An unknown source of reactor radionuclides in the Baltic Sea revealed by multi-isotope fingerprints

A review of natural and anthropogenic radionuclide pollution in marine bivalves

Radionuclide contamination in food is a public health issue. Bivalves are known to accumulate relatively high levels of radionuclides. Despite many relevant reports, this information is poorly organized. Therefore, in this study, we conducted a comprehensive scientific review of radionuclides in marine bivalves. In general, the accumulation of radionuclides in bivalves is highly species and tissue-specific, which may be due to the different biological half-life of radionuclides in different species and tissues. The trophic pathway is the main pathway for the accumulation of most radionuclides in bivalves, with polonium-210 (210Po) and lead-210 (210Pb) potentially selectively accumulating in the digestive glands, while 134Cs and 137Cs selectively accumulating in the adductor muscle and mantle. Some other radionuclides (radium-226 (226Ra) and strontium-90 (90Sr)) are absorbed along with other minerals (e.g. Calcium) and selectively accumulate in bivalve shells. The information in this study can provide an overview of radionuclide contamination in marine bivalves.

Reconstructing the chronology of the natural and anthropogenic uranium isotopic signals in a marin sediment core from beppu bay, Japan

The long-lived U isotopes, ²³³U and ²³⁶U, have been used increasingly in recent years as marine circulation tracers and for identifying sources of uranium contamination in the environment. The sedimentation histories of these two U isotopes in combination with natural ²³⁸U were reconstructed for an anoxic sediment core collected from Beppu Bay, Japan, in the western North Pacific Ocean showing good time resolution (less than 2.6 y/sample). The ²³³U/²³⁶U atom ratio showed a prominent peak of (3.20 ± 0.30) × 10^-2 around 1957 which can be attributed to the input from atmospheric nuclear weapons tests including thermonuclear tests conducting in the Equatorial Pacific. The integrated ²³³U/²³⁶U ratio of (1.64 ± 0.08) × 10^-2 for the sediment was found to be in relatively good agreement with the representative ratio published for global fallout (∼1.4 × 10^-2). A prominent increase in the authigenic ratio of ²³³U/²³⁸U_a,s in the leached fraction (1.39 ± 0.11 × 10^-11) and the bulk digestion (1.36 ± 0.10 × 10^-11) was also observed around 1957. This reflects the input supply of ²³³U to the seawater which is known to have a relatively constant ²³⁸U content. The authigenic ²³⁶U/²³⁸U_a,s ratio (0.18 ± 0.02 × 10^-9) obtained for 1921 increased from the early 1950's to a maximum of (6.59 ± 0.60) × 10^-9 around 1962. The variation in this ratio represents well the introduction history of U into the surface environment without site-specific U contamination and the time profile is also consistent with the ¹³⁷Cs signature. This work thus provides a benchmark for the long-term use of the isotopic U composition as an input parameter for seawater circulation tracers and as a chronological marker for anoxic sediments and sedimentary rocks. Especially the ²³³U/²³⁶U ratio may serve as a key-marker for the new geological age Anthropocene.

Plutonium Signatures in a Dated Sediment Core as a Tool to Reveal Nuclear Sources in the Baltic Sea

Plutonium distribution was studied in an undisturbed sediment core sampled from the Tvären bay in the vicinity of the Studsvik nuclear facility in Sweden. The complete analysis, including minor isotopes, of the Pu isotope composition (²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴²Pu, and ²⁴⁴Pu) allowed us to establish the Pu origin in this area of the Baltic Sea and to reconstruct the Studsvik aquatic release history. The results show highly enriched ²³⁹Pu, probably originating from the Swedish nuclear program in the 1960s and 1970s and the handling of high burn-up nuclear fuel in the later years. In addition, the ²⁴⁴Pu/²³⁹Pu atomic ratio for the global fallout period between 1958 and 1965 is suggested to be (7.94 ± 0.31)·10^-5. In the bottom layer of the sediment, dated 1953-1957, we detected a higher average ²⁴⁴Pu/²³⁹Pu ratio of (1.51 ± 0.11)·10^-4, indicating the possible impact of the first US thermonuclear tests (1952-1958).

