Article pubs.acs.org/est
Temporal Variation of Iodine Isotopes in the North Sea Peng He,*,†,‡ Ala Aldahan,‡,§ Göran Possnert,∥ and Xiaolin Hou⊥,# †
Department of Geochemistry, Chengdu University of Technology, Chengdu 610059, China Department of Earth Sciences, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden § Department of Geology, United Arab Emirates University, PO Box 15551, Al Ain, UAE ∥ Tandem Laboratory, Uppsala University, PO Box 529, 751 20 Uppsala, Sweden ⊥ Center for Nuclear Technology, Technical University of Denmark, Risø Campus, DK-4000 Roskilde, Denmark # Xi’an AMS Center, SKLLQG, Institute of Earth Environment, CAS, Xi’an 710075, China ‡
S Supporting Information *
ABSTRACT: Monitoring temporal variability of 129I in the North Sea, a relatively large reservoir of radioactive discharges from the nuclear fuel reprocessing facilities, is vital for the environmental situation in the region. New information on concentration levels and distribution of 129I and 127I and their species forms (iodide and iodate) are gained here through sampling of surface water in 2010. The results show generally large spatial and temporal (compared to data from 2005) fluctuations of total 129I and 127I, and iodide and iodate. In samples south of 53°N, the level of 127I− in 2010 was generally comparable or higher than in 2005. The results also show total 129I concentrations comparable in the south, but 2−8 times lower in the north, to the analyses made in 2005. Different from total 129I, the 129I−/129IO3− values in the northern part were 2 times higher in 2010 than values observed in 2005. These variations in total 129I and 127I and their species are related to coastal water offshore propagation and surface currents that are linked to long-term and seasonal climatic changes over the North Atlantic and North Sea. Inventory estimation shows that >90% of 129I resides in the Southern and German Bights, which also suggests negligible contribution from the Sellafield facility discharges when compared with that from the La Hague. Variability in discharge rate from La Hague may also affect the distribution patterns of 129I in the North Sea on the monthly scale.
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INTRODUCTION
enters the North Sea via the English Channel and along the British coast via Fair Isle Channel. The other part is injected into the atmosphere and consequently fallout by precipitation all over the entire North Sea areas including its surrounding terrestrial environment. A number of studies on transport and distribution of radionuclides (e.g., 137Cs, 134Cs 99Tc, 125Sb, 90Sr, etc.) released from the Sellafield and La Hague have been carried out in the North Sea during 1990s.2−4 Of the pollutants, the anthropogenic radioisotope 129I has been paid more attention nowadays due to potential environmental hazards and tracer applications that are associated with increased discharges from the two reprocessing facilities since 1990.5,6 129 I inventory in the natural environment is dominated by human activities after the nuclear era and more than 90% of the global anthropogenic 129I was released from the Sellafield and La Hague reprocessing facilities.7 Iodine is easily accumulated in seaweed with a concentration factor as high as 104.8
The North Sea is a marginal shallow continental shelf water body that lies between 51°N and 62°N where the westerlies dominant. It connects to the saline Atlantic Ocean through Orkney−Shetland−Norwegian channels in the north and the English Channel in the southwest, while exchanges with the brackish Baltic water via the Skagerrak and Kattegat. General cyclonic circulation within the North Sea is responds to the strong tidal currents and southwesterly wind, and is modified by specific topography and bathymetry (Figure 1).1 As one of the biologically rich and productive regions, the ecosystem of the North Sea is vulnerable, especially in coastal areas, and thus has aroused great public concern. The intensive exploitation of the North Sea, combined with excessive input of nutrients and hazardous contaminants, causes a number of ecological problems, such as deterioration of water quality and decline in biodiversity. Radionuclide contamination in the North Sea is associated with discharges of radioactive waste from the nuclear reprocessing facilities located at the France (La Hague) and the United Kingdom (Sellafield). Major part of the radioactive pollutants is directly discharged into the marine water that © 2013 American Chemical Society
Received: Revised: Accepted: Published: 1419
May 9, 2013 November 19, 2013 December 20, 2013 December 20, 2013 dx.doi.org/10.1021/es402047s | Environ. Sci. Technol. 2014, 48, 1419−1425
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distribution patterns in 2005. Here we revisited the North Sea, particularly the southern part, during the expedition of the icebreaker Oden to Antarctica in 2010. Surface water samples were collected for the analysis of 127I and 129I as well as their species, aiming to reveal temporal variations compared to the data gained in 2005 as mentioned above. Additionally, inventory estimation of 129I in the North Sea is calculated here using a two-layer box model.
