A 40-year marine record of 137Cs and 99Tc transported into the Danish Straits: Significance for oceanic tracer studies

Half-century trends of radioactivity in fish from Danish areas of the North Sea, Kattegat, and Baltic Sea

This work reports comprehensive time-series datasets over the past 50 years for natural (210Po) and anthropogenic (134Cs and 137Cs) radionuclides in three fish species (cod, herring and plaice) from Danish marine areas covering the North Sea, Kattegat, and Baltic Sea. Impact from the global fallout of atmospheric nuclear weapons testing, radioactive discharges from the European nuclear reprocessing plants and release from Chernobyl accident are clearly detected in the fish samples. While 210Po concentrations in each fish species demonstrated comparable levels across the three regions without notable temporal trends, significantly higher median 210Po concentration was observed in the lower trophic level fish, namely herring and plaice, compared to cod. In contrast, 137Cs concentrations in all three species steadily decrease over time after the Chernobyl-attributed peaks in late 1980s in the entire study area, whereas 137Cs always demonstrated higher concentrations in cod than herring and plaice. Our calculated concentration factors (CFs) for 137Cs in this work indicate that the mean CFs for 137Cs over the past 50 years are significantly different across the three species, following the order of cod < herring < plaice. Based on the time-series data, ecological half-lives (Teco) of 137Cs in fish from Danish marine areas were estimated to evaluate the long-term impact of anthropogenic radioactive contamination in different regions. Our results indicate no significant difference in Teco across different fish species, whereas the weighted mean Teco for fish in the Baltic Sea (29.3 ± 3.9 y) is significantly longer than those of the North Sea (9.8 ± 0.9 y) and Kattegat (11.7 ± 1.2 y), reflecting the strong 'memory effect' of the Baltic Sea due to its slow water renewal. However, the dose assessment demonstrates that the contribution of the natural radionuclide 210Po to ingestion dose from fish consumption is 1-2 order of magnitude higher compared to that of 137Cs.

Sediments as sinks and sources of marine radionuclides: Implications for their use as ocean tracers

A Lagrangian transport model for the North Atlantic has been applied to simulate the historical releases of 137Cs, 129I and 236U from the European nuclear fuel reprocessing plants. Advection by currents, mixing and decay are included, as radionuclide interactions between water, sediments and suspended matter. The model was validated comparing predictions with measured radionuclide concentrations in water and sediments in several areas. 129I and 236U signals entering the Arctic Ocean have been compared with the input terms: the 236U signal is distorted, but the 129I signal preserves its shape. In the first moments after the releases, the sediments act as sinks for 236U, but not significantly for 129I and ultimately they become sources of 236U to the open sea. This results in a weaker correlation between input and output signals for 236U than for 129I. The same effects as for 236U have been found for 137Cs signal into the Arctic.

Long-term environmental risks of the Baltic Sea's "memory effect" revealed by ocean modeling and observation of reprocessing-derived radiotracers

Although previous research indicated that the Baltic Sea has a strong "memory effect" for trapping pollutants/nutrients, the associated environmental risks are not well understood due to the knowledge gaps in the long-term hydrodynamics-driven exchange of pollutants/nutrients between the North Sea and the Baltic Sea. In this work, we exploited ⁹⁹Tc and ¹²⁹I released from the two European nuclear reprocessing plants as oceanic tracers and pollutant proxies, and performed a five-decade hindcast simulation to quantitatively estimate the fluxes and timescales of marine transport of pollutants/nutrients in the North-Baltic Sea. Modeling results underline two potential environmental risks of the Baltic Sea's "memory effect": (1) ∼26 years of environmental half-life for any existing water-soluble pollutants/nutrients in the Baltic Sea driven by its hydrodynamics; (2) the Baltic Sea as a pollutant reservoir continuously exporting 3 % of contaminations per year to the downstream areas after any pollution event. Our findings provide fundamental knowledge for understanding the long-term hydrodynamics-driven pollutant/nutrient transport in the North-Baltic Sea, facilitating the future regional management of the marine environment.

Tracing Atlantic water transit time in the subarctic and Arctic Atlantic using 99Tc-233U-236U

