Pre- and post-Chernobyl accident levels of 129I and 137Cs in the Southern Baltic Sea by brown seaweed Fucus vesiculosus

Pre- and post-Chernobyl accident levels of 129 I and 137 Cs in the Southern Baltic Sea by brown seaweed Fucus vesiculosus J.M. Gómez-Guzmán a , b , * , 1 , E. Holm a , c , S.M. Enamorado-Báez a , d , J.A. Abril d , A.R. Pinto-Gómez a , J.M. López-Gutiérrez a , e , M. García-León a , b a Centro Nacional de Aceleradores (CNA), Avda. Thomas Alva Edison 7, Isla de la Cartuja, 41092 Seville, Spain b Dpto., de Física Atómica, Molecular y Nuclear, University of Seville, Spain c Norwegian Radiation Protection Authority, Østerås, Norway d Dpto., de Física Aplicada I, E.T.S. de Ingeniería Agronómica, University of Seville, Spain e Dpto., de Física Aplicada I, Escuela Universitaria Politécnica, University of Seville, Spain article info Article history: Received 26 March 2012 Received in revised form 12 July 2012 Accepted 9 August 2012 Available online 30 August 2012 Keywords: AMS ICP-MS 129 I 127 I Seaweed Chernobyl accident abstract 129 I is a very long-lived radionuclide (T 1/2 ¼15.7 10 6 years) that is present in the environment both because of natural and anthropogenic sources. In this work 129 I concentration and 129 I/ 127 I ratio have been determined in seaweed Fucus vesiculosus collected in the Southern Baltic Sea during 1982 and 1986 (post-Chernobyl accident). The resulting data were evaluated in terms of 129 I concentrations, 129 I/ 127 I and 129 I/ 137 Cs ratios. 129 I concentrations were found to be in the order of (0.82e5.89) 10 9 atoms g 1 in 1982 and (1.33e38.83) 10 9 atoms g 1 in 1986. The 129 I/ 127 I ratios ranged from (22.7e87.8) 10 10 for seaweed collected in 1982 and from (26.1e305.5) 10 -10 for seaweed collected in 1986. Also a linear relationship was established for 127 I concentrations in seawater and salinity in this area, enabling the estimation of concentration factors for 127 IinF.vesiculosus. The high levels of 129 I and 129 I/ 127 I in the Kattegat and their gradually decreasing trend to the Baltic Sea indicates that the most important contribution to the 129 I inventory in the Baltic Sea area comes from Sellafield and La Hague reprocessing plants. With respect to Chernobyl accident, 129 I concentrations in samples collected in 1986 were not much higher than those expected in less contaminated samples from 1982. This supports the view that the contribution of the Chernobyl accident to 129 I in the Baltic region was not significant. Ó2012 Elsevier Ltd. All rights reserved. 1. Introduction 129 I is a long-lived cosmogenic radionuclide (T 1/2 ¼15.7 10 6 years) for which the natural abundances have been altered in a significant way by human actions. In nature, it is produced by cosmic rays interactions with Xe in the atmosphere, by spontaneous fission of 238 U and 235 U in the lithosphere (Eisenbud and Gesell, 1997) and, to a minor extent, by neutron-induced reactions on 128 Te and 130 Te in the Earth crust (NCRP, 1983). These processes lead to typical isotopic ratios 129 I/ 127 I between 10 12 (Kilius et al., 1992) and 6 10 13 (Fabryka-Martin et al., 1985). The natural inventory of 129 I has been estimated to be w230 kg, most of which resides in the deep oceans (Rao and Fehn,1999). 129 I has also been produced anthropogenically during the last 60 years during fission processes associated with civil and military nuclear activities and accidents, such as the Chernobyl accident (April 26th, 1986). An estimated 50e150 kg of 129 I was released to the atmosphere during the main period (1945e1964) of nuclear weapons testing (Raisbeck et al., 1995;Wagner et al., 1996) and a further w6 kg were released due to the Chernobyl accident (Gallagher et al., 2005). However, the main sources of anthropogenic 129 Iare nuclear fuel reprocessing plants (La Hague-France and Sellafield- England, see Fig. 1). It has been estimated that, by 2004, 1371 kg of 129 I had been discharged into the NE Irish Sea from the Sellafield plant, with an additional 182 kg released directly to the atmosphere in gaseous form (Gray et al., 1995;López-Gutiérrez et al., 2004). By the end of the same year, 3119 kg of 129 I had been released from La Hague plant as liquid discharges into the English Channel, with an *Corresponding author. Centro Nacional de Aceleradores (CNA), Avda. Thomas Alva Edison 7, Isla de la Cartuja, 41092 Seville, Spain. Tel.: þ34 954 46 05 53; fax: þ34 954 46 01 45. E-mail addresses: [email protected],[email protected] (J.M. Gómez- Guzmán). 