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Ministry of the Environment Plans Demonstration Test for Reuse of Decontaminated Soil from Fukushima in Shinjuku Gyoen, Tokyo

Friday, December 9, 2022 11:53

Minister of the Environment Yoshiaki Nishimura announced that the Ministry of the Environment is planning to conduct a demonstration test at the Shinjuku Imperial Garden in Tokyo to see if the “decontaminated soil” generated during the decontamination process after the Fukushima Daiichi Nuclear Power Plant accident can be reused.

The government has indicated that it intends to reuse the large amount of “decontaminated soil” in Fukushima Prefecture for public works projects if the concentration of radioactive materials is below a certain standard value.

At a press conference today, Environment Minister Nishimura announced that the Ministry of the Environment is planning to conduct a demonstration test at the Shinjuku Gyoen, which is managed by the Ministry of the Environment, to demonstrate the reuse of the soil. The plan is to create flower beds using decontaminated soil in areas that are off limits to the general public, and to test the radiation levels in the surrounding areas.

This is the second time that a demonstration test is being planned outside of Fukushima Prefecture, following Tokorozawa City in Saitama Prefecture, and the other is in Tsukuba City, Ibaraki Prefecture.
https://newsdig.tbs.co.jp/articles/-/225831?display=1

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January 4, 2023 Posted by | Fuk 2023 | , , | Leave a comment

Decontaminated soil from the nuclear power plant accident “Cleanup is right in front of my house…” Plans to reuse soil from outside Fukushima emerge in Shinjuku Gyoen, Tokorozawa, and Tsukuba

December 10, 2022
An important move has been made regarding the cleanup of the TEPCO’s Fukushima Daiichi Nuclear Power Plant accident. A demonstration project to reuse decontaminated soil is planned to be conducted for the first time outside of Fukushima Prefecture. The Ministry of the Environment is planning to reduce the amount of decontaminated soil for interim storage in the prefecture by reusing it, and briefing sessions are scheduled for Tokorozawa City, Saitama Prefecture, on December 16 and Shinjuku Ward, Tokyo, on December 21. Why has reuse emerged in these areas? Can we easily proceed with reuse that will lead to the proliferation of pollution? (Special Reporting Division: Takuya Kishimoto, Takeshi Nakayama)

A demonstration project to reuse decontaminated soil is planned at the Environmental Research and Training Institute in Tokorozawa City, Saitama Prefecture.

