We present the first application of multi-isotopic fingerprints (i.e., 236U/238U, 233U/236U, 236U/129I and 129I/127I) for the discovery of unrevealed radioactive sources. Our data indicate that, besides the reactor signature from the two European reprocessing plants and global fallout signature, there must be a previously undiscovered additional reactor 236U source in the Baltic Sea. This reactor 236U may come from unreported discharges from nuclear research facilities in Sweden, or it may come from accidental leakage from disposal of spent nuclear fuel on the Baltic seafloor, either reported or unreported. Such leakage would indicate potential problems with the safety of seafloor disposal, and may be accompanied by leakage of other radionuclides. The results demonstrate the high sensitivity of the multi-isotopic tracer systems, especially the newly accessible 233U/236U signature, to distinguish environmental emissions of unrevealed historical or present radioactive releases for nuclear safeguard and emergency preparedness, as well as tracing environmental processes from the releasing sites.
Article
A previously unknown source of reactor radionuclides in the Baltic Sea, identified by 233, 236, 238U and 127, 129I multi-fingerprinting
https://doi.org/10.21203/rs.3.rs-49352/v1
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published 05 Feb, 2021
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236U (t½= 2.34 × 107 y) is an isotope of uranium, which is produced by thermal neutron capture of the omnipresent 235U via (n, γ)-reactions and through 238U (n, 3n) 236U reactions with fast neutrons. Even though a small amount (about 35 kg in total) of 236U is produced naturally in the Earth’s surface environments, 236U is (by mass) the largest secondary product created in nuclear reactors, estimated totally to be an order of 106 kg 1. 236U is a sensitive tracer of deliberate or accidental leakage from the nuclear fuel/waste cycle 2–5. The known sources of reactor 236U, i.e., deliberate releases from the two European reprocessing plants at La Hague, France (LH) and Sellafield, UK (SF) since 1950s, can be traced throughout the North Atlantic and the Arctic water currents 6. Emissions from other known sources of reactor 236U, e.g., the Springfield nuclear facility and the Fukushima accident, are negligible 5,7.
A significant amount of 236U (estimated at > 1000 kg) was also delivered to the Earth’s surface environments from the global fallout of atmospheric nuclear weapons testing in the 1950s and 1960s 8. This omnipresent fallout source can make identification of unreported sources of reactor 236U challenging, because of methodological difficulties in distinguishing the source of 236U 9. In addition, the 236U/238U ratio does not provide source information because of 238U of natural origin is ubiquitous.
Reactor 236U could be differentiated from fallout 236U because these sources have different and characteristic 233U/236U ratios due to different nuclear production mechanisms. 233U was mostly produced during nuclear weapons testing by fast neutrons via 235U (n, 3n) 233U reactions or directly by 233U-fueled devices, whereas almost no 233U is produced in thermal nuclear power reactors or reprocessing plants 10. Recently 233U measurements at environmental level have become possible 10.
The representative 233U/236U atomic ratio of global fallout from atmospheric nuclear weapons testing was suggested to be (1.40 ± 0.12) × 10− 2 10. This is several orders of magnitude higher than the 233U/236U atomic ratio in nuclear reactors, e.g., 1 × 10-7-1 × 10− 6 in LH discharges 11, which agrees well with reactor model calculations 12. In the Irish Sea, an average 233U/236U atomic ratio of (0.12 ± 0.01) × 10− 2 has been measured 9, reflecting a dominant reactor signal released from SF. The use of the 233U/236U atomic ratio helps better distinguishing the origin of 236U and since being radionuclides of the same element, the 233U/236U ratio will not be affected during the transport pathway. In addition, the combination of 236U with other radionuclides, e.g. 129I, can be useful to trace the transport of 236U from specific source points, e.g., releases from LH and SF 13–16.