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MATERIALS AND METHODS Sampling was carried out from the Swedish icebreaker Oden in late October 2010 during the 2010/2011 Antarctica expedition organized by the Swedish Polar Research Secretariat as a part of joint Sweden−U.S. Antarctica venture. Surface seawater samples were collected from 12 sites located in the southern North Sea (Figure 1). Each 500 mL sample was instantly filtered on boat through a 0.45 μm membrane (Sartorius AG, Gottingen, Germany) and sealed in a clean polyethylene Nansen bottle (Hydro Bios) under cold and dark conditions until analysis. Measurement of other parameters such as salinity, temperature and wind speed were also conducted along the transit. A change in 129I concentration during a short period of storage was proved insignificant elsewhere.22 Procedures for the separation of iodine (129I and 127I) species were based on anion-exchange chromatography method.22 All chemical reagents used were of analytical grade and all solutions were prepared using deionized water (18.2 MΩ·cm−1). Determination of 127I and 129I were carried out using inductively coupled plasma mass spectrometry (ICP-MS) and accelerator mass spectrometry (AMS), respectively. Details of the separation and measurement techniques are documented in the SI. Inventory estimation is based on 2005 data using a two layer model. 129I distribution is established and calculated by ArcGIS Desktop 10.0. For details please see the SI.
Figure 1. Locations of seawater samples collected in November 2010. Topography, bathymetry, and scheme of general water circulation (red arrows) of the North Sea are displayed. The two nuclear fuel reprocessing facilities are highlighted as stars. The green arrows refer to 129I transport pathways from Sellafield.
Variations in a range of 10−7−10−5 have been observed in the isotopic 129I/127I values in seaweeds collected in the English Channel and the Irish Sea.9,10 Even in the coast of the northern North Sea, which is relatively far away from radioactive source waters, the ratio of 129I/127I in seaweed samples was still higher than 10−7 in 1998.11,12 Isotopic ratio of 129I/127I in marine aerosol sampled in southeast of the North Sea in 2002 indicated differences between organic, inorganic and particlebounded species, but all the values were above 10−7.13 To our best knowledge, there are no 129I data in published literatures for precipitation in the open North Sea. However, investigated rainwater samples in Germany and Denmark show that the 129 127 I/ I values vary from 10−8 to 10−7,14−16 which may be considered as a range in precipitation over the North Sea. Similar range was also observed in upper soil samples around coastal areas in the vicinity of the North Sea.17−19 129I/127I levels in those environmental and biological samples mentioned above are several orders of magnitude higher than values reported in samples prior to human nuclear activity era (10−8 (>1010 atoms/L for 129I concentration) in the central North Sea during the early 1990s. 6 Michel et al. 13 deployed a comprehensive survey covering the entire North Sea region in 2005. Their results of 129I/127I of up to 10−6 demonstrated an accelerated accumulation of 129I in the North Sea waters that responded to a huge amount of discharges from Sellafield and La Hague after 1994 (Supporting Information (SI) Figure S-1). Furthermore, species analysis of 129I in the North Sea waters has been applied by Hou et al.21 for investigating their
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RESULTS Distribution of 127I and Its Species. Speciation of 127I in the North Sea surface water are shown in Figure 2a and presented in SI Table S-1. The concentration of 127I ranges from 0.31 to 0.41 μM with an average of 0.37 μM (47 μg/kg). Maximum 127I appears in the Southern Bight, whereas a distinct lowest iodine concentration occurred off the coast of the West Frisian Islands, Netherlands. The mean concentration of iodate is 0.26 μM (33 μg/kg), which accounts for 70% of the total 127I. Iodate concentrations generally follow the same pattern as that of total iodine, decreasing to the lowest level of 0.20 μM from the German Bight to the Dutch coast, and then increasing up to 0.35 μM in the southern sits. Significant quantities of iodide ranging between 0.06 and 0.18 can be found in the surface waters, although the average concentration (0.11 μM, or 14 μg/ kg) is markedly lower than the predominant species, iodate. On the contrary to iodate distribution, highest value of iodide is observed close to the Dutch coast. Distribution of 129I and Its Species. The results of 129I and its species (129I− and 129IO3−) in the North Sea show rather similar patterns, with a significant increase from 54°N to 52°N (Figure 2b and SI Table S-1). The concentrations of 129I indicate a nearly 2 orders of magnitude variation along the transect, ranging from 9.2 × 109 to 2.4 × 1011 atoms/L with an average of 8.15 × 1010 atoms/L. Higher values of over 2 × 1011 atoms/L occurred in the southern samples that are close to the 1420