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.

Retrospective determination of fallout radionuclides and 236U/238U, 233U/236U and 240Pu/239Pu atom ratios on air filters from Vienna and Salzburg, Austria

137Cs and 241Pu (via 241Am) concentrations were measured γ-spectrometrically on air filters from the early 1960s (mainly from 1964-66) from Vienna, Austria, and an alpine station in Salzburg, Austria. Accelerator mass spectrometry (AMS) was used to determine 240Pu/239Pu, 236U/238U and 233U/236U atom ratios as well as 236U, 239Pu and 240Pu atom concentrations. The maximum 236U/238U atom ratio of these unique undisturbed global fallout samples was (1.19 ± 0.31) × 10-5 in spring 1964. The 233U/236U atom ratios were found within (0.15-0.49) × 10-2 and indicate that the weapons tests of the early 1960s can be excluded as 233U source. The 236U/239Pu atom ratios were calculated in the range of 0.22-0.48.

Understanding source terms of anthropogenic uranium in the Arctic Ocean - First 236U and 233U dataset in Barents Sea sediments

This work reports the first dataset of ²³⁶U and ²³³U in sediment cores taken from the Barents Sea, with the aim to better understand the source terms of anthropogenic uranium in the Arctic region. Concentrations of ²³⁶U and ²³³U along with ¹³⁷Cs, and ²³³U/²³⁶U atomic ratio were measured in six sediment profiles. The cumulative areal inventories of ²³⁶U and ²³³U obtained in this work are (3.50-12.7) × 10¹¹ atom/m² and (4.92-21.2) × 10⁹ atom/m², with averages values of (8.08 ± 2.93) × 10¹¹ atom/m² and (1.08 ± 0.56) × 10¹⁰ atom/m², respectively. The total quantities of ²³⁶U and ²³³U deposited in the Barents Sea bottom sediments were estimated to be 507 ± 184 g and 7 ± 3 g, respectively, which are negligible compared to the total direct deposition of ²³⁶U (6000 g) and ²³³U (40-90 g) from global fallout in the Barents Sea. The integrated atomic ratios of ²³³U/²³⁶U ranging in (0.98-1.57) × 10^-2 reflect the predominant global fallout signal of ²³⁶U in the Barents Sea sediments and the highest reactor-²³⁶U contribution accounts for 30 ± 14 % among the six sediment cores. The reactor-²³⁶U input in the Barents Sea sediments is most likely transported from the European reprocessing plants rather than related to any local radioactive contamination. These results provide better understanding on the source term of anthropogenic ²³⁶U in the Barents Sea, prompt the oceanic tracer application of ²³⁶U for studying the dynamics of the Atlantic-Arctic Ocean and associated climate changes. The ²³⁶U-²³³U benchmarked age-depth profiles seem to match reasonably well with the reported input function history of radioactive contamination in the Barents Sea, indicating the high potential of anthropogenic ²³⁶U-²³³U pair as a useful tool for sediment dating.

Deciphering sources of U contamination using isotope ratio signatures in the Loire River sediments: Exploring the relevance of 233U/236U and stable Pb isotope ratios

A broad range of contaminants has been recorded in sediments of the Loire River over the last century. Among a variety of anthropogenic activities of this nuclearized watershed, extraction of uranium and associated activities during more than 50 years as well as operation of several nuclear power plants led to industrial discharges, which could persist for decades in sedimentary archives of the Loire River. Highlighting and identifying the origin of radionuclides that transited during the last decades and were recorded in the sediments is challenging due to i) the low concentrations which are often close or below the detection limits of routine environmental surveys and ii) the mixing of different sources. The determination of the sources of anthropogenic radioactivity was performed using multi-isotopic fingerprints (²³⁶U/²³⁸U, ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb) and the newly developed ²³³U/²³⁶U tracer. For the first time ²³³U/²³⁶U data in a well-dated river sediment core in the French river Loire are reported here. Results highlight potential sources of contamination among which a clear signature of anthropogenic inputs related to two accidents of a former NUGG NPP that occurred in 1969 and 1980. The ²³³U and ²³⁶U isotopes were measured by recent high performance analytical methods due to their ultra-trace levels in the samples and show a negligible radiological impact on health and on the environment. The determination of mining activities by the use of stable Pb isotopes is still challenging probably owing to the limited dissemination of the Pb-bearing material marked by the U-ore signature downstream to the former U mines.

