Tritium in Japanese precipitation following the March 2011 Fukushima Daiichi Nuclear Plant accident

Short-lived natural radionuclides as tracers in hydrogeological studies - A review

Fundamental approaches to the study of groundwater rely on investigating the spatial and temporal distribution of stable and radioactive isotopes and other anthropogenic compounds in natural waterbodies. The most often used tracers for estimating groundwater flow paths and residence times, groundwater/surface water interaction as well as tracing chemical (contamination) sources include stable isotopes of water (δ 18O and δ 2H), radiocarbon (14C; t1/2 = 5730 a), tritium (3H; t1/2 = 12.43 a) as well as unreactive fluorine-containing gases (e.g., chlorofluorocarbons CCl3F or CFC-11; CCl2F3 or CFC-12; C2Cl3F3 or CFC-113; and SF6). While gas tracers are usually referred to as transient tracers and are appropriate for investigating modern flow systems, the isotopic tracers are often used to investigated paleo or regional flow systems. Stable isotopes of water can also be used to investigate groundwater/surface water interactions. Another, thus far been less frequently used group of groundwater tracers, are cosmo- and geo- genic short-lived radioisotopes. These isotopes are uniquely suited for studying a wide range of groundwater problems that have short time scales including high aquifer vulnerability to quantitative and qualitative impacts and groundwater discharge to surface waters. Here, we discuss and compare the applications of radio‑sulphur (35S; half-life t1/2 = 87 d), radio‑beryllium (7Be; t1/2 = 53 d), radio‑phosphorus (32/33P; combined t1/2 = 33 d), natural tritium (3H; t1/2 = 12.43 a), radon (222Rn; t1/2 = 3.8 d) and short-lived radium (224/223Ra; combined t1/2 = 5.2 d). The paper discusses the principles of the individual tracer methods, focusing on the isotopes' input functions or values, on sampling techniques, and on methods of analyses. Case studies that applied a combined use of the tracers are referred to for readers who wish to learn more about the application of the so far underused cosmo- and geo- genic radioisotopes as aquatic tracers.

Temporal variation of tritium concentration in monthly precipitation collected at a Difficult-to-Return Zone in Namie Town, Fukushima Prefecture, Japan

This article discusses tritium concentrations in monthly precipitation in part of the Difficult-to-Return Zone in Namie Town during 2012-2021. The tritium concentrations, which were measured with a low background liquid scintillation counter after carrying out an enrichment procedure, fluctuated seasonally from 0.10 ± 0.02 to 0.85 ± 0.02 Bq L-1. This range of concentrations is concluded to not be unusual based on comparisons with the concentrations at other sites and estimates of the past range of the concentrations. Moreover, no significant variations in observed tritium concentrations were observed due to decommissioning work at the Fukushima Dai-ichi Nuclear Power Plant. These results contribute to understanding the background level of tritium concentration in precipitation before the oceanic discharge of treated water from the Fukushima plant. In addition, this article evaluates the amount of tritium supplied to the ocean by terrestrial rainwater pouring into the Pacific Ocean via Ukedo River, which flows through Namie Town; this information will contribute to the discussion on the impact of the oceanic discharge of treated water.

Record of 3H and 36Cl from the Fukushima nuclear accident recovered from soil water in the unsaturated zone at Koriyama

The opportunity to measure the concentrations of 3H and 36Cl released by the Fukushima nuclear accident in 2011 directly in rain was lost in the early stage of the accident. We have, however, been able to reconstruct the deposition record of atmospheric 3H and 36Cl following the accident using a bore hole that was drilled in 2014 at Koriyama at a distance of 60 km from the accident. The contributions of 3H and 36Cl from the accident are 1.4 × 1013 and 2.0 × 1012 atoms m-2 respectively at this site. Very high concentrations of both 3H (46 Bq L-1) and 36Cl (3.36 × 1011 atoms L-1) were found in the unsaturated soil at depths between 300 and 350 cm. From these, conservative estimates for the 3H and 36Cl concentrations in the precipitation in the ~ 6 weeks following the accident were 607 Bq L-1 and 4.74 × 1010 atoms L-1, respectively. A second hole drilled in 2016 showed that 3H concentrations in the unsaturated soil and shallow groundwater had returned to close to natural levels, although the 36Cl concentrations were still significantly elevated above natural levels.

