD∗ Min Max Loutraki MV ± SD∗ Min Max Milos MV ± SD∗ Min Max
226 Ra (sea water)
226 Ra (sediment)
1.2 ± 1.0 < 0.1 1.9 ± 0.3
212 ± 311 24 ± 14 764 ± 10
0.3 ± 0.18 0.1 ± 0.3 0.5 ± 0.4
228 Th (sea water)
0.5 ± 0.3 0.2 ± 0.2 0.8 ± 0.1
13 ± 2 11 ± 2 16 ± 4
(1.53 ± 0.17) × 10−3 (1.45 ± 0.25) × 10−3 (1.67 ± 0.37) × 10−3
228 Th (sediment)
43 ± 18 18 ± 48 66 ± 3
0.1 ± 0.1 < 0.1 0.1 ± 0.4
6±3 3±4 11 ± 4
(0.09 ± 0.04) × 10−3 (0.06 ± 0.01) × 10−3 (0.13 ± 0.02) × 10−3
15 ± 14 4±3 50 ± 21
13 ± 13 4±2 47 ± 29
∗ Values are given as MV ± SD for 10 samples for seawater and 16 for sediments of each area.
Table 2 Activity concentrations of 226 Ra and 228 Th in algae and P. oceanica – young leaves (Bq kg−1 wet weight)∗ Jania sp.
Ikaria Loutraki Milos
Padina pavonica
Posidonia oceanica
226 Ra
228 Th
226 Ra
Cystoseira sp. 228 Th
226 Ra
228 Th
226 Ra
228 Th
85 ± 2 2.3±0.4 3.3±0.5
9.1 ± 1.9 3.4 ± 0.4 8.8 ± 0.6
59 ± 25 0.36±0.30 3.1 ± 1.1
2.3 ± 0.5 0.1 ± 0.1 8.3 ± 2.2
0.9 ± 0.6 4.8 ± 3.1
1.3 ± 0.9 4.1 ± 1.9
24 ± 7
8.7 ± 4.6
4.4±3.0
8.3 ± 2.0
∗ Values are given as MV ± SD for 10 composite samples for each species of each area.
Table 3 Activity concentrations of 226 Ra and 228 Th in fish (Bq kg−1 wet weight)∗ 226 Ra (flesh)
Boops boops Ikaria Loutraki Milos Trachurus trachurus Ikaria Loutraki Milos
228 Th (flesh)
0.12 ± 0.12 3.1 ± 1.1 3.80 ± 1.12
0.12 ± 0.16 3.6 ± 1.7 2.15 ± 0.55
0.17 ± 0.13 − 0.29 ± 0.09
0.01 ± 0.01 − < 0.05
226 Ra (bone)
228 Th (bone)
0.89 ± 0.16
0.06 ± 0.12
2.2 ± 0.6 −
0.53 ± 0.3 −
∗ Values are given as MV ± SD for 10 composite samples for each species of each area.
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G. Trabidou et al. Table 4 Distribution coefficients (Kd ) of 226 Ra and 228 Th (sediment versus seawater) Sampling station
226 Ra
228 Th
Ikaria Loutraki Milos
106 36 14 615
136 24 145 404
3.3. Distribution coefficients – Kd Based on gamma-spectroscopy measurements and applying the calculation formulae described above, the distribution coefficients are presented in Table 4. The distribution coefficients in Milos are higher than those in Ikaria and Loutraki, where the continuous outflow of the radioactive springs into the sea results in higher concentrations in seawater. Thus, the seawater is continuously enriched, whereas no respective increase is noticed for sediments in due time. This leads to lower distribution coefficients for sediments in the areas of spring influence and the hypothesis of dynamic equilibrium during the exchange procedure between seawater and sediment may not be valid. 3.4. Concentration factors – CF Based on gamma-spectroscopy measurements and applying the calculation formulae described above, the concentration factors for algae, P. oceanica and the fish species studied are presented in Table 5 with the notation (L) for linearity (formula (2)) or (NL) for non-linearity (formula (3)). Elevated concentrations of Ca, the analog element of 226 Ra and 228 Th, were recorded in the abiotic material of the investigated areas. This affects the observed concentrations of 226 Ra and 228 Th in the organisms considered [2,16], and consequently results in the non-linear model of equilibrium between the donor and the receiving compartment. 3.4.1. Jania species The CFs for Jania sp. sampled from Milos, where the relation between the abiotic and biotic materials is non-linear, are higher compared to those from Ikaria, where the linear relation is satisfied (Table 5). The lowest CFs are observed in Loutraki conforming to the non-linear relation. The CFs of 226 Ra in Ikaria are higher than those of 228 Th. This is in agreement with the higher biological mobility reported for 226 Ra compared to that of 228 Th [1,2]. Note that in Milos, where non-linear relations are observed, the CFs for 228 Th are higher than the CFs for 226 Ra. In Loutraki the CFs for the studied radionuclides are comparable. 3.4.2. Cystoseira species The CFs for Cystoseira sp. sampled from Milos, where the relation between the abiotic and biotic materials is non-linear, are higher compared to those from Loutraki and Ikaria, where the linear relation is satisfied (Table 5). Besides, the CFs for 226 Ra are higher than those of 228 Th, whereas in Milos the CFs of 228 Th are higher than 226 Ra, as is observed in Jania sp.
