Tritium and 14 C background levels in pristine aquatic systems and their potential sources of variability

C Aquatic systems Background levels Global fallout Regional scale a b s t r a c t Tritium and 14 C are currently the two main radionuclides discharged by nuclear industry. Tritium integrates into and closely follows the water cycle and, as shown recently the carbon cycle, as does 14 C (Eyrolle-Boyer et al., 2014a, b). As a result, these two elements persist in both terrestrial and aquatic environments according to the recycling rates of organic matter. Although on average the organically bound tritium (OBT) activity of sediments in pristine rivers does not significantly differ today (2007 e2012) from the mean tritiated water (HTO) content on record for rainwater (2.4 ± 0.6 Bq/L and 1.6 ± 0.4 Bq/L, respectively), regional differences are expected depending on the biomass inventories affected by atmospheric global fallout from nuclear testing and the recycling rate of organic matter within watersheds. The results obtained between 2007 and 2012 for 14 C show that the levels varied between 94.5 ± 1.5 and 234 ± 2.7 Bq/kg of C for the sediments in French rivers and across a slightly higher range of 199 ± 1.3 to 238 ± 3.1 Bq/kg of C for fish. This variation is most probably due to preferential uptake of some organic carbon compounds by fish restraining 14 C dilution with refractory organic carbon and/or with old carbonates both depleted in 14 C. Overall, most of these ranges of values are below the mean baseline value for the terrestrial environment (232.0 ± 1.8 Bq/kg of C in 2012, Roussel-Debet, 2014a) in relation to dilution by the carbonates and/or fossil organic carbon present in aquatic systems. This emphasises yet again the value of establishing regional baseline value ranges for these two radionuclides in order to account for palaeoclimatic and lithological variations. Besides, our results obtained from sedimentary archive investigation have confirmed the delayed contamination of aquatic sediments by tritium from the past nuclear tests atmospheric fallout, as recently demonstrated from data chronicles (Eyrolle-Boyer et al., 2014a,b). Thus Sedimentary archives can be successfully used to reconstruct past 14 C and OBT levels. Additionally, sediment repositories potentially represent significant storages of OBT that may account for in case of further remobilisation. We finally show that floods can significantly affect the OBT and 14 C levels within suspended particles or sediments depending on the origin of particles reinforcing the need to acquire baseline value range at a regional scale

Contribution of close-in fallout from the French atmospheric tests in inventories of 137Cs, 241Am and plutonium (238, 239, 240) in Gambier Islands (French Polynesia) – Signatures of stratospheric fallout in the Southern Hemisphere

International audienceThe inventories of 137Cs (503 ± 34 Bq.m−2), 241Am (11.3 ± 1.2 Bq.m−2), 241Pu (33.7 ± 3.4 Bq.m−2), 238Pu (6.82 ± 0.87 Bq.m−2) and 239+240Pu (113.0 ± 5.9 Bq.m−2), sum of 239Pu (100 ± 11 Bq.m−2) and 240Pu (14.5 ± 1.7 Bq.m−2), in the Gambier archipelago (23°S) of the French Polynesia, are well higher the global fallout at this latitude, in unequal proportions for the different radionuclides. (240Pu/239Pu)AR (AR: atomic ratio) of 0.0394 ± 0.0062, and (241Pu/239Pu)AR of (2.03 ± 0.39)10−4, confirm that the overwhelmingly dominant source of these radionuclides comes from local fallout during the 1970s of the French atmospheric tests of Moruroa and Fangataufa located nearly 400 km from Gambier. The signatures of the local fallout were deduced from the excess of its inventory in 137Cs and from the mixing lines established from the signatures of the global fallout, some of the test sites and the isotopic ratios measured in Gambier. Signatures obtained are 2.0 ± 0.4 for 137Cs/239+240Pu, 0.045 ± 0.008 for 238Pu/239+240Pu, 0.031 ± 0.009 for 241Am/239+240Pu, 0.092 ± 0.027 for 241Pu/239+240Pu, 0.0163 ± 0.0049 for (240Pu/239Pu)AR, (0.78 ± 0.23)10−4 for (241Pu/239Pu)AR. The concordance of the mixing lines of the [(240Pu/239Pu)AR, (241Pu/239Pu)AR] and the linear regression of these ratios measured in the stratosphere (40°S) during the 1970s, indicates that the signatures of the close-in deposition are also those of the stratospheric injections of the French tests. The signatures of stratospheric fallout in the Southern Hemisphere were evaluated by considering that the fission energy of these injections represents 11% and that of the Northern Hemisphere represents 89% and that the isotopic ratios of stratospheric injections remained the same over the period 1970–1974. The activity ratios deducted are 21.9 ± 0.1 in 137Cs/239+240Pu, 0.11 ± 0.05 in 238Pu/239+240Pu, 1.03 ± 0.12 in 241Pu/239+240Pu and 0.35 ± 0.04 in 241Am/239+240Pu. The associated atom ratios are 0.157 ± 0.011 for (240Pu/239Pu)AR and (8.33 ± 0.48)10−4 for (241Pu/239Pu)AR. These signatures appear to be consistent with the results of the inventories at Hiva Oa, located more than 1,000 km north of both French test sites, and with those found in the Australian continent, in regions not impacted by UK-test debris. The proportions of close-in tropospheric fallout from the French tests are about 90% in Gambier. They represent a proportion in the inventories of 40% for the 137Cs, 60% for 241Am and in the range between 80 and 90% for Pu isotopes