The Baltic Sea is a highly polluted sea, including potentially for anthropogenic radionuclides. It receives radionuclides from global fallout, discharges from the two European reprocessing plants, potentially from the Chernobyl accident, and from any other local sources. In this study, we use a novel combination of three anthropogenic radionuclides − 233U, 236U, and 129I - to identify a previously unknown local source of radionuclide pollution to the Baltic Sea.
The study area and sampling. The Baltic Sea is a landlocked intracontinental sea in Northern Europe and constitutes one of the largest brackish water environments on Earth with about 80 million inhabitants in the surrounding states 17. The water exchange of this large brackish estuarine-like water mass with the Kattegat and the North Sea takes place through the narrow and shallow Danish Straits (Figure 1). The driving force for the water circulation is fresh water surplus from river run-off estimated at 473 km3 yr-1 together with “recycled” North Sea inflowing water as Baltic outflow that sum up to a total water exchange rate of 753 km3 yr-1 18. A mean residence time for the 21580 km3 Baltic water volume was estimated to be 29 years which is equivalent to a "half-life" for the water volume of 20 years 18.
In the investigation presented here, water and sediments samples were collected from the Baltic Sea and related water masses including the western Danish coast in the years 2011-2016 (Table S1 and S2). The water sampling covers mainly the surface distribution (0-5 m depth), with a few samples from deep water and one riverine water from the Mälaren river, which receives downstream discharges from a nuclear fuel fabrication facility (Westinghouse) in Sweden. In addition to the Baltic Sea water, we analyzed sediment samples to gain some idea about the accumulation trend of the isotopes in the bottom of the Baltic Sea. A more detailed description of the study area and samples can be found in the Methods section.
To facilitate the presentation of results and related discussion, we grouped the sampling locations into five geographical regions (Figure. 1) in the Baltic Sea including 1) KGR: Kattegat-Skagerrak and the region including the Danish west coast nearby the North Sea; 2) DS: Danish Straits including the Belt Seas and the Sound; 3) SBR: South Baltic Sea region including Arkona Basin, Borholm Basin and South Baltic Proper; 4) MBR: Middle Baltic Sea region including Northern Baltic Proper, Western Gotland Basin, Eastern Gotland Basin and Gulf of Riga; and 5) NBR: North Baltic Sea region including Archipelago and Åland Sea, Bothnian Sea and Bothnian Bay.
Distribution of 236U concentration and 236U/238U and 233U/236U atomic ratios
The measured 236U/238U atomic ratios (Table S1, S2) vary within (5-52) × 10-9, with the higher ratios (in the central and northern parts of the Baltic Sea and lower ones in the western parts (Danish Straits, Kattegat/Skagerrak and Danish west coast). The highest value reported here is 6-fold higher than the average value found in the North Sea in 2010 ((7.6 ± 3.7) × 10-9) 19.
The distribution patterns (Figure 2) suggest a decline of 236U concentration that is labeled with discharges from LH and SF in the North Sea at the water crossover into the Kattegat and further into the Baltic Sea. However, high 236U concentrations ((6-9) ×107 atom/L) are observed in the surface water of the Bothnian Sea and Borthnian Bay, which are comparable to the central North Sea values ((3-10) ×107 atom/L) 19. Compared to the Kattegat-Skagerrak region, the average 236U/238U atomic ratio in the middle and north Baltic region increases by a factor of 3, from (10 ± 3) × 10-9 to (32 ± 7) × 10-9. This increasing pattern of 236U/238U ratio points out an additional, likely local, supply of 236U in the Baltic Sea 7.
233U/236U atomic ratios obtained here are in the range of (0.14-0.87) × 10-2, with the lower 233U/236U atomic ratios distributed close to the western parts of the Baltic, including the Danish coast, and the higher ratios in the central Baltic Sea. As the typical 233U/236U ratio for global fallout is (1.4 ± 0.1) × 10-2 9, the high 233U/236U in the central Baltic Sea could indicate either strong influence of global fallout or addition from a local source.
