A previously unknown source of reactor radionuclides in the Baltic Sea, identied by 233, 236, 238U and 127, 129I multi-ngerprinting

We present the rst application of multi-isotopic ngerprints (i.e., 236 U/ 238 U, 233 U/ 236 U, 236 U/ 129 I and 129 I/ 127 I) 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 236 U source in the Baltic Sea. This reactor 236 U 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 seaoor, either reported or unreported. Such leakage would indicate potential problems with the safety of seaoor 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 233 U/ 236 U 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.

Introduction 236 U (t ½ = 2.34 × 10 7 y) is an isotope of uranium, which is produced by thermal neutron capture of the omnipresent 235 U via (n, γ)-reactions and through 238 U (n, 3n) 236 U reactions with fast neutrons. Even though a small amount (about 35 kg in total) of 236 U is produced naturally in the Earth's surface environments, 236 U is (by mass) the largest secondary product created in nuclear reactors, estimated totally to be an order of 10 6 kg 1 . 236 U is a sensitive tracer of deliberate or accidental leakage from the nuclear fuel/waste cycle [2][3][4][5] . The known sources of reactor 236 U, i.e., deliberate releases from the two European reprocessing plants at La Hague, France (LH) and Sella eld, UK (SF) since 1950s, can be traced throughout the North Atlantic and the Arctic water currents 6 . Emissions from other known sources of reactor 236 U, e.g., the Spring eld nuclear facility and the Fukushima accident, are negligible 5,7 .
A signi cant amount of 236 U (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 identi cation of unreported sources of reactor 236 U challenging, because of methodological di culties in distinguishing the source of 236 U 9 . In addition, the 236 U/ 238 U ratio does not provide source information because of 238 U of natural origin is ubiquitous.
Reactor 236 U could be differentiated from fallout 236 U because these sources have different and characteristic 233 U/ 236 U ratios due to different nuclear production mechanisms. 233 U was mostly produced during nuclear weapons testing by fast neutrons via 235 U (n, 3n) 233 U reactions or directly by 233 U-fueled devices, whereas almost no 233 U is produced in thermal nuclear power reactors or reprocessing plants 10 . Recently 233 U measurements at environmental level have become possible 10 .
The representative 233 U/ 236 U 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 233 U/ 236 U 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 233 U/ 236 U atomic ratio of (0.12 ± 0.01) × 10 − 2 has been measured 9 , re ecting a dominant reactor signal released from SF. The use of the 233 U/ 236 U atomic ratio helps better distinguishing the origin of 236 U and since being radionuclides of the same element, the 233 U/ 236 U ratio will not be affected during the transport pathway. In addition, the combination of 236 U with other radionuclides, e.g. 129 I, can be useful to trace the transport of 236 U from speci c source points, e.g., releases from LH and SF [13][14][15][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 − 233 U, 236 U, and 129 I -to identify a previously unknown local source of radionuclide pollution to the Baltic Sea.

