233U/236U signature allows to distinguish environmental emissions of civil nuclear industry from weapons fallout

Accelerator mass spectrometry measurements of 233U in groundwater, soil and vegetation at a legacy radioactive waste site

Low-level radioactive wastes were disposed at the Little Forest Legacy Site (LFLS) near Sydney, Australia between 1960 and 1968. According to the disposal records, 233U contributes a significant portion of the inventory of actinide activity buried in the LFLS trenches. Although the presence of 233U in environmental samples from LFLS has been previously inferred from alpha-spectrometry measurements, it has been difficult to quantify because the 233U and 234U α-peaks are superimposed. Therefore, the amounts of 233U in groundwaters, soils and vegetation from the vicinity of the LFLS were measured using accelerator mass spectrometry (AMS). The AMS results show the presence of 233U in numerous environmental samples, particularly those obtained within, and in the immediate vicinity of, the trenched area. There is evidence for dispersion of 233U in groundwater (possibly mobilised by co-disposed organic liquids), and the data also suggest other sources of 233U contamination in addition to the trench wastes. These may include leakages and spills from waste drums as well as waste burnings, which also occurred at the site. The AMS results confirm the historic information regarding disposal of 233U in the LFLS trenches. The AMS technique has been valuable to ascertain the distribution and environmental behaviour of 233U at the LFLS and the results demonstrate the applicability of AMS for evaluating contamination of 233U at other radioactive waste sites.

Simple and convenient preconcentration procedure for the isotopic analysis of uranium in seawater

The demand for monitoring anthropogenic U isotopes, 236U and 233U, in seawater will continue to increase due to radioecological issues and the need for tools for environmental dynamics research. In response to this growing demand, herein, a novel and simple method was developed for the collection of U isotopes in seawater, both in the laboratory and field, using a fabric-like amidoxime adsorbent. The results from the adsorption studies showed that the optimum conditions for processing seawater in a glass beaker were as follows: seawater pH 4, amidoxime adsorbent 0.20 mmol per 500 g seawater and an adsorption time of 9 hours. Alternatively, when using a closed polyethylene container in experiments on-board a ship and using the same ratio of adsorbent to seawater as in the beaker experiment in the laboratory, the optimum conditions were as follows: seawater pH 8 and an adsorption time of 24 hours. Under the above-mentioned conditions, more than 95% of the U underwent adsorption in both the beaker and the polyethylene container experiments. In the case of analyte desorption, more than 80% of U in seawater was recovered using 2-3 mol dm-3 HCl or HNO3 as the eluent. Thus, it was concluded that the amidoxime adsorbent can serve as a simple and effective pre-concentration method for the ultra-trace monitoring of U isotopes in seawater.

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.

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.

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.

Retrospective determination of U and Pu isotopes and atom ratios in lung samples from Vienna, Austria

Uranium and plutonium isotope concentrations as well as ²³⁶U/²³⁸U and ²⁴⁰Pu/²³⁹Pu atom ratios were measured by AMS in human lung samples from the early 1960s. The ²³⁶U concentrations as well as the ²³⁶U/²³⁸U atom ratios show a maximum in 1964, ²³⁹Pu and ²⁴⁰Pu concentrations are increasing continually from 1962 to 1965. ²³⁶U/²³⁸U atom ratios are lower by two orders of magnitude compared to corresponding aerosol data from Vienna, probably due to older ²³⁸U deposited in the lungs, enhanced ²³⁸U concentrations in the city air, and activity partition within different particle sizes. The ²³⁶U/²³⁹Pu atom ratios in lung samples are also lower than expected from the aerosol data, while ²⁴⁰Pu/²³⁹Pu atom ratios lie well within the range typical for nuclear bomb fallout.

Role of Ectomycorrhizal Symbiosis Behind the Host Plants Ameliorated Tolerance Against Heavy Metal Stress

Soil heavy metal (HM) pollution, which arises from natural and anthropogenic sources, is a prime threat to the environment due to its accumulative property and non-biodegradability. Ectomycorrhizal (ECM) symbiosis is highly efficient in conferring enhanced metal tolerance to their host plants, enabling their regeneration on metal-contaminated lands for bioremediation programs. Numerous reports are available regarding ECM fungal potential to colonize metal-contaminated lands and various defense mechanisms of ECM fungi and plants against HM stress separately. To utilize ECM–plant symbiosis successfully for bioremediation of metal-contaminated lands, understanding the fundamental regulatory mechanisms through which ECM symbiosis develops an enhanced metal tolerance in their host plants has prime importance. As this field is highly understudied, the present review emphasizes how plant’s various defense systems and their nutrient dynamics with soil are affected by ECM fungal symbiosis under metal stress, ultimately leading to their host plants ameliorated tolerance and growth. Overall, we conclude that ECM symbiosis improves the plant growth and tolerance against metal stress by (i) preventing their roots direct exposure to toxic soil HMs, (ii) improving plant antioxidant activity and intracellular metal sequestration potential, and (iii) altering plant nutrient uptake from the soil in such a way to enhance their tolerance against metal stress. In some cases, ECM symbiosis promotes HM accumulation in metal stressed plants simultaneous to improved growth under the HM dilution effect.