Distribution coefficients and concentration factors of 226 Ra and 228 Th in the Greek marine environment 1175 Table 5 Concentration factors (CF) of 226 Ra and 228 Th in algae, P. oceanica and fish 226 Ra
Sampling station Jania sp. Ikaria Loutraki Milos Cystoseira sp. Ikaria Loutraki Milos Padina pavonica Loutraki Milos Sampling station P. oceanica Ikaria Milos Sampling station Boops boops Ikaria Loutraki Milos Trachurus trachurus Ikaria Milos
226 Ra (water)
228 Th
283 (L) 30 (NL) 2200 (NL)
12 (L) 23 (NL) 96 000 (NL)
58 (L) 19 (L) 2062 (NL)
35 (L) 7 (L) 92 000 (NL)
6 (L) 3142 (NL)
30 (NL) 46 000 (NL) 226 Ra (sediment)
228 Th (sediment)
0.6 (L) –
0.5 (L) –
228 Th (flesh)
226 Ra (bone)
228 Th (bone)
0.3 (NL) 10.3 (NL) 2 × 103 (NL)
0.4 (NL) 24 (NL) 24 × 103 (NL)
2.4 (NL) – –
0.2 (NL) – –
0.4 (NL) 0.3 × 103 (NL)
0.12 (NL) 0.6 × 103 (NL)
4.9 (NL) –
1.2 (NL) –
52 (L) 3035 (NL) 226 Ra (flesh)
228 Th (water)
21 (L) 92 200 (NL)
3.4.3. Padina pavonica The concentration factors of Padina pavonica from Milos are higher compared to those from Loutraki, as also observed for Jania sp. and Cystoseira sp. (Table 5). The CFs of 228 Th are higher than those of 226 Ra for both areas. 3.4.4. Posidonia oceanica Considering P. oceanica in Ikaria, the CFs are calculated based on two donor compartments, water and sediment (P. oceanica is a marine angiosperm with a functional root system). The CFs based on sea water as a donor are two orders of magnitude higher than those based on sediment (Table 5). This means that the leaf system plays the major role in radionuclide bioacummulation compared to the root system of the plant. The CFs on the sea water basis from Milos are found to be higher compared to those from Ikaria. 3.4.5. Boops boops and Trachurus trachurus In terms of fish species examined, the CFs of 226 Ra and 228 Th for bone tissue as the receiving compartment are higher than those of flesh (Table 5). This is due to the fact that both 226 Ra and 228 Th are bone seekers. Besides, the CFs for flesh are lower in Ikaria compared to those from Milos and Loutraki, as found for algae and P. oceanica.