1 Present address: Technische Universität München, Physics Department E12, James-Franck-Strasse 1, D-85748 Garching bei München, Germany. Contents lists available at SciVerse ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad 0265-931X/$ esee front matter Ó2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2012.08.007 Journal of Environmental Radioactivity 115 (2013) 134e142 additional 68 kg released in gaseous form (López-Gutiérrez et al., 2004;Aldahan et al., 2007). These large releases of 129 I from reprocessing facilities, along with the conservative behaviour of iodine in seawater (Chai and Zhu, 1994) give a residence time of iodine in the ocean of 4 10 5 years, make 129 I an important oceanographic tracer. Seaweed is useful as a bioindicator for monitoring radioactive contamination of the sea, due to its property of concentrating various elements present in the water. It is well known that it can concentrate iodine to a high level, with concentration factors as high as 10 4 in some brown seaweed (Hou and Yan, 1998). It has therefore been used as a bioindicator of radioiodine, such as 131 I and 129 I(Druehl et al., 1988;Cooper et al., 1998;Hou et al., 2000a). In addition, the relatively high stability of iodine in seaweed and the convenience of storing dry seaweed present a unique possibility to use archived seaweed collected at appropriate times and locations. This provides useful records of the variation of the anthropogenic 129 I levels. It has been well established that the discharged radioactive effluents from the reprocessing facilities at La Hague are first transferred northwards to the coast of north-western Europe via the English Channel, mixed with effluent from Sellafield at the North Sea, and then further transferred by the Norwegian Coastal Current to the Arctic (Alfimov et al., 2004). Meanwhile, there is an inflow of saline sea water from the North Sea that enters the Skagerrak bringing anthropogenic pollutants from the European reprocessing facilities. 129 I in the Baltic Sea is mainly due to fallout from the atmospheric emissions from nuclear facilities and emissions from contaminated seawater as iodide (especially forming organic compounds, such as CH 3 I(Aldahan et al., 2006)), and iodate (Lehto et al., 2012). The Baltic and Black Seas became contaminated by the Chernobyl accident with 90 Sr, 134 Cs, 137 Cs and 239,240 Pu. The dose rates to the public from ingestion of 137 Cs from Chernobyl in aquatic food have been estimated to be low, at least one order of magnitude lower than those due to natural 210 Po (Povinec et al., 1996). The highest dose from the Chernobyl accident was delivered to the critical group in the Baltic Sea region, who received a maximum dose of 0.08 mSv during 1986 (Nielsen and Keith- Roach, 2004). While the distribution and behaviour of caesium isotopes have been widely studied, less is known about the iodine isotopes, which had a substantially different initial release history and also different physical mechanisms governing their deposition. Thus, in the 10 days period after the accident, the initial large release from the Chernobyl reactor contained mainly the more volatile radionuclides such as noble gases, iodine and some caesium. The second large release was associated with the high temperatures reached in the core melt, and contained most of the particulate caesium (Brant et al., 2002). Here we present a study of 129 I concentration in brown seaweed Fucus vesiculosus sampled around the coastline of Sweden at 33 different stations in the years 1982 and 1986 (post-Chernobyl accident). The resulting data were evaluated in terms of 129 I concentrations, 129 I/ 127 I and 129 I/ 137 Cs ratios. 2. Sampling and radiochemical method Brown seaweed F.vesiculosus was sampled at locations selected to provide a representative geographical distribution on all aspects of the Swedish coast (Fig. 2). Sampling took place in years 1982 (during the month of October) and 1986 (during the month of July), and about 1 kg of each sample was collected, dried, ground and homogenised. All samples were chemically prepared using the microwavedigestion method described by Gómez-Guzmán et al. (2011). Typically, 0.5 g of sample and 10 mL of 60% (v/v) HNO 3 suprapure quality (Merck, Darmstadt, Germany) were introduced into the vessels. The operational conditions were: 200  C for 10 min (ramp of 10 min), 200  C for 20 min and 20 min for cooling to room temperature. After digestions, the chemical separation consisted of an organic compound extraction followed by an aqueous solution extraction. Iodine was extracted from digested samples three times with 10 mL CHCl 3 and then back-extracted into aqueous solution by addition of 20 mL 0.1 M NaHSO 3 .Thenthe solution was stirred and the SO 3 2 and SO 4 2 ions were precipitated by addition of 10 mL of saturated Ba(NO 3 ) 2 solution followed by centrifugation. Finally, iodine was precipitated as AgI by adding 10 mL 0.1 M AgNO 3 solution, washed twice with distilled water and dried at 60e80  C. The mean recovery of the radiochemical procedure was found to be 90%. The AgI was mixed with about the same volume of Nb powder and loaded in a copper target holder for the measurement of 129 I. After the digestion and before the addition of 2 mg of Woodward iodine as carrier ( 129 I/ 127 Iz710 14 (Roberts et al., 1997)), a 1 mL aliquot of each sample was used for 127 I measurement. These aliquots were diluted in adequate volume of double deionised water (18 M U cm) to obtain maximum stable iodine concentrations in the order of 20 m gL 1 . 