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Digital Comprehensive Summaries of Uppsala Dissertations
from the Faculty of Science and Technology 1068
Iodine Isotopes and their
Species in Surface Water from
the North Sea to the
Northeastern Atlantic Ocean
PENG HE
ACTA
UNIVERSITATIS
UPSALIENSIS
UPPSALA
2013
ISSN 1651-6214
ISBN 978-91-554-8743-0
urn:nbn:se:uu:diva-206711
Dissertation presented at Uppsala University to be publicly examined in Axel Hambergsalen,
Villavägen 16, Uppsala, Friday, October 18, 2013 at 13:15 for the degree of Doctor of
Philosophy. The examination will be conducted in English.
Abstract
He, P. 2013. Iodine Isotopes and their Species in Surface Water from the North Sea to
the Northeastern Atlantic Ocean. Acta Universitatis Upsaliensis. Digital Comprehensive
Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1068. 73 pp.
Uppsala. ISBN 978-91-554-8743-0.
Huge amounts of anthropogenic 129I have been and still are released to the environment through
liquid and gaseous discharges from the nuclear fuel reprocessing facilities worldwide and in
particular the ones in Europe. Most of this 129I signal has been accumulated in the marine
environment which plays a major role in the iodine natural pool. In this thesis, an overview
of available 129I concentrations in waters of the oceans and marginal seas together with new
data about 129I and 127I spatial distribution in surface seawater along a transect between the
North Sea and the northeastern Atlantic Ocean are presented. After comprehensive chemical
separation, the concentrations of iodine isotopes (127I and 129I) and their species (iodide and
iodate) were analysed using accelerator mass spectrometry and inductively-coupled plasma
mass spectrometry. The results show that, generally, changes in the 127I and 127I-/127IO3- are
comparable to data from other marine waters which are related to natural distribution patterns.
A considerable variation of 129I along the transect is observed with the highest values occurring
in the eastern English Channel and relatively low values obtained in the northeastern Atlantic
Ocean. Inventory estimations of 129I in the North Sea and the English Channel are 147 kg
and 78 kg, respectively, where more than 90% resides in the Southern Bight and the eastern
English Channel. Iodate is the dominant iodine species for both 127I and 129I in most seawater
samples from the North Sea to the Atlantic Ocean. 129I species variability suggests a slow
process of 129I- oxidation in the open sea. It takes at least 10 years for the 129I-/129IO3- pair to
reach their natural equilibrium as the water is transported from the English Channel. The results
suggest a main transport of 129I from the western English Channel via the Biscay Bay into the
northeastern Atlantic Ocean. Further, high 129I/127I and distinctive 129I-/129IO3- values south of 40°N
indicate possible contribution of 129I through Mediterranean Outflow Water. The environmental
radioactive impact of 129I and possible applications in ecosystem studies are also discussed.
Keywords: iodine isotopes, 129I, 127I, oceans, North Sea, speciation, Atlantic Ocean, English
Channel, Celtic Sea, AMS, iodine chemistry, geochemistry
Peng He, Uppsala University, Department of Earth Sciences, LUVAL, Villav. 16,
SE-752 36 Uppsala, Sweden.
© Peng He 2013
ISSN 1651-6214
ISBN 978-91-554-8743-0
urn:nbn:se:uu:diva-206711 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-206711)
In memory of my Grandma Zhang,
Dedicated to my parents
List of Papers
This thesis is based on the following papers, which are referred to in the text
by their Roman numerals.
I
II
III
IV
V
He, P., Aldahan, A., Possnert, G., Hou, X. L. (2013) A summary of global 129I in marine waters. Nuclear Instruments and
Methods in Physics Research Section B: Beam Interactions with
Materials and Atoms, 294:537–541.
He, P., Aldahan, A., Possnert, G., Hou, X. L. (2013) Temporal
variation of iodine isotopes in the North Sea. Environmental
Science & Technology, under review.
He, P., Aldahan, A., Possnert, G., Hou, X. L. (2013) Iodine isotopes in the English Channel: a source region of 129I. Manuscript.
He, P., Aldahan, A., Hou, X. L., Possnert, G. (2013) Radioactive 129I in surface water of the Celtic Sea. Journal of Radioanalytical and Nuclear Chemistry, in press.
He, P., Aldahan, A., Hou, X. L., Possnert, G. (2013) Iodine isotopes species fingerprinting environmental conditions in surface
water along the northeastern Atlantic Ocean. Scientific Reports,
in press.
Reprints were made with permission from the respective publishers.
I have written first draft of all papers, with other authors contributing to further interpretations and discussions. I made all the sampling and chemical
preparation. I also performed data evaluation, calculation and modeling.
Contents
1. Introduction ................................................................................................. 9
1.1 General background .......................................................................... 9
1.2 Objectives of this thesis .................................................................. 11
1.3 Stable iodine (127I) and its sources .................................................. 11
1.4 Radioactive iodine (129I) and its sources ......................................... 12
1.5 Applications of 129I and its species .................................................. 15
2. A short review of iodine (127I and 129I) and its species in natural surface
environments ................................................................................................. 18
2.1 127I and 129I in fresh water (rivers and lakes) ...................................... 18
2.2 127I and 129I in seaweed ....................................................................... 19
2.3 127I and 129I in the atmosphere and precipitation................................. 21
2.4 127I and 129I in soil and sediment ......................................................... 22
2.5 127I and 129I in oceans .......................................................................... 23
3. Sampling sites and analytical techniques .................................................. 25
3.1 Sampling sites .................................................................................... 25
3.1.1 2010/2011 Southern Ocean expedition ....................................... 25
3.1.2 The North Sea samples ............................................................... 26
3.1.3 English Channel and Celtic Sea samples .................................... 27
3.1.4 The northeastern Atlantic Ocean samples .................................. 27
3.2 Measurement technique of iodine (127I and 129I) from seawater ......... 30
3.2.1 Sample preparation and chemical reagents ................................. 30
3.2.2 Separation of iodide and iodate from seawater ........................... 30
3.2.3 Extraction of iodine species (127I and 129I) .................................. 30
3.2.4 ICP-MS determination of 127I ..................................................... 32
3.2.5 AMS determination of 129I .......................................................... 32
4. Results and discussion .............................................................................. 33
4.1 Distribution of 129I in global oceans ................................................... 34
4.2 Variations of iodine (127I and 129I) and its species along the 2010
transect ..................................................................................................... 36
4.2.1 Iodine isotopes species in the North Sea .................................... 36
4.2.2 Iodine isotopes species in the English Channel .......................... 38
4.2.3 Iodine isotopes species in the northeastern Atlantic Ocean ........ 41
4.3 Sources of 129I ..................................................................................... 43
4.3.1 Sources of 129I- in the North Sea ................................................. 43
4.3.2 Sources of 129I in the northeastern Atlantic Ocean ..................... 44
4.4 129I inventories in the North Sea and the English Channel ................. 46
4.5 Transformation among iodine species in seawater ............................. 48
5. Environmental impact of 129I .................................................................... 51
6. Conclusions ............................................................................................... 53
7. Acknowledgements ................................................................................... 55
8. Summary in Swedish ................................................................................ 57
9. References ................................................................................................. 60
1. Introduction
1.1 General background
Iodine is a trace element which ubiquitously exists in the Earth’s surface
environment (i.e. atmosphere, lithosphere, hydrosphere and biosphere). Due
to its biophilic nature, iodine is easily taken up by organisms and therefore
tends to be enriched in organic matters (Vinogradov, 1953). Iodine is found
in high concentrations in marine seaweed and in some brown algae, the concentration by dry weight measurement can reach > 10-3 g·g-1 (Hou and Yan,
1998), while in terrestrial plants, the concentrations are normally lower
(Fuge and Johnson, 1986). As a nutrient element, iodine is indispensable and
essential to all mammals. Many trace elements in the environment have a
close relation to the human health and nutrition. Linkage between iodine
deficiency disorders (IDD) such as goiter and intake amount of iodine in
human daily diet have evoked enormous attentions to this element. In addition, recent studies on atmospheric chemistry showed that iodine plays a
significant role in the depletion of ozone and in aerosol particles for cloud
nucleation, which in turn have a direct impact on global climate change
(Solomon et al., 1994; O'Dowd et al., 2002; von Glasow, 2006).
Although iodine has long been recognized as an important environmental
element, data on distribution of iodine in natural environments were generalized and poorly described in early studies. As one of the most important
reservoirs, the oceans contain more than 70% of the mobile iodine in the
Earth’s surface environment (Wong, 1991; Alfimov, 2005). Iodine in some
sparse seawater samples collected in the Atlantic, Pacific, Arctic and Indian
Oceans, as well as the Mediterranean and Red Seas, were discussed in the
early decades of the last century (Reith, 1930; Barkley and Thompson, 1960).
These studies reported a rather constant concentration of total iodine in
oceans, but varying proportions of iodate with location and depth. The first
extensive survey of iodine and its speciation in the whole Pacific Ocean was
conducted by a Japanese group. They found a latitudinal variation of iodide
in the surface of the Pacific seawaters and they related this feature to the
high biological productivity. In the meantime, they also observed elevated
iodide concentration in bottom waters (Tsunogai, 1971; Tsunogai and Henmi,
1971). Later, a systematic investigation of iodine concentration in near surface waters was deployed along a transect across the Atlantic Ocean, which
presented a rather similar feature in terms of meridional distribution of io9
dine (including its species) as that observed in the Pacific Ocean (Truesdale
et al., 2000).
The only long-lived radioactive isotope of iodine, 129I (T1/2 = 15.7 Myr), is
mainly introduced into the environment anthropogenically through nuclear
activities that started in the 1940s. As a result, the 129I/127I was increased to at
least two orders of magnitude higher than the pre-nuclear era globally. Because of its long half-life and low radiation energy, 129I was suggested as a
potential tracer of iodine geochemistry (Edwards, 1962). Thereafter some
environmental samples were collected for 129I analysis (Oliver et al., 1982;
(Fabryka-Martin et al., 1987). The Chernobyl accident, one of the most serious nuclear accidents, occurred on 26 April 1986, and released many types
of anthropogenic radioactive isotopes including 129I into the environment.
Thus measurements of some soil, rainwater and animal thyroid samples were
reported in order to assess the impact of iodine isotopes (129I and 131I) in the
vicinity environment of the Chernobyl region (Paul et al., 1987b; Kutschera
et al., 1988; VanMiddlesworth and Handl, 1997; Hou et al., 2003; Michel et
al., 2005). Release of 129I from the recent Japan Fukushima Dai-ichi nuclear
accident on 11 March 2011was estimated by Hou et al. (Hou et al., 2013).
Awareness of potential environmental hazards as a result of huge 129I releases from the nuclear fuel reprocessing facilities (NRFs) led to extensive
data collection in the surrounding areas of major NRFs such as Hanford
(USA), Tokaimura (Japan) (Muramatsu and Ohmomo, 1986), Karlsruhe
(WAK, Germany) (Robens et al., 1988), Sellafield (UK), La Hrgue (France)
(Edmonds et al., 1998) and Russian facilities (e.g. Mayak, Seversk and
Zheeleznogorsk). Estimation of early marine discharges of 129I from La
Hague and Sellafield was evaluated by Raisbeck et al. based on times series
analysis of 129I/127I in seaweeds (Raisbeck et al., 1995). Since then, many
studies have been carried out using 129I as an oceanographic tracer in many
129
I-contaminated regions (e.g. English Channel, Irish Sea, North Sea, Arctic
Ocean, etc.) (Beasley et al., 1998; Cooper et al., 1998; Buraglio et al., 1999;
Smith et al., 1999; Hou et al., 2000a; Cooper et al., 2001; Edmonds et al.,
2001; Alfimov et al., 2004b; Alfimov et al., 2004c; Smith et al., 2005; Hou
et al., 2007; Schnabel et al., 2007; Orre et al., 2010; Hansen et al., 2011b;
Smith et al., 2011; Yi et al., 2012b; Yi et al., 2013).
10
1.2 Objectives of this thesis
The use of chemical tracers (natural and anthropogenic) has expanded our
understanding of circulation patterns and ventilation rates in the oceans.
Numerous studies have, however, shown that we are still far away from
complete understanding of most oceanic processes and further development
in tracer technology can, without doubt, substantially increase our
knowledge.
Investigations of 129I geochemical behavior have been carried out by the
Uppsala group for years. These studies were mainly focused on 129I in the
marine environment at high latitudes in the Northern Hemisphere, including
the Arctic Ocean (Buraglio et al., 1999), the Nordic Seas (Alfimov et al.,
2004) and the Baltic Sea (Yi et al., 2010). In the thesis presented here, a
focus is given on exploring distribution of iodine isotopes (127I and 129I) and
their species (iodide and iodate) along a transect across the North Sea passing by the English Channel and into the northeastern Atlantic Ocean. The
objectives of this thesis are:
1. Reviewing of the global distribution of 129I in the oceans and identifying regional variability.
2. Revealing temporal evolution of 129I and its species in surface water of the North Sea in comparison to earlier data.
3. Documenting distribution and sources of iodine isotopes (129I and
127
I) and their species (iodide and iodate) in the surface waters of
the English Channel, the Celtic Sea and the northeastern Atlantic
Ocean.
4. Exploring the potential of 129I species inter-conversion as a tracer
of surface water sources in the northeastern Atlantic.
5. Assessing environmental impact and future effects on the marine
ecological systems.
Before going into details of the thesis work, a summary of iodine chemistry and distribution in the environment is given below.