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4. Conclusions The highest Kd ’s are observed in Milos with the lowest concentrations of 226 Ra and 228 Th in seawater compared to Ikaria and Loutraki, where the continuous enrichment of radionuclides by the springs disturbs the sea water/sediment equilibrium process. The CFs in Milos were found to conform to the non-linear model for all the organisms of the three taxa examined (algae, angiosperm, fish). In Ikaria the CFs of algae and P. oceanica satisfy the linear model. In Loutraki only Jania sp. for both radionuclides and Cystoseira sp. for 226 Ra satisfy the linear model. The linear relation for CFs for marine flora is rather followed in the areas with the highest concentrations in the donor compartment. In this case, the CFs of 226 Ra are higher than those of 228 Th, whereas the CFs of 228 Th were found higher wherever the non-linear relation is satisfied. The CFs in fish conform to the non-linear relation for the three areas. The CFs of 226 Ra and 228 Th of the species examined in Milos are found to be higher than the respective ones in Ikaria and Loutraki. This seems to be connected with the lowest concentrations in the donor compartment and the non-linear relation satisfied. Considering the different tissues in P. oceanica and fish, higher CFs are observed in that tissue of the organism with the major metabolic role for the examined radionuclide. References [1] E.J. McGee, K.J. Johanson, M.J. Keatinge, H.J. Synnott, P.A. Colgan, An evaluation of ratio systems in radioecological studies, Health Phys. 70 (1996) 215–221. [2] F.W. Whicker, V. Schultz, Radioecology: Nuclear Energy and the Environment, vol. 1, CRC Press, Boca Raton, FL, 1982. [3] J.B. Cowart, W.C. Burnett, The distribution of uranium and thorium isotopes decay-series radionuclides in the environment – A review, J. Environ. Qual. 23 (1994) 651–662. [4] G.G. Pyle, F.V. Clulow, Non-linear radionuclide transfer from the aquatic environment to fish, Health Phys. 73 (3) (1997) 488–493. [5] G. Trabidou, H. Florou, A. Angelopoulos, L. Sakelliou, Environmental study of the spas in the Ikaria island, Radiat. Prot. Dosim. 63 (1) (1996) 63–67. [6] H. Florou, P. Kritidis, Natural radioactivity in environmental samples from an island of volcanic origin (Milos, Aegean sea), Mar. Poll. Bull. 22 (8) (1991) 417–419. [7] P. Kritidis, A radiological study of the Greek radon spas, in: Proc. Int. Symp. on Radon and Radon Reduction Technology, vol. 3, session VI(8), 1992. [8] C.A. Ktenas, G. Marinos, La géologie de l’île de Nikaria, Geological and Geophysical Research, Institute for Geology and Subsurface Research, Athens, 1969. [9] M.D. Fytikas, Geological and geothermal study of Milos island, Geol. Geophys. Res. XVIII (1) (1975). [10] P. Kritidis, H. Florou, Natural radioactivity in the environment and radioactive pollution, in: Proc. Natl. Conf. on Environmental Science and Technology, vol. B, Aegean University, Mytilini, September 1989, pp. 24–34. [11] F. Boisson, J.-C. Miquel, O. Cotret, S.W. Fowler, 210 Po and 210 Pb cycling in a hydrothermal zone in the coastal Aegean Sea, Sci. Total Environ. 281 (2001) 111–119. [12] H. Florou, P. Kritidis, Gamma radiation measurements and dose rates in the coastal areas of a volcanic island, Aegean Sea, Greece, Radiat. Prot. Dosim. 45 (1–4) (1992) 277–279. [13] H. Florou, Behavior and dispersion of radionuclides in marine ecosystems (Aegean and Ionian Sea, Greece), PhD thesis, University of Athens, 1992. [14] R.L. Kathren, Radioactivity in the Environment, Harwood Academic, New York, 1984. [15] P.F. Landrum, H. Lee II, M.J. Lydy, Toxikokinetics in aquatic systems – model comparisons and use in hazard assessment, Environ. Toxicol. Chem. 11 (1992) 1709–1725. [16] E.V. der Stricht, R. Kirchmann (Eds.), Radioecology – Radioactivity and Ecosystems, International Union of Radioecology, Belgium, 2001.
Distribution coefficients and concentration factors of 226Ra and 228Th in the Greek marine environment G. Trabidou, H. Florou, P. Kritidis, Ch. Chaloulou, Ch. Lykomitrou Institute of Nuclear Technology and Radiation Protection, NCSR Demokritos, 15310 Aghia Paraskevi, Athens, Greece
The levels of 226 Ra and 228 Th in sea water, sediment and biota were measured in some selected areas in the sublittolar zone of Ikaria Island, Loutraki in continental Greece and Milos Island. The concentrations of 226 Ra and 228 Th were found to be significantly elevated in seawater, sediments and biota in Ikaria Island. The results obtained were used for the evaluation of distribution coefficients and concentration factors by applying a linear and a non-linear regression analysis. The high distribution coefficients estimated for 226 Ra and 228 Th indicate that a large proportion of each radionuclide considered remains in the solid phase. In general, the estimated values of the concentration factors of 226 Ra and 228 Th in Algae and P. oceanica seem to follow the linear model of analysis in Ikaria and Loutraki, where the concentrations in sea water were high. The respective concentration factors from Milos were found to satisfy the non-linear model of analysis. Concentration factors in certain fish species were found to satisfy the non-linear model of analysis for the three areas studied.
