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.