Results
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 km 3 yr -1 together with "recycled" North Sea in owing water as Baltic out ow that sum up to a total water exchange rate of 753 km 3 yr -1 18 . A mean residence time for the 21580 km 3 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 ve geographical regions ( Figure.  Distribution of 236 U concentration and 236 U/ 238 U and 233 U/ 236 U atomic ratios The measured 236 U/ 238 U 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 236 U 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 236 U concentrations ((6-9) ×10 7 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) ×10 7 atom/L) 19 . Compared to the Kattegat-Skagerrak region, the average 236 U/ 238 U 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 236 U/ 238 U ratio points out an additional, likely local, supply of 236 U in the Baltic Sea 7 . 233 U/ 236 U atomic ratios obtained here are in the range of (0.14-0.87) × 10 -2 , with the lower 233 U/ 236 U 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 233 U/ 236 U ratio for global fallout is (1.4 ± 0.1) × 10 - Aldahan et al. 21 reported that the average concentration of 129 I in the rivers around the Baltic Sea was 3.9 × 10 8 atom/L, which suggested some minor contribution of 129 I from riverine water to the Baltic Sea.
The 129 I concentrations show a larger gradient (two orders of magnitude) compared to the 236 U concentrations (15-fold) along the Baltic Sea. 236 U/ 129 I ratios are within the range of (5-133) × 10 -4 and indicate a reversed geographical distribution compared to 129 I concentration and 129 I/ 127 I 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 127 I, 3 µg/L of 238 U, but negligible 129 I, 236 U and 233 U. B) Natural fresh water with salinity < 1‰ and negligible concentrations of 129 I, 236 U and 233 U, and lower 127 I and 238 U than seawater (0.05-10 µg/L for both nuclides). C) Global fallout from atmospheric nuclear weapons testing, with negligible 127 I and 238 U, an average 233 U/ 236 U atomic ratio of (1.4 ± 0.2) × 10 − 2 , and a surface geographical distribution pattern for 236 U and 233 U similar to that of 137 Cs which is not unusually high in the Baltic region 22 . Earlier studies have estimated 236 U concentration (up to 1.4 × 10 8 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 233 U/ 236 U atomic ratio of the global fallout contribution is expected to be constant after 1980 when all countries stopped aboveground nuclear bomb tests. Concentration of 236 U in river runoff is expected to have reduced over the decades, while the 233 U/ 236 U atomic ratio stays constant. D) Marine discharges from European nuclear fuel reprocessing plants (including mainly SF and LH), with known 236 U and 129 I source functions 23,24 , but negligible amounts of 127 I and 238 U. This source dominates the 236 U and 129 I budget of marine water entering the Skagerrak from the North Sea.
Compared to 236 U, almost no 233 U is produced in thermal nuclear reactors, and thus 233 U 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 236 U should have been deposited following a similar pattern as Pu isotopes. Consequently, a Chernobyl signal of 236 U may be present in river runoff and marine waters. Based on the present understanding of the production mechanisms of 233 U, it is expected that Chernobyl fallout is not a signi cant contributor of 233 U in this context. Waters entering the Baltic Sea from the North Sea have 236 U/ 238 U and 233 U/ 236 U 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 236 U and 233 U (and minor 238 U in river waters). Removal of uranium from Baltic water will not alter the ratios. The increase in 236 U/ 238 U observed within the Baltic Sea points clearly to a local source of this anthropogenic radionuclide.