Estimation of Groundwater and Spring Water Residence Times near the Coast of Fukushima, Japan

The massive Tohoku earthquake, colloquially known as "The 2011 Great East Japan Earthquake," occurred off the Pacific coast of Japan on March 11, 2011. The coastal area of the Tohoku region was severely affected by the tsunami, and the tsunami also caused severe damage to the Fukushima Daiichi Nuclear Power Plant (FDNPP) releasing a large amount of radioactive material into the atmosphere and environment. Determining the residence time of groundwater is important for evaluating how long radioactive materials are present after nuclear accidents such as FDNPP and multiple methods are needed to account for mixing between old/young water that can occur. The apparent residence times of spring water, groundwater, and artesian well water samples obtained during 2014 to 2018 from the northern coastal area of Fukushima Prefecture were estimated using tritium (³ H), chlorofluorocarbons (CFCs), and sulfur hexafluoride (SF₆ ) concentrations. The lowest ³ H concentrations were within the background range (1-5 TU) and were observed in artesian wells in Shinch, Soma and Minamisoma. The highest ³ H concentrations (∼8-15 TU) were observed near the FDNPP, and were probably affected by the accident following the 2011 earthquake. The average apparent residence time of spring water/shallow groundwater was 29 years based on the SF₆ concentration and exponential mixing model, whereas that of artesian well water was 62 years based on the CFC-12 concentration and piston flow model. Considering the relatively short apparent residence time of spring water/shallow groundwater, it is important to conduct continuous observations to understand the influence of the FDNPP accident.

Impact of the Fukushima Accident on 3H and 14C Environmental Levels: A Review of Ten Years of Investigation

The investigation of the impact of the Fukushima accident is still going on although more than ten years have passed since the disaster. The main goal of this paper was to summarize the results of tritium and radiocarbon determinations in different environmental samples, possibly connected with the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. A document containing compiled data may serve as a solid basis for further research in the selected fields. To accomplish such effort, we went through dozens of relevant published papers, reporting 3H and 14C activity concentrations in precipitations, groundwater, seawater, river systems, tree rings, and, in some more extraordinary samples, such as herbaceous plants or debris from the damaged reactor buildings. As the referenced results would not be obtainable without adequate analytical techniques, the most common methods for routine measurement of tritium and radiocarbon concentrations are discussed as well. We believe that the correct identification of the affected environmental compartments could help quantify the released 3H and 14C activities and track their following fate, which could be especially important for plans to discharge contaminated water from the FDNPP in the upcoming years.

Monthly Precipitation Collected at Hirosaki, Japan: Its Tritium Concentration and Chemical and Stable Isotope Compositions

Monthly precipitation samples were collected at Hirosaki, Aomori Prefecture from January 2018 to December 2020 to measure the ion species and stable hydrogen and oxygen isotope ratios in order to understand the regional properties. The tritium concentration ranged from 0.28 to 1.20 Bq/L, with mean values (±S.D.) of 0.52 ± 0.18, 0.67 ± 0.25 and 0.63 ± 0.21 Bq/L in 2018, 2019 and 2020, respectively. This concentration level was almost the same as for Rokkasho, Aomori Prefecture. The tritium concentration had clear seasonal variation: high in the spring and low in the summer. This trend was thought to arise from seasonal fluctuations in the atmospheric circulation. On the other hand, the pH tended to be low, and the electrical conductivity (EC) tended to be high from the winter to the spring. The ion components, which major ion species contained in sea salt, also tended to be high in the winter, and these components had a strong influence on EC. The d-excess values were high in the winter and low in the summer, and when this trend was considered from the viewpoint of the wind direction data in Hirosaki, these dust components were attributed to the northwest monsoon in the winter to the spring coming from the Asian continent.

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.

Landside tritium leakage over through years from Fukushima Dai-ichi nuclear plant and relationship between countermeasures and contaminated water

There has been tritium groundwater leakage to the land side of Fukushima Dai-ichi nuclear power plants since 2013. Groundwater was continuously collected from the end of 2013 to 2019, with an average tritium concentration of approximately 20 Bq/L. Based on tritium data published by Tokyo Electric Power Company Holdings (TEPCO) (17,000 points), the postulated source of the leakage was (1) leaks from a contaminated water tank that occurred from 2013 to 2014, or (2) a leak of tritium that had spread widely over an impermeable layer under the site. Based on our results, sea side and land side tritium leakage monitoring systems should be strengthened.