The pathway and transport time of Atlantic water passing northern Europe can be traced via anthropogenic radioisotopes released from reprocessing of spent nuclear fuels at Sellafield (SF) and La Hague (LH). These reprocessing derived radioisotopes, with extremely low natural background, are source specific and unique fingerprints for Atlantic water. This study explores a new approach using ⁹⁹Tc-²³³U-²³⁶U tracer to estimate the transit time of Atlantic water in the coast of Greenland. We isolate the reprocessing plants (RP) signal of ²³⁶U (²³⁶U_RP) by incorporating ²³³U measurements and combine this with ⁹⁹Tc which solely originates from RP, to estimate the transit time of Atlantic water circulating from Sellafield to the coast of Greenland-Iceland-Faroe Islands. Both being conservative radioisotopes, the temporal variation of ⁹⁹Tc/²³⁶U_RP ratio in Atlantic water is only influenced by their historic discharges from RP, thus ⁹⁹Tc/²³⁶U_RP can potentially be a robust tracer to track the transport of Atlantic water in the North Atlantic-Arctic region. Based on our observation data of ⁹⁹Tc-²³³U-²³⁶U in seawater and the proposed ⁹⁹Tc/²³⁶U_RP tracer approach, Atlantic water transit times were estimated to be 16-22, 25 and 25 years in the coast of Greenland, Iceland and Faroe Island, respectively. Our estimates from northeast Greenland coastal waters agree with earlier results (17-22 years). Therefore, this work provides an independent approach to estimate Atlantic water transit time with which to compare estimates from ocean modelling and other radiotracer approaches.

Rapid determination of 99Tc in water samples using Ti(OH)3-TcO2 co-precipitation and TK200 resin by liquid scintillation counting

A novel method for the determination of ⁹⁹Tc in water samples was developed using stable Re as a chemical yield tracer and TiCl₃ as a reducing agent. The influences of several experimental parameters, including TiCl₃ concentration, HCl concentration and reaction time, on the reduction of TcO₄^- and ReO₄^- as well as Ti(OH)₃-TcO₂-ReO₂ co-precipitation were investigated. Tc(VII) and Re(VII) retained on TK200 resin were effectively eluted by 5 mL of 1 mol/L NH₄SCN, which can be directly mixed with the scintillation cocktail for liquid scintillation counting. The results show that the chemical behaviors of Tc and Re are very consistent in the whole procedure. The decontamination factors of potential interferences from β-emitting nuclides mainly released from nuclear fuel reprocessing plants were also evaluated, and the minimum detectable activity concentration was calculated to be 0.08 Bq/L for ⁹⁹Tc in water samples with a counting time of 2 h.

Determination, Separation and Application of 137Cs: A Review

In the context of the rapid development of the world's nuclear power industry, it is necessary to establish background data on radionuclides of different samples from different regions, and the premise of obtaining such basic data is to have a series of good sample processing and detection methods. The radiochemical analysis methods of low-level radionuclides ¹³⁷Cs (Cesium) in environmental and biological samples are introduced and reviewed in detail. The latest research progress is reviewed from the five aspects of sample pretreatment, determination, separation, calculation, application of radioactive cesium and the future is proposed.

Anthropogenic 236U and 233U in the Baltic Sea: Distributions, source terms, and budgets

The Baltic Sea receives substantial amounts of hazardous substances and nutrients, which accumulate for decades and persistently impair the Baltic ecosystems. With long half-lives and high solubility, anthropogenic uranium isotopes (²³⁶U and ²³³U) are ideal tracers to depict the ocean dynamics in the Baltic Sea and the associated impacts on the fates of contaminants. However, their applications in the Baltic Sea are hampered by the inadequate source-term information. This study reports the first three-dimensional distributions of ²³⁶U and ²³³U in the Baltic Sea (2018-2019) and the first long-term hindcast simulation for reprocessing-derived ²³⁶U dispersion in the North-Baltic Sea (1971-2018). Using ²³³U/²³⁶U fingerprints, we distinguish ²³⁶U from the nuclear weapon testing and civil nuclear industries, which have comparable contributions (142 ± 13 and 174 ± 40 g) to the ²³⁶U inventory in modern Baltic seawater. Budget calculations for ²³⁶U inputs since the 1950s indicate that, the major ²³⁶U sources in the Baltic Sea are the atmospheric fallouts (∼1.35 kg) and discharges from nuclear reprocessing plants (> 211 g), and there is a continuous sink of ²³⁶U to the anoxic sediments (589 ± 43 g). Our findings also indicate that the limited water renewal endows the Baltic Sea a strong "memory effect" retaining aged ²³⁶U signals, and the previously unknown ²³⁶U in the Baltic Sea is likely attributed to the retention of the mid-1990s' discharges from the nuclear reprocessing plants. Our preliminary results demonstrate the power of ²³⁶U-¹²⁹I dual-tracer in investigating water-mass mixing and estimating water age in the Baltic Sea, and this work provides fundamental knowledge for future ²³⁶U tracer studies in the Baltic Sea.