1.3 Stable iodine (127I) and its sources
Iodine is a redox sensitive element with oxidation states of -1, 0, +1, +3, +5
and +7. There are 37 known isotopes of iodine in which 127I is the only stable
one. Although iodine is the least abundant halogen element, it is widespread
in Earth’s crust with an average abundance of 0.45 mg/kg (Hou et al.,
2009b). Most iodine is found in igneous and sedimentary rocks, though in
low concentrations (0.24-4 mg/kg) (Fuge and Johnson, 1986). However, a
major part of iodine is bound in rocks, sediments and soils. Only a small
11
amount occurs in the marine pool and the global cycle (Fuge and Johnson,
1986; Moreda-Pineiro et al., 2011).
The ocean is the major mobile iodine pool that interacts with land, atmosphere and biosphere. Some studies suggested that the main mode of iodine
transport from sea to land is through wet (precipitation) deposition
(Truesdalea and Jones, 1996; López-Gutiérrez et al., 2001; Reithmeier et al.,
2010). Therefore it seems reasonable to draw a general pattern of decreasing
iodine content with increased distance from oceans, which coincides with the
prevalence of IDD (Selinus, 2005). However, this hypothesis remains doubtful since some counterexamples have been reported (Krupp and Aumann,
1999). Bearing in mind that iodine is easily incorporated in organic matters
and trapped by soils and sediment, distribution of iodine on land is thus also
affected by the type of soils (Gerzabek et al., 1999; Yoshida, 1999). In addition, iodine may be derived from erosion of bedrock (Hou et al., 2009b).
Emission of iodocarbons from the ocean surface provides a major source
of atmospheric iodine which eventually contributes to reactive iodine in the
marine boundary layer (MBL). Methyl iodide (CH3I) is considered to be the
principal species while recently, high concentration of molecular iodine (I2)
was reported in a coastal marine environment (Chatfield and Crutzen, 1990;
Saiz-Lopez et al., 2006; Küpper et al., 2008). Accordingly, microalgae (phytoplankton) and macroalgae (seaweeds) appear to play a dominant role in the
open sea and coastal areas, respectively (Carpenter, 2003). On land, large
iodine emissions from wetlands, vegetation and soils are most important
(Sive et al., 2007).
Some anthropogenic sources of iodine are related to agricultural and industrial activities. About 5% of methyl iodide in the atmosphere is estimated
to be derived from emission of rice paddies (Redeker et al., 2000). The use
of soil fumigant in pest control may also add iodine into the environment
(Aldahan et al., 2009). Combustion of fossil fuel provides another important
source of iodine to the atmosphere duo to relatively high average concentrations of iodine in coal and petroleum, 4 mg/kg and 1 mg/kg respectively
(Block and Dams, 1974; Valkovic, 1978). Additionally, application of iodine
as a disinfectant makes municipal wastewater treatment plants an important
source of iodine in surface waters (Moreda-Pineiro et al., 2011). However,
the contribution of iodine from anthropogenic sources inland is much less
when compared to natural sources of marine origin and therefore, could be
considered negligible (Chameides and Davis, 1980; Carpenter, 2003).
1.4 Radioactive iodine (129I) and its sources
The only naturally occurring long-lived (T1/2 = 15.7 Myr) radioisotope of
iodine, 129I, which is a beta emitter with relatively low energy radiation
(154.4 keV, Eβmax), is an important soluble fission product in spent nuclear
12
fuel. 129I can be naturally produced by spallation of xenon (Xe) induced by
interaction with cosmic rays in the upper atmosphere, as well as from spontaneous fission of 238U and thermal neutron-induced fission of 235U inside the
Earth (Snyder et al., 2010). Neutron bombardment (130Te) and neutron caption (128Te) of tellurium isotopes are, to a lesser extent, another natural
source of 129I (Hou et al., 2009b). The collective 129I natural flux contributes
to a 129I/127I ratio of 10-13 to 10-12 prior to human nuclear activities (Broecker
and Peng, 1982; Fabryka-Martin et al., 1985; Fehn, 1986; Moran et al., 1998;
Fehn et al., 2007) and the total natural inventory of 129I is therefore estimated
to be 100-260 kg (Fabryka-Martin et al., 1985; Raisbeck and Yiou, 1999;
Snyder et al., 2010), which is insignificant when compared to the anthropogenic sources (Figure 1.1).
Since the start of the nuclear era in the 1940s, 129I has been overwhelmingly introduced into the environment by a variety of anthropogenic sources,
including: 1) aboveground nuclear weapons tests, 2) nuclear accidents, 3)
discharges from nuclear fuel reprocessing plants and possibly 4) the nuclear
power plants during routine operation. Aboveground nuclear weapons testing, which peaked in the 1960s, has released various radionuclides into the
stratosphere, thus eventually contributing to a global fallout, particularly in
the Northern Hemisphere (Wilkins, 1989; Snyder and Fehn, 2004;
(Reithmeier et al., 2006). These tests have introduced 57-135 kg of 129I into
the environment (NCRP, 1983; Wagner et al., 1996; Raisbeck and Yiou,
1999). Amounts of 129I released by some nuclear accidents are difficult to
estimate, but it is considered insignificant, since no evidence of any enhanced 129I has been found in surrounding areas of Windscale (10 Oct. 1957)
(Gallagher et al., 2005) and Three Mile Island (28 Mar. 1979) (Moran et al.,
2002b; Gallagher et al., 2005). Estimations of 129I emitted by the most serious nuclear accident ever, the Chernobyl accident (26 Apr. 1986), is between
1.3 and 6 kg, which is even 10 times lower than releases from nuclear weapons testing (Paul et al., 1987; Aldahan et al., 2007b). Comparable releases of
129
I (1.2 kg) was estimated from the recent nuclear accident in Japan (12 Mar.
2011, Fukushima) (Hou et al., 2013). 129I release from the nuclear power
plants is insignificant (Hou et al., 2002; He et al., 2010). In addition, because
of the highly mobile and soluble nature of iodine, dumping of nuclear radioactive waste could be another potential 129I source to the marine deep layer.
However, previous surveys on radioactive waste dumping sites in the Atlantic and Pacific Oceans did not show any abnormal concentration of 129I that
is above the expected background natural level (Povinec et al., 2000; Cooper
et al., 2001). Thus at present, 129I from radioactive waste dumping source is
considered negligible. It is worth mentioning that an estimated 68 tons of 129I
were produced in routine nuclear power plants operation, mostly waiting for
future reprocessing (Hou et al., 2009b).
13
Figure 1.1: Sources of 129I (% inventory) in the environment based on available
published literatures from (Aldahan et al., 2007; He et al., 2013; Hou et al., 2009b;
Snyder et al., 2010) and references therein.
The function of the nuclear reprocessing facilities (NRFs) is to extract fissionable plutonium (Pu) from irradiated nuclear fuel since the 1940s. Some
of them were decommissioned (e.g. Hanford and Marcoule) and some are
still operating (e.g. Sellafield and La Hague) for commercial use. These
NRFs have discharged more than 95% of total 129I inventory into the Earth’s
surface environments (Figure 1.1). Among them, around 90% of 129I is released from the two major European NRFs Sellafield and La Hague, which
are located in the United Kingdom and France, respectively. Until 2011,
these two NRFs have together discharged 5400 kg and 250 kg 129I directly to
marine environment and the atmosphere, respectively (He et al., 2013;
AREVA; Monitoring our Environment, Sellafield Annual Report). The discharge rate of 129I from these two facilities increased and remained high after
1990 and peaked in 1997, but declined slightly during the past 10 years
(Figure 1.2).
14
Figure 1.2: Annual marine and atmospheric releases from (a) Sellafield (UK) and (b)
La Hague (France) between 1966 and 2011 (literatures refer to the text).
Marcoule (France) is another major European NRF that has released 180 and
75 kg of 129I to the atmosphere and Rhone river, respectively (Hou et al.,
2009b). For comparison, only 1.1 kg of 129I was released from the Karlsruhe
NRF (WAK), which is located in Germany and was decommissioned in
1987 (Robens et al., 1988). The largest gaseous releases of 129I were from the
Hanford NRF (USA), ranging at 300 kg during its operation from 1944 to
1988 (HHIN; Moran et al., 1998). This discharge is even greater than that of
the total amount by Sellafield and La Hague. Furthermore, total gaseous 129I
released from Savannah River NRF and some major former Soviet NRFs
was estimated by an atmospheric transport box model (Reithmeier et al.,
2010). The outcome suggested that about 41 and 215 kg of 129I were emitted
to the atmosphere during their operation times for Savannah River NRF and
former Soviet NRFs (including Mayak, Seversk and Zheleznogorsk) respectively. In Asia, a Japanese NRF located at Tokai, has released about 1.0 kg
of 129I to the environment (JAEA; Shinohara, 2004). Release records of the
other Asian NRFs (i.e. in China and India) are, however, not well documented.
1.5 Applications of 129I and its species
Implementation of the highly sensitive analytical technique of accelerator
mass spectrometry (AMS) allowed determination of ultra-low 129I/127I values
in pre-nuclear era materials. The present detection limit of AMS and the long
half-life of 129I have made the technique a powerful tool in radiometric dating of geochronology within a range of 80 Myr (Nichols Jr et al., 1994; Fehn
et al., 2000).
In environmental sciences, 129I can be used to study the biogeochemical
cycle of stable iodine (Santschi and Schwehr, 2004; Snyder and Fehn, 2004;
Schwehr et al., 2005; Karcher et al.). Duo to high releases from nuclear acci15
dent, 131I is one of the most important and harmful radionuclides to the human health. However, it soon becomes undetectable in the environment due
to its short half-life (T1/2 = 8 days). Therefore 129I can help to reconstruct 131I
dose in order to evaluate 131I deposition patterns after nuclear accidents
(Schmidt et al., 1998; Michel et al., 2005). It is also an important radionuclide to assess the safety of radioactive waste repositories.
In view of the highly soluble and mobile nature of iodine, 129I could be
used as a good tracer in hydrogeological studies. Applications in this field
include the tracking of surface water movement processes from infiltration
to subsurface flow, investigating the sources and migration characteristics of
groundwater, assessment of interactions between surface water and groundwater, as well as estimating age and residence time of groundwater in recharge areas (Fabryka-Martin et al., 1985; Fabryka-Martin et al., 1987;
Lehmann et al., 1993; Moran et al., 1998). The sample volume needed for
129
I analysis is relatively small and the isotope often has a greater magnitude
of anthropogenic signal than any other traditional tracers (e.g. 137Cs, 3H) in
hydrological research (Santschi et al.).
As a highly conservative tracer that maintains a strong and consecutive
signal in seawaters, 129I became a potential tracer in oceanography. Variations of 129I concentration and 129I/127I ratio in oceans clearly reflect movement and pathways of currents, circulation, mixing and exchanges of water
masses, as well as the transport and diffusion patterns of anthropogenic pollutants (Karcher et al., 1998; Smith et al., 1999; Hou et al., 2007; Schnabel et
al., 2007; Orre et al., 2010). Time series analysis of 129I/127I and comparison
of available release functions from specific NRFs, the early discharge records could be established (Raisbeck et al., 1995; Raisbeck and Yiou, 1999;
Gallagher et al., 2005; Reithmeier et al., 2006). In addition to being a retrospective tracer, 129I can also be applied to obtain information of transit time
and transfer factor (TF) of contaminants (Smith et al., 1998; Raisbeck and
Yiou, 1999; Smith et al., 1999; Hou et al., 2000a; Smith et al., 2005; Keogh
et al., 2007; Smith et al., 2011).
Speciation analysis of 129I provides a powerful tool for gaining a detailed
picture of sources and exchange of water masses in the ocean compared to
using total 129I alone. Moreover, speciation of both isotopes offers additional
information by identifying redox rates of I-/IO3- pairs in environmental conditions. The huge discrepancy between iodide/iodate for 127I and 129I reflects
different sources of iodine isotopes. These potentials have been successfully
applied in the North Sea and the Baltic Sea (Hou et al., 2001; Hou et al.,
2007; Hansen et al., 2011b; Yi et al., 2012). Because iodine exhibits different behaviors (i.e. mobility and bioavailability) in terms of species in the
environment, speciation of 129I in soil, atmosphere and biological samples is
therefore helpful in understanding the stable iodine (127I) cycle. However,
determination of the individual species of 129I is difficult and thus speciation
analysis of 129I is mainly focused on the determination of different 129I16
bounded fractions in these samples (Wershofen and Aumann, 1989; Hou et
al., 2000c; Hou et al., 2003).
Continuous release of 129I from NRFs and the tremendous amount of it
(up to 70 tons) that is waiting for reprocessing makes 129I a source of potential environmental hazard in the long run. Therefore, it is important to monitor the levels of 129I in order to evaluate and predict the consequences for
both the environment and humans, since 129I is taken up by organisms and
eventually enters the food chain.
17
2. A short review of iodine (127I and 129I) and
its species in natural surface environments
2.1 127I and 129I in fresh water (rivers and lakes)
Concentrations of 127I in rivers and lakes are not fully investigated and the
available data are scarce, scattered and vary widely (0.01-73 ppb, commonly
at a range of 0.1- 20 ppb) (Fuge and Johnson, 1986; Moran et al., 2002a). As
the main contributor of iodine is expected to be rainfall, concentration of 127I
in non-marine surface waters varies with respect to its locations and local
soil types, with a mean value of 5ppb. Oceanic iodine is delivered to lakes
and rivers through dry/wet deposition and sea-spray. This branch of the iodine cycle tends to lift iodine level in its surrounding fresh water system.