Additional radiation dose due to atmospheric dispersion of tritium evaporated from a hypothetical reservoir

With regard to an inland nuclear power plant bordered by a reservoir, a major concern was that fresh water might be polluted and the human body might be radiation exposed due to the discharge of liquid radioactive effluents. In contrast to other radionuclides in the effluents, tritium has specific dispersion behavior in the aquatic environment such as emission into the air along with water evaporation. Further, the evaporated tritium in the air could go toward the territorial system where the wind blows. As a result, the person staying in the vicinity of the plant discharge point would be exposed with an additional radiation dose. In light of this characteristic, this study first introduced this new exposure pathway and investigated the additional radiation dose on the basis of a hypothetical reservoir. The results indicated that annual tritium evaporation fraction is approximately 2.5%, which is a comparable level with the radioactive decay factor. This would produce an additional radiation dose of 0.63 μSv/a to a person staying 50 m away from the plant discharge point for the case of 1 g/a tritium discharge. Tritium evaporation effects could be decreased through controlling the discharge depth. Thus, a preliminary suggestion to adopt a deep discharge instead of surface discharge was proposed from the ALARA (as low as reasonably achievable) criterion of radiation protection.

Radioactive Games? Radiation Hazard Assessment of the Tokyo Olympic Summer Games

We conducted a comprehensive radiation hazard assessment of the Tokyo Olympic Games (Tokyo 2020, postponed to 2021). Our combined experimental and literature study focused on both external and internal exposure to ionizing radiation for athletes and visitors of the Games. The effective dose for a visit of 2 weeks ranges from 57 to 310 μSv (including flight dose). The main contributors to the dose are cosmic radiation during the flights (approximately 10-81%), inhalation of natural radon (approximately 9-47%), and external exposure (approximately 8-42%). In this complex exposure, anthropogenic radionuclides from the Fukushima nuclear accident (2011) always play a minor role and have not caused a significant increase of the radiological risk compared to pre-Fukushima Japan. Significantly elevated air dose rates were not measured at any of the Tokyo Olympic venues. The average air dose rates at the Tokyo 2020 sites were below the average air dose rates at the sites of previous Olympic Games. The level of radiological safety of foods and water is very high in Japan, even for athletes with increased water and caloric demands, respectively.

Temporal changes in tritium and radiocarbon concentrations in the western North Pacific Ocean (1993-2012)

The western North Pacific is one of the most studied oceanic basins due to its diverse structure and important role in connection with the adjacent reservoirs. Tritium (³H) and radiocarbon (¹⁴C) have been frequently exploited as oceanographic tracers due to their suitable properties; several extensive observation projects, such as GEOSECS, WOCE and WOMARS, used these two radionuclides to investigate different oceanographic processes, pathways, ocean currents and time scales of deep and bottom water formation. Here we evaluate temporal changes in ³H and ¹⁴C levels in seawater of the western North Pacific Ocean from 1993 to 2012. When compared to the background levels from 1993, the data from 2012 suggests significant impact of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on surface and vertical ³H seawater profiles, increasing its water column inventories in the southern part of the 149°E meridian by a factor of 2-7. On the other hand, ¹⁴C content in surface seawater has been steadily decreasing from 1993, with the accelerated rate from 2005, probably due to downwelling of bomb-produced radiocarbon and its transport along isopycnal layers. The influence of the Oyashio current on ¹⁴C levels in the northern part of the investigated transect and formation of its intrusion was also clearly visible in the collected datasets. Regarding bomb-produced radiocarbon, its water column inventories decreased or remained same from 2005 to 2012 at all stations, except the ones located in the coastal areas of the New Guinea island (3.5°S).

Evaluation of public dose from FHR tritium release with consideration of meteorological uncertainties

Tritium management is a potentially significant issue in fluoride-salt-cooled high-temperature reactors (FHRs), as these reactors can produce tritium at high rates. Potential impact of the tritium released into the environment needs to be investigated to help determine the maximum-allowable tritium-release rate from an FHR plant. In this study, a dose assessment on the public resulting from FHR tritium release was performed via computational modeling. Three potential locations for FHR construction, i.e., the Hanford site, Idaho Falls in Idaho, and Oak Ridge in Tennessee, were selected. Atmospheric tritium dispersion was modeled using computer code family GENII and a parametric study of key meteorological variables was carried out. An uncertainty analysis was performed to examine the reliability of the prediction of dose for the year 2020. It is discovered that conditions in favor of lower public dose level from FHR tritium release include low atmosphere temperature, high wind speed, high relative humidity, and high tritium release point. It is also discovered that for different geological locations, the dominance of meteorological parameters differs significantly. Among the three locations modeled, although the Hanford site might be the most suitable location for FHR construction in the past, in the near future, Oak Ridge would possess advantages in the dose assessment aspect over the other two. We assumed that the tritium release rate from an FHR plant is given at 18.5 TBq/day and compared the probability of the maximum individual dose exceeding the regulatory limit (0.4 mSv/y). According to the prediction of dose for the year 2020, this probability is extremely low. While for Idaho Falls, it is 91.62% and for the Hanford site, 44.27%. The results indicate that effective measures should be taken for tritium control in FHR.