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.

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.

Microbial interaction with and tolerance of radionuclides: underlying mechanisms and biotechnological applications

Radionuclides (RNs) generated by nuclear and civil industries are released in natural ecosystems and may have a hazardous impact on human health and the environment. RN-polluted environments harbour different microbial species that become highly tolerant of these elements through mechanisms including biosorption, biotransformation, biomineralization and intracellular accumulation. Such microbial-RN interaction processes hold biotechnological potential for the design of bioremediation strategies to deal with several contamination problems. This paper, with its multidisciplinary approach, provides a state-of-the-art review of most research endeavours aimed to elucidate how microbes deal with radionuclides and how they tolerate ionizing radiations. In addition, the most recent findings related to new biotechnological applications of microbes in the bioremediation of radionuclides and in the long-term disposal of nuclear wastes are described and discussed.

Hydrochemical behaviour of dissolved uranium in selected groundwaters of the Kłodzko Valley (SW Poland) and its application possibilities as an environmental tracer

Waters from the Kłodzko Valley containing pharmacodynamics therapeutic agents are used for drinking and for balneotherapeutical purposes. These medicinal, highly mineralized waters are mostly of deep circulation. Moreover, outflowing water contain admixture of shallow circulation water of contemporary infiltration which negatively effects medicinal water quality. Due to the complicated pathways of groundwater circulation, the authors used uranium as an environmental indicator to gain deeper insight into the factors affecting the quality of exploited medicinal water and groundwater origin. The authors collected samples from 26 groundwater intakes from 3 Spas and from one mineral water bottling plant for analysis of ^234,238U and ²³⁴U/²³⁸U activity ratios using α-spectrometry technique. Uranium was applied as a dilution tracer of medicinal mineral groundwater by shallow circulation low-mineralized water. The results of ²³⁴U/²³⁸U activity ratios in particular water intakes collected in different years were equal in the range of uncertainties although the uranium concentrations varied. The stability ²³⁴U/²³⁸U activity ratio parameter is a consequence of dilution of medicinal water by shallow circulation water of present infiltration with a very small amount or without uranium. Apart from simple water dilution, waters of deep circulation mixed with low mineralized shallow circulation water may also contain an admixture of third deeper circulation component with different both U content and ²³⁴U/²³⁸U ratio. In that case ²³⁴U/²³⁸U ratio may be applied as a hydrogeochemical tracer.

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

We present an application of multi-isotopic fingerprints (i.e., ²³⁶U/²³⁸U, ²³³U/²³⁶U, ²³⁶U/¹²⁹I and ¹²⁹I/¹²⁷I) for the discovery of previously unrecognized sources of anthropogenic radioactivity. Our data indicate a source of reactor ²³⁶U in the Baltic Sea in addition to inputs from the two European reprocessing plants and global fallout. This additional reactor ²³⁶U may come from unreported discharges from Swedish nuclear research facilities as supported by high ²³⁶U levels in sediment nearby Studsvik, or from accidental leakages of spent nuclear fuel disposed on the Baltic seafloor, either reported or unreported. Such leakages would indicate problems with the radiological safety of seafloor disposal, and may be accompanied by releases of other radionuclides. The results demonstrate the high sensitivity of multi-isotopic tracer systems, especially the ²³³U/²³⁶U signature, to distinguish environmental emissions of unrevealed radioactive releases for nuclear safeguards, emergency preparedness and environmental tracer studies.

Anthropogenic activities lead to the accumulation of radioactive substances in the environment. Here the authors use multi-isotopic fingerprints of uranium and iodine to discover a previously unknown source of reactor uranium in the Baltic Sea, likely sourced from a Swedish nuclear facility.