Another source is land-derived, by weathering of soils and rocks (Oktay et
al., 2001). Surface waters with high content of iodine were also reported in
drainage from industrialized and urban rather than rural areas (Whitehead,
1979). Other sources such as vegetation volatilization, degradation of organic matter and groundwater disturbance are, however, regarded as minor
fractions (Moran et al., 2002b). Iodide and dissolved organic iodine (DOI)
are the predominant species in surface fresh waters with generally high iodide concentration in estuaries (Smith and Butler, 1979; Oktay et al., 2001;
Schwehr and Santschi, 2003). However, high iodate was reported in some
lake waters, which contradict traditional points of view (Jones and Truesdale,
1984). Interconversion between iodine species is suggested to be driven by
biological activities in lakes, but this redox process is not pronounced when
compared to some marine waters (Jones and Truesdale, 1984).
Variation of 129I in surface fresh water is even larger than that of 127I, with
the concentrations of 129I spanning from 106 to 1010 atoms/L and 129I/127I
ranges between 10-11 and 10-5 (Snyder et al., 2010). However, the concentration of 129I can be even lower than 106 atoms/L in areas remote from the
anthropogenic effects. High 129I concentrations appeared in fresh water systems of Europe and North America with a typical range of 108-109 atoms/L
(10-7-10-5 for 129I/127I), suggesting that the influence of NRFs is already evident in continental surface water bodies (Beasley et al., 1997; Rao and Fehn,
1999; Cochran et al., 2000; Buraglio et al., 2001b; Moran et al., 2002a;
Keogh et al., 2010). Since a huge amount of 129I has been introduced into the
marine system, oceanic re-emission of 129I could be another important source
18
in terrestrial waters. Positive correlations were found for 129I/127I in lake waters (collected in the English Lake District) as a function of distance from
Sellafield/Irish Sea, which implied a strong influence by both Sellafield and
the Irish Sea (Atarashi-Andoh et al., 2007). Rapid temporal variation of 129I
in English and Irish rivers was a response to the change of gaseous release
from Sellafield NRF, while this change was not pronounced in rivers far
distant from NRFs, such as in Sweden (Buraglio et al., 2001b). Nevertheless,
atmospherically transported 129I from Sellafield NRF was also regarded as a
major source of 129I in Swedish lakes, where concentrations of 129I was up to
1.4×109 atoms/L and a strong latitudinal dependence was observed (Kekli et
al., 2003). Contribution of 129I from the Chernobyl accident seems to be insignificant in relation to elevated 129I level in European lakes, even for those
located in the areas of high Chernobyl fallout (Buraglio et al., 2001a). Similarly, local power plants are also believed to have limited influences (Rao
and Fehn, 1999; Hou et al., 2002). Considering the fact that iodine easily
attaches to suspended particles or binds to organic materials, regional properties such as watershed characteristics (e.g. river flow rate, evapotranspiration rate, catchment areas and vegetation coverage, etc.) and intensity of
agricultural and other anthropogenic activities may be involved in altering
the 129I/127I in rivers (Oktay et al., 2001; Moran et al., 2002b; Kekli et al.,
2003), which can be used to trace organic carbon in estuarine waters
(Schwehr et al., 2005).
Compared to the dramatic 129I value of ~ 1010 atoms/L found in the Savannah River (USA; Moran et al., 2002a), the 129I levels in surface waters of
the southern hemisphere, which is far from the point sources, are 2-4 orders
of magnitude lower (106-108 atoms/L, 129I/127I > 10-11) (Snyder and Fehn,
2004), indicating spread of anthropogenic 129I on a global scale, even in remote Antarctica. This anthropogenic 129I in the southern hemisphere includes
not only bomb fallout, but also NRF signals from the northern hemisphere. It
seems that, however, the distribution of 129I in rivers and lakes is not uniform,
but latitudinal-dependent (Fehn and Snyder, 2000). Higher levels of 129I/127I
in middle latitude of both hemispheres compared to equatorial and polar
regions are probably associated with atmospheric circulation coupled with
precipitation patterns on a global scale. Local ecology and marine productivity are also important factors that affect the terrestrial distribution of 129I/127I
worldwide.
2.2 127I and 129I in seaweed
Iodine can be concentrated in seaweeds with a concentration factor (CF) as
high as 106 in some species (Hou et al., 1997), and thus is an ideal bioindicator that is sensitive to its surrounding environment. Concentration of
iodine in seaweeds is often higher than inland plants (< 10-6 g·g-1)
19
(Rucklidge et al., 1994). This feature might indicate that seaweeds are more
efficient in building up iodine than terrestrial plants, or can be related to
difference of iodine speciation between land surface and seawaters. Iodine
concentration in seaweeds varies remarkably (10-5-10-3 g·g-1 (dry weight))
and it is reported to be species- and seasonal-dependent, but normally at the
level of 10-4 g·g-1 (dry weight) (Hou et al., 1997; Hou and Yan, 1998; Osterc
and Stibilj, 2008). Similarly, iodine speciation in marine algae shows a tremendous difference in relation to species of algae. More than 99% of iodine
in brown algae is water-soluble, while in some green algae it is < 10%. Further, > 60% and < 40% is iodide and organic iodine in the aqueous leachate,
respectively, and the iodate is the least component (< 5%) (Hou et al., 1997).
The iodine of water-insoluble form, is mainly bounded with protein, and a
lesser content with pigment and polyphenol (Hou et al., 2000c). This chemical speciation of iodine in seaweeds is important as it is linked to bioavailability and biological toxicity.
It is suggested that comparable iodine isotopes (129I and 127I) are taken up
and assimilated by algae. Therefore 129I/127I in seaweed is usually used as a
biological indicator that reflects local level of 129I in seawaters. Additionally,
129 127
I/ I in combination with 129I/137Cs and 129I/99Tc are commonly applied to
trace the geochemical cycle of 129I temporally and spatially (Cooper et al.,
1998; Hou et al., 2000a; Nies et al., 2010). Because of the sensitive response
of 129I level in seaweed to 129I release from NRFs, time series of 129I in seaweeds samples were successfully applied as an oceanographic tracer to reconstruct the missing release record of NRFs in early stages of operation.
Furthermore, this approach was coupled with investigations of the ocean
current circulation and estimate of transit times, as well as the initial 129I/127I
ratio prior to anthropogenic nuclear activities (Raisbeck et al., 1995;
Kershaw et al., 1999; Raisbeck and Yiou, 1999; Yiou et al., 2002; Fehn et al.,
2007; Keogh et al., 2007; Hou et al., 2009a).
129
I concentration in seaweeds varies from 108 to 1012 atoms/g (dry weight)
and depends on species and sampling locations, while 129I/127I in marine algae has increased by 3-4 orders of magnitude from ~10-10 to 10-6 in the last
two decades. This increase is mainly due to the increase of 129I discharges
from NRFs since the 1990s. The highest levels were of course found in the
coastal waters of the Irish Sea and the English Channel, where the 129I/127I in
seaweed was >10-5 (Raisbeck et al., 1995; Frechou and Calmet, 2003; Keogh
et al., 2007). Other relatively high levels (10-8-10-7) were reported from
Denmark and the Norwegian coast, which are apparently contaminated by
129
I plumes from the Sellafield and La Hague advected water towards the
north (Hou et al., 2000a; Yiou et al., 2002). In areas remote from the NRFs,
which are less or indirectly influenced by 129I-contaminated marine currents,
the 129I/127I in seaweeds is in the order of 10-9 (Hou et al., 2000a; Keogh et al.,
2007; Osterc and Stibilj, 2008). Extremely low 129I/127I levels of 10-10 or less
were measured in Japan, China and USA, as well as the Beaufort and Bering
20
Seas in the Arctic Ocean. These regions are believed to be less affected by
releases from Sellafield and La Hague thus the 129I/127I in seaweeds are therefore close to the global fallout level (Kilius et al., 1994; Tseng and Chao,
1996; Cooper et al., 1998; Hou et al., 2000b).
2.3 127I and 129I in the atmosphere and precipitation
In the atmosphere, iodine exists as particle/aerosol associated, inorganic
gaseous (e.g. I2, HI, HIO, etc.) and organic gaseous (CH3I, CH2I2, etc) substances. Low concentrations of 129I were found in the atmosphere with an
order of 104-105 atoms/m3 (10-8-10-9 for 129I/127I ratio) (Santos et al., 2005).
The long time needed for 129I and 127I to approach their equilibrium combined with the long time needed to approach their isotopic equilibrium, suggest a different distribution of 129I and 127I speciation in atmosphere
(Wershofen and Aumann, 1989). In general, for both 129I and 127I, gaseous
organic iodine is the predominant form in the atmosphere, followed by the
gaseous inorganic form, while the particle associated iodine content comprises normally < 20% (Wershofen and Aumann, 1989; Hou et al., 2009b;
Michel et al., 2012). Furthermore, for 129I, the closer to the NRFs, the higher
the content of the gaseous organic form, which is not observed for 127I (Hou
et al., 2009b) and photochemical processes were suggested to change the
chemical forms of atmospheric iodine. 129I/127I values in all iodine species
were at a similar level (~ 10-7). Differently, the highest 129I/127I was measured
in particle associated form, and in organic and inorganic gaseous forms,
129 127
I/ I were 3.1 and 1.2×10-7, respectively (Michel et al., 2012). Particle
associated iodine is suggested to play a role in cloud formation. High IO3-/Iwas found in oceanic-derived aerosols (1.5-3.0), which was attributed to the
accumulation of iodate, as well as to the depletion of iodide depending on
aerosol pH (I- + HIO + H+ = IX↑ + H2O) (Vogt et al., 1999; Baker, 2004).
However, the rate of this reaction, as well as the formation of iodate was
suggested to be slow since a significant proportion of iodide was observed in
precipitation (Baker et al., 2001).
Available data on concentration of 129I in precipitation varies temporally
and spatially (~ 107-1010 atoms/L) while 127I is normally around 1-2 μg/L. In
rainwater, iodide is the major specie for 129I while 127I mainly exists as iodate
(Hou et al., 2009a; Lehto et al., 2012). In general, concentrations of 129I in
rain waters were observed to be higher in coastal areas compared to inland,
and are strongly determined by whether the air masses have passed the NRFs
areas before precipitation (Bachhuber and Bunzl, 1992; Aldahan et al., 2009;
Keogh et al., 2010). In addition, it seems that the concentration of 129I could
be continuously lowered by washing out during a rainfall event (LópezGutiérrez et al., 2001; López-Gutiérrez et al., 2004). Extremely high 129I
concentrations of > 1010 atoms/L were found in Germany and Ireland, im21
plying a significant influence by atmospheric release of 129I from NRFs,
nuclear accidents, as well as the influence from highly 129I-contaminated
ocean water (Paul et al., 1987; Krupp and Aumann, 1999b; Keogh et al.,
2010). It is suggested that, re-emission of 129I from the ocean surface is the
major source that contributed to 129I of rainwater in the European continental
coast areas (Hou et al., 2009a).
In most areas of Europe, 129I in precipitation is in the order of 108-109 atoms/L (~ 10-7 for 129I/129I) without any specific correlation to latitude
(Buraglio et al., 2001b; Persson et al., 2007; Reithmeier et al., 2005), implying a large coverage of NRFs influences. Moreover, 129I released into the
atmosphere from NRFs has enough time to travel over long distances based
on a suggested residence time of 10-18 days in the atmosphere (Duce et al.,
1963), and thus provides a significant source of 129I in regions far away from
those directly influenced by Sellafield and La Hague. Rainwater samples
collected from USA demonstrate the possible influence from Europe, although the concentrations were 1-2 orders of magnitude lower (Oliver et al.,
1982; Moran et al., 1999; Rao and Fehn, 1999).
2.4 127I and 129I in soil and sediment
Iodine in soil mainly originates from the sea after wet/dry deposition of atmospheric iodine, or by weathering of rocks. Concentrations of iodine in
soils are suggested to decrease with increasing distance from the coast, although some controversies about this issue exist (Fuge and Johnson, 1986;
Moran et al., 2002a). Iodine concentration in European surface soil varies at
1-10 μg·g-1 which is affected by rock/soil types, hydrological conditions and
land use practices. Eh/pH conditions may play a major role in iodine distribution since iodide tend to volatilize in acid soils, while iodate is more stable
in alkaline soils (Rucklidge et al., 1994).
Soil is the largest 129I reservoir of the terrestrial environment. Most 127I resides in the top 0-50 cm depth, and 129I is primarily retained within the upper
10 cm at the order of 107-1010 atoms/g (10-9-10-7 for 129I/129I) in Europe
(Ernst et al., 2003; Michel et al., 2005; Hansen et al., 2011). Because of the
injection of anthropogenic 129I from major NRFs, surface soils in most adjacent areas are contaminated (Muramatsu and Ohmomo, 1986; Rao and Fehn,
1999). High levels of 129I were observed in many samples of European soil
which is influenced by Sellafield and La Hague. Besides, sources of Chernobyl fallout is evident in surface soils collected from Ukraine, Belarus and
Russia (Hou et al., 2003; Michel et al., 2005). Soil depth profiles show different depth dependencies of the 127I and 129I concentrations and exponential
decrease was observed for both isotopes, though the slope of 129I was much
steeper in upper 25 cm (Ernst et al., 2003).