A method for the determination of organically bound tritium in marine biota based on an improved tubular-combustion system

A method for the measurement of organically bound tritium (OBT) in marine biota has been developed using a combustion device designed with three independent temperature control zones and a two-stage intake mode to ensure full combustion of samples. The versatility of the combustion device and combustion water recoveries were tested on nine different types of marine biota, with recoveries varying from 85.15% to 92.67%, with an average of 89.4 ± 11.34%. This indicates that our developed combustion method (combustion program and apparatus) provides stable results. Finally, the OBT activity of the marine samples measured varied from 1.88 to 12.9 Bq/L, with an average of 5.61 ± 3.21 Bq/L.

Seasonal and Spatial Distribution of Atmospheric Tritiated Water Vapor in Mainland China

To reveal the distribution of atmospheric tritium water (HTO) vapor and provide a baseline for tritium pollution control, a subnational survey was conducted in mainland China. As the largest study on HTO vapor in China that has ever been formally reported, this study provides a macroimpression of the atmospheric HTO specific activity from March 2017 to March 2018. A total of 102 passive samplers were deployed at 34 sites in 30 provinces to determine the seasonal and spatial distributions of HTO vapor. In general, the HTO specific activity in the atmosphere ranged from lower than the minimum detectable activity (0.18 Bq·L^-1) to 5.5 Bq·L^-1. Spatially, the specific activity of HTO was positively correlated to the latitude and the distance to proximal coastline. Seasonally, significantly higher HTO specific activities were observed in spring and relatively lower in summer. Based on correlation analysis, the atmospheric HTO distributions were considered to be the consequence of combined factors of the stratospheric-tropospheric net mass flux, the distance from the tropopause to the ground, the fraction of air mass that originated from ocean re-evaporation and long-distance transport from high-latitude continents.

Monitoring of Tritium Internal Exposure Doses of Heavy-Water Reactor Workers in Third Qinshan Nuclear Power Plant

To analyze the tritium internal exposure dose of workers in the Third Qinshan Nuclear Power Plant over the past 15 years. Urine samples provided by workers are tested directly to analyze the tritium concentrations and estimate internal exposure dose. Since 2004, an average of approximately 1600 workers have been monitored annually, with an average annual monitoring frequency of approximately 11 000. Since 2004, the average annual collective dose of tritium internal exposure was 149.62 person·mSv, accounting for 19.07% of the total annual collective dose. A total of 18 workers’ annual individual internal tritium radiation doses exceeded 2 mSv, of which 5 workers’ internal tritium radiation doses in a single intake exceeded 2 mSv. The occupational population with the largest total internal tritium radiation doses consists of maintenance personnel, fuel operators, and radiation protection personnel, whose collective doses of internal exposure account for 75.51% of the total collective doses within the plant. Over 15 years of operation, the internal tritium radiation doses of workers in the Third Qinshan Nuclear Power Plant have been strictly controlled within the national regulatory limit and power plant management target, ensuring the health and safety of the workers.

Radionuclides in surface waters around the damaged Fukushima Daiichi NPP one month after the accident: Evidence of significant tritium release into the environment