Transfer of Tritium Released into the Marine Environment by French Nuclear Facilities Bordering the English Channel

International audienceControlled amounts of liquid tritium are discharged as tritiated water (HTO) by the nuclear industry into the English Channel. Because the isotopic discrimination between 3H and H is small, organically bound tritium (OBT) and HTO should show the same T/H ratio under steady-state conditions. We report data collected from the environment in the English Channel. Tritium concentrations measured in seawater HTO, as well as in biota HTO and OBT, confirm that tritium transfers from HTO to OBT result in conservation of the T/H ratio (ca. 1 × 10 −16). The kinetics of the turnover of tritium between seawater HTO, biota HTO, and OBT was investigated. HTO in two algae and a mollusk is shown to exchange rapidly with seawater HTO. However, the overall tritium turnover between HTO and the whole-organism OBT is a slow process with a tritium biological half-life on the order of months. Nonsteady-state conditions exist where there are sharp changes in seawater HTO. As a consequence, for kinetic reasons, the T/H ratio in OBT may deviate transiently from that observed in HTO of samples from the marine ecosystem. Dynamic modeling is thus more realistic for predicting tritium transfers to biota OBT under nonsteady-state conditions

The unravelling of radiocarbon composition of organic carbon in river sediments to document past anthropogenic impacts on river systems

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A brief history of origins and contents of Organically Bound Tritium (OBT) and 14C in the sediments of the Rhône watershed

(IF 5.59; Q1)International audienceTritium (3H) and Carbon-14 (14C) are radionuclides of natural (cosmogenic) origin that have also been introducedinto the environment by humans since the middle of the last century. They are therefore not compoundsthat have only recently been released into the environment and they do not pose a recognized health threat dueto their lowradiotoxicity. However, they hold an important placeamong current concerns because they are beingdischarged into the environment by the nuclear industry in large quantities compared to other radionuclides.Those both radionuclides partly integrate organic matter during metabolic processes (i.e., photosynthesis) leadingto organically bound forms that can be found in sediments. Organically bound tritium(OBT) analyses carriedout on the sediments of the Rhône and its tributaries indicate a significant and historical tritiumlabelling of sedimentaryparticles all along the Rhône river, aswell as in several northern tributaries, in particular the Ognon andthe Tille rivers (tributaries of the Saone), the Doubs River and the Loue River (a tributary of the Doubs) and theArve river. The recorded levels (10 to over 20,000 Bq/L) are very likely to be related to the presence of synthetictritiated particles (technogenic tritium),whichwere used in the past inwatchmakingworkshops. Although overallcontamination levels decrease from north to south in the Rhône watershed and fade over time, particularlydue to the radioactive decay of tritium, this contamination source of technogenic tritiumin the Rhônewatershedsis currently still not negligible. Carbon-14 analyses show that the Rhône sediments generally display 14C levelsclose to the atmospheric reference values (231 Bq·kg−1 of C in 2015) or even lower in most of cases, andshow sporadic and weak labelling near nuclear facilities. The low 14C levels in the Rhône sediments are mostlikely related to the solid contributions from tributaries draining areas that are rich in fossil organic matter,and therefore devoid of 14C. In the Rhône watershed, the presence in solid particles of tritium in a form organically bound to synthetic compounds and of petrogenic (fossil) organic carbon, can potentially alter the apparentassimilation rates to the food chain of these two radionuclides