22
127
I in the top part of sediments is generally 5-10 times higher than that in
soils (> 10 μg·g-1) (Englund et al., 2008; Hansen et al., 2011; Osterc and
Stibilj, 2012; Qiao et al., 2012). 129I/127I in lake sediments and marine sediments varies considerably and can provide a time marker for 129I emissions
and depositions (Moran et al., 1998; Gallagher et al., 2005). Inventories of
iodine in sediments varies a lot depending on sorption coefficients, dynamic
sedimentation and erosion processes (Snyder et al., 2010). In many 129Icontaminated seas, accumulated 129I was found in the top 5-10 cm of marine
sediments in the range of 109-1011 atoms/g (Wilkins, 1989; López-Gutiérrez
et al., 2004a; Aldahan et al., 2007b). However, in marine sediments collected
off the USA coast that are remote from European NRFs, the 129I/127I is in
good agreement with the pre-nuclear era natural level (Fehn, 1986; Moran et
al., 1998).
Sequential extraction is widely used in fractionation analysis of iodine in
soil and sediment samples and a similar distribution of 129I in soil and sediment is observed (Hou et al., 2003). Generally, around 80% of 129I is bound
to oxides and organic matter, and less than 10% of soil leachate are water
soluble and exchangeable fractions (Hou et al., 2003; Hansen et al., 2011;
Qiao et al., 2012). An exception was reported in soil collected in Germany,
where ~ 50% of readily available 129I form was observed, which was attributed to specific soil properties and chemical speciation of deposited 129I
(Schmitz and Aumann, 1995). In the organic fraction, 129I was mainly associated with humic and fulvic acids. In addition, it is suggested that the anoxic/oxic conditions may influence the mobility of soil iodine (Hansen et al.,
2011) as well as the iodine speciation in the water soluble fraction (Yuita,
1992). For 127I, though the major fraction is bound to oxides and organic
matter, the composition is different when compared with 129I because a significant fraction of 127I is not extractable.
2.5 127I and 129I in oceans
The ocean is the major iodine reservoir and concentration of 127I in seawater
is generally rather uniform (~ 60 ppb). The initial natural ratio of 129I/127I
introduced into the ocean is estimated to be 1.5×10-12 without spatial difference, in theory, due to much longer residence time of iodine (30,000 yr) than
seawater turnover time (1000 yr) (Broecker and Peng, 1982; Fehn et al.,
2007). However, because of human nuclear activities since the 1940s, more
than 6000 kg of 129I have been introduced into the environment, leading to
dramatic increase of 129I in the world oceans. Nuclear fuel reprocessing is
regarded as the major anthropogenic iodine contributor and releases from
those facilities dominate the 129I inventory nowadays. Sellafield (UK) and La
Hague (France) are the main point sources of 129I, and together they account
for 90% of total global emissions. The temporal and spatial variability of
23
anthropogenic 129I is strongly linked to the major point sources in the Irish
Sea and the English Channel and the global marine spreading pathways are
partly outlined from these sources. However, most of the available 129I data
are assembled from small territories and there is no comprehensive picture
about the magnitude of 129I distribution in different parts of the world oceans
or with respect to water depth (Schink et al., 1995; Carmack et al., 1997;
Beasley et al., 1998; Buraglio et al., 1999; Cooper et al., 2001; Smith et al.,
2005; Suzuki et al., 2008; He et al., 2010; Povinec et al., 2010). A review of
the 129I variability in the world oceans is compiled in paper I of this thesis
and is presented in section 4.1.
24
3. Sampling sites and analytical techniques
3.1 Sampling sites
3.1.1 2010/2011 Southern Ocean expedition
The 2010/2011 Antarctica two-ship expedition was an international scientific cruise jointly funded by the Swedish Polar Research Secretariat and the
US National Science Foundation (NSF). This Antarctica scientific expedition was devoted to studies of physical and chemical oceanography, sea ice,
the carbon cycle, the dynamics of Amundsen Sea Polynya, and ecosystems.
Investigation of iodine was one of the expedition’s projects and was conducted through a joint cooperation between the Uppsala group (Department
of Earth Sciences and The Tandem Laboratory) and the Danish group at the
Center for Nuclear Technologies, Technical University of Denmark, Risø
Campus. The aims of the project are focused on the distribution of the iodine
isotopes (129I and 127I) and species of iodine (I- and IO3-) along the expedition
transect. During October-December 2010, the Swedish icebreaker Oden
sailed from Landskrona (Sweden) to Punta Arenas (Chile) via the North Sea,
the English Channel and the Atlantic Ocean (Figure 3.1). Surface water
samples along the first transect of the expedition (North Sea - Canary Islands)
represent the material for this thesis.
Surface water samples were continuously collected through the Teflon
pipes at the Oden water lab. The water samples were planned for analysis of
tracer elements, such as iodine, uranium and cesium. Meanwhile, other sampling devices (i.e. Denuder sampler, PM 2.5 sampler, etc.) were set up
onboard to collect air samples along the cruise from Sweden to South America, which were aimed for the investigation on organic and inorganic halogen species in the marine boundary layer (MBL). Further, a standard CTD
sampler was launched to automatically measure water temperature and salinity along the transect. Meteorological parameters such as wind velocity and
humidity were also measured at the same time.
Atlantic surface water was continuously sampled at varying sampling
density and volume depending on location and expected 129I concentration.
Sampling strategy along the Oden route from north to south (Figure 3.1)
varied depending on expected variability of 129I concentration in the different
water bodies. In addition, the volume of water samples was gradually in-
25
creased from 0.5 L to 2.0 L along the transect in response to the expected 129I
concentration in the water.
Figure 3.1: Atlantic transect of icebreaker Oden from Sweden to Chile. Different
colors represent different strategies of surface seawater sampling. Sampling at each
5 km with 0.5 L for each seawater sample was implemented within the English
Channel (purple). Sampling at each 20 km was performed in the North Sea (red, 1.0
L each sample) and each 50 km in the Celtic Sea and the northeastern Atlantic
Ocean (yellow, 2.0 L each sample), while 100 km a part sampling was carried out
in the rest of the transect (green, 2.0 L each sample).
3.1.2 The North Sea samples
The North Sea is a semi-enclosed shallow continental shelf sea with an average of water depth 90 m and lies between 51°N and 62°N where prevailing
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 it exchanges with brackish Baltic water via the Danish
straits. General cyclonic circulation within the North Sea relates to its strong
tidal currents and southwesterly wind, and is modified by specific topography and bathymetry (Howarth and Oceanographic, 2001).
In its southern part and the coastal areas of the German Bight where the
water is well mixed throughout the year, the depth is less than 40 m. Depth
26
in the central and north parts gradually increases from 40 m to 200 m, while
in the Norwegian Trench, northeast, deeper regions occur with the deepest,
about 750 m, in the Skagerrak. Therefore in these regions, the water column
usually stratifies in May with a mean thermocline of 30 m, in particular in
the Norwegian Trench (Richardson and Pedersen, 1998; Howarth, 2001).
Due to the relatively high 129I level in the North Sea, only 0.5 L surface seawater was collected at each sampling site for 129I separation and analysis.
3.1.3 English Channel and Celtic Sea samples
The English Channel is a narrow shallow passage that separates southern
England from the European continent. It connects to the Atlantic Ocean in
the west and the North Sea through the Dover Strait in its eastern end. The
width of the English Channel narrows gradually from the west to east, while
the average depth decreases from 120 m to 45 m. The Celtic Sea is an extension of the European continental shelf that opens to the deep Atlantic Ocean.
Its northern and eastern limits are bound by the southern Irish Sea and the
English Channel, respectively, while the western and southern parts are bordered by the continental slope. The water is relatively shallow (< 100 m) in
its northern portion and increases towards south and west with the longest
open boundary normally defined by the 200 m isobathymetry.
Saline Atlantic water partly enters the English Channel north of Ushant
Island (France) and flows eastwards within the channel. Accordingly, an
evident gyre is isolated from the main flow in the Gulf of Saint Malo
(Salomon and Breton, 1993). Another fraction of Atlantic water moves north
along the coast from the Isles of Scilly to Lundy Island in the Celtic Sea and
returns southwestward through the St. George’s Channel. This flow pattern
continues along the Irish coast thus a counterclockwise current is suggested
(Brown et al., 2003). In addition, a weak residual current flows poleward
along the west continental slope (Southward et al., 2005). The currents in the
Celtic Sea mainly responds to the variation of wind systems in this region,
where in summer the westerly winds dominate while wind direction is more
southwesterly in winter (Pingree and Le Cann, 1989).
It is expected that large variations in 129I concentration occur between the
east and west parts of the English Channel. Therefore frequent sampling
density was applied within the English Channel, in all 34 samples of 0.5 L
each. A small number of samples, only six, were collected in the Celtic Sea,
with a volume of 1.0 liter each.
3.1.4 The northeastern Atlantic Ocean samples
Although in general the surface ocean waters flow eastward, the whole region of the northeastern Atlantic Ocean is considered to be stagnant (Pingree,
1993). This region is located between subpolar and subtropical gyres where
27
seasonal and regional variations of wind systems in this transition zone (between westerlies and trade winds) highly influence the surface circulation
pattern in most parts of the sampling transect. A branch of the North Atlantic
Current (NAC) borders the transect in the north, while the southern limit is
influenced by a combination of the Azores, Portugal and Canary Currents
systems, as well as the injection of Mediterranean Outflow Water (MOW)
(Figure 3.2).
Figure 3.2: General bathymetric chart of the investigated area in the northeastern
Atlantic Ocean. (a) Sampling transect of 129I along the northeastern Atlantic Ocean.
Sampling locations are expressed as black dots. Location of Sellafield and La Hague
nuclear reprocessing facilities (NRFs) is highlighted with stars. (b) General scheme
of surface water circulation in the Bay of Biscay and (c) the Gulf of Cadiz.
NAC=North Atlantic Current, IPC=Iberian Poleward Current, PC=Portuguese Current, ACN=Azores Current, northern branch, ACS=Azores Current, southern branch
and CC=Canary Current.
Intensive in situ hydrographic data and sea surface temperature (SST) satellite observations, as well as numerical models reveal that the eastward
Azores Current, combined with the equatorial Portuguese Current that flows
parallel to the west Iberia coast, enters the Gulf of Cadiz along the continental slope towards the Strait of Gibraltar. The easternmost part of this anticyclonic circulation eventually feeds the Canary Current. This general fea28
ture of circulation pattern is thought to be weakened in winter and enhanced
in summer (Johnson and Stevens, 2000; Relvas and Barton, 2002; Sánchez
and Relvas, 2003).
Saline and dense MOW mixed with overlying North Atlantic Central Water (NACW) descends as the Mediterranean Undercurrent in the Gulf of
Cadiz along the continental slope and interacts with bottom topography.
High velocity Mediterranean Water arising from the release of potential energy makes this region highly unstable (Baringer and Price, 1999). On its
way to Cape St. Vincent, the plume becomes neutrally buoyant and divides
into two parts with their main cores lie at 800 m and 1200 m depth (Zenk,
1970). These Mediterranean waters can be traced across the North Atlantic
Ocean along 40°N and even extend to the south of the Iceland-Scotland
Ridge (Reid, 1978). Ambar (Ambar, 1983) also identified a shallow Mediterranean core of 400-700 m depth, stretching from the vicinity of the Strait of
Gibraltar to the area off the Cape Roca of the western Portugal coast.
Mauritzen et al. (Mauritzen et al., 2001) make a point that a salinity detrainment process (towards low-density levels fluxes) occurs in the eastern Gulf
of Cadiz, which partly results in a higher salinity of East North Atlantic Central Water compared to that in the west.
Many models and direct observations show that the Atlantic Water not
only flows eastward towards the Mediterranean Sea, but also has a recirculation within the upper layers in the Gulf of Cadiz (Johnson and Stevens,
2000; Mauritzen et al., 2001; Machín et al., 2006) (Figure 3.2). During winter, an Iberian Poleward Current was observed along the Portugal coast as a
recirculation of the Azores Current (AC) which centered at about 35°N
(Mazé et al., 1997; Peliz et al., 2005). Although the origin and variability of
this slope-flow current is still unclear, it may be connected with the coastal
counter flow in the western Cadiz. The main route of this westward flowing
Atlantic Water is along the slope of the European continent and it is well
defined west of Cape Santa Maria and strengthened around Cape St. Vincent. Thus the modified surface water in the Gulf of Cadiz can subsequently
spread westward and northward.
Considering long-term currents circulation patterns in the northeastern
Atlantic Ocean, as well as the relatively long distance from Sellafield and La
Hague, concentration of 129I should be low in this region (31.07-47.36 °N,
7.96-14.52°W). Therefore, the sampling frequency was reduced and the volume of each sample was increased to 2 liters.
29
3.2 Measurement technique of iodine (127I and 129I) from
seawater
3.2.1 Sample preparation and chemical reagents
All collected samples were instantly filtered onboard through a 0.45 μm
membrane (Sartorius AG, Gottingen, Germany) and filled in clean polyethylene containers under cold and dark conditions. 129I standard (NIST-SRM4949c) was purchased from National Institute of Standard and Technology
(Gaithersburg, MD, USA). Carrier-free 125I was purchased from Amersham
Pharmacia Biotech (Little Chalfout, Buckinghamshire, UK) and carrier 127I
(Woodward iodine) from MICAL Specialty Chemicals (New Jersey, USA).
Bio-Rad AG1- ×4 anion exchange resin (50-100 mesh, Cl- form, Bio-Rad
laboratories, Richmond, CA) was used for iodine separation because of different affinities of iodide and iodate on the resin. All chemical reagents used
were of analytical grade and all solutions were prepared using deionised
water (18.2 MΩ·cm-1).