U source identi cation via binary mixing
The concentration of 238 U (Fig. 3A) demonstrates a strong positive correlation (R 2 = 0.91) with salinity.
The intercept corresponds to the average riverine input with a 238 U concentration of 0.33 ± 0.05 µg/L, which falls in the range (0.2-0.7 µg/L) of 238 U for some rivers in the Baltic Sea region 26 . There is more scatter in the 238 U concentration for low salinities, which might be attributed to differences in regional riverine input. I-129 also shows a positive linear correlation (R 2 = 0.69) with salinity ( Fig. 3B), but strong scatter occurs at the high salinity end. This trend can be attributed to the mixing of 129 I enriched North Sea coast water with 129 I depleted North Atlantic water in the Kattegat-Skagerrak region ( Fig. 1 A). The 238 U and 129 I 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 236 U/ 238 U and 236 U/ 129 I atomic ratios increase with the decreasing salinity as water mix in the interior of the Baltic Sea. The 236 U/ 238 U ratio increases by a factor of 3, while the 236 U/ 129 I ratio increases from an average of (8 ± 2) × 10 − 4 in the Kattegat-Skagerrak region, corresponding to reprocessing derived 236 U and 129 I, to 1 × 10 − 2 in the central Baltic Sea. Both ratios indicate addition of 236 U from a local source. The difference in increases between the two ratios can be explained by the presence of 238 U and possibly 129 I in that local source in addition to 236 U. If the source does not contain any 129 I, the 10-fold increase in 236 U/ 129 I suggests that ca. 90% of 236 U in the central Baltic Sea is from local sources. If the source does contain 129 I, the portion of 236 U derived locally must be still larger.
To understand the source terms of 236 U in the Baltic Sea, a binary mixing model is applied with two respective end members representing 236 U input from the North Sea and freshwater input via river runoff. Parameters for the rst end member representing the North Sea water entering from the west Baltic Sea are well de ned by previous studies (Table S3) 19,27 . The deviation of the observed 236 U/ 238 U atomic ratio from the binary mixing (line L1, Fig. 4A) between the North Sea water and an assumed freshwater end member containing no 236 U (neither 233 U) re ects additional 236 U sources besides North Sea water. The spatial distribution of deviations in the 236 U/ 238 U atomic ratio allow locating the additional 236 U source ( Figure S2). The distribution pattern in Figure S2 is compatible with the hypothesis of additional riverine 236 U input from the north Baltic region, which has most river runoff.
Nevertheless, it is challenging to de ne the 236 U/ 238 U ratio of the riverine input to the Baltic because some global fallout may still be washing from the land surface. The 236 U/ 238 U and 236 U/ 129 I ratios therefore do not directly indicate whether the excess 236 U is only from global fallout, or from an additional, previously undiscovered, source that has directly released 236 U to the Baltic Sea.
Application of 233 U/ 236 U atomic ratio for 236 U source identi cation If we assume that the excess 236 U originates only from global fallout, the 236 U/ 238 U atomic ratio of the riverine input in the best-t binary mixing is 6 × 10 − 8 (line L, Fig. 4A). However, there is a clear deviation of the observation from the model for 233 U/ 236 U atomic ratios (Fig. 5A). A subgroup of samples from the Kattegat-Skagerrak reveal a relatively stable 233 U/ 236 U atomic ratio of 0.2 × 10 − 2 (blue dash-dotted line in Therefore, . With = 0.5 × 10 -2 , = 1.4 × 10 -2 and = 0.12 × 10 -2 , we obtain that the 236 U contribution from our assumed reactor source is 2.4 times that of global fallout. To locate this additional reactor 236 U source, we apply another binary mixing line L2 (Figure 4 A) Figure 6. The data indicate that the extra reactor 236 U 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 236 U into these locations.

Properties of the 236 U unknown source
To identify the source of the excess 236 U, the order of magnitude of 236 U inventories and uxes 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 236 U 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 out ow water 18 ), meaning that the ratio of seawater to riverine water is 1:4.
An average excess of 6 × 10 7 atom/L 236 U in riverine water is obtained based on the deviation of the 236 U concentration from L1 in samples from NBR ( Figure S2). The volume of the Baltic Sea of 21700 km 328 with 80% riverine water corresponds to 400 g of 236 U. Taking into account that ca. 71% (i.e., =2.4) of this excess 236 U 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 236 U 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 × 10 5 km 2 (without the catchment area) in comparison to the Northern Hemisphere (half of the Earth's surface area, i.e. 5.10 × 10 8 km 2 ), then the total 236 U 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 236 U was transported out after 60 years, leaving behind 0.19 kg. In addition, some particle-associated 236 U fraction from global fallout might be incorporated into the Baltic sediment 29 . Therefore, the above estimation of 120 g remaining 236 U in the Baltic seawater from global fallout (using salinity data) seems justi ed.
Emissions from the Chernobyl accident may be an additional 236 U source in the Baltic Sea, yet it is di cult to be identi ed. 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 233 U and 236 U release records from these installations (Table S4) 11 . Data on 236 U is available from Westinghouse during 1998-2017, with a total reported release of 1.06 × 10 6 Bq of 236 U, 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 236 U/ 238 U 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 233 U/ 236 U atomic ratio of (0.18 ± 0.05) × 10 − 2 , a signature of reactor material.
The amount of 0.44 g of 236 U 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 238 U concentration was measured to be 1.5 ± 0.1 µg/L in this work, together with a ux of 166 m 3 /s 28 , it means an input of 0.1 g/yr of 236 U, which is negligible also.
Another candidate we assume is reactor fuel, dumped into the sea; the atomic ratio of 236 U/ 238 U 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). 235 U 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 235 U is used; the Russian submarine cores reportedly contain some 50 to 200 kg of 235 U 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 236 U/ 238 U 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 236 U content ((2.02 ± 0.12) × 10 13 atom/kg), which is three orders of magnitude higher than sediment collected from the North Baltic Sea region (Table S2). The 233 U/ 236 U atomic ratio ((0.36 ± 0.05) × 10 − 2 ) for the Studsvik sediment clearly indicates a higher contribution of reactor input compared to the other ve sediments collected in the Baltic Sea with 233 U/ 236 U ratios between 0.59 × 10 − 2 to 0.83 × 10 − 2 .
Even though the release of 236 U from Studsvik is not well documented due to its low speci c radioactivity, it is not surprising that waste discharges from Studsvik contain 236 U. The high 236 U levels in the sediment samples measured most likely originate from scavenging of waterborne 236 U from liquid waste discharges by particles into the sediment. Waste dumping/discharges in the Studsvik area are our most plausible candidate for the excess 236 U in the Baltic Sea.

