Tritium in river waters from French Mediterranean catchments: Background levels and variability

Tritium: Its relevance, sources and impacts on non-human biota

Tritium (³H) is a radioactive isotope of hydrogen that is abundantly released from nuclear industries. It is extremely mobile in the environment and in all biological systems, representing an increasing concern for the health of both humans and non-human biota (NHB). The present review examines the sources and characteristics of tritium in the environment, and evaluates available information pertaining to its biological effects at different levels of biological organisation in NHB. Despite an increasing number of publications in the tritium radiobiology field, there exists a significant disparity between data available for the different taxonomic groups and species, and observations are heavily biased towards marine bivalves, fish and mammals (rodents). Further limitations relate to the scarcity of information in the field relative to the laboratory, and lack of studies that employ forms of tritium other than tritiated water (HTO). Within these constraints, different responses to HTO exposure, from molecular to behavioural, have been reported during early life stages, but the potential transgenerational effects are unclear. The application of rapidly developing "omics" techniques could help to fill these knowledge gaps and further elucidate the relationships between molecular and organismal level responses through the development of radiation specific adverse outcome pathways (AOPs). The use of a greater diversity of keystone species and exposures to multiple stressors, elucidating other novel effects (e.g., by-stander, germ-line, transgenerational and epigenetic effects) offers opportunities to improve environmental risk assessments for the radionuclide. These could be combined with artificial intelligence (AI) including machine learning (ML) and ecosystem-based approaches.

Development and calibration of a modifiable passive sampler for monitoring atmospheric tritiated water vapor in different environments

Anthropogenic release of tritium from nuclear facilities is expected to increase significantly in the coming decades, which may cause radiation exposure to humans through the contamination of water and food chains. It is necessary and urgent to acquire detailed information about tritium in various environments for studying its behavior and assessing the potential radiation risk. In the atmosphere, although the passive sampling technique provides a low-cost and convenient way to characterize the dynamics of tritiated water vapor (HTO), a single, simple sampler configuration makes it difficult to collect sufficient and representative samples within the expected period from different environments. In this study, we systematically studied the impacts of sampler configurations on sampling performance and proposed a modifiable sampler design by scaling sampler geometry and adjusting absorbent to achieve different monitoring demands. The samplers were subsequently deployed at five sites in China and Germany for the field calibration and the measured results exhibited a good agreement between the adsorption process obtained in sites corrected with diffusion coefficient and the one calibrated in Shanghai. This suggests the feasibility of predicting sampling performance in the field based on known data. Finally, we developed a strategy for sampler modification and selection in different environments and demonstrated that using easily obtainable environmental data, our sampler can be optimized for any area without any time-consuming preliminary experiments. This work provides a scientific basis for establishing high-resolution atmospheric HTO database and expands the conventional empirical sampler design paradigm by demonstrating the feasibility of using quantitative indices for sampler performance customization.

A modeling approach to estimate 3H discharge from rivers: Comparison of discharge from the Fukushima Dai-ichi and inventory in seawater in the Fukushima coastal region

Estimation of ³H discharge from river catchments is important to evaluate the effect of Fukushima Dai-ichi discharge and future planned ³H release to the ocean on the coastal environment. Using a previously developed model based on the tank model and observed ³H concentration in river water, the ³H discharge from the Abukuma River and 13 other rivers in the Fukushima coastal region were estimated from June 2013 to March 2020. The ³H discharge from catchments of the Abukuma River and 13 other rivers in the Fukushima coastal region during 2014-2019 were estimated to be 1.2-4.0 TBq/y. These values were approximately 2-22 times larger than the annual ³H discharge from the Fukushima Dai-ichi after 2016, indicating the significance of ³H discharge from the catchments through the rivers. This estimation is expected to be useful to evaluate and predict ³H concentrations and inventories in the Fukushima coastal region for consideration of planned ³H release to the ocean.