3. Experimental 3.1. Measurement of 129 I by AMS Determination of 129 I/ 127 I ratios in seaweed samples was conducted at the 1 MV multielement compact Accelerator Mass Spectrometry (AMS) system at the Centro Nacional de Aceleradores (CNA) in Seville (Spain). Details of the measurement and the system have been previously described (Gómez-Guzmán et al., 2012;Klein et al., 2006,2007), so only a short description will be given here. A beam of negative iodine ions is extracted from the AgI þNb target using the Cs þ high-intensity sputter source at 35 keV. At the terminal of the tandem, negative iodine ions are changed to positive iodine ions by the stripper (pressure at 6 10 3 mbar of argon, which corresponds to a mass thickness of about 0.15 m gcm 2 ) and they are then accelerated. Following acceleration, selected positive ions (charge state þ3) are analyzed by mass spectrometer. The 127 I Fig. 1. Annual 129 I release to the sea from the reprocessing facilities in Sellafield and La Hague (compilation until 2004 from López-Gutiérrez et al. (2004) and the rest by Monitoring our Environment (2006) and Areva Rapport Environnement (2006)). J.M. Gómez-Guzmán et al. / Journal of Environmental Radioactivity 115 (2013) 134e142 135 current beam is measured in a Faraday cup, and 129 I 3þ ions are counted in a gas ionization chamber. AMS measurements were performed relative to a standard with known 129 I/ 127 I isotopic ratio which was made by repeated dilutions from NIST SRM 3230 Iodine Isotopic Standard Level I. The isotopic ratios obtained directly by AMS measurement of the samples (including carrier) were typically of the order of 129 I/ 127 Iw10 10 e10 11 . Background effects were evaluated through frequent digestions of iodine blanks (Woodward Iodine, from Woodward Iodine Corporation, USA), which measured a 129 I/ 127 I isotopic ratio of (3e4) 10 13 when 2 mg carrier was used, meanwhile the limit of detection of the CNA AMS system was found to be 129 I/ 127 I¼(3.11 0.16) 10 13 (Gómez-Guzmán et al., 2012). This is typically 2 to 3 orders of magnitude lower than the isotopic ratios measured for real samples. 3.2. Measurement of 127 I by ICP-MS Measurements of 127 I content in seaweed samples were performed at the Inductively Coupled Plasma Mass Spectrometry (ICPMS) Unit of the Servicio de Investigación Agraria (University of Seville, Spain). Details of the measurement and the system have been previously described (Gómez-Guzmán et al., 2011), so only a short description will be given here. A Quadrupolar ICP-MS (Thermo X7, Thermo, Madrid, Spain) equipped with a concentric quartz nebulizer (Meinhard Associates, Golden, USA) was used for 127 I determination. The instrument was used at normal resolution and set to detect the signal intensity at m/z127. Samples were analyzed using recommendations provided by US-EPA 200.8 methodology adapted for iodine. Various data quality tests were used during every instrumental running, such as replicate, matrix matching, memory effects, and both digestion analysis and laboratory reactive blanks checking. The instrumental detection limit of 127 I calculated as three times the standard deviation of the blank, was 0.045 m gL 1 . The methodology used to measure the 127 I content in seaweed samples by this technique was previously validated by measuring 127 I content in standard reference materials from the National Institute of Standards and Technology (NIST), the International Atomic Energy Agency (IAEA) and the Community Bureau of Reference (BCR) (Gómez-Guzmán et al., 2011). 4. Results Concentrations of 129 I and 127 I, 129 I/ 127 I ratios and the activity of 137 Cs in seaweed samples collected in 1982 and 1986 around the Swedish coastline are shown in Table 1. It can be seen that the concentration of stable iodine ( 127 I) in the seaweed varies significantly from 54 to 178 m gg 1 for 1982 and from 37 to 372 m gg 1 for 1986 dry weight. This variation has also been observed in other species of seaweed (Hou and Yan,1998;Hou et al., 2000b). The ratio 129 I/ 127 I instead of 129 I concentration in seaweed could be used as an index of the 129 I level in the surrounding seawater of sampling location, which is based on the assumption that seaweed has a similar capability to take up and assimilate 129 I and 127 I, as previously reported by Hou et al. (2000b). 