Distribution of 129I concentration, 129I/127I and 236U/129I atomic ratios
The measured 129I concentrations ((3-232) × 109 atom/L) and 129I/127I atomic ratios ((101-1286) × 10-9) in the seawater collected in this work show comparable values and distribution trends as observed in an earlier investigation 20, with the highest values in the North Sea-Skagerrak-Kattegat which decrease toward the Sound and remain relatively constant in the Baltic Proper. The distributions of 129I concentrations and 129I/127I atomic ratios indicate that the major source of 129I in the Baltic Sea are marine discharges from the two nuclear reprocessing plants at LH and SF. The water mass pathways from these plants have been shown to contain appreciable amounts of 129I along the passage to the Baltic Sea 21.
Aldahan et al. 21 reported that the average concentration of 129I in the rivers around the Baltic Sea was 3.9 × 108 atom/L, which suggested some minor contribution of 129I from riverine water to the Baltic Sea. The 129I concentrations show a larger gradient (two orders of magnitude) compared to the 236U concentrations (15-fold) along the Baltic Sea. 236U/129I ratios are within the range of (5-133) × 10-4 and indicate a reversed geographical distribution compared to 129I concentration and 129I/127I atomic ratio (Figure 2).
Potential sources of uranium and iodine in the Baltic Sea
The overall sources of uranium and iodine in the Baltic Sea can be summarized as follows:
A) Natural ocean water, with salinity of 35‰, which contains about 60 µg/L of 127I, 3 µg/L of 238U, but negligible 129I, 236U and 233U.
B) Natural fresh water with salinity < 1‰ and negligible concentrations of 129I, 236U and 233U, and lower 127I and 238U than seawater (0.05-10 µg/L for both nuclides).
C) Global fallout from atmospheric nuclear weapons testing, with negligible 127I and 238U, an average 233U/236U atomic ratio of (1.4 ± 0.2) × 10− 2, and a surface geographical distribution pattern for 236U and 233U similar to that of 137Cs which is not unusually high in the Baltic region 22. Earlier studies have estimated 236U concentration (up to 1.4 × 108 atom/L peaking in 1960s) in surface water of the North Sea to be related to global fallout, which may have been partly masked by discharges from the nuclear reprocessing of LH and SF 21,22. In the Baltic Sea, that has an average depth of 55 m, the dilution by vertical dispersion is limited, and a ten times higher concentration is expected for the same inventory, which might mimic higher input. The 233U/236U atomic ratio of the global fallout contribution is expected to be constant after 1980 when all countries stopped aboveground nuclear bomb tests. Concentration of 236U in river runoff is expected to have reduced over the decades, while the 233U/236U atomic ratio stays constant.
D) Marine discharges from European nuclear fuel reprocessing plants (including mainly SF and LH), with known 236U and 129I source functions 23,24, but negligible amounts of 127I and 238U. This source dominates the 236U and 129I budget of marine water entering the Skagerrak from the North Sea. Compared to 236U, almost no 233U is produced in thermal nuclear reactors, and thus 233U should also be absent from marine discharges of the reprocessing plants.
E) The Chernobyl accident. Pu from Chernobyl has been found in fallout over central Europe 25 and, as Pu and U are refractory elements transported similarly by atmospheric dispersion, Chernobyl 236U should have been deposited following a similar pattern as Pu isotopes. Consequently, a Chernobyl signal of 236U may be present in river runoff and marine waters. Based on the present understanding of the production mechanisms of 233U, it is expected that Chernobyl fallout is not a significant contributor of 233U in this context.
Waters entering the Baltic Sea from the North Sea have 236U/238U and 233U/236U atomic ratios set by the balance of reprocessing discharge and global fallout 9,19. As they distribute in the Baltic and mix with waters from various rivers, these ratios can be altered by addition from local sources of 236U and 233U (and minor 238U in river waters). Removal of uranium from Baltic water will not alter the ratios. The increase in 236U/238U observed within the Baltic Sea points clearly to a local source of this anthropogenic radionuclide.
