Introduction

236U (t½ = 2.34 × 107 years) is an isotope of uranium that is produced by thermal neutron capture of 235U via (n, γ)-reactions and through 238U (n, 3n) 236U reactions with fast neutrons. Even though a small amount of 236U (~35 kg) occurs naturally in the Earth’s crust, 236U is (by mass) the largest secondary product created in nuclear reactors, estimated to be ~106 kg1. 236U is therefore a sensitive tracer of deliberate or accidental leakage from the nuclear fuel/waste cycle2,3,4,5. 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 currents6. Emissions from other known sources of reactor 236U, e.g., the Springfield nuclear facility and the Fukushima accident, are negligible5,7.

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 1960s8. This ubiquitous fallout signature can make identification of sources of reactor 236U challenging because of methodological difficulties in distinguishing the source of 236U9. In addition, the 236U/238U ratio does not provide source information because of the prevalence of 238U in nature.

Reactor 236U can 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 plants10. Recently 233U measurements at environmental levels have become possible with advanced accelerator mass spectrometry10.

The representative 233U/236U atomic ratio of global fallout from atmospheric nuclear weapons testing was suggested to be (1.40 ± 0.15) × 10−2 9. 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 discharges11, which agrees well with reactor model calculations12. In the Irish Sea, an average 233U/236U atomic ratio of (0.12 ± 0.01) × 10−2 has been measured9, reflecting a dominant reactor signal released from SF. The use of the 233U/236U atomic ratio helps to deconvolve the origin of 236U based on the characteristic 233U/236U fingerprint from different source terms. 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 SF13,14,15,16.

The Baltic Sea is a highly polluted sea, with anthropogenic radionuclides demanding specific attention because of the risk to ecosystem and humans from radioactivity in the environment. It receives radionuclides from global fallout, discharges from the two European reprocessing plants, releases 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.

Results and discussion

Study area and sampling

The Baltic Sea is a landlocked intracontinental sea in Northern Europe with about 80 million inhabitants in the surrounding states and constitutes one of the largest brackish water environments on Earth17. 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 (Fig. 1). The driving force for the water circulation is freshwater surplus from river runoff, estimated at 473 km3 per year, together with “recycled” North Sea inflowing water as Baltic outflow that sum to a total water exchange rate of 753 km3 per year18. A mean residence time for the 21,721 km3 Baltic water volume19 was estimated to be 29 years, which is equivalent to a “half-life” for the water volume of 20 years18.

Fig. 1: Study region and sampling map.
figure 1

Overview of schematic circulation water mass in North Sea-Baltic Sea region (A) and sampling stations in this work as well as nuclear installations around the Baltic Sea (B). The symbols in A are CS Celtic Sea, EC English Channel, ECW English Channel Waters, NAC North Atlantic Current, NCC Norwegian Coastal Current, BB Bothnian Bay, BS Bothnian Sea, AS Archipelago and Åland Sea, GF Gulf of Finland, NB Northern Baltic Proper, WG Western Gotland Basin, EG Eastern Gotland Basin, GR Gulf of Riga, SB Sourth Baltic Proper, BMB Bornholm Basin, AB Arkona Basin, S The Sound, BTS Belt Sea, KG Kattegat, SKG Skagerrak, KGR Kattegat–Skagerrak region including the Kattegat, Skagerrak and Danish west coast nearby the North Sea, DS Danish Straits including the Belt Seas and the Sound, SBR South Baltic Sea region including Arkona Basin, Borholm Basin, and South Baltic Proper, MBR Middle Baltic Sea region including Northern Baltic Proper, Western Gotland Basin, Eastern Gotland Basin and Gulf of Riga, and NBR North Baltic Sea region including Archipelago and Åland Sea, Bothnian Sea and Bothnian Bay) Nuclear installations including: RH Ringhals NPP, BB Barseback NPP, GW Greifswald NPP, OS Oskarshamn NPP, SV Studsvik AB site, WH Westinghouse Electric Sweden AB, FM Forsmark NPP, OL Olkiluoto NPP, LO Loviisa NPP, LG Leningrad NPP, IL Ignalina NPP, SM Sillamäe site, PD Paldiski site, SP Salaspils research reactor. The stations marked with cross in B are either lake water or sediment samples (1—Lake Mälaren water; 2—Studsvik sediment; 3—sediment BY15; 4—sediment LL17; 5—sediment LL3a; 6—sediment EB1; 7—sediment CVI), all the other samples are seawaters collected in different years during 2011–2016 as marked with different symbols. Red arrows refer to bottom water movement and green arrows refer to surface water movement.