129 I concentrations in seaweed samples were found to be in the order of (0.82e5.89) 10 9 atoms g 1 in 1982 and (1.33e 38.83) 10 9 atoms g 1 in 1986 (Table 1,Fig. 3b). The 129 I/ 127 I ratios ranged from (22.7e87.8) 10 10 for seaweed collected in 1982 and from (26.1e305.5) 10 10 for seaweed collected in 1986 (Table 1,Fig. 3a). For both years these 129 I/ 127 I levels are still two or three orders of magnitude higher than the fallout background level of w10 10 (Edmonds et al., 1998;Cooper et al., 1998). From Table 1 and Fig. 3 it can be seen that there is a geographical enhancement of 129 I concentrations and 129 I/ 127 I ratios in samples collected in the Kattegat area (positions 27e33) with respect to seaweed samples collected in the Baltic Sea coastline (positions 1e26). The effect of the horizontal gradients in salinity has to be Fig. 2. Sampling sites (blue dots) of Fucus vesiculosus extracted from Carlson and Holm (1992). Red dots represent the nuclear facilities in the drainage are of the Baltic Sea. The small map in the left upper corner shows the location of Sellafield and La Hague reprocessing facilities with respect to the sampling sites. [For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.] J.M. Gómez-Guzmán et al. / Journal of Environmental Radioactivity 115 (2013) 134e142136 evaluated, but previous results indicate that a significant amount of discharge from the reprocessing facilities had been transferred from the North Sea to the inner Danish waters and reached the west coast of Sweden through the Kattegat and Skagerrak. However, only a minor fraction of this contaminated seawater is then transferred through the Kattegat further to the Baltic Sea; most of it returns to the North Sea (Hou et al., 2000b). To compare these 129 I concentrations and 129 I/ 127 I ratios on a global scale, Table 2 presents a comparison of 129 I levels and 129 I/ 127 I ratios in seaweed from other locations worldwide. For example, Osterc and Stibilj (2008) reported results obtained in seaweed Fucus virsoides sampled during the summers of 2005 and 2006 in the coastline of the Adriatic Sea. 129 I concentrations and 129 I/ 127 I ratios in these samples ranged between (1.5e3.4) 10 9 atoms g 1 and (8.3e21.1) 10 10 , respectively. Results reported by Fréchou and Calmet (2003) and obtained in seaweed sampled between 1998 and 1999 in Goury, sited in the vicinity of La Hague nuclear reprocessing plant, show very high 129 I concentrations and 129 I/ 127 I ratios. Both range between (1.4e3.2) 10 13 atoms g 1 and (1.01e1.94) 10 5 , respectively, between four and five orders of magnitude higher to that found in this study. Hou et al. (1999a) reported 129 I concentrations and 129 I/ 127 I ratios in seaweed sampled in Denmark and Iceland between 1988 and 1992. These values ranged between (0.6e115) 10 9 atoms g 1 and (4e640) 10 10 , respectively, and were in the same order of magnitude as results found in this study. Results reported by Cooper et al. (1998) showed 129 I/ 127 I ratios ranged between (2.2e80) 10 10 in seaweed sampled in Novaya Zemlya between 1967 and 1993. This Russian archipelago has been influenced by the impact of the Soviet nuclear tests and, indirectly, its waters are contaminated by Sellafield and La Hague liquid emissions that arrive through the NCC (Norwegian Coastal Current) (Arctic Monitoring and Assessment Programme, 2003). Activity concentrations of 137 Cs varied from 600 Bq kg 1 at the northernmost locality (1), to 20e25 Bq kg 1 at the southeast coast in July 1986. For 1982, the mean activity concentration of 137 Cs was 92Bqkg 1 . Thus, the 137 Cs concentration increased by 2e5 times in most of the seaweed samples (Table 1). 5. Discussion 5.1. 127 I The Baltic Sea is a semi-enclosed water body connected to the North Sea through the Strait of Kattegat. Salinity distributions in its surface layer (0e10 m) show strong horizontal gradients, with values close to 30 psu at the Kattegat area. This value decreases towards the Bothnian Bay and the gulfs of Finland and Riga, where values below 3 psu are found (Feistel et al., 2010). Vertical stratification is also noticeable (Hou et al., 2001). 127 I is naturally present in seawater with concentrations that increase with the salinity, as shown in Fig. 4, with data from a set of water samples from the Baltic Sea area (Hou et al., 2001;Lehto et al., 2012). Fig. 4 shows that iodine behaviour is conservative, as expected taking into account that this radionuclide is generally soluble in seawater and poorly adsorbed onto particulates and sediment. Both iodide (I  ) and/or iodate (IO 3  ) anions can be present, but the latter predominates (Pentreath, 1985). Table 1 129 I and 127 I concentrations (dry weight), 137 Cs activities and 129 I/ 127 I ratios in Fucus vesiculosus sampled around the Swedish coastline in 1982 and 1986. Quoted uncertainties are 1 s . Uncertainties related to the 129 I/ 127 I ratios come from the error propagation theory. 