Application of a liquid scintillation system with 100-ml counting vials for environmental tritium determination: Procedure optimization, performance test, and uncertainty analysis

For more efficient and accurate determination of airborne tritium in the environment, the procedure optimization, performance test and uncertainty analysis of a commercially available low-background liquid scintillation counting (LSC) system with 100-ml counting vials were studied in this work. The results showed that 50 ml water sample mixed with 50 ml scintillation cocktail (Ultima Gold uLLT, PE) could achieve the optimal counting condition after a dark adaption time longer than 1440 min. The minimum detectable activity (MDA) of the 100-ml vial system was estimated to be 0.18 Bq·L^-1 in a continuous counting time of 3600 min, which was approximately 3.5 times lower than that of 20-ml vial system, and its determination uncertainty was also generally lower provided the collected samples was more than 15 ml. It indicates that the LSC system with 100-ml counting vials is preferable for environmental tritium determination. However, for more accurate determination, the electrolytic enrichment is still needed for the sample with the specific activity lower than 0.4 Bq·L^-1. On the other hand, considering the cost and potential environmental impact of present available cocktails, the system with 20-ml vials is recommended for determining the sample with the specific activity higher than 2 Bq·L^-1.

Spatial and temporal variation of tritium concentrations during a dam flushing operation

Tritium is a radionuclide commonly observed worldwide in riverine systems. In the Rhône River downstream the Lake Geneva (Switzerland and France), its occurrence is also related to its use for its luminescent properties in watchmaking paints. In fact, tritium is regularly observed at anomalous levels in this river and extreme events such as flushing operations might conduct to its transport downstream. In the Rhône River, characterized by 21 dams downstream the Geneva Lake, such operations are regularly organized to remove the sediments and limit problematic consequences such as siltation and increased flooding hazards. The consequences of dam flushing operations on tritium concentrations were thus investigated. Samples of Suspended Particulate Matter (SPM) and water were collected in the Rhone River downstream of Geneva in June 2012, during a planned flushing operation of three upstream reservoirs (Verbois, Chancy-Pougny and Génissiat). The concentrations of tritiated water (HTO) and organically bound Tritium (OBT) were measured and compared to reference concentrations. The flushing operations had no impact on the HTO concentration while the increases observed were related to the authorized releases of HTO from a nuclear power plant located downstream the dams. High increases of OBT concentrations in SPM were observed at two stations (Creys-Malville and Jons) without clear spatial or temporal trends. These anomalous peaks could be explained by the heterogeneous spatial distribution of technogenic tritium leading to large variations of tritium concentrations within the samples even though collected in areas close to each other. The results highlight the need to investigate the amount of such technogenic tritium currently stored in the upstream Rhone River as it might be significant.

Radionuclides in waters and suspended sediments in the Rhone River (France) - Current contents, anthropic pressures and trajectories

The Rhone River is one of the most nuclearized river in the world. Radionuclide concentrations in water and suspended sediments transferred to the marine environment were intensively monitored in this river over the last decades (2002-2018). Over this period of time, >12 and 25 time integrating samples were collected each year in filtered waters and suspended sediments, respectively, and analyzed for their radionuclide contents at ultra-trace levels by using top performance analytical tools. While >60% of plutonium, americium, cesium, cobalt, silver, beryllium and actinium radioisotopes are carried by sedimentary particles, sodium, tritium, antimony and strontium are mainly exported as dissolved species (>90%) due to their low affinity with particles. Most natural radionuclides contents show low seasonal variation. No significant trends are observed over the last two decades for these elements, even for ⁴⁰K widely used in fertilizers after the middle of the last century, indicating that the basin has currently converged towards geochemical equilibrium for all of them. In contrast, the concentrations of numerous anthropogenic radionuclides originating from nuclear industries significantly declined since the beginning of the 2000s. Assuming no change of the current anthropic and climatic pressures over the next decades, apparent periods, i.e. the time required for a reduction by half the concentrations in the downstream part of the Rhône River, would be close to 6 years for most artificial radionuclides, except for tritium and other artificial radionuclides conveyed to the river by soil leaching and erosion (⁹⁰Sr, ²⁴¹Am, plutonium isotopes) which would be far longer. Referring to regional referential backgrounds, only few anthropogenic radionuclides specifically produced by nuclear industries are still detectable at the downstream part of the Rhone River and excess contents of tritium, ²³⁸Pu and ²⁴¹Am are observed in filtered waters.