A large column was loaded with AG1- × 4 anion exchange resin (50-100
mesh, in Cl- form) and washed with 300 ml of 2 mol/L NaNO3 until Cl- ion
was washed off from the resin. Afterwards the column was washed with 50
ml deionised water. Then the prepared converted resin (in NO3- form) was
transferred to a column (Ø 1.0 × 20 cm) for separation of iodine.
3.2.2 Separation of iodide and iodate from seawater
A method for separation of iodine species (iodate and iodide) developed by
Hou et al. (Hou et al., 2001) was used in the experiment (Figure 3.3). The
filtered seawater, mixed with 0.1 ml of 125I- tracer (250 Bq), was loaded to
the prepared AG1- × 4 NO3- form column (Ø 1.0 × 20 cm) with a flow rate
2-4 ml/min. After the seawater pass through, the column was washed with
30 ml deionised water and followed by 50 ml of 0.2 mol/L NaNO3. The effluent was collected for solvent extraction of iodate. The iodide (including
125 I ), however, is adsorbed onto the resin, and can be eluate with 60 ml of
10% NaClO solution. The eluate is used for iodide extraction. Meanwhile,
1.0 ml of the effluent seawater, as well as the eluate was used for determination of 127I using ICP-MS (Inductively Coupled Plasma Mass Spectrometry).
3.2.3 Extraction of iodine species (127I and 129I)
To extract iodate and total inorganic iodine, 1.0 ml of 127I carrier (Woodward
iodine, 2 mg/ml) and 0.1 ml tracer (125I-, 250 Bq) were added to the filtered
seawater and the effluent of the iodate fraction. In order to reduce all the
iodine to iodide form, 10 ml 3 mol/L HNO3 and 1.0 ml of 1 mol/L Na2S2O5
were added in turn, and the pH was maintained at < 2 to ensure a fast reduc30
tion. Then the mixed solution was transferred to a suitable separation funnel
with addition of 20-50 ml CHCl3. Iodide was oxidized to iodine (as I2) by
using 2-5 ml of 1.0 mol/L NaNO2 and extracted to the CHCl3 phase. This
procedure is repeated two times to extract all iodine to organic phase. 0.5 ml
of 3 mol/L HNO3 is added to the funnel to provide enough hydrogen ions
before oxidizing iodide in each extraction. The extraction of iodate from the
eluate is much the same. The differences are that we use a 1.0 mol/L
NH2OHHCl solution to reduce iodate to molecular iodine. Finally, 0.2 ml of
0.05 mol/L Na2S2O5 was added to the funnel and iodine in the organic phase
was back-extracted to the water phase, then transferred to a vial to which 0.1
ml of NH3·H2O was added to remain iodide stable before precipitation. The
precipitation of AgI was achieved through addition of 1.0 mol/L AgNO3. The
AgI precipitate was dried at 70oC.
Figure 3.3: Analytical scheme for chemical speciation of 127I and 129I species from
seawater. Modified from (Hou et al., 2001).
The chemical separation yield is obtained by measurement of 125I in the final
separated solution and the 125I added to the solution (as standard) using a NaI
31
gamma detector (well type, Bicron) at channels 25-115 in 26-36 keV for 60
seconds. The chemical yield (Y) is calculated by: Y = (counts of sample) /
(counts of 125I standard). The chemical yield of total iodine, iodate and iodide during the separation processes were calculated to be 81-99%, 97-99%
and 53-80%, respectively.
3.2.4 ICP-MS determination of 127I
Measurement of 127I was performed using X-Series inductively coupled
plasma mass spectrometry (ICP-MS) system under hot plasma conditions
with Xt interface. Before measurements, each 1.0 mL of raw sample was
spiked with Cs+ as internal standard, and then diluted to 20 mL with 1% of
ammonium solution. The detection limit for 127I calculated as 3 SD of blanks
was 0.05 ng/mL.
3.2.5 AMS determination of 129I
The dried silver iodide precipitation (AgI) was mixed with niobium powder
(~7.4 mg) in a mass ratio of 1:2. The mixed sample was then transferred and
pressed into a copper holder for the AMS measurement in the Tandem Laboratory, Uppsala University. Negative ions beam of iodine is sputtered out
from the copper holder and selected by a 90° bouncer magnet. A voltage of
3.5 MV was applied in the terminal to measure 129I. The 129I /127I isotopic
ratio in the standard (NIST-SRM-4949C) is (1.1±0.1) ×10-11 and the ultralow background of the AMS system is 4×10-14. The statistical error of the
measurements was < 10% (one standard deviation).
32
4. Results and discussion
The whole data sets of 129I and 127I and their species along the transect are
shown in Figure 4.1 and 4.2. Because of the huge number of data and the
large variability in oceanographic characteristics of the transect areas, the
data were divided into four sets representing the regions: the North Sea (NS,
paper II), the English Channel (EC, paper III), the Celtic Sea (CS, paper IV)
and the northeastern Atlantic Ocean (NEAO, paper V). Detailed discussion
of each region is given in this section. Furthermore, a review of 129I distribution in the marine waters is given in paper Ⅰand summarized in section 4.1.
Figure 4.1: Analytic results of 129I and its species along the transect of Oden Antarctica expedition used in this thesis. NS = North Sea, EC = English Channel, CS =
Celtic Sea, NEAO = northeastern Atlantic Ocean.
33
Figure 4.2: Analytic results of 127I and its species along the transect of Oden Antarctica expedition used in this thesis. NS = North Sea, EC = English Channel, CS =
Celtic Sea, NEAO = northeast North Atlantic Ocean.
4.1 Distribution of 129I in global oceans
The 129I concentration spans from 6.0×106 atoms/L (southern Indian Ocean)
to 1.3×1012 atoms/L (Irish Sea) which is 1.3-6 orders of magnitude compared
to pre-anthropogenic background value. The Nordic Seas and the Arctic
Ocean present the major 129I reservoir, while the Pacific and Indian Oceans
have low concentrations close to the nuclear fallout level, reflecting less
influence from nuclear reprocessing facilities (Figure 4.3). Highest values
(>1×1010 atoms/L) are restricted along the 50-70°N bands and near to the
vicinity of two point sources in Western Europe. Relatively higher levels
could be also found in the Arctic Ocean and Nordic Seas region where the
average 129I is 1×109 atoms/L, about 100 times above nuclear weapons testing fallout value, with a gradual decrease to ~1×108 atoms/L in the northwest
of Atlantic Ocean. 129I concentrations in south of 40°N in Northern Hemisphere decrease abruptly, ranging from ~106 to ~108 atoms/L which is generally 2-3 orders of magnitude lower compared to north of 40°N.
34
Figure 4.3: Concentrations of 129I in (a) Arctic Ocean, (b) North Atlantic Ocean and
Nordic Seas, (c) North Pacific Ocean, (d) Indian Ocean. Solid dots indicate the sampling sites, data in He et al.
Concentration of 129I in the Nordic Seas and the Arctic Ocean clearly reflects
main 129I pathways after its release from Sellafield and La Hague. 129I carried
by the Norwegian Coastal Current (NCC) further mixed with the North Atlantic Current (NAC) into the Norwegian Sea and the Arctic Ocean, resulting in a dramatic increase of 129I (~3.5×1010 atoms/L) along the Norwegian
coast. A small part of the NCC westward current combined with water masses from southeastward branch of East Greenland Current (EGC) contribute
to an enhanced level of 129I in the Norwegian Sea (~1.5-10×108 atoms/L).
Concentrations in western Nordic Seas are normally one order of magnitude
lower than east, surface 129I concentrations in the Greenland Sea and EGC
are typically 5-10×108 atoms/L, with higher levels above the eastern Greenland shelf and decrease towards the central Greenland Gyre. Within the Arctic Ocean, the 129I-bearing water enriched in the Barents and Kara Seas
moves along the continental margin, with a small component entering the
35
Canadian Basin and undergoing further bifurcation at Chukchi Plateau,
whereas a large part returns along the Lomonosov Ridge and flows out of the
Arctic through Fram Strait. This near continental high-129I flow seems not to
incorporate into the fresher Siberia Coastal Current eastward towards Chukchi Sea because a remarkable 129I gradient is observed, suggesting an encounter of the Pacific- and Atlantic-origin water masses within the Chukchi
Sea.
4.2 Variations of iodine (127I and 129I) and its species
along the 2010 transect
4.2.1 Iodine isotopes species in the North Sea
The concentrations of total iodine in the North Sea range from 0.31 to 0.41
μM with an average of 0.37 μM (Figure 4.4a). Maximum 127I appears in the
Southern Bight whereas the distinctly lowest iodine concentration occurs off
the coast of the West Frisian Islands, Netherlands. Iodate concentrations
generally follow the same pattern as that of total iodine, which decrease to
the lowest level of 0.20 μM from the German Bight to the Dutch coast, and
then increase up to 0.35 μM in southern seawater samples. Iodide ranges
between 0.06 and 0.18 μM in the surface waters, although the average concentration (0.11 μM) is markedly lower than the predominant species of
iodate. The highest iodide value is observed close to the Dutch coast.
The results of 129I and its species (129I- and 129IO3-) in the North Sea show
a rather similar pattern, with a significant increase from 54°N to 52°N (Figure 4.4b). The concentrations of 129I indicate a nearly two orders of magnitude variation along the transect, ranging from 9.21×109 to 2.45×1011 atoms/L with an average of 8.15×1010 atoms/L. Higher values of over 2×1011
atoms/L occurred in the south close to the Dover Strait, whereas the low
concentrations around 1×1010 atoms/L was observed in the north part of the
sampling campaign, relatively distant from the English Channel. The distributions of 129I species depict a similar behavior with that of total 129I, and the
concentrations of 129IO3- (3.2×109-1.5×1011 atoms/L) show a relatively wider
range than 129I- (6.9×109-1.2×1011 atoms/L). It is worthwhile to observe that
compared to speciation of 127I, iodide is generally the dominant species of
129
I in most samples, except the two in the south.
Similar to the pattern of 129I, values of 129I/127I atomic ratio increased by
almost two orders of magnitude from north to south (4.4×10-8 to 1.0×10-6)
with an average of 3.5×10-7 (Figure 4.4c). For species of 129I/127I, the average
levels of 129I-/127I- and 129IO3-/127IO3- are 5.9×10-7 and 2.7×10-7, respectively.
The atomic ratios of 129I/127I for iodide along the transect are all higher than
that of 127I, as well as iodate, even in southern samples which have relatively
high 129I-iodate concentration.
36
Figure 4.4: Variations of (a) total iodine concentrations and its species, (b) 129I concentrations and its species, (c) ratio of 129I/127I and its species and (d) iodide/iodate
for 127I and 129I along the transect in the North Sea.
Both of measured 129I concentration and 129I/127I ratio are 5-6 orders of magnitude higher than pre-anthropogenic levels, and 1-4 orders higher than values reported in any other oceans (He et al., 2013). Extremely high 129I concentrations in Southern Bight were clearly linked to the English Channel
water (one branch of the North Atlantic Water), this water is mixed with the
relatively low 129I of the North Sea water and is diluted as the water parcel
moves northward along the continental coast in a short distance. A nearly
20-fold decrease of 129I concentrations appeared in the German Bight and the
level remained rather constant (~1×1010 atoms/L).
Concentrations of 127I varied to a much lesser extent than 129I. Most of the
127
I concentrations are comparable with other North Sea data, but generally
lower than those reported in any other oceans (Elderfield and Truesdale,
1980; Hou et al., 2007; Michel et al., 2012). This is probably caused by discharge of fresh riverine water from the UK and the European continent in the
south, as well as brackish water from the Baltic Sea through the Kattegat in
the north. Moreover, the abundance of phytoplankton biomass inhabiting in
37
this temperate shallow shelf sea, especially in the Southern and German
Bights, imply a potential organisms uptake of iodine in the North Sea (Joint
and Pomroy, 1993). A positive correlation between salinity and the 127I concentration (r = 0.62, P < 0.05) was observed in all seawater samples investigated here. This feature also suggests that continuous dilution of saline and
high iodine content of Atlantic water by continental runoff occurred from
south to the north. Compared to total 127I, the species (iodide and iodate)
show a considerable variation. The lowest total 127I concentration was observed close to the west Wadden Sea, which might demonstrate a relatively
strong influence from the fresher west Wadden Sea (Zimmerman, 1976).
This feature is also confirmed by lowest salinity and low iodate content in
this sample.
4.2.2 Iodine isotopes species in the English Channel
Higher concentration of total iodine was observed in the English Channel
compared to that in the Celtic Sea (mean 0.38 μM), particularly in the
southwestern part (off Plymouth), where the average iodine concentration
reaches 0.48 μM. In contrast, concentrations of iodate is generally increased
from east to west along the transect and do not show pronounced variation
between the Channel water and the open sea. Although salinity data is not
available for some of the samples located in the Celtic Sea, the rationalized
iodate (normalized to a salinity of 35) was rather constant (around 0.33 μM)
west of La Hague. High iodide and 127I-/127IO3- in the English Channel suggest a more productive water compared to the Celtic Sea, which is confirmed
by high concentration of chlorophyll a (Gentilhomme and Lizon, 1997).