127I and 129I species in the English Channel and its adjacent areas: Uncovering impact on the isotopes marine pathways

Radioactive iodine-129 has been released from the La Hague nuclear fuel reprocessing facility (NRF) into the English Channel, but the distribution and transformation of the isotope species, and environmental consequences have not been fully characterized in the Channel. Here we present data on iodine isotopes (¹²⁹I and ¹²⁷I) species in surface water of the English Channel and the southern Celtic Sea. Compared to ¹²⁷I species, the concentrations of ¹²⁹I^- and ¹²⁹IO₃^- show more variations, but iodate is the major species for both ¹²⁹I and ¹²⁷I. Our data provide new information regarding iodide-iodate inter-conversion showing that water dilution and mixing are the main factors affecting the ¹²⁷I and ¹²⁹I species distribution in the Channel. Some reduction of iodate occurs within the English Channel and mainly in the west part because of biotic processes. The ¹²⁹I species transformation is overall insignificant, especially in the eastern Channel, where a constant value of ¹²⁹IO₃^-/¹²⁹I is observed, which might characterize the La Hague wastewater signal. In the Celtic Sea, oxidation of iodide can be traced by ¹²⁷I and ¹²⁹I species. On a larger scale, ¹²⁹I generally experienced an oxidation process in the Atlantic Ocean, while in the coast of shallow shelf seas, new produced ¹²⁹I^- can be identified, especially in the German Bight and the Baltic Sea. The data of ¹²⁹I species in the English Channel can provide estimate of redox rates in a much broader marine areas if the transit time of ¹²⁹I from La Hague is well-defined. Furthermore, estimate of inventories for ¹²⁹I and its species in the Channel, and fluxes of ¹²⁹I species from the English Channel to the North Sea add important information to the geochemical cycle of ¹²⁹I.

A review of anthropogenic radionuclide 236U: Environmental application and analytical advances

²³⁶U is an anthropogenic radionuclide that is produced from nuclear reactions of ²³⁵U(n, γ) and ²³⁸U(n, 3n). It has gained extensive attention in the field of environment, geology, nuclear emergency, and nuclear forensics. Due to the unique physical and chemical character and the distinct fingerprint character from different sources, ²³⁶U has been successfully applied in the environmental tracer, nuclear material source appointment, and environmental assessment. Until now, few reviews were published about the database, application, and the latest analytical technology development of ²³⁶U. In this review, the ²³⁶U concentration and ²³⁶U/²³⁸U isotope ratio were summarized, and the data were classified into four categories, including soil and seawater samples affected by global fallout and nuclear incidents. Furthermore, the development of environmental application and pretreatment methods were also summarized. The advanced pretreatment technology using alkali fusion and flow injection was especially discussed to introduce the development of a rapid analytical method. Finally, the research challenge and direction of ²³⁶U were proposed for further research, such as the tracer application combining ²³⁶U with other radionuclides in the terrestrial environment and the precise analysis of minor isotopes in ultra-trace uranium samples. We hope this review will help scholars to have a deep research on the analysis and application of ²³⁶U.

Distribution of Anthropogenic 129I in the Western South China Sea and Its Application for Tracing the Sources and Movement of Pollution

Anthropogenic ¹²⁹I has been dispersed all over the world and could be utilized as an oceanographic tracer based on its conservative nature in the ocean. The first datasets of ¹²⁹I and ¹²⁷I were obtained by analysis of seawater of 36 water columns collected in the western South China Sea during August-September 2018. The measured ¹²⁹I concentrations decreased with depth from (0.93-1.61) × 10⁷ atoms/L in the upper 200 m to (0.04-0.14) × 10⁷ atoms/L at 1500 m, indicating a clear anthropogenic source in the upper layer, mainly originated from the global fallout. The riverine input of the deposited ¹²⁹I on the catchment area of the Mekong River is an important source besides the direct deposition in the seas. The water mass with high ¹²⁹I from the Mekong River water moves to the east at 11°N by the North Nansha Current in the surface layer (2-25 m). The exponentially decreasing ¹²⁹I level with depth indicates that the vertical dispersion of ¹²⁹I from the upper to the lower layer was mainly through slow diffusion, and the deep water at more than 1500 m was not significantly contaminated by the upper layer water at least in the past 70 years.