236U source identification via binary mixing
The concentration of 238U (Fig. 3A) demonstrates a strong positive correlation (R2 = 0.91) with salinity. The intercept corresponds to the average riverine input with a 238U concentration of 0.33 ± 0.05 µg/L, which falls in the range (0.2–0.7 µg/L) of 238U for some rivers in the Baltic Sea region 26. There is more scatter in the 238U concentration for low salinities, which might be attributed to differences in regional riverine input. I-129 also shows a positive linear correlation (R2 = 0.69) with salinity (Fig. 3B), but strong scatter occurs at the high salinity end. This trend can be attributed to the mixing of 129I enriched North Sea coast water with 129I depleted North Atlantic water in the Kattegat-Skagerrak region (Fig. 1 A). The 238U and 129I trends with salinity suggest that their concentrations in the Baltic Sea are mainly controlled by the saline water input from the North Sea via Kattegat-Skagerrak, mixing with river-waters in the basin.
Both the 236U/238U and 236U/129I atomic ratios increase with the decreasing salinity as water mix in the interior of the Baltic Sea. The 236U/238U ratio increases by a factor of 3, while the 236U/129I ratio increases from an average of (8 ± 2) × 10− 4 in the Kattegat-Skagerrak region, corresponding to reprocessing derived 236U and 129I, to 1 × 10− 2 in the central Baltic Sea. Both ratios indicate addition of 236U from a local source. The difference in increases between the two ratios can be explained by the presence of 238U and possibly 129I in that local source in addition to 236U. If the source does not contain any 129I, the 10-fold increase in 236U/129I suggests that ca. 90% of 236U in the central Baltic Sea is from local sources. If the source does contain 129I, the portion of 236U derived locally must be still larger.
To understand the source terms of 236U in the Baltic Sea, a binary mixing model is applied with two respective end members representing 236U input from the North Sea and freshwater input via river runoff. Parameters for the first end member representing the North Sea water entering from the west Baltic Sea are well defined by previous studies (Table S3) 19,27. The deviation of the observed 236U/238U atomic ratio from the binary mixing (line L1, Fig. 4A) between the North Sea water and an assumed freshwater end member containing no 236U (neither 233U) reflects additional 236U sources besides North Sea water. The spatial distribution of deviations in the 236U/238U atomic ratio allow locating the additional 236U source (Figure S2). The distribution pattern in Figure S2 is compatible with the hypothesis of additional riverine 236U input from the north Baltic region, which has most river runoff.
Nevertheless, it is challenging to define the 236U/238U ratio of the riverine input to the Baltic because some global fallout may still be washing from the land surface. The 236U/238U and 236U/129I ratios therefore do not directly indicate whether the excess 236U is only from global fallout, or from an additional, previously undiscovered, source that has directly released 236U to the Baltic Sea.
Application of233U/236U atomic ratio for236U source identification
If we assume that the excess 236U originates only from global fallout, the 236U/238U atomic ratio of the riverine input in the best-fit binary mixing is 6 × 10− 8 (line L, Fig. 4A). However, there is a clear deviation of the observation from the model for 233U/236U atomic ratios (Fig. 5A). A subgroup of samples from the Kattegat-Skagerrak reveal a relatively stable 233U/236U atomic ratio of 0.2 × 10− 2 (blue dash-dotted line in Fig. 5) independent of 236U/238U and salinity. This behavior can be explained by assuming an end member of North Sea water with 233U/236U atomic ratio = 0.2 × 10− 2 (a mixed signal of global fallout plus nuclear reprocessing) and salinity 35‰, which is mixed with natural uranium or water with neither 236U nor 233U. This feature shows the notable impact of nuclear reprocessing from SF and LH in the region.
On the other hand, a cluster of samples with the majority from the middle and north Baltic Sea region indicate a typical 233U/236U atomic ratio of 0.5 × 10− 2 (the green dash-dotted line in Fig. 5), uncorrelated with both 236U/238U and salinity. This cluster lies significantly below the binary mixing model L, indicating an additional local 236U sources besides the global fallout, which is characterized by low 233U/236U atomic ratio. A low 233U/236U atomic ratio is typical for releases from nuclear reactors, thereby we assume such a reactor-related source of 236U with negligible 233U in the following.