Tritium and C-14 background levels in pristine aquatic systems and their potential sources of variability

Tritium and 14C currently are the two main radionuclides discharged by nuclear industry. Tritium integrates into and closely follows the water cycle and, as shown recently the carbon cycle, as is the case with 14C (Roussel-Bebet; 2014a, 2014b, Eyrolle-Boyer et al. 2014). As a result, these two elements persist in both terrestrial and aquatic environments according to the recycling rates of organic matter. Although on average the OBT activity of sediments in pristine rivers does not significantly differ today (2007-2012) from the mean HTO content on record for rainwater (2.4±0.6 Bq/L and 1.6±0.4 Bq/L, respectively), regional differences are expected based on the biomass inventories affected by atmospheric global fallout from nuclear testings and the recycling rate of organic matter within watersheds. The results obtained between 2007 and 2012 for 14C show that the levels vary between 94.5±1.5 and 234±2.7 Bq/kg of C for the sediments in French rivers and across a slightly higher range of 199±1.3 to 238±3.1 Bq/kg of C for fish. This variation is most probably due to preferential uptake of organic carbon. These ranges of values overall are below the mean baseline value for the terrestrial environment (232.0±1.8 Bq/kg of C in 2012, Roussel-Debet; 2014b) in relation with dilution by carbonates and/or fossil organic carbon present in aquatic systems. This emphasises yet again the value of establishing for these two radionuclides ranges of regional baseline values in order to account for paleoclimatic and lithological variations. We also show that sedimentary archives can be used to reconstruct past 14C and OBT levels. These results have made it possible to confirm delayed contamination of aquatic sediments by atmospheric fallout from past nuclear tests, as recently demonstrated (Eyrolle-Boyer et al., 2014). Lastly, our results show that the OBT and 14C levels within the suspended particles or sediments downstream of liquid discharges from nuclear facilities are affected by inputs from tributaries, and thus by hydrology, floods origin and chronologies

Trajectories of technogenic tritium in the Rhône River (France)

International audienceTritium is a radioisotope of hydrogen with a half-life of 12.32 years and was used for its luminescent properties by the watchmaking industry from 1962 to the 2008. Tritiated luminescent salts were integrated in the paints applied on the index and dial of watches and clocks. French and Swiss watchmaking workshops used more than 28 000 TBq of tritium over this period of time and produced almost 350 million watches. Despite the end of tritiated salts use in watchmaking workshops in 1992 in France and 2008 in Switzerland, high level of organically bound tritium (OBT) are still observed in sediments of the Rhône River downstream the Lake Geneva. Contamination of the Rhône River by tritiated hot particles since 1962 up to nowadays remains poorly documented. In order to assess the long term behavior and fate of technogenic tritium in this river and its trajectories in the river system, two sediment cores were collected at the upstream (UC) and downstream (DC) part of the Rhône River in France and OBT contents were determined. For both sedimentary cores, maximum OBT contents were registered over the 1980s when tritium was intensively used by watchmaking industries. These residual OBT contents are 1 000 to 10 000 fold higher than current natural background levels in riverine sediments. The OBT contents progressively decreased since 1989 with close effective half-life in upstream and downstream area (5 ± 2 years). The OBT contents were lower in DC than in UC due to the dilution by uncontaminated sediments delivered by tributaries not affected by the watchmaking industries. Trajectories analysis indicates that the resiliency of the Rhône River system in regards to this contamination would be reached in 14 to 70 years and in 14 to 28 years respectively for the upstream and downstream part of the river

Evidence for tritium persistence as organically bound forms in river sediments since the past nuclear weapon tests