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.

Deciphering anthropogenic uranium sources in the equatorial northwest Pacific margin

This work reports the first high-resolution deposition records of anthropogenic uranium (²³⁶U and ²³³U) in a sediment core taken at the continental slope of the Philippine Sea off Mindanao Island in the equatorial northwest Pacific Ocean. Two notable peaks were observed in both profiles of ²³⁶U and ²³³U concentrations, with a narrower peak in 1951-1957 corresponding to close-in Pacific Proving Grounds (PPG) signal, and a broader peak in 1960s-1980s corresponding to the global fallout from nuclear weapons testing. ²³⁶U and ²³³U areal cumulative inventories in the studied sediment core are (2.79 ± 0.20) ∙ 10¹² atom ∙ m^-2 and (3.12 ± 0.41) ∙ 10¹⁰ atom ∙ m^-2, respectively, about 20-30% of reported ²³³U and ²³⁶U inventories from the direct global fallout deposition. The overall ²³³U/²³⁶U atomic ratios obtained in this work vary within (0.3-3.5) ∙ 10^-2, with an integrated ²³³U/²³⁶U atomic ratio of (1.12 ± 0.17) ∙ 10^-2. The contribution from global fallout and close-in PPG fallout to ²³⁶U in the sediment core is estimated to be about 69% and 31%, respectively. We believe the main driving process for anthropogenic uranium deposition in the Philippine sediment is continuous scavenging of dissolved ²³⁶U from the surface seawater by sinking particles.

Sorption methods in marine radiochemistry

The review presents the general methodology of using sorption methods to solve problems of marine radiochemistry, including sampling, preconcentration and radiochemical preparation and methods for measuring the activity of radionuclides. The possible methodological errors at various stages of sampling and sample concentration are discussed. The most widely used artificial (90Sr, 134Cs, 137Cs, 239Pu, 240Pu), natural (210Pb, 210Po; radium quartet: 223Ra, 224Ra, 226Ra, 228Ra; thorium isotopes, mainly 234Th) and cosmogenic (7Be, 32P, 33P) radiotracers are considered. The sorption of uranium from seawater is not addressed, since its concentration in seawater is usually calculated from the known dependence of uranium concentration on seawater salinity.

The bibliography includes 200 references.

Enhanced Bioreduction of Radionuclides by Driving Microbial Extracellular Electron Pumping with an Engineered CRISPR Platform

Dissimilatory metal-reducing bacteria (DMRB) with extracellular electron transfer (EET) capability show great potential in bioremediating the subsurface environments contaminated by uranium through bioreduction and precipitation of hexavalent uranium [U(VI)]. However, the low EET efficiency of DMRB remains a bottleneck for their applications. Herein, we develop an engineered CRISPR platform to drive the extracellular electron pumping of Shewanella oneidensis, a representative DMRB species widely present in aquatic environments. The CRISPR platform allows for highly efficient and multiplex genome editing and rapid platform elimination post-editing in S. oneidensis. Enabled by such a platform, a genomic promoter engineering strategy (GPS) for genome-widely engineering the EET-encoding gene network was established. The production of electron conductive Mtr complex, synthesis of electron shuttle flavin, and generation of NADH as intracellular electron carrier are globally optimized and promoted, leading to a significantly enhanced EET ability. Applied to U(VI) bioreduction, the edited strains achieve up to 3.62-fold higher reduction capacity over the control. Our work endows DMRB with an enhanced ability to remediate the radionuclides-contaminated environments and provides a gene editing approach to handle the growing environmental challenges of radionuclide contaminations.

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.