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In the investigation presented here, water and sediment samples were collected from the Baltic Sea and related water masses including the western Danish coast, from 2011 to 2016 (Supplementary Tables 1 and 2). The majority of water samples are from the surface (0–5 m depth), with a few samples from deep water, and one lake water from the Lake Mälaren, which receives downstream discharges from a nuclear fuel fabrication facility (Westinghouse) in Sweden and finally drains into the Baltic Sea. In addition to the Baltic Sea water, we analyzed sediment samples to assess the accumulation trend of the isotopes in 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 (Fig. 1) in the Baltic Sea including (1) KGR: Kattegat–Skagerrak region including the Kattegat, Skagerrak and 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.

Spatial pattern of 236U concentration and 236U/238U and 233U/236U atomic ratios

The measured 236U/238U atomic ratios (Supplementary Tables 1 and 2) vary within (5–52) × 10−9, with the higher ratios in the central and northern parts of the Baltic Sea and lower ratios in the western parts (Danish Straits, Kattegat, Skagerrak, and Danish west coast). The highest value reported here is sixfold greater than the average value found in the North Sea in 2010 ((7.6 ± 3.7) × 10−9)20.

The spatial patterns (Fig. 2) suggest a general decline of 236U with distance from higher values in the North Sea which is expected to be dominated by discharges from LH and SF. 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 values ((3–10) × 107 atom/l) in the central North Sea20. 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 pattern of increasing in 236U/238U ratio highlights an additional, likely local, source of 236U in the Baltic Sea7.

Fig. 2: Results of anthropogenic radionuclides.
figure 2

Distribution of 236U and 129I concentrations, and 236U/238U, 129I/127I, 233U/236U, and 236U/129I atomic ratios in the Baltic Sea surface water during 2011–2016.

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233U/236U atomic ratios obtained here are in the range of (0.14–0.87) × 10−2, with the lowest 233U/236U atomic ratios in the western parts of the Baltic, including the Danish coast, and the highest ratios in the central Baltic Sea. As the typical 233U/236U ratio for global fallout is (1.4 ± 0.1) × 10−29, 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 investigation21, with the highest values in the North Sea-Skagerrak–Kattegat, decreasing values toward the Sound and relatively constant values 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 Sea21.

Aldahan et al. 22 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 obtained in this work 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 (Fig. 2).

Potential sources of uranium and iodine in the Baltic Sea

Five different sources of uranium and iodine in the Baltic Sea are:

  1. (1)

    Natural ocean water, with salinity of 35‰, which contains ~60 µg/l 127I, 3 µg/l 238U, but negligible 129I, 236U, and 233U.

  2. (2)

    Natural freshwater with salinity <1‰, negligible 129I, 236U, and 233U, and significantly lower 127I and 238U than seawater (0.05–10 µg/l for both nuclides).

  3. (3)

    Global fallout from atmospheric nuclear weapons testing, with negligible 127I and 238U, an average 233U/236U atomic ratio of (1.40 ± 0.15) × 10−2, and a surface geographical distribution pattern for 236U and 233U similar to that of other actinides (e.g., Pu) from global fallout23. 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 SF24,25. In the Baltic Sea, with 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.

  4. (4)

    Marine discharges from European nuclear fuel reprocessing plants (including mainly SF and LH), with known 236U and 129I source functions24,26, 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.

  5. (5)

    The Chernobyl accident. Pu from Chernobyl has been found in fallout over central Europe27 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 fallout9,20. As they are distributed in the Baltic and mix with waters from various rivers, 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 region28. We will use the typical value 0.4 µg/l in the following calculations. There is more scatter in the 238U concentration for low salinities, which might be attributed to differences in regional riverine input. 129I also shows a general positive linear correlation with salinity demonstrated by two mixing lines for the western (KGR-DS, R2 = 0.89) and interior (SBR-MBR-NBR, R2 = 0.97) region (Fig. 3B). The scatter at the high salinity end can be attributed to the mixing of 129I enriched North Sea coast water with 129I depleted North Atlantic water in the Kattegat–Skagerrak region. 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 fresh waters in the basin.