Following the Fukushima nuclear accident (2011), radionuclides mostly of volatile elements (e.g., ¹³¹I, ^134,137Cs, ¹³²Te) have been investigated frequently for their presence in the atmosphere, pedosphere, biosphere, and the Pacific Ocean. Smaller releases of radionuclides with intermediate volatility, (e.g., ⁹⁰Sr), have been reported for soil. However, few reports have been published which targeted the contamination of surface (fresh) waters in Japan soon after the accident. In the present study, 10 surface water samples (collected on April 10, 2011) have been screened for their radionuclide content (³H, ⁹⁰Sr, ¹²⁹I, ¹³⁴Cs, and ¹³⁷Cs), revealing partly unusually high contamination levels. Especially high tritium levels (184 ± 2 Bq·L^-1; the highest levels ever reported in scientific literature after Fukushima) were found in a puddle water sample from close to the Fukushima Daiichi nuclear power plant. The ratios between paddy/puddle water from one location only a few meters apart vary around 1% for ¹³⁴Cs, 12% for ¹²⁹I (¹³¹I), and around 40% for both ³H and ⁹⁰Sr. This illustrates the adsorption of radiocesium on natural minerals and radioiodine on organic substances (in the rice paddy), whereas the concentration differences of ³H and ⁹⁰Sr between the two waters are mainly dilution driven.

Isotope Composition and Chemical Species of Monthly Precipitation Collected at the Site of a Fusion Test Facility in Japan

The deuterium plasma experiment was started using the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS) in March 2017 to investigate high-temperature plasma physics and the hydrogen isotope effects towards the realization of fusion energy. In order to clarify any experimental impacts on precipitation, precipitation has been collected at the NIFS site since November 2013 as a means to assess the relationship between isotope composition and chemical species in precipitation containing tritium. The tritium concentration ranged from 0.10 to 0.61 Bq L^-1 and was high in spring and low in summer. The stable isotope composition and the chemical species were unchanged before and after the deuterium plasma experiment. Additionally, the tritium concentration after starting the deuterium plasma experiment was within three sigma of the average tritium concentration before the deuterium plasma experiment. These results suggested that there was no impact by tritium on the environment surrounding the fusion test facility.

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.

Groundwater age and mixing process for evaluation of radionuclide impact on water resources following the Fukushima Dai-ichi nuclear power plant accident

Radionuclide contamination of groundwater causes critical impacts on water resources, human lives, and ecosystems. The intrusion of radionuclides into the groundwater flow system in Fukushima, Japan, could be illuminated by determining groundwater age and mixing processes. To do this, periodical field surveys were conducted in catchments contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Sampling began in May 2011, which was 2 months after the disaster, and continued through June 2012. Chlorofluorocarbon (CFCs), tritium, and oxygen and hydrogen stable isotopes were used as environmental tracers. The observed tritium concentrations suggested that the water contained accident-derived radionuclides that exceeded the natural background baseline. Groundwater ages in the selected two headwater catchments were estimated to be between 10 and 26 years by combined use of multiple CFCs concentrations. In addition, the governing groundwater flow system was mostly approximated by a piston flow model; however, modern water fraction was also suggested based on the relationship between CFC-11 and CFC-12. The estimated water age and isotopic signals among stream water, spring water, and groundwater revealed that the intrusion of radionuclides into the groundwater was caused by the mixing between groundwater and modern water sources such as soil water and precipitation with relatively high radionuclide concentrations. This mixing was facilitated by a weathered and fractured granite bedrock and a thin unsaturated subsurface layer in the study area. Continued long-term monitoring of radionuclides in the groundwater will be necessary for water resources management in the future. CAPSULE: Radionuclide intrusion into the groundwater is related to the mixing between radionuclide-poor groundwater and modern water with relatively high radionuclide concentration.

Evaluating anthropogenic and environmental tritium effects using precipitation and Hokkaido snowpack at selected coastal locations in Asia

Tritium dating requires a good understanding of the tritium and water inputs into hydrologic systems, including their main trends due to latitudinal, seasonal and altitudinal effects. Although tritium reached ambient levels at the end of the 20th century, tritium released from nuclear facilities and bomb tests since then has the potential to confound use of tritium for age dating. We therefore collected precipitation and snowpack samples for tritium analysis to confirm that tritium levels in Japanese precipitation had not exceeded ambient levels following the North Korean nuclear tests in January 6th 2016 and September 3rd 2017. As the result, the highest tritium concentration was 5.52(±0.27)TU at samples collected from January 8 to 11th at one Honshu and four Hokkaido locations and samples collected at six Honshu locations had 8.01(±1.5)TU from September 6 to 19th 2017. Confirming ambient tritium concentrations after both events we investigated the latitude tritium effect at selected coastal stations in Asia, indicating a break of latitude trend around Tokyo area, and established the latitude scaling factors to the north and south of the Tokyo area data. The seasonal trend was investigated during the winter-spring 2016 in precipitation samples confirming the higher spring tritium compared with winter continental tritium values. The altitude effect on tritium and stable (¹⁸O and ²H) isotopes was observed in Hokkaido snowpack, which had tritium concentrations ranging between 4.08 and 5.93 TU during March-April, and demonstrated two trends for western and central Hokkaido mountain ranges. Using established latitude and altitude scaling factors with the long-term continuous time-series of monthly Tokyo area tritium we estimated the annual weighted tritium at 110 meteorological stations in Japan with monthly precipitation demonstrating the applicability of this approach for future tritium-tracer studies across Asia.