1. Introduction 1.1. A brief state of the art As natural ecosystems are functional units composed by different parts of biotic and abiotic integrated compartments, concentrations of environmental components and transfer factors among them are parameters for the evaluation of the major pathways of radionuclide distribution and behavior. Radionuclide transfer between two different environmental components, of which one is considered as the source, is used to study the selective accumulation, magnification and/or bioaccumulation through the various levels of the environmental chain [1]. Concentration, biotransfer, biomagnification and bioaccumulation factors are important for the organisms because they reflect the response of different taxa to the varying lithospheric RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07143-8
© 2005 Elsevier Ltd. All rights reserved.
1168
G. Trabidou et al.
Fig. 1. Conceptual model of radionuclide kinetics.
composition of their habitats. Besides, these factors are the major parameters determining the role of non-familiar materials present or introduced that affect the ecosystem sustenance. Concentration and bioaccumulation factors can be used as a tool for studying the radionuclide kinetics through the various levels of ecosystem organization (Fig. 1). Furthermore, the bioaccumulation factor is the main parameter considered for the selection of an organism as an indicator for environmental quality assessment, in terms of a specific radionuclide. Considering an ecosystem as a conceptual model of interactions among various components, the use of transport and/or transfer factors results in better understanding of its inherent structure and processes. In addition, prediction of radiation dose to plants, animals and man from radioactive materials present in or released to the environment can support countermeasures for protection of man and non-humans. In the present study, the distribution coefficients (Kd ) for the abiotic elements (sediments) and concentration factors (CF) in marine organisms for 226 Ra and 228 Th are studied in three areas of relatively high natural radioactivity, in Greece. Radium-226 (T1/2 = 1622 y) is one of the most biologically significant radionuclides. In terms of toxicity, it is considered as a bone seeker by replacing its analog Ca and being retained for a long time on bones and teeth. It is distributed in the environment by forming soluble salts and shows a reasonable biological mobility [2]. Thorium-228 (T1/2 = 1.91 y) is, relative to 226 Ra, unavailable for biological uptake with low mobility and low adsorption by organisms, because of its strong adsorption and adhesion onto inorganic material. It shows a tendency to form mostly insoluble compounds, which do not seem to have metabolic significance for organisms [2]. However, it is retained tenaciously by bones, following the Ca distribution. It is noteworthy that bone-deposited 228 Th has been cited to result in greater carcinogenic effects than 226 Ra [2,3]. The aim of this study is: (a) to obtain and present some important results on the levels of natural radioactivity in three selected coastal areas in Greece, (b) to select the appropriate model between linear and non-linear analysis [4] in order to apply this for CF estimations. 1.2. The investigated areas The natural radiation regime has been evaluated comparatively in three selected coastal areas with characteristic features. The areas considered present elevated levels of natural
Distribution coefficients and concentration factors of 226 Ra and 228 Th in the Greek marine environment 1169
Fig. 2. The areas investigated.
radioactivity, which are attributed to the local geological background. Furthermore, these areas are characterized by the presence of geothermal springs and vents [5–7]. The springs and vents provide a continuous flow of continental water into the sea. The investigated areas are shown in Fig. 2 and are described as follows: (i) The island of Ikaria (37◦ 59 N, 22◦ 58 E), with an area of 267 km2 , is located in the Eastern Aegean Sea in Greece. A mountainous area dominates this island. The island is divided into two equal parts, which are geologically distinct: (a) the eastern part that consists of largely metamorphosed sedimentary formations and (b) the western part mainly consisting of granite formations [8]. In the littoral zone around the island, there are several geothermal springs and in the sublittoral zone some springs emerge under the strata through the bottom to the seawater layer above. (ii) The island of Milos (36◦ 42 N, 24◦ 27 E), with an area of 150 km2 , is part of the Hellenic volcanic arc, which is located in the South Aegean Sea in Greece. The arc is parallel to the subduction zone of the lithospheric plates of the Eastern Mediterranean [9]. The island is characterized by the presence of geothermal vents, which are used experimentally for energy production by the Public Power Corporation. The underground hydrothermal fluids emitted from the vents reportedly have a direct and an indirect influence on the abiotic material and organisms of the coastal areas of Milos [10,11].
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Fig. 3. Maps of the sampling stations – S – and the results obtained by car-borne scintillometer.