137 Cs data extracted from Carlson and Holm (1992). Sampling site 1982 1986 129 I(10 9 atoms g 1 ) (measured) 127 I( m gg 1 ) (measured) 129 I/ 127 I(10 9 ) (calculated) 137 Cs (Bq kg 1 ) (measured) 129 I(10 9 atoms g 1 ) (measured) 127 I( m gg 1 ) (measured) 129 I/ 127 I(10 9 ) (calculated) 137 Cs (Bq kg 1 ) (measured) 1 3.49 0.16 38 5 19.6 3 600 29 2 1.77 0.09 61.6 1.6 6.0 0.3 110 6 3 3.87 0.22 40 4 20.2 2.4 138 7 4 1.80 0.11 35.9 0.2 10.6 0.7 91 5 5 1.33 0.06 54 4 5.2 0.5 41 2 6 1.55 0.21 51 3 6.4 0.9 46 3 7 1.03 0.06 75 6 2.9 0.3 8.2 0.5 1.81 0.15 51.4 0.6 7.4 0.6 26 1 8 0.82 0.05 77 3 2.27 0.16 7.3 0.7 1.59 0.07 74 6 4.5 0.4 30 2 9 1.18 0.06 85 5 2.92 0.23 9.3 0.5 1.88 0.18 53 4 7.4 0.9 46 3 10 0.93 0.05 61 7 3.2 0.4 7.5 0.4 1.72 0.10 41.9 2.0 8.6 0.7 26 1 11 0.96 0.05 67 5 3.0 0.3 7.0 0.4 12 1.12 0.09 90 4 2.66 0.24 7.5 0.7 1.60 0.08 129 5 2.60 0.17 20 1 13 1.44 0.10 102 4 2.97 0.24 7.2 0.5 1.50 0.09 78.4 4.1 0.3 28 2 14 1.9 0.3 104 3 3.8 0.7 28 2 15 2.55 0.13 84 3 6.4 0.4 57 3 16 2.11 0.12 132 12 3.4 0.4 7.5 0.4 1.82 0.11 71 4 5.4 0.4 24 1 17 2.09 0.10 84 3 5.3 0.3 8.3 0.7 1.97 0.19 35 3 12.0 1.5 27 2 18 1.9 0.3 106 8 3.8 0.7 42 2 19 2.19 0.11 115 10 4.0 0.4 29 2 20 2.48 0.13 144 6 3.65 0.24 8.6 0.7 2.81 0.16 163 3 3.64 0.22 89 5 21 3.39 0.15 131 2 5.5 0.3 9.3 0.5 22 2.51 0.11 134 9 3.9 0.3 8.7 0.9 2.89 0.21 142 5 4.3 0.4 23 1 23 2.81 0.16 68 7 8.8 1.0 14.6 0.8 2.1 0.4 67 6 6.8 1.3 24 1 24 2.18 0.09 72 4 6.4 0.4 8.0 0.7 1.88 0.13 50 3 7.9 0.7 352 19 25 2.05 0.10 54 8 7.9 1.3 8.2 0.9 2.66 0.12 55 4 10.2 0.9 19 1 26 3.22 0.23 118 8 5.7 0.6 6.9 0.4 3.36 0.17 68.4 1.9 10.4 0.6 107 5 27 4.02 0.21 109 8 7.8 0.7 10.4 0.6 6.9 0.3 114 5 12.7 0.8 40 2 28 5.89 0.15 178 15 6.9 0.6 13.0 1.3 18.0 0.8 132 5 28.6 1.6 36 2 29 29.6 1.2 341 24 18.3 1.5 34 2 30 28.8 1.2 331 12 18.4 1.0 34 2 31 31.0 1.6 372 18 17.6 1.3 33 2 32 29.4 2.0 279 7 22.2 1.6 27 1 33 38.8 2.1 269 15 30.5 2.4 41 2 J.M. Gómez-Guzmán et al. / Journal of Environmental Radioactivity 115 (2013) 134e142 137 Measured values of 127 I concentrations in F.vesiculosus sampled around the southern Swedish coastline in 1982 and 1986 along with salinity are shown in Fig. 5 (mean annual values for the period 1900e2005 and for 1  1  10 m size cells, taken from Feistel et al., 2010). The distribution of 127 I concentrations, C, as a function of the sampling point were similar in both campaigns, with an increasing trend towards the Strait of Kattegat that can be linearly correlated with salinity, S, at 99% confidence level (R 2 ¼0.84, p<0.01; C¼20.4$Se60.4, in units of m gg 1 ). 127 I concentrations in seawater can be inferred from the salinity data (Fig. 5) and from Fig. 4, which allows the estimation of concentration factors (defined as the ratio between concentrations in F.vesiculosus and in seawater). The so obtained values for the two sampling campaigns are compatible with a normal distribution of mean value 6540 and a standard deviation of 400. When excluding the sampling points from the Kattegat area (those with higher salinity, and index >27), the concentration factors follow a normal distribution with mean value 5900 and standard deviation of 355. 5.2. 129 I, 137 Cs and 129 I/ 137 Cs ratio The possible origins of man-made radionuclides in the Baltic Sea can be classified as follows: natural occurrence, fallout from atmospheric nuclear weapon tests, the release from the Chernobyl nuclear power plant accident, authorised discharges of radioactivity into the sea occurring during routine operation of nuclear installations in the Baltic Sea region and discharges from European nuclear fuel reprocessing plants. The natural inventory of 129 I has been estimated to be w230 kg, most of which resides in the deep oceans (Rao and Fehn, 1999). It is well established that the ratio 129 I/ 127 I in the pre-nuclear era had typical values between 10 12 (Kilius et al., 1992) and 6 10 13 (Fabryka-Martin et al., 1985). These levels, compared with the ones measured in F.vesiculosus in this work, account for <0.05% of the total 129 I inventory in this region. An estimated amount of 50e150 kg of 129 I were released to the atmosphere during the main period (1945e1964) of nuclear weapons testing (Raisbeck et al., 1995;Wagner et al., 1996), which enhanced the natural 129 I/ 127 I ratio to 10 10 . This level, compared with the isotopic ratios measured in this work, still account for <5% of the total 129 I inventory in the Baltic Sea region. According to Salo et al. (1986), the inventory of 137 Cs in the Baltic Sea due to this source, based on measured concentrations in water and sediments, is 620 TBq (calculated to 1981). Fig. 3. (a) 129 I/ 127 I ratios and (b) 129 I concentration (dry weight) in seaweed Fucus vesiculosus sampled around the Swedish coastline in 1982 and 1986. Table 2 129 I concentrations and 129 I/ 127 I reported in various seaweeds sampled worldwide. Seaweed Sampling zone 129 I(10 9 atoms g 1 ) 129 I/ 127 I (10 10 ) Reference Date Location F. Vir. Sept. 2005 Slovenia 2.0 0.6 424.5 34.0 (Osterc and Stibilj, 2008)F. Vir. June 2006 Italy 1.5 1.1 405.3 28.9 F. Vir. Oct. 2006 Croatia 3.4 1.1 330.7 26.8 F. Ves. Oct. 1998 Goury, France 14,000 900 99,434 985 (Fréchou and Calmet, 2003)F. Ves. March 1999 Goury, France 32,000 2300 190,992 17781 F. Ves. July 1967 Kruglaya Levaya 21.4 1.1 (Cooper et al., 1998) F. Ves. July 1972 Sosnovetz 71.8 2.0 F. Ves. August 1993 Gribovaya Fjord 789 18 F. Ves. Dec. 1988 Iceland 0.6 0.04 3.90 0.24 (Hou et al., 1999a) F. Ves. Nov. 1989 Klint, Denmark 62.1 4.6 393 6 F. Ves. April 1992 Klint, Denmark 115.3 10.4 638 11 1982 8 0.82 0.05 22.7 0.16 This work F. Ves. 1982 28 5.89 0.15 88 10 a F. Ves. 1986 5 1.33 0.06 26.1 1.7 a F. Ves. 1986 33 38.8 2.1 305 24 a Highest 129 I/ 127 I ratio in 1982 and lowest 129 I/ 127 I ratio in 1986 were found in positions 23 and 12, respectively. Fig. 4. Correlation between 127 I concentration and salinity in seawater. 