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atoms/L) and do not exhibit sharp 129I gradient. Furthermore, levels of both 129I and 129I/127I increase from sample 5 and a sharp ascent occurs south of sample 8 (Figure 2c). This feature may be attributed to the fact that the major plume of 129I in this area is transported eastward along the Dutch and German coast, and the turning point is believed to happen between sample 5 and 8. Thus the nearly 90° change of the water current direction from the English Channel outflow, together with recirculation of south North Sea water and incorporation of continental coastal water (Figure 1), suggest a complex water masses mixing between samples 5 and 8. Concentrations of 127I varied to a much lesser extent than 129 I. Most of 127I results are comparable with early North Sea investigations, but generally lower than those reported in any other ocean.13,21,24 This can be probably ascribed to discharge of fresh riverine water from the UK and European continent in the south, as well as brackish water from the Baltic Sea through the Skagerrak in the North. Moreover, abundance of phytoplankton biomass inhabiting this temperate shallow shelf sea, especially in the Southern and German Bights imply that in the North Sea, uptake of iodine by organisms may occur in the euphotic zone.25 A positive correlation between salinity and the 127I (r = 0.62, P < 0.05, SI Figure S-2) was observed in all seawater samples investigated here. This feature also suggests that continuous dilution of saline and high 127I of Atlantic water by continental runoff occurred from south to the north. The lowest iodine concentration was observed in sample 6 that is near to the west Wadden Sea. The Wadden Sea is connected to the artificial lake IJsselmeer, which is mainly fed by fresh water from the Rhine River. Thus this low iodine concentration in sample 6 might demonstrate relatively strong influence by the fresher west Wadden Sea,26 which is also confirmed by lowest salinity and low iodate content in this sample. Sources of Iodide. The marked difference between iodide/ iodate for 129I and 127I (Figure 2d) suggests high proportion of 129 I−iodide in the source water. This feature indicates a disequilibrium behavior for the species of 129I in the North Sea. Considerable variation of iodide/iodate values for 129I was observed compared with that for 127I. Furthermore, all 129 − 129 I / IO3− values analyzed here are higher than that of 127 − 127 I / IO3− and the differences become even larger in the northern samples (samples 1−8). This may attribute to either the transport of water with high 129I− concentrations from coastal areas, or the reduction of 129IO3− in the North Sea as the English Channel water parcels move northward. Despite the positive correlation between total 127I and salinity (SI Figure S-2), the insignificant correlation between iodide and salinity indicates that part of the iodate is locally reduced. The 127 − 127 I / IO3− values in surface water sampled in 2010 are higher than those found in open seas,24,27 though there are no data for comparison of 129I−/129IO3−. Therefore, a week reductive environment may occur in the southeastern North Sea, possibly mediated by bioactivities. The previous investigation21 suggests a rather sluggish iodine redox process in the open sea. Thus, local reduction of 129IO3− cannot fully explain the remarkable increase of 129I−/129IO3− in the transect presented here, especially in the region between samples 8 and 9, where a nearly 2-fold increase occurred. Hou et.al.21 reported a rapid reduction of 129IO3− in the continental coastal areas and the German Bight. They related the 129 − 129 I / IO3− high values (up to 50) in the estuary of Elbe
Figure 2. Variations of, (a) 127I concentration and species, (b) 129I concentrations and species, (c) ratio of 129I/127I and species, and (d) iodide/iodate, along the sampled transect.
Dover Strait, whereas the low concentrations, around 1 × 1010 atoms/L, can be observed in the northern part of the transect, relatively distant from the English Channel. The distributions of 129 I species depict a similar behavior with that of total 129I, and the concentrations of 129IO3− (3.2 × 109 to 1.5 × 1011 atoms/ L) show a relatively wider range than 129I− (6.9 × 109 to 1.2 × 1011 atoms/L). It is worth mentioning that unlike 127I speciation, iodide is generally the dominant species of 129I in most samples, except the two in the south. Distribution of 129I/127I. Similar to the pattern of 129I, 129 127 I/ I increase by almost 2 orders of magnitude from north to south, and vary between 4.4 × 10−8 and 1.0 × 10−6 with an average of 3.5 × 10−7 (Figure 2c and SI Table S-1). For species of 129I/127I, the average level of 129I−/127I− and 129IO3−/127IO3− are 5.9 × 10−7 and 2.7 × 10−7, respectively. The ratios of 129 127 I/ I for iodide along the transect are all higher than that of iodate, even in the southern samples which have relatively high 129 I−iodate concentration.