(iii) Loutraki (37◦ 36 N, 26◦ 17 E) is located in Korinthiakos gulf in Central Greece. It occupies the western part of the Hellenic volcanic arc. Several geothermal springs are located in the littoral and sub-littoral zone and in the part of the area considered here [7]. The gamma-radiometry mapping of the three investigated areas was used as a guide for the selection of sampling stations, as shown in Fig. 3.
2. Materials and methods The methodologies used for both the in-situ and laboratory measurements are described in detail in a number of our published papers [5,12]. A car-borne total gamma-scintillometer
Distribution coefficients and concentration factors of 226 Ra and 228 Th in the Greek marine environment 1171
with a 2 × 2 NaI (Tl) cylindrical detector (sensitivity 1 cpm per 3.5 × 10−3 µR h−1 for 226 Ra at 1 m above ground) was used for in-situ gamma-radiometry in the investigated areas. Marine abiotic material and biota were sampled in a five-year period under warm and cold weather conditions (Fig. 3). The samples were physically treated and measured by gamma-spectroscopy in the laboratory, whereas seawater samples were treated radiochemically [13]. For gamma spectroscopy, two high-resolution systems with HpGe detectors of 20% relative efficiency were used. The statistical error (1 σ ) of the measurements did not exceed 18%. 2.1. Distribution coefficient Based on the radionuclide levels in seawater and sediment, the distribution coefficients can be evaluated as follows: Kd = Ysed /Xsw ,
(1)
where Kd is distribution coefficient, Ysed is concentration of radionuclide in sediment (Bq kg−1 ), and Xsw is concentration of radionuclide in seawater (Bq L−1 ). The calculation of Kd was performed on the assumption that concentrations are at dynamic equilibrium. 2.2. Concentration factor To reveal biological pathways for radionuclide transfer to biota, the concentration factor CF can be evaluated by considering a dual-compartment uptake system, consisting of a donor and receiving compartment, as follows: CF = Yorg /Xdonor
(2)
where CF is concentration factor, Yorg is concentration of radionuclide in organism (Bq kg−1 wet weight), and Xdonor is concentration of radionuclide in the donor compartment (Bq L−1 or Bq kg−1 ). The donor compartment Xdonor (i.e., seawater, sediment, or fish diet) provides the radionuclide to the receiving compartment Yorg (i.e., fish, algae, angiosperm). The calculation of CF is generally performed on the basis of three assumptions [1,14,15]: – The donor and receiving compartment must be at a dynamic equilibrium. – The radionuclide concentrations in the receiving compartment are linearly correlated with the radionuclide concentrations in the donor compartment. – Only one radionuclide is available for biological uptake by the receiving compartment – competitive or synergetic actions are not taken into account. In the case that these assumptions are not satisfied, the evaluation of CF by this linear relation may lead to underestimations or overestimations, as shown in Fig. 4 [14,15]. Therefore, in non-linear equilibrium conditions due to weathering processes, re-suspension, bioturbation and seasonal variation of spring current, which may not be covered by a reasonable time series sampling, a non-linear relation for CF may be used, as follows [4]: b Yorg = (CF)Xdonor ,
(3)
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Fig. 4. The underestimation and the overestimation of a linear model [4]. Y for radionuclide in receiving compartment, X for radionuclide in donor compartment.
where CF is concentration factor, Yorg is concentration in organism (Bq kg−1 wet weight), Xdonor is concentration in donor (Bq L−1 or Bq kg−1 ) and b is exponential term. The exponential term b is given by regression analysis. If b is statistically different from 1 at the 95% significance level, the relation (3) is applied instead of (2), thus allowing for a more accurate evaluation of CF.