127 I and salinity data extracted from Hou et al. (2001) and Lehto et al. (2012). J.M. Gómez-Guzmán et al. / Journal of Environmental Radioactivity 115 (2013) 134e142138 The Chernobyl accident, which happened in April 1986, released a large amount of radioactive material, including about 6 kg of 129 I (Gallagher et al., 2005). The total input of 137 Cs from the Chernobyl accident into the Baltic Sea area was estimated at 4500 TBq by CEC (1991). This estimate was later adjusted to 4700 TBq by Nielsen et al. (1999). The deposition fallout from Chernobyl was very unevenly distributed in the drainage area of the Baltic Sea; the most contaminated areas were surrounding the Bothnian Sea and the eastern Gulf of Finland. This behaviour is shown in Table 1, where it can be seen that the highest 137 Cs activities in samples from 1986 are in the north-eastern area of the Baltic Sea. Concentration factors for 127 I can be used to reconstruct the 129 I concentration in seawater from the measured concentrations in F. vesiculosus collected in 1982. In the second sampling campaign, samples were collected in July 1986, three months after the Chernobyl accident. The biological half-life of iodine in F.vesiculosus has been measured and found to be quite short, between 5 and 10 days (Vives i Batlle et al., 2007;Druehl et al., 1988). Using a CF of 14.7 m 3 kg 1 and a typical mass of 0.261 kg (Vives i Batlle et al., 2008), a simple two-compartmental model shows that equilibrium with a water body of w10 10 m 3 should be attained in two months. Although in the real world a steady-state is rarely encountered, under our particular conditions, it seems reasonable to use concentration factors derived from 127 I for estimating the 129 I concentrations in seawater in July 1986. Furthermore, from the results shown in Table 1, 137 Cs concentrations in F.vesiculosus sharply increased in samples collected in July 1986 (it is worth noting that radionuclide discharges from Sellafield and Cap de la Hague continued their decreasing trend in the period between sampling campaigns, and thus, activity concentrations lower than those measured in 1982 could be expected just prior the Chernobyl accident). On the other hand, the application of models with concentration factors constant in time (although varying from site to site) for 137 Cs, could reasonably reproduce the observed time series of 137 Cs concentrations in F.vesiculosus in the Baltic Sea, prior to and after the Chernobyl accident (HELCOM, 2009). Results are depicted in Fig. 6 (double panel, with the detail for the common area covered by the two sampling campaigns). Concentrations in 1982 show an increasing trend towards the Kattegat Strait, consistent with the expected pattern of 129 I dispersion following liquid discharges from Sellafield and Cap de la Hague (Povinec et al., 2003)ewe note that the effect of salinity was already incorporated in the estimation of concentration factors e. Within this common area, 129 I concentrations in 1986 are slightly increased in some locations, as it could be expected of non-uniform atmospheric deposition from the Chernobyl radioactive cloud (HELCOM, 2009), and the limited homogenization of concentrations promoted by water circulation during this elapsed time (Toscano and García-Tenorio, 2004). At the two edges of the expanded area of sampling in 1986, relatively higher concentrations of 129 I were found. To distinguish what extent they are due to localized deposition from the Chernobyl accident or if they are the result of the dispersion pattern from the two reprocessing plants, it is necessary to study the 137 Cs distribution and the 129 I/ 137 Cs isotopic ratios. From Table 1, 137 Cs concentrations in F.vesiculosus in 1982 were rather uniform, with a mean value and standard deviation of 8.7 and 0.5 Bq kg 1 , respectively. It is well established that the Chernobyl accident was the main source of 137 Cs to the Baltic Sea. For example, Carlson and Holm (1992) measured activity ratios 134 Cs/ 137 Cs of (0.45 0.05) in seaweed sampled in these locations, in good agreement with the ratio (0.47e0.48) in July 1986, calculated by IAEA (1986). Thus, 137 Cs concentrations in Fucus collected in 1986 were very high in samples 1e4 and that can be linked with high local deposition from Chernobyl, and is also consistent with the high concentrations of 129 I found in this area. Isolated peaks in 137 Cs concentration also appear in samples 24 and 26. By excluding these six samples, the mean concentration and standard deviation become 35 and 3 Bq kg 1 , respectively, a factor 4 higher than the Fig. 5. 