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DISCUSSION Iodine (127I and 129I) in 2010. Relatively isolation from the open sea and proximity to the major European nuclear reprocessing facilities, as well as the specific northward ocean currents system, make the North Sea one of the most 129Icontaminated areas in the world. Both 129I concentration and 129 127 I/ I ratios are 5−6 orders of magnitude higher than preanthropogenic levels, and 1−4 orders of magnitude higher than values reported in any other sea or ocean,23 except the values in the English Channel. Extremely high 129I concentrations in the Southern Bight were clearly linked to the English Channel water (one branch of the North Atlantic Water), which is mixed with the North Sea water of relatively low 129I. Strong dilution continues as the water mass moves northward along the continental coast in a short distance, where a nearly 20-fold decrease appeared in the German Bight. Concentrations of 129I in the German Bight seem rather constant (∼1 × 1010 1421
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Figure 3. Comparison of, (a) total iodine, (b) iodide, (c) iodate, (d) 129I, (e) 129I−, (f) 129IO3−, (g) 129I/127I, (h) 127I−/127IO3−, and (i) 129I−/129IO3−, between the 2005 and 2010 expeditions.
shown in SI Figure S-3. In order to examine five year variations compared with our 2010 sampling, total 129I and 127I concentrations are interpolated to the corresponding 2010 locations from Michel et al.13 The species of 129I and 127I in the same sites are interpolated using the data of Hou et al.21 The results of iodine isotopes (127I and 129I) and species (iodide and iodate) in the North Sea between 2005 and 2010 are shown in Figure 3. In general, concentrations of 127I, iodide and iodate in 2010 show large fluctuations. Compared to 2005, 127I concentrations were low in north of 53°N and generally high in the south. Speciation analysis exposed that the level of iodide in 2010 was comparable or higher than 2005 in samples south of 53°N except sample 8, which exhibited much lower 127 − 127 I / IO3− value (0.16, Figure 3h). The reason of low iodide and high iodate occurring in this sample is unclear, but may relate to the episodic strong Atlantic water intrusion and/or fast local iodide oxidation in 2010. Unlike 127I, concentrations of 129I in the northern samples in 2010 were 2−8 times lower than that in 2005, and the discrepancy increased toward the north (Figure 3d). Only in the most southern three samples, which are close to the Dover Strait, the 129I concentrations were comparable with those in 2005. The same pattern also appears for the 129I species (129I− and 129IO3−) and 129I/127I values (Figure 3e, f, g). Due to the
River to the combined effects of chemical and biological processes. Our samples investigated here were generally located in the open North Sea and distant from the coastline. Therefore, a significant part of 129I− found in our northern samples might be associated with diffusion and transportation of 129I-rich water that originated from coastal areas, particularly from hypoxic German Bight. The differences between 129I/127I isotopic ratios for iodide and iodate are significantly larger in the southern samples than those in the north. This feature also reflects strong water mixing events between the continental coastal water and the southern North Sea water. Relatively low 129 − 129 I / IO3− in sample 5 compared to the surrounding areas implies a lesser influence from the German Bight water parcel. The west Wadden Sea is also a shallow area with estuarine properties characterized by relatively low salinity and large loading of organic matter. Additionally, very low oxygen saturation on its tidal mudflat was reported and seasonal episodic oxygen deficiency occurred in its interior basin.28,29 Thus the western Wadden Sea might be another potential source of newly produced 129I−iodide under anoxic conditions and a further investigation of 129I species within this region is needed to confirm this finding. Comparison With 2005 Data. Data of 129I distribution in the North Sea in samples collected in August of 2005 are 1422
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relatively lower discharge of 129I and specific radionuclide transport route, the influence of 129I from Sellafield water within the interior of the North Sea, particularly the southern portion, is insignificant. Furthermore, compared to high 129I in the English Channel, low 129I concentrations along the British east coast endorse marine discharges from La Hague as the dominant source in the investigated region.21,30 No significant variation of 129I liquid discharge from La Hague occurs in the annual record between 2005 and 2010 (SI Figure S-1). This is also established by the comparable 129I concentrations in the most southern samples (samples 10−12) during the five years. Therefore, the large differences in 129I and its species between 2005 and 2010 in samples 1−9, seem to be strongly influenced by water masses mixing and modification rather than variation in the discharges from La Hague. Iodine existed in seawater mainly as iodate, iodide and a minor of dissolved organic iodine (DOI). According to 2005 investigation, concentration of DOI in the North Sea is 90% was accumulated in 1423
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the upper layer. In the Norwegian Trench, the 129I inventory is comparable in surface and deep layers. The inventory estimation involves contributions from both atmospheric and marine releases from the nuclear reprocessing facilities. However, atmospheric releases of 129I accounts for 4− 5% of total release from the Sellafield and La Hague until 2005 and these percentages were even decreased to