3. Results and discussion 3.1. Gamma radiometry The results of the dose rates of gamma-radiometry measurements in Ikaria, Loutraki and Milos are shown in Fig. 3. The dose rates in Ikaria are in the range of 0.05–0.21 µGy h−1 . The highest values were measured in the vicinity of geothermal springs, with an average of 0.14 µGy h−1 . The dose rates in Loutraki areas are in the range of 0.01–0.04 µGy h−1 . The dose rates in Milos range at 0.05–0.20 µGy h−1 . The derived annual dose rate equivalent in Ikaria is 307–1226 µSv y−1 , in Loutraki 60–167 µSv y−1 and in Milos is 307–1226 µSv y−1 . One can note that the geothermal springs in Loutraki do not affect the background radiation of the coastal environment. Considering Greece, a mean value of 0.08 µGy h−1 for dose rates and a consequent annual dose rate equivalent of 490 µSv y−1 have been reported for other regions [10]. 3.2. Concentrations of natural gamma emitters in abiotic material and biota The results of gamma-spectroscopy measurements of 226 Ra and 228 Th in seawater, sediment, algae and fish are given in Tables 1, 2, 3, respectively. These results show that elevated concentrations of 226 Ra and 228 Th are detected in sea water and sediments in the coastal areas of Ikaria in comparison to the respective values from Loutraki and Milos (Table 1). The elevated concentrations in the abiotic environmental materials of Ikaria are reflected in the concentrations of 226 Ra and 228 Th in the examined species of algae compared to those from Milos and Loutraki (Table 2). The concentrations of 226 Ra in P. oceanica are found elevated, whereas those of 228 Th are found in the same levels for Milos and Ikaria. Considering the pelagic fish species Boops boops and Trachurus trachurus, higher concentrations have been found in flesh tissue in Boops boops from Loutraki and Milos (Table 3).
Distribution coefficients and concentration factors of 226 Ra and 228 Th in the Greek marine environment 1173 Table 1 Activity concentrations of 226 Ra and 228 Th in seawater (Bq L−1 ) and sediments (Bq kg−1 ) in Ikaria, Milos and Loutraki
Ikaria MV ± SD∗ Min Max Loutraki MV ± SD∗ Min Max Milos MV ± SD∗ Min Max
226 Ra (sea water)
226 Ra (sediment)
1.2 ± 1.0 < 0.1 1.9 ± 0.3
212 ± 311 24 ± 14 764 ± 10
0.3 ± 0.18 0.1 ± 0.3 0.5 ± 0.4
228 Th (sea water)
0.5 ± 0.3 0.2 ± 0.2 0.8 ± 0.1
13 ± 2 11 ± 2 16 ± 4
(1.53 ± 0.17) × 10−3 (1.45 ± 0.25) × 10−3 (1.67 ± 0.37) × 10−3
228 Th (sediment)
43 ± 18 18 ± 48 66 ± 3
0.1 ± 0.1 < 0.1 0.1 ± 0.4
6±3 3±4 11 ± 4
(0.09 ± 0.04) × 10−3 (0.06 ± 0.01) × 10−3 (0.13 ± 0.02) × 10−3
15 ± 14 4±3 50 ± 21
13 ± 13 4±2 47 ± 29
∗ Values are given as MV ± SD for 10 samples for seawater and 16 for sediments of each area.
Table 2 Activity concentrations of 226 Ra and 228 Th in algae and P. oceanica – young leaves (Bq kg−1 wet weight)∗ Jania sp.
Ikaria Loutraki Milos
Padina pavonica
Posidonia oceanica
226 Ra
228 Th
226 Ra
Cystoseira sp. 228 Th
226 Ra
228 Th
226 Ra
228 Th
85 ± 2 2.3±0.4 3.3±0.5
9.1 ± 1.9 3.4 ± 0.4 8.8 ± 0.6
59 ± 25 0.36±0.30 3.1 ± 1.1
2.3 ± 0.5 0.1 ± 0.1 8.3 ± 2.2
0.9 ± 0.6 4.8 ± 3.1
1.3 ± 0.9 4.1 ± 1.9
24 ± 7
8.7 ± 4.6
4.4±3.0
8.3 ± 2.0
∗ Values are given as MV ± SD for 10 composite samples for each species of each area.
Table 3 Activity concentrations of 226 Ra and 228 Th in fish (Bq kg−1 wet weight)∗ 226 Ra (flesh)
Boops boops Ikaria Loutraki Milos Trachurus trachurus Ikaria Loutraki Milos
228 Th (flesh)
0.12 ± 0.12 3.1 ± 1.1 3.80 ± 1.12
0.12 ± 0.16 3.6 ± 1.7 2.15 ± 0.55
0.17 ± 0.13 − 0.29 ± 0.09
0.01 ± 0.01 − < 0.05
226 Ra (bone)
228 Th (bone)
0.89 ± 0.16
0.06 ± 0.12
2.2 ± 0.6 −
0.53 ± 0.3 −
∗ Values are given as MV ± SD for 10 composite samples for each species of each area.