127 I concentration ( m gg 1 )inFucus vesiculosus from the Baltic Sea in 1982 (black) and 1986 (red), and salinity (mean annual values for the period 1900e2005 for 1110 m size cells, taken from Feistel et al. (2010)). [For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.] Fig. 6. 129 I concentrations in seawater in the different sampling sites inferred by concentration factors for 127 I. J.M. Gómez-Guzmán et al. / Journal of Environmental Radioactivity 115 (2013) 134e142 139 levels measured in 1982 (and likely higher than the pre-Chernobyl levels, due to the known decreasing trend in 137 Cs releases from Sellafield and Cap de la Hague). Finally, 137 Cs concentration in Fucus in samples 29e33 remained at the level of the previous mean value, which in turn suggests that the high concentrations of 129 I found in this area should be related to the effluents from the two reprocessing plants. The concentration factors for 137 Cs in F.vesiculosus varies widely from site to site in the Baltic Sea (HELCOM, 2009), most likely linked to changes in salinity. This could mask to some extent the information from isotopic ratios. Thus, 137 Cs concentrations in seawater have been estimated from their recommended concentration factors in F.vesiculosus in the different regions covered by the sampling campaign (HELCOM, 2009; concentration factors ranging from 188 39 L kg 1 dry weight in Kattegat to 453 145 L kg 1 in the western Baltic Sea and 622 100 L kg 1 in the Bothnian Sea). This allows estimation of the 129 I/ 137 Cs isotopic ratios in both matrices, F.vesiculosus and seawater using, for the last one, the following equation, 129I 137CsW ¼129I 127IF ½ 127IWCFð137CsÞ ½137CsF where: [ 129 I/ 137 Cs] W is the 129 I/ 137 Cs atom ratio in seawater at the various Fucus sampling sites; [ 129 I/ 127 I] F is the atom ratio in Fucus, using 129 I and 127 I concentrations obtained in this work, and assuming it to be the same as the [ 129 I/ 127 I] ratio in seawater (Hou et al. (2000b)); [ 127 I] W is the stable iodine concentration in coastal water around Sweden, taken from salinity data (Figs. 3 and 4); [ 137 Cs] F is the 137 Cs concentration in Fucus given by Carlson and Holm (1992) and CF( 137 Cs) is the recommended concentration factor for radiocaesium from seawater to Fucus in Swedish coastal waters given by HELCOM (2009). These ratios are shown in Fig. 7. From the estimated composition of the Chernobyl emissions (Hou et al., 2002), the expected 129 I/ 137 Cs isotopic ratio was 0.18, much lower than the values depicted in Fig. 7. Previous levels of 137 Cs concentrations in seawater contribute to decrease this ratio, while the previous existing levels of 129 I increase it. If the distribution and deposition of both isotopes did not largely differ, the isotopic ratios in seawater depicted in Fig. 7 have to be understood in terms of the relatively low contribution of 129 I from Chernobyl to enhance previously existing levels (as shown in Fig. 6), what also has been reported elsewhere (Hou et al., 2002;Buraglio, 2000). Furthermore, from Fig. 7 it can be concluded that as the deviation of this isotopic ratio in seawater becomes larger with respect to the 1982 levels, the higher will be the local atmospheric deposition of the Chernobyl effluents. Again we can see that for samples 29e33, the isotopic ratio remains at the same level as that in 1982, which suggests that the origins of 129 I in this region are the Sellafield and Cap de La Hague reprocessing plants. The 129 I/ 137 Cs isotopic ratios in F.vesiculosus are mediated by the different concentration factors (those for 129 I being on average, larger by a factor 20 than those for 137 Cs) and by their different trends of variation with salinity (concentration factors for 129 I slightly increase with increasing salinity, while those for 137 Cs sharply decrease with increasing salinity). In fact, these data can be used to infer a relationship for the concentration factor for 137 Cs in F.vesiculosus as a function of the salinity in the Baltic Sea area, following a logelog relationship (R 2 ¼0.79, confidence level 99%; data set for 1986): log ½CFCs¼3:55 1:03log ½S This relationship can be understood in terms of the effect of competitive ions (Abril and Fraga, 1996). Effectively, 137 Cs is usually present in seawater as a monovalent cation at tracer concentrations, in competition with the dominant cations Na þ and K þ , whose concentrations increase with salinity. It is worth noting in contrast that iodine exists in seawater mainly as dissolved anions iodate and iodide (Hou et al., 2001). From data shown in Table 1 it can be seen that the 137 Cs concentrations in seaweed samples collected in 1986 are much higher than those collected in 1982. This also explains the fact that the atomic ratios 129 I/ 137 Cs in