Insignificant correlation between iodate and iodide (r = -0.34 and P = 0.22)
and the nearly constant rationalized iodate concentration (0.33 μM) between
the western English Channel and the open sea suggests that, to a large extent,
the changes in iodine speciation within this region is not primarily a result of
interconversions between iodate and iodide. Higher concentration of total
iodine in the western English Channel seems directly linked to the elevation
of iodide (r = 0.89, P < 0.0001; slope = 0.8) and thus the net increase of iodine is mainly attributed to direct addition of iodide in this area. However,
no similar pattern was observed in 129I and its species. This is because the
129
I is overwhelmingly influenced by the source point of La Hague in the
English Channel, as well as a relatively long time for 129I to reach open sea
equilibrium.
38
Both 129I concentrations and 129I/127I in the English Channel are 4-5 orders
of magnitude higher than pre-nuclear values. The changes of 129I and 129I/127I
along the transect show a rather similar trend that is continuously decreasing
towards the west (Figure 4.5). There is a ‘great sink’ starting just west of the
La Hague tip, as 129I concentrations dramatically drop from ~1010 to ~108
atoms/L. This is attributed to the major Atlantic water pathway in the English Channel, which results in a much more serious contamination of seawater in the North Sea in the east rather than the Atlantic Ocean in the west.
Within the English Channel, the highest 129I concentration (4.6×1011 atoms/L) did not occur in samples that are close to the La Hague tip, but in the
surrounding area of the Dover Strait. This is due to that the sampling transect
was slightly towards the British Isles and the major water movement along
the Channel causes dispersion from the French to the British side of the
Channel. The same pattern was also reported in studies of other radionuclides such as 125Sb, 134+137Cs and 90Sr (Herrmann et al., 1995; Bailly du Bois
and Guéguéniat, 1999).
Figure 4.5: Concentration of
Shelf in 2010.
129
I in the English Channel and the southern Celtic
Similar to 127I, iodate in the English Channel is the dominant species for 129I
with a few exceptions, and this observation can generally be extended to the
Celtic shelf (Figure 4.6). However, the ratio of iodide to iodate for 129I is
quite different when compared with that of 127I. All of the 129I-/129IO3- values
39
were significantly higher than the corresponding 127I-/127IO3-, which may be
attributed to the presence of ‘old’ 129I that reached its species equilibrium.
This is because the major source of 129I in the English Channel originated
from La Hague and thus the observed 129I-/129IO3- should directly reflect the
proportion of 129I species in liquid discharges. It is worthwhile to mention
that the average 129I-/(129IO3- + 129I-) in the English Channel in 2010 (0.43) is
comparable to that in 2005 (0.41), though the data available in 2005 were
sparse (Hou et al., 2007). This finding is vital for the estimate of 129I redox
rate in marine water as the percentage of 129I- in the source water could be
regarded constant. Moreover, there was no significant difference for 129I/(129IO3- + 129I-) in the west and east of the English Channel (P=0.054) in
2010. Despite more than one order of magnitude lower 129I concentration
west of La Hague, this feature again confirms that the western English
Channel has also significantly been contaminated by the La Hague discharges and that the oxidation of iodide is a slow process within the English
Channel. Compared to the data presented here, up to 20 times higher 129I/129IO3- values were reported in the offshore of Fukushima surface water that
was contaminated by the Japanese nuclear power plant (NP) accident in
March 2011 (Hou et al., 2013). This suggests that a higher 129I-/129IO3- level
is always expected in discharges of 129I through anthropogenic nuclear activities and evidently, different NRFs/NPs have different initial 129I-/129IO3values in their source waters.
Figure 4.6: Ratio of iodide/iodate for
southern Celtic Sea in 2010.
40
129
I and
127
I in the English Channel and the
4.2.3 Iodine isotopes species in the northeastern Atlantic Ocean
It is apparent that, as a sensitive radioactive tracer, the concentration of 129I
(including 129I- and 129IO3-) shows a larger variation in the seawater compared with that of 127I (Figure 4.7). More than one order of magnitude difference in 129I concentration is shown in the northeastern Atlantic where the
highest occurred in the middle of Biscay Bay (12.67×108 atoms/L) and all
the 129I data here are higher than 4.0×107 atoms/L. Five pronounced peaks of
129
I (>2×108 atoms/L) are observed along the surface water transect. However, if these distinct peaks are not taken into consideration, average concentration of 129I in the sampled transect is less than 1×108 atoms/L (mean 6.9×107
atoms/L) and also does not vary a lot in the open sea surface water. Despite
nearly similar behavior for 129I- and 129IO3- in the sampled surface water, the
concentrations of 129I- show a relatively wider range (0.07-5.73×108 atoms/L). Additionally, 129IO3- concentrations are normally higher than that of
129 I in the sampled region with a few exceptions. The isotopic ratio of
129 127
I/ I has the same trend as that of 129I, which varies between 1.82×10-10
and 5.69×10-9 with an average value of 6.63×10-10. The same pattern also
appears for the ratio of 129I-/127I- and 129IO3-/127IO3- and the atomic ratios of
129 127
I/ I for iodide is significantly higher than those for iodate in the investigated region.
Figure 4.7: Iodine isotopes (127I and 129I) and their species in the Atlantic seawaters.
(a) Variations of isotopic ratio of 129I/127I, 127I-/129I- and 127IO3-/129IO3- in the sampled
transect along the northeastern Atlantic Ocean. (b) Concentration of 129I and its species (129I- and 129IO3-) along the sampled transect in the northeastern Atlantic Ocean.
The error bars show the analytical uncertainty of one sigma.
Higher values of 129I-/129IO3- compared with 127I-/127IO3- are found in all seawater samples (Figure 4.8). Some 129I-/129IO3- values, however, are close to
the corresponding 127I-/127IO3- levels which are interpreted to reflect a local
effect. This may either be related to older water ventilated to the surface
during winter, or the small-scale accelerated redox cycle induced by local
41
chemical or biological characters in these waters. The 129I-/129IO3- value varies from 0.14 to 2.02 in samples where the highest level is located in the
region between Madeira and the African coast. Most of the ratio for 129I lies
at 0.3-0.7 while the 127I-/127IO3- ratio is normally below 0.2. High values of
129 - 129
I / IO3- (above 0.7) are thought to be directly linked to the large-scale
circulation pattern as described in the next section. Similar to 129I and
129 127
I/ I, there are five peaks observed for 129I-/129IO3- which shows decreasing values towards the north. Unlike the 129I-/129IO3-, the ratios of iodide to
iodate for 127I remain nearly constant in the sampled surface water transect.
On average the value for 127I-/127IO3- is 0.14, which is a factor of five lower
than that of 129I-/129IO3- (0.65).
Figure 4.8: Variations of 129I-/129IO3- and 127I-/127IO3- along the sampled transect of
the northeastern Atlantic Ocean. Colored zones represent 129I species in samples that
is close to (< 0.3, pink), approaching (0.3-0.7, green) and far from reaching (> 0.7,
yellow) the iodine redox equilibrium in seawaters.
Concentrations of 129I show about 30 times difference in surface seawater
and with an average of 1.53×108 atoms/L in the sampled transect in 2010. In
addition, the ratio of 129I/127I also shows a 30 times variation between the low
and high values (1.82-56.85×10-10 atoms/atoms). Both of these values are 2-4
orders of magnitude higher than the natural 129I levels in the ocean (estimated at ~105 atoms/L for 129I and ~10-12 for 129I/127I), which implies strong influence of anthropogenic nuclear activities along the European Coasts. Accordingly, our data suggest that, even the lowest 129I value measured
(4.3×107 atoms/L and 1.82×10-10 atoms/atoms for 129I/127I ratio) is higher
42
than the background level of the nuclear weapon testing. This again indicates
that the surface water of the northeastern Atlantic Ocean had been labeled by
a nuclear fuel reprocessing signal, either from direct marine discharge or
from atmospheric 129I releases. In general, iodate is the dominant species of
dissolved iodine for both 127I and 129I in the surface waters of the sampled
northeastern Atlantic. More than 75% of 127I-iodate is observed in these surface waters, which is comparable to seawaters collected in any other ocean
regions (Tsunogai and Henmi, 1971; Truesdale, 1978; Elderfield and
Truesdale, 1980; Jickells et al., 1988; Wong, 1995; Campos et al., 1996;
Wong and Cheng, 1998; Campos et al., 1999; Farrenkopf and Luther Iii,
2002; Waite et al., 2006; Bluhm et al., 2011). However, for the species of
129
I, there is evidently a higher concentration of iodide compared to that of
127
I. This huge discrepancy of iodine species for 127I and 129I probably reflects different sources of iodine isotopes.
4.3 Sources of 129I
4.3.1 Sources of 129I- in the North Sea
Considerable variation of iodide/iodate values for 129I in the North Sea was
observed compared with that for 127I. This feature implies a reduction of
iodate in the North Sea as the English Channel water parcels moves northward. Despite the positive correlation between 127I and salinity, the insignificant correlation between iodide and salinity indicates that part of the 129IO3is locally reduced. All these features provide evidence for a reductive environment within the North Sea, possibly mediated by bio-activities.
An earlier study (Hou et al., 2007) reported a rapid reduction of 129IO3- in
the continental European coastal areas and the German Bight. 129I-/129IO3values as high as 50 were observed in the estuary of Elbe River and they
related this feature to the combination of chemical and biological processes.
Our samples were generally located in the open North Sea, 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 the 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 indicates that the major source of 129I- in northern samples was water
masses bringing other than local production.
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 (Van
43
der Veer and Bergman, 1986; Hoppema, 1991). Thus the western Wadden
Sea should be another potential source of newly produced 129I-iodide under
anoxic conditions and further investigation of 129I within this region is needed.
4.3.2 Sources of 129I in the northeastern Atlantic Ocean
For the 129I inventory, the influence of direct marine discharges and later
transport by ocean current systems play a major role for the concentration in
the surface water of the investigated area. The 129I concentration and 129I/127I
value in samples north of Cape Finisterre (Spain) analyzed here also confirm
the influence from north, where a gradually decreasing pattern is documented southward. Therefore, the 129I level in the north part of the investigated
region exhibited a combined contribution from Sellafield and La Hague that
could be further advected southward.
Seawaters with high 129I concentration south of 40°N clearly imply a different source other than the influence of Sellafield and La Hague from the
north. These three isolated peaks do not seem linked to waters from the north
due to the general pattern of meridional water movement with respect to the
Portugal Current system in this region (Fiúza et al., 1998). This is also confirmed by speciation analysis of 129I that shows in the southern three peaks,
iodide as the main species in the surface water column.
In addition to the two NRFs mentioned above, the Marcoule NRF
(France), which is located on the banks of Rhone river, directly released 45
kg of liquid 129I to the river and 145 kg of 129I to the atmosphere during its
operation until 1997 (Hou et al., 2009b). Considering 50% of atmospheric
129
I deposited in the vicinity of the plant and assume all 129I has eventually
injected into the Mediterranean Sea through continent runoff, about 120 kg
of 129I was calculated in the Mediterranean Sea. As a result, the average concentration of 129I would be about 1.5×108 atoms/L if the 129I is homogeneously mixed in the Mediterranean water. Considering the large amount of 129I
released by Marcoule reprocessing plant and the relatively long residence
time of iodine (~1800 y) compared with the water turnover time (~1000 y) in
the Mediterranean Sea, the Mediterranean water could be a source of 129I to
the North Atlantic Ocean. Seawater and algae samples taken from the Mediterranean Sea show a comparable 129I level with our samples that show high
129
I level in the south, suggesting a Mediterranean origin of these waters
(Osterc and Stibilj, 2008; Pham et al., 2010). To our best knowledge, there is
no 129I speciation in the Mediterranean Sea. However, 129I-/129IO3- values in
the English Channel suggest that the source water of 129I-/129IO3- from nuclear reprocessing facilities should contain high values (Hou et al., 2007) and it
is reasonable to conclude a high 129I-/129IO3- in the Mediterranean water.
Moreover, high iodide concentration is normally found in coasts, bays, estuaries and semi-enclosed basin (Wong, 1995; Tian et al., 1996; Hou et al.,
44
2007). Therefore, seawater samples south of 40°N that are characterized by
high 129I and 129I-/129IO3- may represent direct influence from the Mediterranean overflow plume that was captured during the cruise.
Figure 4.9: Concentrations of 129I (108 atoms/L) along the transect and suggested
surface 129I pathways from the English Channel and the Strait of Gibraltar (dashed
line). Red regions represent major coastal upwelling and solid blue lines show the
spreading pathways of deep Mediterranean water. The nuclear reprocessing facilities
(NRFs) are marked as stars.
We suggest that the surface waters at 36.5°N may reflect the propagation of
a branch of this onshore water westwards towards the open ocean after leaving Cape St. Vicente when easterlies dominated (Figure 4.9). Moreover, a
poleward surface current has been reported off the west coast of Portugal
during winter when the wind is blowing northward (Frouin et al., 1990).
When this water reaches the Cape Espichel, local coastal morphology,
shelf/slope bathymetry as well as propagation of eddies make the 129I plume
extend seaward. The other two high-129I samples, which occur in the middle
45
of Madeira and African coast, however, are related to the onshore flow of the
Canary Current, which with its one branch separated from the northern Morocco coast (Johnson and Stevens, 2000), may transport 129I from the Strait
of Gibraltar to further west around 14°W.
4.4 129I inventories in the North Sea and the English
Channel
Here we made a simple calculation of 129I inventory based on data from 2005
(Michel et al., 2012) and applying the release function from the NRFs (Figure 1.2). The entire North Sea is divided into three compartments and for the
relatively deep regions (i.e. the Norwegian Trench and the central and northern part of the North Sea), the compartments are subdivided into two layers
separated by mean thermocline depth of 30 m. Distribution of 129I inventory
in each 10×10 km water column is shown in Figure 4.10. The results show
that about 114 kg 129I resided in the North Sea water. Among this, more than
90% of 129I inventory resides in the German Bight and the southern North
Sea.