Spatial and temporal variations of 14C in Fucus spp. in Swedish coastal waters

Carbon-14 (14C) dominates the collective effective dose from globally dispersed long-lived radionuclides produced by and released from the nuclear power industry. Literature data on the discharge of 14C to the marine environment from nuclear power plants (NPPs) and its dispersion in the marine ecosystem are sparse. The local marine 14C background must be determined before the 14C enrichment in the marine environment from a NPP can be estimated. This is not trivial since marine activity concentrations of 14C vary spatially, partly due to long-range transport of 14C from other anthropogenic sources. We have analysed 14C in samples of several species of brown algae (Fucus spp.) collected at 45 sites along the Swedish coast in 2020. At sites remote from NPPs, the 14C activity concentrations per unit mass of carbon (here expressed as Fraction Modern, F14C) were significantly higher on the west coast than on the east coast (F14C up to about 1.10 in Skagerrak, and about 1.01, close to atmospheric levels, in the Baltic Sea and the Gulf of Bothnia). On the west coast, F14C showed a strong correlation with salinity, both of which increased towards the north. This indicates that 14C is carried from other anthropogenic sources (e.g. from the nuclear fuel reprocessing plants at La Hague and Sellafield). The highest value of F14C observed was close to the Ringhals NPP on the west coast, F14C ≈ 1.3, which is higher than expected in the terrestrial environment of this NPP. We also report on temporal variations of F14C in Fucus spp. collected at Särdal on the Swedish west coast during the period 1967-2020. The values of F14C in the Särdal marine samples collected after the 1990s are clearly higher than F14C in clean air CO2, indicating contributions of 14C of anthropogenic origin.

70-Year Anthropogenic Uranium Imprints of Nuclear Activities in Baltic Sea Sediments

A strongly stratified water structure and a densely populated catchment make the Baltic Sea one of the most polluted seas. Understanding its circulation pattern and time scale is essential to predict the dynamics of hypoxia, eutrophication, and pollutants. Anthropogenic ²³⁶U and ²³³U have been demonstrated as excellent transient tracers in oceanic studies, but unclear input history and inadequate long-term monitoring records limit their application in the Baltic Sea. From two dated Baltic sediment cores, we obtained high-resolution records of anthropogenic uranium imprints originating from three major human nuclear activities throughout the Atomic Era. Using the novel ²³³U/²³⁶U signature, we distinguished and quantified ²³⁶U inputs from global fallout (45.4-52.1%), Chernobyl accident (0.3-1.8%), and discharges from civil nuclear industries (46.1-54.3%) to the Baltic Sea. We estimated the total release of ²³³U (7-15 kg) from the atmospheric nuclear weapon testing and pinpointed the ²³³U peak signal in the mid-to-late 1950s as a potential time marker for the onset of the Anthropocene Epoch. This work also provides fundamental ²³⁶U data on Chernobyl accident and early discharges from civil nuclear facilities, prompting worldwide ²³³U-²³⁶U tracer studies. We anticipate our data to be used in a broader application in model-observation interdisciplinary research on water circulation and pollutant dynamics in the Baltic Sea.

Uptake of Radionuclides 60Co, 137Cs, and 90Sr with α-Fe2O3 and Fe3O4 Particles from Aqueous Environment

In the paper, investigation results of the uptake efficiency of radionuclides ⁶⁰Co, ⁹⁰Sr, and ¹³⁷Cs dissolved in water onto iron oxides α-Fe₂O₃ and Fe₃O₄ are presented. It was found that sorption efficiency increased for higher pH values. Independent of the oxide nature, the uptake characteristics are the best toward ⁶⁰Co and the worst toward ¹³⁷Cs, forming the row as follows: ⁶⁰Co > ⁹⁰Sr > ¹³⁷Cs. The highest sorption ability at pH 9 was found for magnetite Fe₃O₄, which was 93%, 73%, and 26% toward ⁶⁰Co, ⁹⁰Sr, and ¹³⁷Cs, respectively, while the respective percentages for hematite α-Fe₂O₃ were 85%, 41%, and 18%. It was assumed that the main sorption mechanism was ion exchange. That may explain some decrease of the sorption efficiency in drinking water due to the interfering presence of magnesium and calcium cations. The obtained results indicated the feasibility of the tested sorbents and their merits, especially in terms of relatively high uptake coefficients, low costs, availability, and lack of toxicity.

Determination of low-level 135Cs and 135Cs/137Cs atomic ratios in large volume of seawater by chemical separation coupled with triple-quadrupole inductively coupled plasma mass spectrometry measurement for its oceanographic applications