About two-third of the anthropogenic uranium observed in the middle and north Baltic Sea region seems to originate from this third source (Equation (1)), indicating a strong contribution of 236U without 233U, e.g. a nuclear reactor sourced 236U.
Where 
and 
represent respectively the 233U/236U atomic ratio of the Baltic seawater, global fallout and nuclear reactor; 
and 
refer to the atomic number of 233U from global fallout and reactor, respectively; 
and 
refer to the atomic number of 236U from global fallout and reactor, respectively. Therefore, 
. With 
= 0.5 × 10-2, = 1.4 × 10-2 and = 0.12 × 10-2, we obtain that the 236U contribution from our assumed reactor source is 2.4 times that of global fallout.
To locate this additional reactor 236U source, we apply another binary mixing line L2 (Figure 4 A) of the North Sea water with riverine water, the latter carrying global fallout which accounts for 1/(1+2.4) of the average 236U concentration in the Baltic Sea ((6.0 ±1.7) × 107 atom/L). Thus, the riverine endmember is characterized by salinity =0, 238U = 0.4 µg /L, 236U =1/(1+2.4) × (6 × 107) = 2 × 107 atom/L, and 236U/238U atomic ratio = 2 × 10-8. The excesses of 236U/238U atomic ratio from the mixing curve L2 and their spatial distribution are demonstrated in Figure 6. The data indicate that the extra reactor 236U source input is not from places where salinity is particularly low or where there are rivers, but in the middle and north basins of the Baltic Sea which is probably linked to direct releases of 236U into these locations.
Properties of the 236U unknown source
To identify the source of the excess 236U, the order of magnitude of 236U inventories and fluxes must be estimated. It should be noted this calculation is based only on our data on surface waters, and a precise interpretation will require substantially more data, and to account for many different effects such as vertical distribution of 236U in the Baltic water columns and on the scavenging of uranium into the sediment (especially in the anoxic regions).
The median salinity of the Baltic Sea seawater analyzed here is about 8.3 ‰ (comparable to the reported value of 8.6 ‰ for the Baltic outflow water 18), meaning that the ratio of seawater to riverine water is 1:4. An average excess of 6 × 107 atom/L236U in riverine water is obtained based on the deviation of the 236U concentration from L1 in samples from NBR (Figure S2). The volume of the Baltic Sea of 21700 km328 with 80% riverine water corresponds to 400 g of 236U. Taking into account that ca. 71% (i.e., 
=2.4) of this excess 236U is from the additional reactor source (~280 g) as discussed above, the remainder (~120 g) is related to global fallout.
It is estimated that a total inventory of 1000 kg of anthropogenic 236U was distributed via global fallout during the 1950s and 1980s mainly on the Northern Hemisphere 7. Considering the surface area of the Baltic Sea of 3.77 × 105 km2 (without the catchment area) in comparison to the Northern Hemisphere (half of the Earth’s surface area, i.e. 5.10 × 108 km2), then the total 236U deposition from direct global fallout is estimated as 1.5 kg.
However, when considering the 29-year mean residence time of Baltic seawater, then most of the 1.5 kg 236U was transported out after 60 years, leaving behind 0.19 kg. In addition, some particle-associated 236U fraction from global fallout might be incorporated into the Baltic sediment 29. Therefore, the above estimation of 120 g remaining 236U in the Baltic seawater from global fallout (using salinity data) seems justified.