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226 Ra
228 Th
Ikaria Loutraki Milos
106 36 14 615
136 24 145 404
3.3. Distribution coefficients – Kd Based on gamma-spectroscopy measurements and applying the calculation formulae described above, the distribution coefficients are presented in Table 4. The distribution coefficients in Milos are higher than those in Ikaria and Loutraki, where the continuous outflow of the radioactive springs into the sea results in higher concentrations in seawater. Thus, the seawater is continuously enriched, whereas no respective increase is noticed for sediments in due time. This leads to lower distribution coefficients for sediments in the areas of spring influence and the hypothesis of dynamic equilibrium during the exchange procedure between seawater and sediment may not be valid. 3.4. Concentration factors – CF Based on gamma-spectroscopy measurements and applying the calculation formulae described above, the concentration factors for algae, P. oceanica and the fish species studied are presented in Table 5 with the notation (L) for linearity (formula (2)) or (NL) for non-linearity (formula (3)). Elevated concentrations of Ca, the analog element of 226 Ra and 228 Th, were recorded in the abiotic material of the investigated areas. This affects the observed concentrations of 226 Ra and 228 Th in the organisms considered [2,16], and consequently results in the non-linear model of equilibrium between the donor and the receiving compartment. 3.4.1. Jania species The CFs for Jania sp. sampled from Milos, where the relation between the abiotic and biotic materials is non-linear, are higher compared to those from Ikaria, where the linear relation is satisfied (Table 5). The lowest CFs are observed in Loutraki conforming to the non-linear relation. The CFs of 226 Ra in Ikaria are higher than those of 228 Th. This is in agreement with the higher biological mobility reported for 226 Ra compared to that of 228 Th [1,2]. Note that in Milos, where non-linear relations are observed, the CFs for 228 Th are higher than the CFs for 226 Ra. In Loutraki the CFs for the studied radionuclides are comparable. 3.4.2. Cystoseira species The CFs for Cystoseira sp. sampled from Milos, where the relation between the abiotic and biotic materials is non-linear, are higher compared to those from Loutraki and Ikaria, where the linear relation is satisfied (Table 5). Besides, the CFs for 226 Ra are higher than those of 228 Th, whereas in Milos the CFs of 228 Th are higher than 226 Ra, as is observed in Jania sp.
Distribution coefficients and concentration factors of 226 Ra and 228 Th in the Greek marine environment 1175 Table 5 Concentration factors (CF) of 226 Ra and 228 Th in algae, P. oceanica and fish 226 Ra
Sampling station Jania sp. Ikaria Loutraki Milos Cystoseira sp. Ikaria Loutraki Milos Padina pavonica Loutraki Milos Sampling station P. oceanica Ikaria Milos Sampling station Boops boops Ikaria Loutraki Milos Trachurus trachurus Ikaria Milos
226 Ra (water)
228 Th
283 (L) 30 (NL) 2200 (NL)
12 (L) 23 (NL) 96 000 (NL)
58 (L) 19 (L) 2062 (NL)
35 (L) 7 (L) 92 000 (NL)
6 (L) 3142 (NL)
30 (NL) 46 000 (NL) 226 Ra (sediment)
228 Th (sediment)
0.6 (L) –
0.5 (L) –
228 Th (flesh)
226 Ra (bone)
228 Th (bone)
0.3 (NL) 10.3 (NL) 2 × 103 (NL)
0.4 (NL) 24 (NL) 24 × 103 (NL)
2.4 (NL) – –
0.2 (NL) – –
0.4 (NL) 0.3 × 103 (NL)
0.12 (NL) 0.6 × 103 (NL)
4.9 (NL) –
1.2 (NL) –
52 (L) 3035 (NL) 226 Ra (flesh)
228 Th (water)
21 (L) 92 200 (NL)
3.4.3. Padina pavonica The concentration factors of Padina pavonica from Milos are higher compared to those from Loutraki, as also observed for Jania sp. and Cystoseira sp. (Table 5). The CFs of 228 Th are higher than those of 226 Ra for both areas. 3.4.4. Posidonia oceanica Considering P. oceanica in Ikaria, the CFs are calculated based on two donor compartments, water and sediment (P. oceanica is a marine angiosperm with a functional root system). The CFs based on sea water as a donor are two orders of magnitude higher than those based on sediment (Table 5). This means that the leaf system plays the major role in radionuclide bioacummulation compared to the root system of the plant. The CFs on the sea water basis from Milos are found to be higher compared to those from Ikaria. 3.4.5. Boops boops and Trachurus trachurus In terms of fish species examined, the CFs of 226 Ra and 228 Th for bone tissue as the receiving compartment are higher than those of flesh (Table 5). This is due to the fact that both 226 Ra and 228 Th are bone seekers. Besides, the CFs for flesh are lower in Ikaria compared to those from Milos and Loutraki, as found for algae and P. oceanica.