the Baltic Sea shown in Fig. 7 were higher in 1982 than in 1986. This was because the deposition of additional 137 Cs from the Chernobyl accident was much more important than the additional 129 I deposition from the same source. The highest 137 Cs activity concentration in 1986 was observed at location 1, the northernmost locality, and adjacent to the land area which received the highest fallout deposition in Sweden from the Chernobyl accident (Carlson and Holm, 1992). The unexpected high 137 Cs activity concentration found at localities 24 and 26 could be influenced by the nearly presence of two nuclear power plants, located at Barsebäck and Oskarshamn, both releasing 137 Cs in the drainage area of the Baltic Sea. Concerning other possible contributions to 129 I and 137 Cs in this area, around the Baltic Sea there are nuclear power plants in Sweden, Finland, Russia, Lithuania and Germany (as shown in Fig. 2) that could directly or indirectly release 129 I and 137 Cs to its drainage area. No data of 129 I discharge from these power plants are available, but some authors (Buraglio, 2000;Hou et al., 1999b, 2002) have concluded that their contribution to the 129 I inventory in the Baltic Sea is insignificant. HELCOM (2009) has determined the total aquatic 137 Cs discharges from these local nuclear facilities into the Baltic Sea to be 2.4 TBq until the end of 2006, compared to the 4700 TBq released from the Chernobyl accident in the same area. So the contribution of these facilities to the total inventory of 137 Cs in the Baltic Sea area is also insignificant, although some authors have related 137 Cs concentrations measured in Fucus collected near Barsebäck nuclear power plant with its released cooling water (Erlandsson and Matsson, 1988;Mattsson et al., 1980). From our present results, it is clear that the most important contribution to the 129 I inventory in the Baltic Sea area comes from the releases of the two reprocessing facilities at Sellafield and La Fig. 7. 129 I/ 137 Cs atom ratio in seawater (open circles and squares) and in Fucus vesiculosus (filled circles and squares) at the various sampling sites for 1982 (black) and 1986 (red). Uncertainties come from the error propagation theory. [For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.] J.M. Gómez-Guzmán et al. / Journal of Environmental Radioactivity 115 (2013) 134e142140 Hague. HELCOM (2009) has also estimated the total input of 137 Cs originating from these two nuclear reprocessing plants to be 250 TBq, which is even less than the 137 Cs released as a result of fallout from atmospheric nuclear weapon tests (620 TBq calculated to 1981). Thus, for this radionuclide the most important source to the inventory in the Baltic Sea area is the Chernobyl accident. 6. Conclusions 129 I and 129 I/ 127 I ratios in seaweed samples F.vesiculosus collected in 1982 (during the month of October) and 1986 (during the month of July) have been measured. For both years, 129 I/ 127 I ratios are much higher than expected for pre-nuclear era (between 10 12 and 6 10 13 ) and due to atmospheric nuclear weapons testing (10 10 ). These levels account for <0.05% and <5% of the total 129 I inventory in this region respectively. A linear relationship has been established for 127 I concentrations in seawater and salinity in the Baltic Sea, enabling the estimation of concentration factors for 127 IinF.vesiculosus, ranging from 2500 to 13,300, and showing a slightly decreasing trend with salinity. It has been assumed that these concentration factors equally apply to 129 I, enabling the reconstruction its concentrations in seawater and the study of their spatial distribution. The high levels of 129 I and 129 I/ 127 I in the Kattegat and their gradually decrease towards the Baltic Sea indicate that the 129 I discharged from the reprocessing plants at Sellafield and La Hague has been transported to the Kattegat from the North Sea and partly further transported to the Baltic Sea via the Belt Sea. Hence, the most important contribution to the 129 I inventory in the Baltic Sea area comes from the releases of both reprocessing plants. This result was also supported by the 129 I/ 137 Cs ratios in F.vesiculosus and seawater. This ratio in Fucus was mediated by concentration factors for 137 Cs, which decreased with salinity as a result of the effect of the dominant competitive cations. With respect to the Chernobyl accident, 129 I concentrations in samples collected in 1986 are not much higher than those expected in less contaminated samples from 1982. This supports the view that the contribution of the Chernobyl accident to 129 I in the Baltic region was not significant. In contrast, the 137 Cs activity concentrations in samples collected in 1986 are much higher than those in corresponding samples collected in 1982, confirming that the Chernobyl accident was the main source of 137 Cs to the Baltic Sea area. 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