Fig. 3: Variations of 238U and 129I with salinity.
figure 3

238U (A) and 129I (B) concentrations vs. salinity in the Baltic Sea. KGR Kattegat–Skagerrak region including the Kattegat, Skagerrak and Danish west coast nearby the North Sea, DS Danish Straits including the Belt Seas and the Sound, SBR South Baltic Sea region including Arkona Basin, Borholm Basin, and South Baltic Proper, MBR Middle Baltic Sea region including Northern Baltic Proper, Western Gotland Basin, Eastern Gotland Basin and Gulf of Riga, NBR North Baltic Sea region including Archipelago and Åland Sea, Bothnian Sea and Bothnian Bay. The zones (1–3) in B refer to dominant water mass: 1—North Sea-North Atlantic water, 2—Kattegat–Skagerrak water, and 3—Baltic Sea water. The intercept for linear regression line of 129I concentration vs. salinity was constrained to 0.6 × 109 atom/l according to the reported minimum 129I concentration in the Baltic river water22. Uncertainties are expanded uncertainties using a coverage factor of k = 1.

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Both the 236U/238U and 236U/129I atomic ratios increase with the decreasing salinity as waters mix in the interior of the Baltic Sea. The 236U/238U ratio increases by a factor of 3, while the 236U/129I ratio increases greater than an order of magnitude 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. If the source does not contain any 129I, the tenfold 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 (Supplementary Table 3)20,29. The deviation of the observed 236U/238U atomic ratio in the binary mixing (line L1, Fig. 4A) of the North Sea water and an assumed freshwater end member containing no 236U (neither 233U) from the best-fit model L reflects additional 236U sources besides North Sea water. The spatial distribution of deviations in the 236U/238U atomic ratio enable determination of the location of the additional 236U source (Supplementary Fig. 2). The distribution pattern is compatible with the introduction of additional riverine 236U input from the north Baltic region, which has most river runoff.

Fig. 4: Variations of 236U/238U and 236U/129I with salinity.
figure 4

236U/238U atomic ratio (A) and 236U/129I atomic ratios (B) vs. salinity. KGR Kattegat–Skagerrak region including the Kattegat, Skagerrak and Danish west coast nearby the North Sea, DS Danish Straits including the Belt Seas and the Sound, SBR South Baltic Sea region including Arkona Basin, Borholm Basin, and South Baltic Proper, MBR Middle Baltic Sea region including Northern Baltic Proper, Western Gotland Basin, Eastern Gotland Basin, and Gulf of Riga, NBR North Baltic Sea region including Archipelago and Åland Sea, Bothnian Sea, and Bothnian Bay, L (blue solid line) the best-fit 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 = (6.79 ± 0.75) × 10−8, and 236U = (6.87 ± 0.76) × 107 atom/l, L1 (black dashed line) the binary mixing line between the North Sea water and an assumed freshwater end member containing no 236U (salinity = 0, 238U = 0.4 µg/l, 236U/238U atomic ratio = 0 and 236U = 0, L2 (red dashed line) the binary mixing line between the North Sea water and the best-fit freshwater end member with salinity = 0, 238U = 0.4 µg /L, 236U = (3.56 ± 0.39) × 107 atom/l, and 236U/238U atomic ratio = (3.52 ± 0.39) × 10−8. The area marked in yellow represents the estimated excess mass of 236U in the Baltic Sea (X236), average salinity \(\bar S\) = 7.36‰. Uncertainties are expanded uncertainties using a coverage factor of k = 1.

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Nevertheless, it is challenging to define the 236U/238U ratio of the riverine input to the Baltic because a component of global fallout may still be present in runoff from the land surface. The 236U/238U and 236U/129I ratios cannot be used to determine the extent to which the excess 236U is from global fallout or an additional, previously undiscovered, source that has directly released 236U to the Baltic Sea.

Application of 233U/236U atomic ratio for 236U 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 (6.79 ± 0.75) × 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 0.20 × 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 = 0.20 × 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.