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).

Tritium and radiocarbon in the western North Pacific waters: post-Fukushima situation

Impact of the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident on tritium (³H) and radiocarbon (¹⁴C) levels in the water column of the western North Pacific Ocean in winter 2012 is evaluated and compared with radiocesium (^134,137Cs) data collected for the same region. Tritium concentrations in surface seawater, varying between 0.4 and 2.0 TU (47.2-236 Bq m^-3), follow the Fukushima radiocesium trend, however, some differences in the vertical profiles were observed, namely in depths of 50-400 m. No correlation was visible in the case of ¹⁴C, whose surface Δ¹⁴C levels raised from negative values (about -40‰) in the northern part of transect, to positive values (∼68‰) near the equator. Homogenously mixed ¹⁴C levels in the subsurface layers were observed at all stations. Sixteen surface (from 30 in total) and 6 water profile (from 7) stations were affected by the Fukushima tritium. Surface and vertical profile data together with the calculated water column inventories indicate that the total amount of the FNPP1-derived tritium deposited to the western North Pacific Ocean was 0.7 ± 0.3 PBq. No clear impact of the Fukushima accident on ¹⁴C levels in the western North Pacific was observed.

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

Tritium background levels in various environmental compartments are deeply needed in particular to assess radiological impact, especially in river systems where most of releases from nuclear facilities are performed. The present study aims to identify the main environmental factors that influence tritium background levels in rivers at the regional scale. 41 samples were collected from 2014 to 2016 along 17 small rivers in the south of France. All were located out of the influence of direct releases from nuclear facilities. Tritiated water (HTO) concentrations measured in water samples ranged from 0.12±0.11 to 0.86±0.15BqL^-1 and HTO concentrations in rains were modelled between 2015 and 2016 over the study period referring to time series acquired from 1963 to 2014 at Thonon-les-Bains monitoring station. The results of tritium concentrations in rivers studied present a significant variability and are more than twice lower than forecasted values in rain. Multiple linear regressions allowed identifying that HTO concentration in rains, watershed area and altitude were the main tested parameters that are linked to the variability of HTO concentrations in the studied rivers. Finally, HTO fluxes delivered to the Mediterranean Sea by French coastal rivers out of influence of nuclear releases were estimated. The results highlight that those account for around 1% of HTO exported while 99% are transferred by the nuclearized Rhone River.

Analysis of air mass trajectories to explain observed variability of tritium in precipitation at the Southern Sierra Critical Zone Observatory, California, USA

Understanding the behavior of tritium, a radioactive isotope of hydrogen, in the environment is important to evaluate the exposure risk of anthropogenic releases, and for its application as a tracer in hydrology and oceanography. To understand and predict the variability of tritium in precipitation, HYSPLIT air mass trajectories were analyzed for 16 aggregate precipitation samples collected over a 2 year period at irregular intervals at a research site located at 2000 m elevation in the southern Sierra Nevada (California, USA). Attributing the variation in tritium to specific source areas confirms the hypothesis that higher latitude or inland sources bring higher tritium levels in precipitation than precipitation originating in the lower latitude Pacific Ocean. In this case, the source of precipitation accounts for 79% of the variation observed in tritium concentrations. Air mass trajectory analysis is a promising tool to improve the predictions of tritium in precipitation at unmonitored locations and thoroughly understand the processes controlling transport of tritium in the environment.