Emissions from the Chernobyl accident may be an additional 236U source in the Baltic Sea, yet it is difficult to be identified. Nuclear dumping and/or nuclear installations around the Baltic countries are also possible source candidates. As marked in Fig. 1, there are many nuclear installations in surrounding Baltic countries, but there is limited documentation about the 233U and 236U release records from these installations (Table S4) 11. Data on 236U is available from Westinghouse during 1998–2017, with a total reported release of 1.06 × 106 Bq of 236U, equal to 0.44 g. In addition, we measured two seawater samples collected in Mälaren River (Table S2), which receives waste discharges from the Westinghouse facility. The results show that the 236U/238U ratios is at the level of 2 × 10− 8, which is comparable with the seawater samples collected in the central Baltic Sea. The river water shows a 233U/236U atomic ratio of (0.18 ± 0.05) × 10− 2, a signature of reactor material.
The amount of 0.44 g of 236U released from the Westinghouse installation is negligible compared to the above estimated 280 g of the unknown reactor source in the Baltic Sea. For the Mälaren river, the 238U concentration was measured to be 1.5 ± 0.1 µg/L in this work, together with a flux of 166 m3/s 28, it means an input of 0.1 g/yr of 236U, which is negligible also.
Another candidate we assume is reactor fuel, dumped into the sea; the atomic ratio of 236U/238U can be as high as 1 × 10− 2 in conventional nuclear reactors, which would require only 27 kg of dumped/dissolved fuel (a commercial nuclear reactor contains ~ 100000 kg of fuel). 235U enrichment in reactor fuel is 3% for light-water reactors, up to 10% for thermal gas-cool reactors and up to 20% for fast reactors 30. The concentration will be even higher in the core of a nuclear reactor for marine applications, where enriched or highly enriched 235U is used; the Russian submarine cores reportedly contain some 50 to 200 kg of 235U 31. The former Soviet Union (USSR) was accused for dumping radioactive waste in the Baltic Sea, yet it is not possible to assess the dumped amount 32,33.
The geographical distribution of 236U/238U atomic ratio in surface seawater of central Baltic Sea shows high values nearby the Swedish coast close to Stockholm, where a nuclear research company Studsvik AB, Nyköping (100 km south of Stockholm), Sweden, has been in operation since 1950s. It was reported that during 1959 and 1961, 64 tons of radioactive waste with total radioactivity of 14.8 GBq were dumped into the coastal area nearby Studsvik 34. Our measurement on some sediment samples from the Studsvik area show very high 236U content ((2.02 ± 0.12) × 1013 atom/kg), which is three orders of magnitude higher than sediment collected from the North Baltic Sea region (Table S2). The 233U/236U atomic ratio ((0.36 ± 0.05) × 10− 2) for the Studsvik sediment clearly indicates a higher contribution of reactor input compared to the other five sediments collected in the Baltic Sea with 233U/236U ratios between 0.59 × 10− 2 to 0.83 × 10− 2.
Even though the release of 236U from Studsvik is not well documented due to its low specific radioactivity, it is not surprising that waste discharges from Studsvik contain 236U. The high 236U levels in the sediment samples measured most likely originate from scavenging of waterborne 236U from liquid waste discharges by particles into the sediment. Waste dumping/discharges in the Studsvik area are our most plausible candidate for the excess 236U in the Baltic Sea.
Detailed description of the study area and sampling. The Baltic Sea features three major basins, the Bothnian Bay, the Bothnian Sea and the Baltic Proper. The two northerly basins (Bothnian Bay and Bothnian Sea) are characterized by low salinity water mass (1-3 ‰ and 3-7 ‰, respectively) and rather weak vertical salinity stratifications, although strong thermoclines usually develop during the summer 35. The Bothnian Sea represents a large reservoir of brackish water mass that can be divided into two layers blocked by a weak halocline around a depth of 60 m. The long term circulation of the Bothnian Sea water is dominated by an estuary circulation where the bottom dense waters can be traced as surface water in the Baltic Proper 36. The Baltic Proper is the largest basin in the Baltic Sea, permanently stratified in the central part with a strong halocline around a depth of 75 m separating the surface water (salinity 7-8 ‰) from the deep water (salinity 9-20 ‰) and a long-term cyclonic current circulation pattern 37. Water exchange in the Baltic Proper happens through renewing of the deep water during extreme inflow events from the Kattegat. The water mass circulation is further associated with outflow of surface water to the Kattegat and inflow of fresher surface waters from the Bothnian Sea, the Gulf of Finland and the Gulf of Riga (Figure 1).