(IF 4.01 [2018]; Q1)International audienceTritium of artificial origin was initially introduced to the environment from the global atmospheric fallout after nuclear weapons tests. Its level was increased in rainwaters by a factor 1000 during peak emissions in 1963 within the whole northern hemisphere. Here we demonstrate that tritium from global atmospheric fallout stored in sedimentary reservoir for decades as organically bound formsin recalcitrant organic matter while tritium released by nuclear industries in rivers escape from such storages. Additionally, we highlight that organically bound tritium concentrations in riverine sediments culminate several years after peaking emission in the atmosphere due to the transit time of organic matter from soils to river systems. these results were acquired by measuring both free and bound forms of tritium in a 70 year old sedimentary archive cored in the Loire river basin (France). Suchtritium storages, assumed to be formed at the global scale, as well as the decadal time lag of tritium contamination levels between atmosphere and river systems have never been demonstrated until now. our results bring new lights on tritium persistence and dynamics within the environment and demonstrate that sedimentary reservoir constitute both tritium sinks and potential delayed sources of mobile and bioavailable tritium for freshwaters and living organisms decades after atmospheric contamination

Inventory and distribution of tritium in the oceans in 2016

International audienceTritium concentrations in oceans were compiled from the literature, online databases and original measurements in order to determine the global distribution of tritium concentrations according to latitude and depth in all oceans. The total inventory of tritium decay corrected in 2016 has been estimated using evaluation of the natural and artificial contributions in 23 spatial subdivisions of the total ocean. It is determined equal to 26.8 ± 14 kg including 3.8 kg of cosmogenic tritium. That is in agreement with the total atmospheric input of tritium from nuclear bomb tests and the natural inventory at steady-state estimated from natural production rates in the literature (25.6 – 28 kg in the Earth). A global increase in tritium concentration was observed according to latitude in the Northern hemisphere with a maximum in the Arctic Ocean. The minimum tritium concentrations observed in the Southern Ocean were close to steady-state with known natural tritium deposition. We focused on the temporal evolution of surface (0 to 500 m) tritium concentrations in a selected area of the North Atlantic Ocean (30°N - 60°N) where we found the 2016 concentration to be 0.60 ± 0.05 TU (1σ). Results showed that in that area, between 1988 and 2013, tritium concentrations: i) decreased faster than the sole radioactive decay, due to a mixing with lower and lateral less concentrated waters, and ii) decreased towards an apparent steady state concentration. The half-time mixing rate of surface waters and the steady state concentration were respectively calculated to be 23 ± 5 years (1σ) and 0.38 ± 0.07 TU (1σ). This apparent steady-state concentration in the North Atlantic Ocean implies a mean tritium deposition of 1870 ± 345 Bq.m-2 (1σ), five folds higher than the known inputs (natural, nuclear tests fallout and industrial releases, ~367 Bq.m-2) in this area. Keywords: Radionuclide, Tracer, Database, Background concentration

A comprehensive assessment of radioactivity monitoring in the Channel Islands

International audienceThe Channel Islands are located in the Normand-Breton Gulf (NBG), in the mid-part of the English Channel (France, Normandy). In the northern part, off Cap La Hague, controlled amounts of radioactive liquid waste are discharged by the ORANO La Hague nuclear fuel reprocessing plant (RP). Radionuclides were monitored in the NBG to assess the dispersion of radioactive discharges from the RP in the marine environment. The temporal and spatial distribution of the data are consistent with the history of the discharges, with most gamma emitter radionuclide environmental levels being close to or below the current limits of detection. A clear fingerprint of H-3, C-14 and I-129 radionuclides discharged from the RP is measured. The hydrodynamics in the NBG do not yield a simple gradient with linear distance from the outfall of the RP. Modelling tools were used to understand how radioactive discharges spread from the source of input. Dispersion patterns clearly illustrate the different behaviours of soluble and non-soluble radionuclides. The study indicated that the footprint of radioactive liquid discharges by French nuclear facilities was still measurable in species collected from the NBG for the mostly dissolved radionuclides. The less conservative ones, with a high affinity for suspended matter, are potentially influenced by old releases. These pathways could be investigated by dedicated hydrodynamic dispersion models. Overall, in the Channel Islands the levels are low and consistent with the general decrease in liquid radionuclide discharges by the RP since the 1990s