Fig. 5: Results of 233U/236U.
figure 5

233U/236U atomic ratio vs. 236U/238U atomic ratio (A) and salinity (B). KGR Kattegat–Skagerrak region including the Kattegat, Skagerrak and Danish west coast nearby the North Sea, DS Danish Straits including the Belt Seas and the Sound, SBR South Baltic Sea region including Arkona Basin, Borholm Basin, and South Baltic Proper, MBR Middle Baltic Sea region including Northern Baltic Proper, Western Gotland Basin, Eastern Gotland Basin and Gulf of Riga, NBR North Baltic Sea region including Archipelago and Åland Sea, Bothnian Sea, and Bothnian Bay, L (blue solid line) the best-fit 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 = (6.79 ± 0.75) × 10−8, and 236U = (6.87 ± 0.76) × 107 atom/l. Uncertainties are expanded uncertainties using a coverage factor of k = 1.

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On the other hand, a cluster of samples with the majority from the south, middle and north Baltic Sea region, representative for a large part of the Baltic surface water and with median salinity (6.92 ± 0.29)‰, show a typical 233U/236U atomic ratio of (0.53 ± 0.03) × 10−2 (the green dash-dotted line in Fig. 5). 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-thirds of the anthropogenic uranium observed in the middle and north Baltic Sea region seems to originate from this additional local source (Eq. (1)), indicating a strong contribution of 236U without 233U, i.e., from a thermal nuclear reactor 236U.

$${{R}}_{\mathrm{s}} = \frac{{{\mathrm{N}}_{233,{\mathrm{f}}} + {\mathrm{N}}_{233,{\mathrm{r}}}}}{{{\mathrm{N}}_{236,{\mathrm{f}}} + {\mathrm{N}}_{236,{\mathrm{r}}}}} = \frac{{{\mathrm{N}}_{236,{\mathrm{f}}} \cdot {{R}}_{\mathrm{f}} + {\mathrm{N}}_{236,{\mathrm{r}}} \cdot {{R}}_{\mathrm{r}}}}{{{\mathrm{N}}_{236,{\mathrm{f}}} + {\mathrm{N}}_{236,{\mathrm{r}}}}} = \frac{{{{R}}_{\mathrm{f}} + {\mathrm{N}}_{236,{\mathrm{r}}}/{\mathrm{N}}_{236,{\mathrm{f}}} \cdot {{R}}_{\mathrm{r}}}}{{1 + {\mathrm{N}}_{236,{\mathrm{r}}}/{\mathrm{N}}_{236,{\mathrm{f}}}}}$$
(1)

where Rs, Rf, and Rr represent, respectively, the 233U/236U atomic ratio of the Baltic seawater, global fallout, and nuclear reactor; N233, f and N233,r refer to the atomic number of 233U from global fallout and nuclear reactor, respectively; N236, f and N236, r refer to the atomic number of 236U from global fallout and nuclear reactor, respectively. Therefore, \(\frac{{{\mathrm{N}}_{236,{\mathrm{r}}}}}{{{\mathrm{N}}_{236,{\mathrm{f}}}}} = \frac{{{{R}}_{\mathrm{f}} - {{R}}_{\mathrm{s}}}}{{{{R}}_{\mathrm{s}} - {{R}}_{\mathrm{r}}}}\). With Rs = (0.53 ± 0.03) × 10−2, Rf = 1.4 × 10−2, and Rr = 0.12 × 10−2 (the Irish Sea ratio), we calculate the 236U contribution from our assumed reactor source to be 2.1 ± 0.2 times that of global fallout.

To locate this additional reactor 236U source, we apply another binary mixing line L2 (Fig. 4A) of the North Sea water with riverine water, the latter carrying global fallout that accounts for 1/(1 + 2.1) of the average 236U concentration of our samples in the Baltic Sea. Thus, the freshwater end member is characterized by salinity = 0, 238U = 0.4 µg/l, 236U = (3.56 ± 0.39) × 107 atom/l, which is calculated to match the 233U/238U atomic ratio ((1.70 ± 0.18) × 10−10) for the cluster of samples from SBR, MBR, and NBR at the media salinity of (6.92 ± 0.29)‰ (Supplementary Fig. 2). The resultant 236U/238U atomic ratio of the freshwater end member is (3.52 ± 0.39) × 10−8. The excesses of the 236U/238U atomic ratio from the mixing curve L2 and their spatial distribution are shown in Fig. 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.