Background and fingerprint characteristics of anthropogenic 236U and137Cs in soil and road dust samples collected from Beijing and Zhangjiakou, China

²³⁶U has attracted more attention as an environmental tracer in recent years. However, in-depth study of ²³⁶U in terrestrial environments is still rare in China. Data on ²³⁶U and ¹³⁷Cs concentrations in soil and road dust samples collected from Beijing and Zhangjiakou, China were obtained to demonstrate the background and distinct characteristics of anthropogenic ²³⁶U and ¹³⁷Cs. ²³⁶U and ¹³⁷Cs were detected in the range of (1.10-7.90) × 10⁷ atoms g^-1 and below the method limits of detection to 5.30 Bq kg^-1. A clear characteristic was observed in road dust, where ²³⁶U concentrations increased with decreasing of sample particle size. Soil samples showed an irregular characteristic, but the highest ²³⁶U concentrations were observed in particle size fraction of <0.053 mm in both samples. This phenomenon was caused by U chemical properties, higher specific surface areas and organic compounds in fine particles. Anthropogenic radionuclides fingerprint characteristics in <0.053 mm samples were specially discussed. ²³⁶U/²³⁸U atom ratios were detected in the range of (0.627-3.38) × 10^-8. A weak correlation between anthropogenic ²³⁶U and natural U isotopes were observed. The intermediate correlation between ²³⁶U and ¹³⁷Cs indicated somewhat distinct migration behavior of these two radionuclides in soil after release to the environment. The released amount of ²³⁶U from global fallout during the period of atmospheric nuclear weapons testing was roughly estimated to be 1300 ± 448 kg. These results could be used as fingerprint information for anthropogenic ²³⁶U migration behavior and tracer application in environment.

First dataset of 236U and 233U around the Greenland coast: A 5-year snapshot (2012-2016)

We report the first combined dataset of ²³⁶U and ²³³U in the Greenland marine environment during the period of 2012-2016. Results are discussed in terms of time evolution and spatial distribution of ²³⁶U concentration, and atomic ratios of ²³⁶U/²³⁸U and ²³³U/²³⁶U. ²³⁶U concentrations along the Greenland coast are distributed within a relatively narrow range of (0.7-12.9) × 10⁷ atom/L, corresponding to ²³⁶U/²³⁸U atomic ratios of (1.1-15.5) × 10^-9. The ²³³U/²³⁶U atomic ratios obtained vary from 0.12 × 10^-2 to 1.16 × 10^-2, with the majority distributed in the range of (0.2-0.7) × 10^-2. We applied ²³³U/²³⁶U and ²³⁶U/²³⁸U atomic ratios in a binary mixing model to identify possible ²³⁶U source terms. The results indicate that anthropogenic ²³⁶U and ²³³U in Greenland surface seawater originated from the direct global fallout (DGF) and the Sellafield and La Hague reprocessing plants (RP) is diluted by a third endmember, mostly likely natural ocean water (NOW), containing marginal ²³⁶U and ²³³U. A preliminary estimation of the source terms of ²³⁶U using ²³³U/²³⁶U atomic ratios indicate that, for both eastern and western Greenland seawater, contributions from global fallout (GF) constitute about 30% of ²³⁶U. The dominating source for ²³⁶U, i.e. 70 %, is associated to reactor ²³⁶U including discharges from RP and local reactor input in the Arctic Ocean.

Plutonium isotopes in Northern Xinjiang, China: Level, distribution, sources and their contributions

Plutonium in the environment has drawn significant attentions due to its radiotoxicity in high concentration and source term linked with nuclear accidents and contaminations. The isotopic ratio of plutonium is source dependent and can be used as a fingerprint to discriminate the sources of radioactive contaminant. ²³⁹Pu, ²⁴⁰Pu and ¹³⁷Cs in surface soil and soil cores collected from Northern Xinjiang were determined in this work. The concentrations of ^239,240Pu and ¹³⁷Cs are in the range of 0.06-1.20 Bq kg^-1, and <1.0-31.4 Bq kg^-1 (decay corrected to Sep. 2017), respectively, falling in the ranges of global fallout in this latitude zone. The ²⁴⁰Pu/²³⁹Pu atomic ratios of 0.118-0.209 and ^239,240Pu/¹³⁷Cs activity ratios of 0.039-0.215 were measured. Among the investigated sites, distinctly lower ²⁴⁰Pu/²³⁹Pu atomic ratios of 0.118-0.133 and higher ^239,240Pu/¹³⁷Cs activity ratios of 0.065-0.215 compared to the global fallout values were observed in the northwest part, indicating a significant contribution from other source besides the global fallout. This extra source is mainly attributed to the releases of atmospheric nuclear weapons testing at Semipalatinsk Nuclear Test Site, which was transported by the west and northwest wind through the river valley among mountains in this region. This contribution is estimated to account for 28-43% of the global fallout in the northwest part of Northern Xinjiang. The contribution from the Chinese atmospheric nuclear weapons testing to this region is negligible due to the lack of appropriate wind direction to transport the radioactive releases to this region.