Methods
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 strati cations, 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 strati ed 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  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 InCl 3 ) was used as an internal standard and 0.5 mol/L HNO 3 solution was used as a washing solution between consecutive assays.
The radiochemical method for 233 U and 236 U 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 puri cation with a 2-mL UTEVA column. A 100-µL aliquot of U eluate from the column separation was taken for measurement of 238 U 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 236 U/ 238 U and 233 U/ 236 U. For sediments, 5-10 g of each dried sample was ashed overnight at 450 °C in a mu e 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 238 U by ICP-MS, which was used to calculate the original 238 U 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 233 U and 236 U has been reported elsewhere 40 .
For the determination of 129 I in seawater, 100 ml of sample was transferred a separation funnels. 2.0 mg of 127 I carrier (prepared using iodine crystal purchased from Woodward company, USA, with a 129 I/ 127 I ratio of 2 x 10 -14 ), 500 Bq of 125 Itracer and 0.5 mL of 0.5 mol/L Na 2 S 2 O 5 solution were added to the funnel, and then the pH of the solution was adjusted to 1-2 using 3 mol/L HNO 3 to convert all iodine species to iodide. With addition of 20-50 mL chloroform (CHCl 3 ) and 2-5 mL 1.0 mol/L NaNO 2 , iodide was oxidized to I 2 and extracted to CHCl 3 phase by shaking. The extraction procedure was repeated three times to extract all iodine. The CHCl 3 phases were combined to a new funnel, 20 mL H 2 O and 0.3-0.5 mL 0.05 mol/L Na 2 S 2 O 5 solution was added to the funnel to reduce I 2 in chloroform phase to iodide and back-extracted iodine into water phase. This extraction and back extraction were repeated once for further puri cation.
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 AgNO 3 solution and 1 mL of 3.0 mol/L HNO 3 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 HNO 3 and two aliquots of 10 mL deionized water to remove possibly formed Ag 2 SO 3 and Ag 2 SO 4 which are soluble in acidic solution. The precipitate was transferred to a 1.5 mL centrifuge tube. 125 I 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 ne powder and mixed with ve times by mass of niobium powder (325 mesh, Alfa Aesar, Ward Hill, MA), which was nally pressed into a copper holder using a pneumatic press (Zhenjiang Aode Presser Instruments Ltd.). 129 I/ 127 I 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 129 I. 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 129 I has been reported elsewhere 41 . It should be noted that only the samples collected in 2015 by research vessel Argos were analysed for 129 I. Other samples were not feasible for 129 I analysis, since the samples have been acidi ed before receiving, resulting in loss of iodine due to its high volatility in acidic conditions.    L (blue solid line): the best-t binary mixing line between the North Sea water and a freshwater end member with salinity = 0, 238U = 0.4 µg/l, 236U/238U atomic ratio = 6×10-8 and 236U = 6×107 atom/L).

Figure 6
Deviations of 236U/238U atomic ratio from binary mixing line L2 (A) and their respective geographical distribution on the map (B) (L2 refers to the red dashed line in Figure 4A)