Fig. 6: Deviation of 236U/238U from L2.
figure 6

Deviations of 236U/238U atomic ratio from binary mixing line L2 (A) and their respective geographical distribution on the map (B). KGR Kattegat–Skagerrak region including the Kattegat, Skagerrak and Danish west coast nearby the North Sea, DS Danish Straits including the Belt Seas and the Sound, SBR South Baltic Sea region including ArkonaBasin, Borholm Basin, and South Baltic Proper, MBR Middle Baltic Searegion including Northern Baltic Proper, Western Gotland Basin, EasternGotland Basin and Gulf of Riga, NBR North Baltic Sea region includingArchipelago and Åland Sea, Bothnian Sea, and Bothnian Bay. Average salinity \(\bar S\) = 7.36‰. Uncertainties are expanded uncertainties using a coverage factor of k = 1.

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Properties of the unknown 236U source

To narrow down the possible sources of the excess 236U, 236U inventories and fluxes need to be estimated. It should be noted this calculation is a first-order approximation based only on our data on surface waters from a multi-year survey. A precise interpretation will require more data, and to account for many different effects such as vertical distribution of 236U in the Baltic water columns, inter-annual variation in distribution pattern and on the scavenging of uranium into the sediment (especially in the anoxic regions).

The existence of an additional source of anthropogenic 236U in the Baltic Sea is indicated by the difference between the models L2 and L (Fig. 6). The amount of 236U required to explain this difference can be calculated by the following approximation and with the uncertainty estimated according to Müller30:

$$X_{236} = \, {\int}_0^{35} \left( {{\mathrm{L}}(S) - {\mathrm{L}}2(S)} \right)\left[ {{\,}^{238}{\mathrm{U}}(S)} \right]\frac{{dV\left( S \right)}}{{dS}}dS\\ \approx \, \left( {{\mathrm{L}} - {\mathrm{L2}}} \right)\left( {\bar S} \right)\left[ {{\,}^{238}{\mathrm{U}}\left( {\bar S} \right)} \right]V_{BS} = 200 \pm 47{\mathrm{g}}$$
(2)

where X236 is the excess mass of 236U in the Baltic Sea, S is the salinity, \(\left[ {{\,}^{238}{\mathrm{U}}\left( S \right)} \right]\) is the 238U concentration corresponding to S taken from Fig. 3A. \(\bar S\) is the average salinity of the Baltic Sea. \(\overline S\) was taken as 7.36‰ based on the reported mean salinity of the Baltic Sea during 1902–199831. Our data from SBR, MBR, and NBR, which comprise the bulk of Baltic Sea water, show an average of salinity of 7.06‰ and a median of 6.92%, comparable to the reported value. \(\left[ {{\,}^{238}{\mathrm{U}}\left( {\bar S} \right)} \right]\) is 0.9 µg/l. VBS is the volume of the Baltic Sea (21,721 km3)19 and \(({\mathrm{L}} - {\mathrm{L2}})(\bar S)\) = (1.02 ± 0.24) × 10−8 is the difference of the model curves at the average salinity. The approximation in the formula is possible because in the models L and L2, the 236U concentration is a linear function of S. Therefore, 200 ± 47 g of 236U is from the additional reactor source.

This calculation is a snapshot in time based on the uranium isotope ratios and salinity. While uranium concentrations in water may be altered in the partly anoxic Baltic Sea by precipitation of inorganic U(IV) or binding to organics in the sediment, the uranium isotopic ratios will only change by mixing of difference sources. Total salinity is slightly affected by precipitation (rain and snow) and evaporation (net balance 63 km3 per year)32, which may, be neglected at the present level of precision. Large intrusions of the North Sea water can change salinity patterns and introduce anthropogenic uranium from the North Sea. These intrusions add up to 5.2 × 109 metric tons of salt33, which is about 3% of the salt inventory of the Baltic32. The spatial pattern may not be constant throughout a multi-year survey, nevertheless, a minor change in the calculation is expected as we use only the average salinity for our estimate.