Water samples analysed in the present investigation were collected via different cruises during 2011-2016. Samples of 2011 were obtained from GEOTRACES cruise on board of research vessel R/V Oceania. Samples from 2013-2014 were collected through the environmental monitoring program for Helsinki Commission (HELCOM). Samples from 2015 were collected on board the research vessel Argos, operated by the marine division of the Swedish Metrological and Hydrological Institute (SMHI). Samples from 2016 were obtained from the Radiation and Nuclear Safety Authority (STUK), Finland, through sampling cruise COMBINE 2 on the research vessel R/V Aranda. One riverine water sample from Mälaren river (in Sweden: 59.33 °N, 18.04 °E) was also sampled for the radioisotope analyses, as this river receives downstream discharges from a nuclear fuel fabrication facility (Westinghouse) in Sweden.
Five surface (0-2 cm) sediments in the middle and north parts of Baltic region were collected (Figure 1 and Table S2) during the COMBINE 2 cruise in 2016. One sediment sample collected outside Studsvik in Bergasundet, Bergas strait (58.75 °N, 17.40 °E) in 2014, which was obtained by pooling 25 sediment plugs (0-10 cm) and homogenized at Swedish Radiation Safety Authority (SSM). The Bergasundet, Bergas strait was the drainage area of the nuclear research facility (Studsvik). Details of the sampling campaigns and location of samples are summarized in Table S1, S2 and Figure 1.
Standards and reagents. Uranium standard solution (1.000 g/L in 2 mol/L HNO3) was purchased from NIST (Gaithersburg, MD), which was used after dilution as a standard for the ICP-MS measurement to quantify 238U in seawater. All reagents used in the experiment were of analytical reagent grade and prepared using ultra-pure water (18 MΩ·cm). UTEVA resin (100-150 µm particle size) was purchased from Triskem International, Bruz, France and packed in 2-mL Econo-Columns (0.7 cm i.d. × 5 cm length, Bio-Rad Laboratories Inc., Hercules, CA) for the chemical purification of uranium isotopes.
Analytical methods for determination of 238U, 236U, 233U, 127I and 129I. The concentration of 238U and 127I in seawater was measured by ICP-MS (X SeriesII, Thermo Fisher Scientific, Waltham, MA) after 10-50 times dilutions with 0.5 M HNO3 and 0.1 M NH3·H2O, respectively. The ICP-MS instrument was equipped with an Xt-skimmer core and a concentric nebuliser under hot plasma conditions. The typical operational conditions of the instrument have been given elsewhere 38. Indium (as InCl3) was used as an internal standard and 0.5 mol/L HNO3 solution was used as a washing solution between consecutive assays.
The radiochemical method for 233U and 236U determination in seawater was applied according to Qiao et al.39. In short, uranium in each seawater sample (0.8-10 L) was co-precipitated with Fe(OH)3, followed by purification with a 2-mL UTEVA column. A 100-µL aliquot of U eluate from the column separation was taken for measurement of 238U by ICP-MS to evaluate the analytical chemical yields by dividing the values in the original samples. The remaining fraction was prepared as target for the AMS measurement of 236U/238U and 233U/236U. For sediments, 5-10 g of each dried sample was ashed overnight at 450 °C in a muffle oven and leached with 100 mL of aqua regia on a hotplate for 30 minutes at 150 °C and then 2 hours at 200 °C. A 100-µL aliquot was taken from the leachate for directly measurement of 238U by ICP-MS, which was used to calculate the original 238U concentrations in the sediment sample. The remaining leachate was processed following the same procedure (i.e., Fe(OH)3 co-precipitation and UTEVA column separation) as for seawater samples. The AMS measurement was carried out at the 3-MV tandem accelerator facility VERA (Vienna Environmental Research Accelerator) at the University of Vienna, Austria. The detailed procedure for the sample preparation and AMS measurement of 233U and 236U has been reported elsewhere 40.