Spatial variations of tritium concentrations in groundwater collected in the southern coastal region of Fukushima, Japan, after the nuclear accident

Spatial variations in tritium concentrations in groundwater were identified in the southern part of the coastal region in Fukushima Prefecture, Japan. Higher tritium concentrations were measured at wells near the Fukushima Daiichi Nuclear Power Station (F1NPS). Mean tritium concentrations in precipitation in the 5 weeks after the F1NPS accident were estimated to be 433 and 139 TU at a distance of 25 and 50 km, respectively, from the F1NPS. The elevations of tritium concentrations in groundwater were calculated using a simple mixing model of the precipitation and groundwater. By assuming that these precipitation was mixed into groundwater with a background tritium concentration in a hypothetical well, concentrations of 13 and 7 TU at distances of 25 and 50 km from the F1NPS, respectively, were obtained. The calculated concentrations are consistent with those measured at the studied wells. Therefore, the spatial variation in tritium concentrations in groundwater was probably caused by precipitation with high tritium concentrations as a result of the F1NPS accident. However, the highest estimated tritium concentrations in precipitation for the study site were much lower than the WHO limits for drinking water, and the concentrations decreased to almost background level at the wells by mixing with groundwater.

The valley system of the Jihlava river and Mohelno reservoir with enhanced tritium activities

The Dukovany nuclear power plant (NPP Dukovany) releases liquid effluents, including HTO, to the Mohelno reservoir, located in a deep valley. Significantly enhanced tritium activities were observed in the form of non-exchangeable organically bound tritium in the surrounding biota which lacks direct contact with the water body. This indicates a tritium uptake by plants from air moisture and haze, which is, besides the uptake by roots from soil, one of the most important mechanisms of tritium transfer from environment to plants. Results of a pilot study based on four sampling campaigns in 2011-2015 are presented and discussed, with the aim to provide new information on tritium transport in the Mohelno reservoir - Jihlava River - plants ecosystems.

Impact of the Fukushima accident on tritium, radiocarbon and radiocesium levels in seawater of the western North Pacific Ocean: A comparison with pre-Fukushima situation

Tritium, radiocarbon and radiocesium concentrations in water column samples in coastal waters offshore Fukushima and in the western North Pacific Ocean collected in 2011-2012 during the Ka'imikai-o-Kanaloa (KoK) cruise are compared with other published results. The highest levels in surface seawater were observed for ¹³⁴Cs and ¹³⁷Cs in seawater samples collected offshore Fukushima (up to 1.1 Bq L^-1), which represent an increase by about three orders of magnitude when compared with the pre-Fukushima concentration. Tritium levels were much lower (up to 0.15 Bq L^-1), representing an increase by about a factor of 6. The impact on the radiocarbon distribution was measurable, but the observed levels were only by about 9% above the global fallout background. The ¹³⁷Cs (and similarly ¹³⁴Cs) inventory in the water column of the investigated western North Pacific region was (2.7 ± 0.4) PBq, while for ³H it was only (0.3 ± 0.2) PBq. Direct releases of highly contaminated water from the damaged Fukushima NPP, as well as dry and wet depositions of these radionuclides over the western North Pacific considerably changed their distribution patterns in seawater. Presently we can distinguish Fukushima labeled waters from global fallout background thanks to short-lived ¹³⁴Cs. However, in the long-term perspective when ¹³⁴Cs will decay, new distribution patterns of ³H, ¹⁴C and ¹³⁷Cs in the Pacific Ocean should be established for future oceanographic and climate change studies in the Pacific Ocean.

An Analytical Method to Measure Free-Water Tritium in Foods using Azeotropic Distillation

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant has raised concerns about the discharge of contaminated water containing tritium ((3)H) from the nuclear power plant into the environment and into foods. In this study, we explored convenient analytical methods to measure free-water (3)H in foods using a liquid scintillation counting and azeotropic distillation method. The detection limit was 10 Bq/L, corresponding to about 0.01% of 1 mSv/year. The (3)H recoveries were 85-90% in fruits, vegetables, meats and fishes, 75-85% in rice and cereal crops, and less than 50% in sweets containing little water. We found that, in the case of sweets, adding water to the sample before the azeotropic distillation increased the recovery and precision. Then, the recoveries reached more than 75% and RSD was less than 10% in all food categories (13 kinds). Considering its sensitivity, precision and simplicity, this method is practical and useful for (3)H analysis in various foods, and should be suitable for the safety assessment of foods. In addition, we examined the level of (3)H in foods on the Japanese market. No (3)H radioactivity was detected in any of 42 analyzed foods.