Radioisotopes of cesium are powerful tracer for oceanographic studies. In this work, a novel method was developed for determination of ultra-low level ¹³⁵Cs and ¹³⁷Cs in seawater using triple-quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). Cesium was pre-concentrated from up to 45 L seawater samples using ammonium molybdophosphate (AMP) adsorption, following a selective leaching of cesium using Sr(OH)₂. The cesium was further purified from interfering elements using AMP-PAN and cation-exchange chromatography. Sr(OH)₂ leaching was found to be an effective approach for selective exchange of cesium from the AMP sorbent without dissolution, which avoids the problem of separation of huge amount of NH₄⁺ and MoO₄^2- in the following steps. The decontamination factors for barium and rubidium with the developed method were more than 4 × 10⁷ and 800, respectively. The separated ¹³⁵Cs and ¹³⁷Cs were measured using ICP-MS/MS by employing N₂O as reaction gas to further elimination of isobaric (i.e. ¹³⁵Ba and ¹³⁷Ba) and polyatomic ions interferences. A detection limit of 1.5 × 10^-16 g L^-1 for ¹³⁵Cs in seawater was achieved. The concentrations of ¹³⁵Cs in seawater from Baltic Sea, Danish straits and Roskilde Fjord were determined using the developed method to identify the sources of ¹³⁵Cs, the water masses exchange in this region was investigated using ¹³⁵Cs and ¹³⁷Cs.

Correlations between 7Be, 210Pb, dust and PM10 concentrations in relation to meteorological conditions in northern Poland in 1998-2018

Analysis of a twenty-year (1998-2018) data series on ⁷Be concentrations in weekly collected aerosol samples in northern Poland showed a clear pattern of seasonal changes in ⁷Be with a maximum in the summer period associated with the most intensive thermal convection and vertical mixing. Activity concentrations of ⁷Be ranged from 480 μBq m^-3 to 9370 μBq m^-3. A strong relationship has been shown between ⁷Be concentrations observed in years and the activity of the Sun related to the sunspot number. Activity concentrations of ²¹⁰Pb in aerosol ranged from 17 μBq m^-3 to 1490 μBq m-3 with maximum occurring in the winter. The difference in the seasonal pattern in ⁷Be and ²¹⁰Pb concentrations were directly related to the different sources of both isotopes, as an additional source of ²¹⁰Pb was the products of combustion during the heating season. Similar pattern with maximum concentrations in winter was observed for PM10, as the main source is the same as in the case ²¹⁰Pb. A content of PM10 was in the range from 6.5 to 81.7 μg m^-3. A statistically significant correlation between both isotopes occurs. At the same time, ⁷Be, ²¹⁰Pb and PM10 are visibly related to the dust concentrations ranged from 7.3 μg m^-3 in winter to 134.8 μg m^-3 in spring. Statistical analysis carried out with simple regression model, stepwise multiple regression, and Random Forest models showed that the sunspots number, air temperature and sunshine duration have the most substantial impact on transport, and hence the concentration of ⁷Be in the surface layer of the atmosphere. The increase in relative humidity and precipitation and higher wind speed have a statistically significant effect on the reduction of ⁷Be concentrations in surface air.

Determination of Ultratrace Level 135Cs and 135Cs/137Cs Ratio in Small Volume Seawater by Chemical Separation and Thermal Ionization Mass Spectrometry

The atomic ratio of ¹³⁵Cs/¹³⁷Cs is a powerful fingerprint for distinguishing the source terms of radioactive contamination and tracing the circulation of water masses in the ocean. However, the determination of the ¹³⁵Cs/¹³⁷Cs ratio is very difficult due to the ultratrace level of ¹³⁵Cs (<0.02 mBq/m³) and ¹³⁷Cs (<2 Bq/m³) in the ordinary seawater samples. In this work, a sensitive method was developed for determination of ¹³⁵Cs concentration and ¹³⁵Cs/¹³⁷Cs ratio in seawater using chemical separation combined with thermal ionization mass spectrometry (TIMS) measurement. Cesium was first preconcentrated from seawater using ammonium molybdophosphate-polyacrylonitrile column chromatography and then purified using cation exchange chromatography to remove the interferences. With this method, decontamination factors of 6.0 × 10⁶ for barium and 1800 for rubidium and a chemical yield of more than 60% for cesium were achieved. By using glucose as an activator, the ionization efficiency of cesium was significantly improved to 50.6%, and a constant high current of Cs⁺ (20 V) can be maintained for more than 180 min, which ensures sensitive and reliable measurement of low level ¹³⁵Cs and ¹³⁷Cs. Detection limits of 4.0 × 10^-17 g/L for both ¹³⁵Cs and ¹³⁷Cs for 200 mL seawater were achieved, which enables the accurate determination of ¹³⁵Cs concentration and ¹³⁵Cs/¹³⁷Cs ratio in a small volume of seawater samples (<200 mL). The developed method has been validated by analysis of seawater reference material IAEA-443. Seawater samples collected from the Greenland Sea, Baltic Sea, and Danish Straits have been successfully analyzed for ¹³⁵Cs concentrations and ¹³⁵Cs/¹³⁷Cs ratios, and the results showed that ¹³⁵Cs concentrations in the seawater of the Baltic Sea is much higher than that in the Greenland Sea, which is attributed to the high deposition of Chernobyl accident derived radiocesium in the Baltic Sea region.