Figure 4.10: Distribution of gridded 129I inventory (in 1022 atoms) in the North Sea
in 2005.
129
I inventories in the central and northern portion of the North Sea were
more than two times higher than that in the Norwegian Trench and >90%
was accumulated in the upper layer whereas in the Norwegian Trench, the
46
129
I inventory is comparable in surface and deep layers. However, inventories of gaseous (atmospheric) 129I release only account for 5% and 4% of the
liquid (marine) discharges from Sellafield and La Hague until 2005, respectively. These percentages were even decreased to < 3% and < 1% in the recent five years, respectively. Therefore, atmospheric release of 129I is relatively insignificant compared to the marine discharge. Accordingly, the estimated 129I inventory in the North Sea accounts for about 3.4% of the individual La Hague marine discharges, and 2.4% of the combined marine
discharges from La Hague and Sellafield until 2005.
During 2006-2010, the 129I marine discharges from La Hague were 956 kg,
which will add about 32.5 kg of 129I to the inventory in the North Sea until
2010 based on the estimated 3.4% mentioned above. Thus if we simply ignored the 129I contribution from Sellafield and the loss of 129I to the Baltic
Sea in 2010, 129I inventory in the North Sea would be 147 kg until 2010.
For inventory calculation of the English Channel, gridding with 10
km×10 km and subdivision into western and eastern parts was applied. The
relatively deep western English Channel is further subdivided into two layers
separated by mean thermocline depth of 20 m. The estimated inventory of
129
I in the English Channel is 78 ± 12 kg, in which over 90% resides in the
east of La Hague (Figure 4.11). The inventory in the western English Channel, however, is much less. Even when the well mixed water column in the
whole western part is considered, the inventory in the west English Channel
is still only one-fifth of that in the east. Additionally, though the volume of
the surface layer is nearly three times less than that in the bottom, it accounts
for about 84% of the total inventory of the western part. Apart from total 129I,
a first inventory estimation of 129I species in the English Channel is also calculated. Despite the high 129I-/129IO3- that occasionally occurred in some seawater samples, most of the inventory is iodate-dominated in the English
Channel. Furthermore, distribution and proportion of 129I- and 129IO3- in each
compartment generally follow the same pattern as total 129I. This distribution
behavior of 129I and its species agrees well with the major water movement
within the channel.
Direct marine release from La Hague plays a major role in terms of 129I
inventory in the English Channel and the estimated 129I in 2010 constitutes
1.77% of liquid release by La Hague since the start of its operation until
2010. Compared to neighboring areas, the inventory of 129I in the English
Channel is nearly 4 times higher than that in the Baltic Proper (Yi et al.,
2010), but about half of the estimated inventory in the North Sea.
47
Figure 4.11: Inventories of 129I (in 1020 atoms) in each gridded cell in the English
Channel in 2010.
In general, there is a total of 255 kg of 129I in the English Channel, North Sea
and the Baltic Sea, which constitutes only 5.8% of marine discharges from
La Hague. This feature implies that most of 129I released from La Hague has
left the North Sea and partly resides in the sediments.
4.5 Transformation among iodine species in seawater
In marine waters, iodine mainly exists as iodate, iodide and a minor fraction
dissolved organic iodine (DOI). In the state of thermodynamic equilibrium,
iodate should be the only detectable form of iodine in oxygenated seawater.
However, significant iodide is often observed in the ocean surface layer. In
some coastal bays, or anoxic basins such as the Baltic Sea and the Black Sea,
48
iodide could be the major form (Ullman et al., 1988; Luther Iii and Campbell,
1991; Luther Iii et al., 1991; Truesdale et al., 2001). Concentration of iodine
species varies a lot with respect to depth and locations in the sea. Although
the conversion between iodine species in natural water is not well understood, it might be attributed to chemical, biological and photochemical processes.
Several abiotic mechanisms have been suggested to reduce iodate to iodide under anoxic conditions, with the presence of reducing agents such as
iron, manganese and sulfur species (e.g. thiols, bisulphides, etc.) (Jia-Zhong
and Whitfield, 1986; Anschutz et al., 2000):
1/2 Mn2+ + 1/6 IO3- + 1/2 H2O ↔ 1/2 MnO2 + 1/6 I- + H+
Photochemical reduction of iodate may also be important which was supported by laboratory experiments (Spokes and Liss, 1996) and this process
seems strongly catalyzed by dissolved organic matter:
hv IO3- + 6H+ + 6e- →
I + 3H2O
The presence of iodide in the oxic euphotic zone is suggested by reduction of iodate mediated biologically. These biotic processes include bacterial
reduction with the enzyme nitrate reductase (Tsunogai and Sase, 1969;
Wong and Hung, 2001), and the phytoplankton activities (Moisan et al.,
1994; Tian et al., 1996; Wong et al., 2002). The reduction of iodate to iodide does not seem to be related to concentration of primary production but
more linked to cell senescence (Tian et al., 1996; Bluhm et al., 2010). In
addition, iodide may be released from sediment at the sediment/seawater
interface where remineralization of organic materials is intensified (Ullman
and Aller, 1980; Aldahan et al., 2007):
C-H-O-N-I + SO42- → CH4 + CO + H2S + NOx + 2IConversely, some experiments by laboratory cultures argued that neither
the photochemical process nor organic activity were a substantial effect on
iodate reduction. In these experiments, little change of iodine species was
reported under phytoplankton bloom or high light intensity conditions
(Brandão et al., 1994; Wong et al., 2002; Truesdale et al., 2003). Thus a
hypothesis was proposed that iodate may be used to mop up electrons which
present in excess of photosynthetic needs (Waite and Truesdale, 2003).
When the iodide is produced, then oxidation into iodate is sluggish, because the redox of iodine in marine waters is suggested to be a slow process.
The mechanisms that govern the iodide oxidation are still unclear. Photochemical oxidation may occur with the help of strong UV radiation or oxidants (e.g. H2O2, O3, etc.) (Wong and Zhang, 2008):
49
hv
I2 + 2OH2I- + 1/2 O2 + H2O →
I + H2O2 → HOI + OH-
The reactive transients of I2 and HOI tend to return to I- or incorporated
into organic materials rather than the further oxidation of HOI to iodate.
Therefore, the transformation of iodide to iodate in seawater might also be
biologically mediated (Luther George et al., 1995).
The significant quantities of dissolved organic iodine (DOI) in some
coastal and estuarine waters may add a new loop of iodine geochemistry in
seawaters (Wong and Cheng, 2001). DOI can be formed from iodate and
iodide with reactive oxidants, or released from decomposition of organic
matter. Conversely, decomposition of DOI can introduce iodide to the water
column and this process was suggested to be photochemical or biological
(bacterial mediated) (Luther Iii et al., 1991; Luther George et al., 1995;
Wong and Cheng, 2001b).
In most of our seawater samples along the transect, although iodate confirmed to be predominantly occurring, significant iodide can also be observed, particularly in the North Sea and the English Channel. This is in
accord with previous reports and it is likely influenced by biotic processes.
However, since the redox of iodine is suggested to be a slow process, it
seems that relatively large variations of iodine species composition in some
regions, such as the North Sea and the southwestern English Sea, mainly
reflect water mass mixing and migration rather than interconversions between iodine species. For 129I, in most cases high 129I-/129IO3- were found,
especially in the eastern North Sea, compared to 127I-/127IO3-. This behavior is
attributed to propagation of inshore waters that bear a high concentration of
129 I into the German Bight, where strong iodate reduction happens. High 129Iin some Atlantic water samples may also occur through reduction in the
coastal areas of the northwest Mediterranean Sea, which might be driven by
mixed mechanisms mentioned in this section. In addition, DOI may play an
insignificant role since the values of (129I- + 129IO3-) / 129I in all samples range
between 0.90 and 1.10, which lie within the analytical error range. Moreover,
in open seas, DOI content is normally low and this was confirmed by seawater samples taken from the North Sea (Hou et al., 2013).
50
5. Environmental impact of 129I
At present, 129I is not regarded to pose a significant human health risk. However, because of its high mobility and long half live, as well as the fact that
more than 90% of spent nuclear fuel is still waiting for reprocessing, concentrations of anthropogenic 129I are expected to increase in the studied regions.
Due to its low beta-decay energy, 129I may not constitute a significant external exposure hazard. Soils from Sweden indicated about 1.0 Bq/m2 for 129I,
which was much less radioactivity than that from 137Cs (103-105 Bq/ m2)
(Aldahan et al., 2006). However, elevated levels of 129I in water, atmosphere
and soil will finally find its way into the food chain, which can result in internal exposure effects.
Iodine is generally concentrated in organisms which are associated with
the human diet. Osterc et al. (Osterc and Stibilj, 2012) investigated 129I concentrations in mussel tissues collected in the Mediterranean Sea and their
results showed a high 129I/127I values (10-9-10-8). A wider ratio range (10-1110-8) was observed in milk from central USA (Oliver et al., 1982). It is
known that seaweed can concentrate iodine to extremely high levels, and the
129 127
I/ I is normally higher than 10-8 in seaweeds sampled from western and
northern European coast (Hou et al., 2000a; Yiou et al., 2002; Keogh et al.,
2007). The bioavailability of iodine depends on its species in the environment. Water soluble and exchangeable fractions of 129I in soil are preferentially taken up by plants (Hou et al., 2009b). For mammals and human beings, KI is more easily assimilated than iodine bonded to organic macromolecules (Hou et al., 2009b). Although most of iodine uptake is relatively
quickly excreted out of human body (4-5 months), the transformation path is
commonly through the mammal thyroid (Hou et al., 2009b; Jahreis et al.,
2001). 129I/127I in animal (or human) thyroid present a low level of ~10-10-10-9
in areas far from known-129I sources, such as NRFs and the Chernobyl region
(Chao and Tseng, 1996; Handl, 1996; Hou et al., 2000b). Accordingly, much
lower values of 129I/127I that are close to pre-anthropogenic level were reported from Argentina (Negri et al., 2012). However, in Chernobyl fallout
regions or vicinity areas of NRFs, the 129I/127I ratio dramatically increased to
10-8-10-5 (VanMiddlesworth and Handl, 1997; Fréchou et al., 2002). Considering about 10 mg of iodine in human thyroid, the highest 129I/127I ratio of
~10-5 (0.65 Bq of 129I) results in equilibrium annual dose equivalent to the
thyroid of ~ 10-2 mSv/y (Soldat et al., 1973). This is about two orders of
magnitude lower than annual maximum effective dose for an adult which is
51
the suggested permissible level of European Nuclear Society (1mSv/y)
(http://www.euronuclear.org/info/encyclopedia/d/doselimitvalue.htm).
The 129I in table salts that are used in Sweden varies at 1-4×10-6 Bq/kg
(Aldahan et al., 2006), which is equivalent to 7.3-29.2×10-7 Bq/y if we assume 2 g of daily uptake of salt, thus the radioactivity from daily table salt
can be neglected. Similarly, insignificant radiation of 129I through inhalation
is estimated (10-7-10-6 Bq/y) if the average 129I concentration of 104-105 atoms/m3 in atmosphere is applied (Santos et al., 2005). However, it is radioactive hazard of water due to large amount of daily water consumption possible (~2L/day for adult). Aldahan et al. (Aldahan et al., 2006) reported that
129
I in Swedish tap water is ~108 atoms/L. Therefore, daily intake of 129I
through drinking water is 2×108 atoms and the corresponding radioactivity is
200 times higher than that from table salt and inhalation. These features suggest an insignificant radiation hazardous presently contributed by 129I intake.
Concentration of 129I in seawaters collected in the northeastern Atlantic
Ocean was slightly lower than in the tap water in Sweden, while the water
samples taken from the North Sea and the English Channel showed a much
higher 129I level (1×10-5 – 6×10-4 Bq/L). However, the external radiation of
129
I is less harmful because 129I is a soft beta-emitter. Thus, the potential radiological risk for human beings in the western European coast and the Baltic regions is accompanied with internal dose, when humans consume seafood which concentrates surrounding 129I to an extremely high level.
Fish otolith has been used as an environmental marker for fish lifetime
and migration pathway (Campana, 1999). The majority of elements deposited in otolith originate from water, with a minor fraction coming from food
sources. 127I was already found in otolith and since 129I has been detected in
fish tissues and flesh (reference IAEA), 129I may also be incorporated into
the otolith (Gemperline et al., 2002; Pham et al., 2006). Thus, variations of
iodine isotopes (129I and 127I) and their species in seawater are expected to be
reflected in the growth of otoliths (e.g. Atlantic cod and Atlantic herring).
This possibility can open a new interesting approach where growth and migration pathways of fish can be traced using the ratio of 129I/127I.
52
6. Conclusions
This thesis concerns the distribution and sources of iodine isotopes (127I and
129
I) and their species (iodide and iodate) in ocean waters collected from the
North Sea, the English Channel, the Celtic Sea and the northeastern Atlantic
Ocean. Additionally, a review of 129I distribution in the oceans was performed. The main conclusions and findings are summarized below:
1. Concentrations of 129I in the world oceans range several orders of
magnitude, but were all at least 130 times higher than the pre-nuclear
era level. The highest values were restricted to the English Channel
and the Irish Sea, whereas low levels occurred in regions that are far
from major NRFs. Currently, most available oceanic 129I data come
from areas north of 60°N in the Atlantic Ocean, Nordic Seas and the
Arctic Ocean, while in most parts of the Pacific, Indian and Southern
Oceans, data are insufficient which hinder tracking global oceanic
distribution of 129I. Moreover, although it would be an additional
powerful tool in iodine geochemistry, few 129I data on speciation of
129
I was available for the world oceans.