Taking into account that the ratio between the additional source and global fallout is N236,r/N236,f = 2.1, it suggests that 95 ± 22 g of 236U is related to global fallout introduced into the Baltic Sea directly or via riverine input. It is estimated that a total inventory of 1000 kg of anthropogenic 236U was distributed via global fallout mainly on the Northern Hemisphere7. Considering the surface area of the Baltic Sea of 3.77 × 105 km2 (without the catchment area) in comparison to the Northern Hemisphere (i.e., 2.55 × 108 km2), the total 236U deposition from direct global fallout is estimated as 1.5 kg. However, if considering the 29-year mean residence time (equivalent to 20-year half-life) of Baltic seawater, then most of the deposited 1.5 kg 236U was transported out of the region after 60 years (i.e., three half-lives), leaving behind ~0.19 kg. In addition, some 236U fraction from global fallout might be removed from the water body and incorporated into the Baltic sediment34. Therefore, the above estimation of 95 ± 22 g remaining 236U in the Baltic seawater from global fallout seems plausible, considering the uneven distribution of global fallout.

If we include the Baltic catchment area (1.64 × 106 km²) in the calculation, the input of global fallout 236U in the Baltic region can be up to 8 kg (1.5 kg in seawater + 6.4 kg in catchment area). However, only a small fraction of the particle associated 236U deposited on land can be leached and transported to the Baltic Sea through river runoff. If we assume this fraction accounts for 10% of the 6.4 kg of 236U deposited in the catchment, the total amount of global fallout 236U in the Baltic Sea might be about 0.64 + 1.5 = 2.14 kg.

Emissions from the Chernobyl accident may contribute additional 236U to the Baltic Sea, but it is difficult to quantify. 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 very limited documentation with poor, unreleased or missing data about the 233U and 236U release records from these installations (Supplementary Table 4)11. Data for 236U are 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 one lake water sample collected in Lake Mälaren (Supplementary Table 2), which receives waste discharges from the Westinghouse facility and finally drains into the Baltic Sea. 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 lake water shows a 233U/236U atomic ratio of (0.18 ± 0.05) × 10−2, a signature of reactor material.

The amount of 236U released from the Westinghouse installation (0.44 g) is negligible compared to the above estimated 280 g of the unknown reactor source in the Baltic Sea. For the Lake Mälaren, the 238U concentration was measured to be 1.5 ± 0.1 µg/l in this work, together with a flux of 166 m3/s, it means an input of 0.1 g per year of 236U, which is negligible also.

Another candidate for the additional source may be reactor fuel, dumped into the Baltic. 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 ~100,000 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 reactors35. The concentration will be even higher in the core of a nuclear reactor for marine applications, where enriched or highly enriched 235U is used; Russian submarine reactors were reported to contain 50–200 kg of 235U36. The former Soviet Union (USSR) was accused of dumping radioactive waste in the Baltic Sea, but it is not possible to assess the dumped amount37,38.

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, which is within ~100 km of a nuclear research company Studsvik AB, Nyköping that 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 Studsvik39. The aquatic discharges of radionuclides (except 3H) from Stusvik into the Baltic Sea in 1999–2010 were reported to be 0.45 TBq with the majority consisting of 90Sr, 137Cs, 60Co, and 134Cs40. Our measurements 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 (Supplementary Table 2). 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.

Outlooks for future study

The radiological risk associated with 233U, 236U, and 129I observed in this work is negligible due to their low specific activities and radiotoxicities. However, the observed unknown 236U reactor source may be an indication of leakage from a previously unrecognized (or unreported) additional radioactive source in the Baltic Sea, e.g., disposed nuclear waste in the seabed. Such source could potentially contain 137Cs and many other radionuclides imposing high radiological risks. Recent studies of the distribution of 137Cs inventories in the Baltic Sea indicated that 137Cs deposited in surface sediments is not permanently buried, but may be re-suspended and re-deposited by currents, bioturbation, or anthropogenic activities41. This leads us to suggest that radioactive release from such a source although currently low, might become more significant in the future with climate and environmental changes (e.g., sea level, temperature, and pH) in the Baltic Sea. It will be important to further understand the sources of anthropogenic radioisotopes in the Baltic regions, so that prediction and monitoring can prevent any associated radiological risk in the future. Further observation and forensic work will be needed to tighten the constraints in the binary mixing models, provide clear source terms and radiation risk assessment.