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4. Conclusions The highest Kd ’s are observed in Milos with the lowest concentrations of 226 Ra and 228 Th in seawater compared to Ikaria and Loutraki, where the continuous enrichment of radionuclides by the springs disturbs the sea water/sediment equilibrium process. The CFs in Milos were found to conform to the non-linear model for all the organisms of the three taxa examined (algae, angiosperm, fish). In Ikaria the CFs of algae and P. oceanica satisfy the linear model. In Loutraki only Jania sp. for both radionuclides and Cystoseira sp. for 226 Ra satisfy the linear model. The linear relation for CFs for marine flora is rather followed in the areas with the highest concentrations in the donor compartment. In this case, the CFs of 226 Ra are higher than those of 228 Th, whereas the CFs of 228 Th were found higher wherever the non-linear relation is satisfied. The CFs in fish conform to the non-linear relation for the three areas. The CFs of 226 Ra and 228 Th of the species examined in Milos are found to be higher than the respective ones in Ikaria and Loutraki. This seems to be connected with the lowest concentrations in the donor compartment and the non-linear relation satisfied. Considering the different tissues in P. oceanica and fish, higher CFs are observed in that tissue of the organism with the major metabolic role for the examined radionuclide. References [1] E.J. McGee, K.J. Johanson, M.J. Keatinge, H.J. Synnott, P.A. Colgan, An evaluation of ratio systems in radioecological studies, Health Phys. 70 (1996) 215–221. [2] F.W. Whicker, V. Schultz, Radioecology: Nuclear Energy and the Environment, vol. 1, CRC Press, Boca Raton, FL, 1982. [3] J.B. Cowart, W.C. Burnett, The distribution of uranium and thorium isotopes decay-series radionuclides in the environment – A review, J. Environ. Qual. 23 (1994) 651–662. [4] G.G. Pyle, F.V. Clulow, Non-linear radionuclide transfer from the aquatic environment to fish, Health Phys. 73 (3) (1997) 488–493. [5] G. Trabidou, H. Florou, A. Angelopoulos, L. Sakelliou, Environmental study of the spas in the Ikaria island, Radiat. Prot. Dosim. 63 (1) (1996) 63–67. [6] H. Florou, P. Kritidis, Natural radioactivity in environmental samples from an island of volcanic origin (Milos, Aegean sea), Mar. Poll. Bull. 22 (8) (1991) 417–419. [7] P. Kritidis, A radiological study of the Greek radon spas, in: Proc. Int. Symp. on Radon and Radon Reduction Technology, vol. 3, session VI(8), 1992. [8] C.A. Ktenas, G. Marinos, La géologie de l’île de Nikaria, Geological and Geophysical Research, Institute for Geology and Subsurface Research, Athens, 1969. [9] M.D. Fytikas, Geological and geothermal study of Milos island, Geol. Geophys. Res. XVIII (1) (1975). [10] P. Kritidis, H. Florou, Natural radioactivity in the environment and radioactive pollution, in: Proc. Natl. Conf. on Environmental Science and Technology, vol. B, Aegean University, Mytilini, September 1989, pp. 24–34. [11] F. Boisson, J.-C. Miquel, O. Cotret, S.W. Fowler, 210 Po and 210 Pb cycling in a hydrothermal zone in the coastal Aegean Sea, Sci. Total Environ. 281 (2001) 111–119. [12] H. Florou, P. Kritidis, Gamma radiation measurements and dose rates in the coastal areas of a volcanic island, Aegean Sea, Greece, Radiat. Prot. Dosim. 45 (1–4) (1992) 277–279. [13] H. Florou, Behavior and dispersion of radionuclides in marine ecosystems (Aegean and Ionian Sea, Greece), PhD thesis, University of Athens, 1992. [14] R.L. Kathren, Radioactivity in the Environment, Harwood Academic, New York, 1984. [15] P.F. Landrum, H. Lee II, M.J. Lydy, Toxikokinetics in aquatic systems – model comparisons and use in hazard assessment, Environ. Toxicol. Chem. 11 (1992) 1709–1725. [16] E.V. der Stricht, R. Kirchmann (Eds.), Radioecology – Radioactivity and Ecosystems, International Union of Radioecology, Belgium, 2001.