Characterization of the fate and changes of post-irradiance fluorescence signal of filtered anthropogenic effluent dissolved organic matter from wastewater treatment plant in the coastal zone of Gapeau river

Anthropogenic effluent dissolved organic matter (DOM) plays an important role in coastal zone pollution. The objectives of the present study were to characterize the fluorescence signal of anthropogenic effluent DOM from wastewater treatment plant and to evaluate the effect of solar irradiation on the fluorescence signal in the coastal zone. Solar irradiation experiments were conducted to evaluate the effect photochemical degradation using excitation-emission matrix (EEM) method combined with parallel factor analysis (PARAFAC). Results showed high fluorescence of DOM before irradiation and the intensity tends to decrease after 4th and 15th day of irradiation. Rapid photochemical degradation of humic-like fluorophores and appearance of a post-irradiance dominant anthropogenic effluent DOM fluorophores were also observed after irradiation. Our experiments showed a sharp reduction in fluorescence intensity which occurred after 4th day of solar irradiation and the fluorescence signal did not disappeared after 15th day indicating the formation of a specific signal due to solar irradiation. PARAFAC model divided the bulk EEM spectra into three individual fluorescent components with C1 "terrestrial humic-like" and C2 "humic-like of longer wavelength" and C3 is a noisy component with two emission maxima. Multilinear regression of PARAFAC components contribution with mixing composition was most suitable according to the equation C*i = A^WW_i,0 + A^WW_i,1.f_SW + A^WW_i,2.f_RW, where C*_i is the normalized contribution of PARAFAC component number i in a given irradiation day; A^WW_i,0, A^WW_i,1, A^WW_i,2 are the multilinear regression coefficients and contain implicitly the effect of f_WW; and WW, SW, and RW are treated wastewater, sea water, and river water respectively. The values of A^WW_i,0, A^WW_i,1, and A^WW_i,2 fitted second-order kinetics with irradiation process with kinetic constant of 9.68, - 987.35, and - 977.67 respectively for C1 equation and the same trend for C2 and no values for C3 due to its noisy character indicating the rapid degradation with increase of f_SW and f_RW and the predominance of the residual fluorescence coming from f_WW which is the content fraction of anthropogenic effluent DOM because A^WW_i,0 was 100 times less sensitive to photobleaching. A suitable model for predicting the fluorescence EEMs as a function of mixing composition was developed.

Effects of tritiated water on locomotion of zebrafish larvae: a new insight in tritium toxic effects on a vertebrate model species

Tritium (³H), a radioactive isotope of hydrogen, is ubiquitously present in the environment. In a previous study, we highlighted a mis-regulation of genes involved in muscle contraction, eye transparency and response to DNA damages after exposure of zebrafish embryo-larvae from 3 hpf to 96 hpf at 0.4 and 4 mGy/h of tritiated water (HTO). The present study aimed to link this gene mis-regulation to responses observed at higher biological levels. Analyses on spontaneous tail movement, locomotor activity and heart rate were performed. Histological sections of eyes were made to evaluate the impact of HTO on eye transparency and whole embryo immunostainings were realized to assess DNA double strand breaks repair using gamma-H2AX foci. We found a decrease of basal velocity as well as a decrease of response in 96 hpf larvae exposed at 0.4 mGy/h after a tactile stimulus as compared to controls. Histological sections of larvae eyes performed after the exposure to 4 mGy/h did not show obvious differences in lens transparency or retinal development between contaminated and control organisms. Gamma-H2AX foci detection revealed no differences in the number of foci between contaminated organisms and controls, for both dose rates. Overall, results highlighted more detrimental effects of HTO exposure on locomotor behavior in 96 hpf larvae exposed at the lowest dose rate. Those results could be linked to mis-regulation of genes involved in muscle contraction found in a previous study at the same dose rate. It appears that not all effects found at the molecular scale were confirmed using higher biological scales. These results could be due to a delay between gene expression modulation and the onset of physiological disruption or homeostatic mechanisms to deal with tritium effects. However, crossing data from different scales highlighted new pathways to explore, i.e. neurotoxic pathways, for better understanding HTO effects on organisms.