◆Local residents voiced their confusion, and the city office was reluctant to go ahead with the project.
 A 10-minute walk from Koku Koen Station on the Seibu Shinjuku Line in Tokorozawa City, we came upon a corner lot adjacent to the National Defense Medical College. This is the Environmental Research and Training Institute, one of the facilities where a demonstration project to reuse decontaminated soil is planned. Across the main street to the west was a residential area.
 How do local residents perceive the plan?
 What is right in front of our house? A woman in her 50s who lives across the street from the training center voiced her confusion. I remember hearing on the news that there was going to be some kind of experiment in Tokorozawa, but…. But have you already decided? I’m not absolutely against it, but there are so many things I don’t understand that I can’t say for sure.
 The training center is a Ministry of the Environment facility used to train personnel involved in environmental conservation. The plan for the demonstration project was explained by Environment Minister Akihiro Nishimura at a press conference on April 6. Decontaminated soil will be used to create a lawn at the facility to confirm its safety.
 He also visited the city hall, which is a few minutes’ walk from the training center. Mr. Kazuto Namiki, director of the Environment and Clean Environment Department, was open to accepting the project.
 The reuse of decontaminated soil is a nationwide issue, not just in Fukushima. We would like to cooperate with them on the premise of ensuring the safety and security of residents. The Ministry of the Environment approached the city in June of this year, and discussions have continued. Naturally, we are proceeding with the project after consulting with the mayor.
 Mayor Masato Fujimoto. He seems to have such strong feelings about the project that he wrote on the city’s website, “The Great East Japan Earthquake and the nuclear power plant accident were the starting point of my desire to become mayor.
 In 2012, the year after the earthquake, the city once announced a policy to cancel the installation of air conditioners in junior high schools, saying, “Now that we have experienced the disaster, we need to be patient. Although he later retracted the policy in response to a referendum in which the majority of residents opposed it, a source familiar with the city government described him as “the type of person who goes his own way, without regard for criticism.
 In August of this year, he revealed that he had attended an event of an organization affiliated with the Family Coalition for World Peace and Unification (formerly the Unification Church), causing controversy when he said, “I don’t feel that much remorse” and “My personality is such that I can’t say I won’t go anymore.
◆Residents’ explanatory meeting was limited to 50 people, many of whom were unaware of the event.
 The lack of explanation about the demonstration project to reuse decontaminated soil seems to be conspicuous.
 The Ministry of the Environment plans to hold a briefing session for residents at the training center on the evening of the 16th of this month. The details of the project will be revealed there for the first time. However, the number of participants is limited to 50 residents of the neighborhood, and pre-registration is required. The city was in charge of the briefing, but it was only announced on 28 bulletin boards in the area.
 A local man (81) said, “I didn’t know about the briefing. I don’t usually look at bulletin boards. Another woman said, “I thought it would be announced in the city’s newsletter. When we asked about 10 residents, none of them knew that the information about the briefing was posted on the bulletin board.
 After the plans for the demonstration project came to light, the city received about 40 inquiries, the majority of which were negative. Yoichi Sugiura, who has been involved in the local anti-base movement and has confronted the government and the city, said, “Even if it is a national project, the city is not going to accept it.
 Even if it is a government project, the city should confirm the wishes of the citizens before taking any action, but they are proceeding with the project without informing us well. If the city is going ahead with the project by fiat without listening to citizens’ opinions, it will be the same as when the air conditioner was installed.
◆Decontaminated soil in Fukushima also failed due to local opposition.
 Why is the Ministry of the Environment trying to reuse decontaminated soil?
 Interim storage facilities in Fukushima Prefecture (Futaba and Okuma towns) began receiving decontaminated soil in 2015, and the amount is expected to reach about 14 million cubic meters. The government has stated that final disposal will take place outside the prefecture by 45 years for both towns. In June 2004, the Ministry of the Environment set a standard for reusing decontaminated soil with a level of 8,000 becquerels per kilogram or less. This is considerably looser than the standard for reusing materials from decommissioned nuclear power plants (100 becquerels per kilogram).

Interim storage facilities for temporarily storing contaminated soil spread out around the Fukushima Daiichi Nuclear Power Plant in the town of Okuma, Fukushima Prefecture, on January 25, 2012, from the “Oozuru” helicopter operated by the head office of the company (photo by Ryo Ito).