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 weak vertical salinity stratification, although strong thermoclines usually develop during the summer42. 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 Proper43. 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 pattern44. 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 (Fig. 1).

Water samples analyzed in the present investigation were collected on different cruises during 2011–2016. Samples of 2011 were obtained from the Baltic GEOTRACES Process Study on board research vessel R/V Oceania. Samples from 2013 to 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. 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 lake water sample from Lake Mälaren (in Sweden: 59.33 °N, 18.04 °E) was also sampled for the radioisotope analyses, as this lake receives downstream discharges from a nuclear fuel fabrication facility (Westinghouse) in Sweden, which finally drains into the Baltic Sea.

Five surface (0–2 cm) sediments in the middle and north parts of Baltic region were collected (Fig. 1 and Supplementary Table 2) during the COMBINE 2 cruise in 2016. One sediment sample collected outside Studsvik AB 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 AB). Details of the sampling campaigns and location of samples are summarized in Supplementary Tables 1 and 2 and Fig. 1.

Standards and reagents

Uranium standard solution (1.000 g/l in 2 M HNO3) purchased from NIST (Gaithersburg, MD) 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. The in-house 236U standards Vienna-KkU (236U/238U = (6.89 ± 0.32) × 10−11)1 and Vienna-US8 (236U/238U = (1.01 ± 0.05) × 10−8)45 diluted by ion (U/Fe = 1:30) were used to monitor the accuracy of the AMS measurement. Five standard samples (3 × Vienna-US8 and 2 × Vienna-KkU) were measured with a batch of around 30 environmental samples. The Vienna-KkU also serve as machine blank for the detection of 233U by AMS.

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 dilution 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 nebulizer under hot plasma conditions. The typical operational conditions of the instrument have been given elsewhere46. Indium (as InCl3) as an internal standard and 0.5 M HNO3 as a washing solution between consecutive assays were applied for 238U, and caesium (as CsCl) as an internal standard and 0.1 M NH3·H2O as a washing solution were used for 127I.

The radiochemical method for 233U and 236U separation from seawater was applied according to Qiao et al.47. Each seawater sample (0.8–10 l) were filtrated with filter paper (Munktell 00 K, particle retention 5–6 µm) to remove large particles and then acidified to pH 2 with concentrated HNO3. Purified FeCl3 solution (0.05 g/ml of Fe) was added to a final Fe concentration of 0.1 g/l. The sample was vigorously stirred with air bubbling for 5–10 min in order to decompose carbonate complexes. In total, 10% NH3·H2O was slowly added to adjust the pH to 8–9 for the co-precipitation of U with Fe(OH)3. The precipitate was allowed to settle for 0.5–1 h in order to decant most of the supernatant. The sample slurry was centrifuged at 3000 × g for 5 min and the supernatant was discarded. The final residue was dissolved with 15 ml of 3 M HNO3 and the solution was loaded onto a 2-ml UTEVA column which was preconditioned with 20 ml of 3 M HNO3. The UTEVA column was rinsed with 40 ml of 3 M HNO3, followed by 20 ml of 6 M HCl. Uranium absorbed on the column was eluted with 10 ml of 0.025 M HCl. The flow rate for the chromatographic separation was controlled manually to 1.0–1.5 ml/min.