Fukushima-derived radionuclides in sediments of the Japanese Pacific Ocean coast and various Japanese water samples (seawater, tap water, and coolant water of Fukushima Daiichi reactor unit 5)

We investigated Ocean sediments and seawater from inside the Fukushima exclusion zone and found radiocesium (¹³⁴Cs and ¹³⁷Cs) up to 800 Bq kg⁻¹ as well as ⁹⁰Sr up to 5.6 Bq kg⁻¹. This is one of the first reports on radiostrontium in sea sediments from the Fukushima exclusion zone. Seawater exhibited contamination levels up to 5.3 Bq kg⁻¹ radiocesium. Tap water from Tokyo from weeks after the accident exhibited detectable but harmless activities of radiocesium (well below the regulatory limit). Analysis of the Unit 5 reactor coolant (finding only ³H and even low ¹²⁹I) leads to the conclusion that the purification techniques for reactor coolant employed at Fukushima Daiichi are very effective.

Nuclear accident-derived (3)H in river water of Fukushima Prefecture during 2011-2014

During 2011-2014, we measured (3)H concentrations in river water samples collected during base flow conditions and during several flood events from two small rivers in a mountainous area in Fukushima Prefecture, which received deposition of (137)Cs from the Fukushima Dai-ichi Nuclear Power Plant accident. (3)H concentrations above background levels were found in water samples collected during both base flow conditions and flood events in 2011. The (3)H concentrations during flood events were generally higher than those during base flow conditions. The (3)H concentrations in both rivers during base flow conditions and flood events decreased with time after the accident and reached almost background levels in 2013. We also measured (3)H concentrations in freshwater samples from 16 other rivers and one dam in eastern Fukushima Prefecture from 2012 to 2014 during base flow conditions. The measured (3)H concentrations were higher than the background level in 2012 and decreased with time. The (137)Cs inventory in the catchment area at each sampling point was estimated from air-borne monitoring results in the literature and compared with the (3)H concentrations. We found surprisingly good correlations between (137)Cs inventories in the catchment areas and (3)H concentrations in the water samples. Further studies will be necessary to clarify the reason for the good correlation.

Scavenging of radioactive soluble gases from inhomogeneous atmosphere by evaporating rain droplets

We analyze effects of inhomogeneous concentration and temperature distributions in the atmosphere, rain droplet evaporation and radioactive decay of soluble gases on the rate of trace gas scavenging by rain. We employ a one-dimensional model of precipitation scavenging of radioactive soluble gaseous pollutants that is valid for small gradients and non-uniform initial altitudinal distributions of temperature and concentration in the atmosphere. We assume that conditions of equilibrium evaporation of rain droplets are fulfilled. It is demonstrated that transient altitudinal distribution of concentration under the influence of rain is determined by the linear wave equation that describes propagation of a scavenging wave front. The obtained equation is solved by the method of characteristics. Scavenging coefficients are calculated for wet removal of gaseous iodine-131 and tritiated water vapor (HTO) for the exponential initial distribution of trace gases concentration in the atmosphere and linear temperature distribution. Theoretical predictions of the dependence of the magnitude of the scavenging coefficient on rain intensity for tritiated water vapor are in good agreement with the available atmospheric measurements.

Fukushima's forgotten radionuclides: a review of the understudied radioactive emissions

In environmental monitoring campaigns for anthropogenic radionuclides released in the course of the Fukushima nuclear accident (2011), most focus had been on gamma-emitting radionuclides. More than 99% of the released activity was due to radionuclides of the elements Kr, Te, I, Xe, and Cs. However, little work had been done on the monitoring of radionuclides other than (131)I, (132)Te, (134)Cs, (136)Cs, and (137)Cs. Radionuclides such as those of less volatile elements (e.g., (89)Sr, (90)Sr, (103)Ru, (106)Ru, plutonium), pure beta-emitters ((3)H, (14)C, (35)S), gaseous radionuclides ((85)Kr, (133)Xe, (135)Xe) or radionuclides with very long half-lives (e.g., (36)Cl, (99)Tc, (129)I, some actinides such as (236)U) have been understudied by comparison. In this review, we summarize previous monitoring work on these "orphan" radionuclides in various environmental media and outline further challenges for future monitoring campaigns. Some of the understudied radionuclides are of radiological concern, others are promising tracers for environmental, geochemical processes such as oceanic mixing. Unfortunately, the shorter-lived nuclides of radioxenon, (103)Ru, (89)Sr and (35)S will no longer exhibit detectable activities in the environment. Activity concentrations of other radionuclides such as tritium, (14)C, or (85)Kr will become blurred in the significant background of previous releases (nuclear explosions and previous accidents). Isotope ratios such as (240)Pu/(239)Pu will allow for the identification of Fukushima plutonium despite the plutonium background.