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

Tritium 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 forms in 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). Such tritium 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.

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

Tritium (³H) and Carbon-14 (¹⁴C) are radionuclides of natural (cosmogenic) origin that have also been introduced into the environment by humans since the middle of the last century. They are therefore not compounds that have only recently been released into the environment and they do not pose a recognized health threat due to their low radiotoxicity. However, they hold an important place among current concerns because they are being discharged 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) leading to organically bound forms that can be found in sediments. Organically bound tritium (OBT) analyses carried out on the sediments of the Rhône and its tributaries indicate a significant and historical tritium labelling of sedimentary particles all along the Rhône river, as well as in several northern tributaries, in particular the Ognon and the Tille rivers (tributaries of the Saone), the Doubs River and the Loue River (a tributary of the Doubs) and the Arve river. The recorded levels (10 to over 20,000 Bq/L) are very likely to be related to the presence of synthetic tritiated particles (technogenic tritium), which were used in the past in watchmaking workshops. Although overall contamination levels decrease from north to south in the Rhône watershed and fade over time, particularly due to the radioactive decay of tritium, this contamination source of technogenic tritium in the Rhône watersheds is currently still not negligible. Carbon-14 analyses show that the Rhône sediments generally display ¹⁴C levels close to the atmospheric reference values (231 Bq·kg^-1 of C in 2015) or even lower in most of cases, and show sporadic and weak labelling near nuclear facilities. The low ¹⁴C levels in the Rhône sediments are most likely 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 apparent assimilation rates to the food chain of these two radionuclides.

Distribution of environmental tritium in rivers, groundwater, mine water and precipitation in Goa, India

Tritium concentration in rivers, groundwater, precipitation and mine pits water, all over Goa state was characterized to find out spatial and temporal variability of tritium. Twenty four water samples were collected during pre-monsoon and post-monsoon and analyzed for their tritium concentration. The mean tritium concentration in surface and sub-surface hydrosphere is 2.5 (±0.6) TU. The mean concentration of tritium in rivers, groundwater, mines pits water and rain water are 2.9 (±0.5) TU, 1.95 (±0.5) TU, 2.5(±0.3) TU and 3.1(±0.1) TU respectively. The tritium distribution in all the samples shows modern precipitation (post-1950) component in surface and sub-surface hydrosphere of Goa. The HYSPLIT4.0 air mass trajectory model and atmospheric circulation pattern suggest that the moisture origin was from the Arabian Sea and this low tritium moisture is diluting the tritium concentration of surface hydrosphere near the coastal area. The tritium concentration in surface hydrosphere shows more and more enrichment as one move inland (i.e. away from the coast). Significant seasonal change is observed in the surface hydrosphere. The pre-monsoon samples showed higher tritium concentration than post-monsoon samples. This may be due to high rate of re-evaporation of water and a reduction in the supply of oceanic moisture during the summer (pre-monsoon).

An updated review on tritium in the environment

Various studies indicated more or less recently that organically bound tritium (OBT) formed from gaseous or liquid tritium releases into the environment potentially accumulates in organisms contradicting hypotheses associated to methods used to assess the biological impact of tritium on humans (ASN, 2010). Increasing research works were then performed during the last decade in order to gain knowledge on this radionuclide expected to be increasingly released by nuclear installations in the near future within the environment. This review focusses on publications of the last decade. New unpublished observations revealing the presence of technogenic tritium in a sedimentary archive collected in the upper reaches of the Rhône river and findings from the Northwestern Mediterranean revealing in all likelihood the impact of terrigenous tritium inputs on OBT levels recorded in living organisms are also presented. Identifying and understanding the physicochemical forms of tritium and the processes leading to its persistence in environmental compartments would explain most observations regarding OBT concentrations in organisms and definitively excludes that tritium would "bio accumulate" within living organisms.