A 100-µl aliquot of U eluate from the column separation was taken for measurement of 238U by ICP-MS to evaluate the chemical yields by comparison with ICP-MS analysis on diluted seawater samples. The 238U content measured in the eluate was also used for blank subtraction to calibrate the actual 236U/238U and 233U/238U atomic ratios47. The remaining fraction was used to prepare 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 min at 150 °C and then 2 h at 200 °C. A 100-µl aliquot leachate was taken for direct measurement of 238U by ICP-MS, which was used to calculate the 238U concentration 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 Vienna Environmental Research Accelerator (VERA) at the University of Vienna, Austria9,10,48. To summarize, U, which is extracted as UO− from a cesium sputter ion source, has to pass a first mass separation stage before it is injected into a tandem accelerator. For the analysis of actinides, the accelerator is operated at a terminal voltage of 1.65 MV and a rather high helium pressure in the terminal stripper is used to suppress molecular background49. The relatively high stripper gas pressure causes losses of a significant fraction of U3+ ions to angular scattering and change exchange outside of the stripper assembly. This gives an effective stripping yield of around 21% for the charge state 3+50, which is selected by the subsequent 90° analysing magnet. The combination of the analysing magnet with a Wien filter, an electrostatic analyzer, and a second 90° magnet, efficiently suppresses isotopic background on the high-energy side. Possible isotopic background is mainly caused by 235U and 232Th that are injected into the accelerator as 235U16O1H and 232Th16O1H, respectively. At the end of the AMS set-up, a Bragg-type ionization chamber is installed in order to detect and identify the remaining ions.

238UH3+ which escapes destruction in the stripping process gives a background to mass 239, 3+ lower than 238UH3+/238U3+ = 10−14. A similar suppression is expected for 235UH3+/235U3+, which suggests an instrumental background for 236U below 235UH3+/238U3+ = 10−16, which is negligible compared to the background of real 236U extracted from the ion source. The mass 239, 3+ background is monitored for every sputter sample. The situation is different for 233U3+, where the potentially interfering molecular isobar is 232ThH3+. In fact, an even higher intensity of these molecules was found from a similar ion source51. As thorium is a different chemical element, the behavior of hydride during stripping cannot be predicted from uranium ions. However, because thorium is only a trace element in our sputter samples, much less suppression than for 235UH3+ would be sufficient to render 232ThH3+ insignificant as background for 233U3+. For quality control, 232Th3+ is monitored for all sputter samples, which is extracted as 232ThO−. Though substantial rate above 100 kHz (too high for quantification by our detector) were observed in some cases, no correlation with the mass 233, 3+ count rates were found. This suggests that 232ThH3+ is also sufficiently supressed by the high stripper gas pressure.

A detection efficiency of 2 × 10−4 for environmental samples and a detection limit for 236U/U below 10−14 has been reported for the VERA set-up10. Because of the small relative mass difference (ca. 1%), fractionation effects between233U and 236U are negligible, therefore a detection efficiency comparable to 236U is assumed for 233U. In samples with low 236U content, e.g., procedure blanks, the uncertainty of 236U/238U atomic ratio measured by AMS is mainly attributed to the counting statistics, while for environmental samples the precision usually is limited by the reproducibility of multiple measurements which is taken into account in the overall uncertainty of 1–5% as well. Due to the low count rates of environmental 233U, the uncertainty of the 233U/238U atomic ratio is dominated by counting statistics of 233U. As the 238U content in the sample determined by ICP-MS was used for blank correction of the atomic ratios measured by AMS, the overall uncertainty of the blank corrected values presented in Supplementary Table 1 is therefore a combination of the corresponding AMS and ICP-MS uncertainties calculated by error propagation.

For the determination of 129I in seawater, 100 ml of sample was transferred into separation funnels. In total, 2.0 mg of 127I carrier (prepared using iodine crystal purchased from Woodward company, USA, with a 129I/127I ratio of 2 × 10−14), 500 Bq of 125I− tracer, and 0.5 ml of 0.5 M Na2S2O5 solution were added to the funnel, and then the pH of the solution was adjusted to 1–2 using 3 M HNO3 to convert all iodine species to iodide. With addition of 20–50 ml chloroform (CHCl3) and 2–5 ml 1.0 M 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 M Na2SO3 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 processes 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 M AgNO3 solution and 1 ml of 3.0 M HNO3 were added to form AgI precipitate. The AgI precipitate was separated using centrifugation at 2300 × g for 3–5 min, and washed in sequence using 10 ml 3 M HNO3 and two times 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. 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 prepared by dilution of 129I standard (NIST-SRM-4949c) and mixed with 127I carrier to a ratio of 129I/127I of 1.0 × 10−11. All samples, blanks, and standards were measured for six cycles and 5 min per sample in each cycle. It should be noted that only the samples collected in 2015 by research vessel Argos were analyzed 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.