2. A more than one order of magnitude difference in 129I concentrations
was observed in the English Channel water between west and east of
La Hague peninsula. This observation suggests strong impact of water
movement towards the east and into the North Sea. The results show
that 129I in the southern Celtic Sea is mainly originated from La
Hague, although some clues implied influences from Sellafield.
3. In general, changes in the 127I and 127I-/127IO3- are comparable to data
from other marine waters which are related to natural distribution patterns. In addition, occurrence of deepwater upwelling in the western
English Channel was detected through enhanced 127I concentration in
surface water.
4.
129
I inventories in the North Sea and the English Channel were estimated as 147 kg and 78 kg until 2010, respectively. Of these, more
than 90% of 129I resides in the eastern English Channel, the southern
North Sea and the German Bight. Total inventory of 129I in the Eng53
lish Channel, the North Sea and the Baltic Sea is about 255 kg, which
constitutes only 5.8% of marine discharges from La Hague.
5. Iodate is the dominant iodine species for both 127I and 129I in most
seawater samples from the North Sea to the Atlantic Ocean. Besides,
iodide to iodate ratio for 129I was normally higher than that of 127I,
which reflects different sources of the two isotopes. Temporal variation of 129I and its species in the North Sea show an off-shore propagation and dilution of coastal water, which can be coupled with
interannual climate oscillation over this region.
6. The average value of 129I-/(129IO3- + 129I-) in source water (English
Channel) shows little temporal variation which was crucial for estimation of iodine species interconversion. Use of this constant ratio
resulted in identifying water transport from the western English
Channel via Biscay Bay as the main source of 129I in the northeastern
Atlantic Ocean.
7. Speciation analysis of 129I suggests a slow process of 129I- oxidation in
the open sea, where about 9% of 129I- is annually oxidized. This feature means that it takes at least 10 years for the 129I-/129IO3- pair to
reach their natural equilibrium as the water is transported from the
English Channel.
8. The results of 129I and its species suggest that the Mediterranean Sea
is another source contributing to the inventory of 129I in the North Atlantic Ocean. Therefore, our research proposes a potential prospective
of labeling MOW (Mediterranean Outflow Water) using 129I speciation. Depth profiles of 129I in the Gibraltar Strait and the Gulf of Cadiz
are needed for further investigations.
9. The environmental impact of 129I can be a multi-axis tool, one is related to radioactivity hazards which is presently seems not harmful.
The other axis is a potential tracer of water masses exchange and circulation. The third axis is a tool for ecosystem variability, where migration of fish and other marine species can be traced.
54
7. Acknowledgements
There are many people whom I wish to thank during my life in Uppsala.
First I would like to thank my supervisor Ala Aldahan, who has offered me
invaluable support during my academic studies as a PhD student. I have
profited greatly from his expert guidance and encouragement throughout
these years. Thank you for teaching me a lot about research and how to express ideas in a structured and scientific way. The completion of this thesis
would not have been possible without his help.
I would also like to extend my sincere thanks to my co-supervisor Göran
Possnert, for your kind help with AMS analysis, as well as the patience and
useful suggestions in the course of writing papers. I have benefited so much
from good discussions with you, and impressed a lot with your good sense of
humor.
I would like to express my gratitude to my co-supervisor Rickard
Pettersson, who patiently guided me with the application of ArcGIS step by
step. Also thanks for your great course of Geoinformatics.
I gratefully acknowledge Xiaolin Hou for his support during these years. I
appreciate his great scientific attitude and thanks for the excellent advice
about research. Thank you for your help with chemical experiment in Denmark and I have learnt a lot from you in both laboratory work and scientific
writing. Gratitude also goes to my friends Keliang Shi, Jixin Qiao, Haijun
Dang and Violeta Hansen, who helped me a lot and shared the wonderful
time in Denmark.
Special thanks to my dear friends Peng Yi and Biao Wang, for your close
friendship and considerable help. You have always been there when I needed
you and our real friendship never ends.
I would like to thank Inger Påhlsson and Ann-Marie Berggren for your
assistance during my laboratory work in Uppsala. Additional gratitude extends to Ann-Marie Berggren for your constructive comments on my thesis.
Warm thanks go to all my present and former PhD students and colleagues in LUVA, including Diana, Lebing, Agnes, Zhibing, Estuardo, Liang,
Anna, Elias, Carmen, Maria, Kristina, Beatriz, Martin, Fritjof, Lichuan, Sergey, Byeongju, Wasana and so many more. I would like to thank all of you
for the friendly environment created at the department. Thanks to Chongyu
Xu, Hemin Koyi, and Kevin Bishop who helped and encouraged me in the
department.
55
I would like to thank my Chinese friends and colleagues within and outside the department, for helping me to adapt the life in Uppsala and make the
days interesting and fun: Fengjiao Zhang, Juntian Zheng, Jianliang Wang,
Chunling Shan, Dixiao Bao, Ping Yan, Yue Hong, Haizhou Wang, Can
Yang, Jiaqi Li, Taigang Zhou, Xu Quan, Zhina Liu, Zhihong Zhao, Fuguo
Tong, Zhifei Zhang, Yan Wang, Ting Zhang, Fei Huang, Hongling Deng,
Tang Xu, Chi Zhang, Lei Zhang.
Thanks to Tomas Nord for excellent help dealing with computer problems.
Thanks to Leif Nyberg, Anna Callerholm and Taher Mazloomian for their
help during office hours. I gratefully acknowledge Susanne, Eva, Fatima and
the administrative stuff who made life easier at the department.
I would like to thank Lars Andersson, Bodil Thorstensson and all members on the research vessel Argos from the Swedish Meteorological and Hydrological Institute (SMHI), for their help during the one-week sampling
campaign in the Baltic Sea.
Thanks to Anna Sturevik Storm, who shared a nice time with me during
the Antarctica expedition. Thanks to Kenneth Nilsson for the interesting
story about the Arctic cruise and I like the good sense of humor. I am also
deeply indebted to Stefan Karlsson, Erik Andersson and all the crew members on icebreaker Oden. Thanks for their help which made the Antarctica
expedition a great memory for life. Thanks to Henrik Kylin for kindly provide us with some of the electrical transformers needed during the expedition.
The Antarctica expedition was funded by the Swedish Polar Research Secretary and the US National Sciences Foundation (NSF).
Special thank to the financial support from the China Scholarship Council
(CSC). The analysis of iodine isotopes was funded by the Tandem Laboratory, Uppsala University, and the Center for Nuclear Technologies, Technical
University of Denmark.
I would like to thank my former supervisor in China, Ruyun Wang, for
his encouragement and giving me the opportunity to study in Sweden.
Last but not least, I wish to thank my girlfriend Sha Wei, and my beloved
family for their continuous support and encouragement. I will try my best to
make you proud of me.
56
8. Summary in Swedish
129
I är en radioaktiv jodisotop med lång halveringstid som sedan 1940-talet
tillförts miljön i stora mängder som en följd av antropogena verksamheter.
Eventuella risker till följd av isotopens radioaktivitet är inte undersökta i
detalj. 129I har däremot framgångsrikt använts som spårämne i marina system
för att undersöka havsströmmar, utbyte av vattenmassor, vattnets transportvägar och allmän jodgeokemi. Undersökningar av hur isotopen sprids i vatten har mestadels fokuserat på Nordsjön, de nordiska haven, Östersjön och
Ishavet eftersom de utgör potentiella transportvägar för 129I-utsläpp från de
två stora kärnbränsleupparbetningars anläggningar i Engelska kanalen (La
Hague, Frankrike) och Irländska Sjön (Sellafield, Storbritannien). Kunskaperna om 129I i andra marina miljöer är dock fortfarande bristfälliga. I denna
avhandling presenteras nya data om spridningsmönster och källor för 129I i
Atlanten genom analyser av ytvatten längst en transekt från Nordsjön, genom Engelska kanalen, vidare in i Keltiska havet och genom nordöstra Atlantiska oceanen. Ytvattensprovtagningen genomfördes under oktober och
november 2010, som en del av Svenska Polarforskningssekretariatet och
amerikanska National Science Foundations (NSF) Antarktis expedition.
Dessa prover analyserades både vad avser totala mängder av två jodisotoper
(127I och 129I) samt deras olika kemiska former (jodat och jodid), med hjälp
av en omfattande kemisk separation, följt av mätningar med accelerator
masspektrometri (AMS) och induktivt kopplad plasma masspektrometri
(ICP-MS).
Avhandlingen innehåller en sammanfattning av data om 129I i världshaven (Paper I), där det framgår att 129I koncentrationer i ytvatten från Atlanten, Stilla havet och Indiska oceanen är minst 1-3 magnituder lägre än i Ishavet. Resultaten av provtagningarna på jodisotoper (127I och 129I) och deras
kemiska former längs transekten presenterade i fyra artiklar (Papers II-V).
Av dessa analyser framgår att 129I-halten i havsvatten längs transekten uppvisar en betydande förändring med en ökning på cirka 40 gånger från Nordsjön
till Engelska kanalen, där det högsta värdet påträffades nära Doverkanalen.
Denna höga 129I-halt följs av en dramatisk minskning på mer än fyra magnituder från Cap de La Hague till det öppna havet. Alla 129I-koncentrationer
och 129I/127I-kvoter i de insamlade ytvattensproverna låg över nivåerna för
det globala nedfallet från kärnvapenprov.
Förutom analyser av 129I-koncentrationer och 129I/127I-kvoter, visar resultaten av jodisotopernas olika kemiska former på en ny möjlighet att förfina
57
användande av 129I som spårämne i marina miljöer. Fram till idag har det
enbart funnits data som visar på bildande av de olika formerna av 129I i
Nordsjön och Östersjön men i denna avhandling utökas denna viktiga information till andra havsområden. Tydliga geografiska källorområden för dessa
två jodisotoper framträder av den stora skillnaden mellan jodid/jodat för
båda 127I och 129I. Dessa karaktärsdrag antyder att den naturliga termodynamiska jämvikten mellan isotoperna inte föreligger i haven och att oxidationsprocessen där 129I omvandlas till 129IO3- är långsam i det öppna havet.
Förändringen av 129I-/129IO3- i havsvatten kan således användas för att identifiera 129I-källor vilket i sin tur leder till slutsatsen att den huvudsakliga källan
till 129I i nordöstra Atlanten kan hänföras till Keltiska havet och inte Labradorhavet. Skillnader i 129I-/129IO3- kvoten i proverna från Keltiska sjön tyder
på en påverkan från utsläppen från Sellafields kärnbränsleupparbetning.
Utsläppen från La Hague å andra sidan förefaller främst bidra till 129I i västra Engelska kanalen. Denna förekomst tillsammans med ett möjligt södergående vattenflöde från Irländska sjön, har medfört att Keltiska sjön kan ses
som ett 129I-förorenat område. Om en 129I-plym förflyttar sig ytterligare, vid
gynnsamma vindförhållanden, kan den även komma att påverka nordöstra
Atlanten och då särskilt Biscayabukten.
Höga 129I-/129IO3- halter i havsvatten från södra Nordsjön visar på en icke
kustnära spridning av europeiska kustvatten, vilket kan vara kopplat till extrema klimatvariationer. De observerade 129I/129I värdena från Engelska kanalen ger en tydlig bild av de troliga ingångsvärdena till La Hagues avloppsledningar och proportionerna visar på små temporala och rumsliga förändringar inom kanalen. Fem vattenprover från nordöstra Atlanten karakteriserades av höga 129I-koncentrationer och skillnader i 129I-/129IO3-halten att dessa
vattenmassor kan förklaras med att olika källorområden föreligger, där de
norra proverna associerades med utsläpp från Sellafield och La Hague medan de övriga proverna antyder Medelhavsvatten som källa till 129I. Förslagsvis kan 129I och dess kemiska former (129I- och 129IO3-) användas för att undersöka spridningen av Medelhavsvattnets utflöde, Mediterranean Outflow
Water (MOW). Potentiella tillämpningar av 129I/127I i marina ekosystem diskuteras även i avhandlingen. Slutligen görs uppskattningar av totala mängden 129I i Nordsjön och Engelska kanalen där resultaten tyder på att > 90 %
av 129I förekomer i östra Engelska kanalen och södra bukten. Största delen av
de 129I-utsläppen som kommer från La Hague och Sellafield har dock borttransporterat med konvektivt flöde från Engelska kanalen-Nordsjön-systemet
och kan därigenom nå Atlantens subtropiska havsströmsområde. För närvarande anses inte 129I skadligt påverka människors hälsa, men på grund av
dess höga mobilitet och långa halveringstid, liksom det faktum att mer än 90
% av allt använt kärnbränsle fortfarande väntar på upparbetning kan en framtida potentiell risk föreligga. Fiskotolit, som uppvisar årsringar och därmed
visar på fiskens livslängd och hur den migrerar, har används som ett intressant miljöarkiv med möjlighet att även återspegla jod och dess isotoper efter58
som både 127I och 129I har påträffats i denna vävnad. Därmed kan kunskaper
om jodisotoperna 129I och 127I och deras olika kemiska former i havsvatten
öppna för en ny intressant metod där tillväxt och migrationsvägar för fiskar
kan studeras.
59
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