For the determination of 129I in seawater, 100 ml of sample was transferred a separation funnels. 2.0 mg of 127I carrier (prepared using iodine crystal purchased from Woodward company, USA, with a 129I/127I ratio of 2 x 10-14), 500 Bq of 125I- tracer and 0.5 mL of 0.5 mol/L Na2S2O5 solution were added to the funnel, and then the pH of the solution was adjusted to 1-2 using 3 mol/L HNO3 to convert all iodine species to iodide. With addition of 20-50 mL chloroform (CHCl3) and 2-5 mL 1.0 mol/L NaNO2, iodide was oxidized to I2 and extracted to CHCl3 phase by shaking. The extraction procedure was repeated three times to extract all iodine. The CHCl3 phases were combined to a new funnel, 20 mL H2O and 0.3-0.5 mL 0.05 mol/L Na2S2O5 solution was added to the funnel to reduce I2 in chloroform phase to iodide and back-extracted iodine into water phase. This extraction and back extraction were repeated once for further purification.
The separated iodine (in iodide form) in a small volume (5-7 mL) was transferred to a centrifuge tube, 1.0 mL of 0.5 mol/L AgNO3 solution and 1 mL of 3.0 mol/L HNO3 were added to form AgI precipitate. The AgI precipitate was separated using centrifugation at 3500 rpm for 3-5 min, and washed in sequence using 10 mL 3 mol/L HNO3 and two aliquots of 10 mL deionized water to remove possibly formed Ag2SO3 and Ag2SO4 which are soluble in acidic solution. The precipitate was transferred to a 1.5 mL centrifuge tube. 125I in the precipitate was measured using a NaI gamma detector for calculating the chemical yield of iodine. The prepared AgI precipitate in small tube was dried at 70˚C and weighed, The dried precipitate was ground to fine powder and mixed with five times by mass of niobium powder (325 mesh, Alfa Aesar, Ward Hill, MA), which was finally pressed into a copper holder using a pneumatic press (Zhenjiang Aode Presser Instruments Ltd.). 129I/127I atomic ratios in the prepared targets were measured by the 5 MV AMS system at the Tandem Laboratory, Uppsala University. The standard used in the measurement was the NIST-SRM-4949c 129I. All samples, blanks and standards were measured for 6 cycles and 5 minutes per sample in each cycle. A detailed description of AMS system and measurement of 129I has been reported elsewhere 41. It should be noted that only the samples collected in 2015 by research vessel Argos were analysed for 129I. Other samples were not feasible for 129I analysis, since the samples have been acidified before receiving, resulting in loss of iodine due to its high volatility in acidic conditions.
Acknowledgements
J. Q. is grateful to all colleagues in the Radioecology and Tracer Studies Section, Department of Environmental Engineering, Technical University of Denmark and Professor Robin Gloser at VERA, University of Vienna. The authors also wish to thank Danish Navy, STUK, GEOTRACES, SSM and SMHI (Argos cruise) for sample collection. A. A. acknowledges the UPAR funding from the UAEU.
Author contributions
J. Q. initiated and coordinated the study, wrote manuscript, performed chemical analysis for uranium and made data evaluation. H. Z. performed chemical analysis for iodine. K. H and P. S performed AMS measurement for uranium. A.A. and G. P. coordinated AMS measurement for iodine. H. Z., G. H., X. H., V. V., A.A. and M. E. contributed to sample collection. G. H. and P. S. have valuable input to the outline of the discussion. All contributed to interpretation and manuscript reviewing.
Competing interests
The authors declare no competing interests.
Additional information
Supporting information includes Tables S1-S4 and Figure S1, which is available online.
Correspondence and request for materials should be addressed to J. Q.
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There is NO Competing Interest.
- Supportinginformationfinal.docx
Supporting information