However, it is difficult to say that recycling is on track.
 In Fukushima Prefecture, a plan to use the soil for filling city roads in Nihonmatsu City was abandoned due to opposition from local residents. In Minamisoma City, the city built fill and measured the radioactivity concentration of seepage water, but a plan to reuse the soil for construction of the Joban Expressway did not materialize due to local opposition. Now, only an experiment in crop cultivation is underway in Iitate Village. Tsunehide Chino, associate professor of environmental sociology at Shinshu University, said, “It is difficult to obtain broad public agreement, and the demonstration project has nowhere to go.
 Nevertheless, in August, the Ministry of the Environment announced a policy to implement the demonstration project outside of Fukushima Prefecture. In addition to the Environmental Research and Training Institute in Tokorozawa City, Shinjuku Gyoen (Shinjuku-ku, Tokyo), a facility affiliated with the Ministry of the Environment, and the National Institute for Environmental Studies (Tsukuba City, Ibaraki Prefecture) are also being discussed as possible sites. A Ministry of the Environment official explained, “We took into consideration the fact that there is space in the area that is not accessible to the general public. In the demonstration project, flowerbeds, grass plazas, parking lots, etc. will be built, and data on changes in radiation levels in the surrounding area will be collected.
◆Tokorozawa, Tsukuba, and Shinjuku Gyoen…places with close ties to the country
 Tokorozawa was the site of an army airfield before World War II and is now the site of a U.S. military communications base, so it is closely related to national security policy. Tsukuba has the face of an academic city, and research institutes with close ties to the national government are also prominent. Shinjuku Gyoen was the site of the “Cherry Blossom Viewing Party” hosted by the prime minister.
 The connection with the government may remind one of the government-led recycling of decontaminated soil, but Mr. Chino said, “If a facility has a relationship with the Ministry of the Environment, it may be easier to conduct a demonstration project. In other words, it is only possible there. He then goes on to express his concern, “It is not clear to what extent the project will be agreed upon, including with the residents of the surrounding area, and they are trying to move forward without finding a way out.
 On the other hand, an official in charge of Shinjuku City, the home of Shinjuku Gyoen, where the demonstration project was announced, remains calm, saying, “The Ministry of the Environment should take the responsibility of explaining the project to the local residents and gain their understanding.
 Chia Yoshida, a freelance writer who has been covering the Fukushima nuclear power plant accident, is suspicious of the Ministry of the Environment’s preoccupation with the issue, saying, “I suspect that they are trying to proceed with the project by putting the residents’ wishes second.
 This attitude of the national government can be seen in the ongoing reconstruction project in Hamadori, Fukushima. In the area, the Reconstruction Agency will launch the Fukushima International Research and Education Organization in the next fiscal year, bringing together industries such as robotics, drones, and radiation science. Many of the technologies being handled are “dual-use” technologies that can be used by both the military and civilian sectors.
 In the name of ostensibly “reconstruction,” the idea of the business community and some research institutes that share its intentions may be taking precedence. The government’s policies, including the reuse of decontaminated soil, are proceeding without sufficient explanation to the local communities, and the residents are being left behind.
 Yoshiharu Monma, 65, chairman of the “30-year Interim Storage Facility Landowners Association,” said, “It is out of the question that decontaminated soil, which should be confined to one place, is being spread around the country by using the sound of reusing it. As with the restarting of nuclear power plants, we are seeing the government move in such a way as to make people think that they can do whatever they want after so much time has passed since the Fukushima accident,” he continued.
 In the first place, TEPCO, which caused the accident, should take responsibility for the final disposal of the decontaminated soil. For example, the government should consider condensing the decontaminated soil on TEPCO’s land, and the government should shoulder the shortage of funds and manpower. I would like to see these disposal methods discussed in a forum open to the public as a problem for Japan as a whole.”
Related article] Where to in 2045? Contaminated soil generated by the nuclear power plant accident: The current location of intermediate storage facilities in Fukushima
◆Desk Memo
 Decontaminated soil should be cleaned up by those who caused the accident. However, the Ministry of the Environment tries to bring it to various places in the name of reusing it. Without regard to pre-accident standards, a system will be set up to allow use even if considerable contamination remains, and a demonstration project for vegetable cultivation will also be carried out. The wild story looms over the Tokyo metropolitan area. This is no time to be distracted by the World Cup. (Sakaki)
https://www.tokyo-np.co.jp/article/219058

December 11, 2022 Posted by | Fuk 2022 | , , | Leave a comment

Tokyo park to be used in Fukushima soil recycling demonstration

Dec. 9, 2022

Japan’s environment ministry has announced plans to demonstrate the reuse of decontaminated soil from Fukushima at a Tokyo park.

Environment Minister Nishimura Akihiro announced on Friday that the project will take place at the Shinjuku Gyoen National Garden.

Soil exposed to radioactive fallout from the accident at the Fukushima Daiichi nuclear power plant has been decontaminated and kept in intermediary storage in the prefecture.

The government plans to reuse the soil for public works projects as long as the concentration of radioactive substances falls below a certain threshold.

Nishimura said the ministry will use the soil in a flower bed in an area normally closed to the public and later hold public flower-viewing events.

Ministry officials are to meet with nearby residents to explain about the project on December 21. The project is due to start early next year.

Nishimura said the ministry hopes to use the project to gain public understanding for the recycling of the decontaminated soil.

Earlier in the week, the ministry announced a plan to test soil recycling at the National Environmental Research and Training Institute in Saitama Prefecture, near Tokyo.

Trials to reuse the soil have so far only taken place in Fukushima.

https://www3.nhk.or.jp/nhkworld/en/news/20221209_17/

December 11, 2022 Posted by | Fuk 2022 | , , | Leave a comment

Ministry plans tests on reusing Fukushima soil in Tokyo area

A temporary storage site for soil contaminated from the 2011 Fukushima nuclear disaster in February. The bags of radioactive waste were due to be shipped to an interim storage facility.

December 7, 2022

The Environment Ministry is eyeing the Tokyo metropolitan area for its first trial runs outside Fukushima Prefecture on reusing soil decontaminated after the 2011 nuclear disaster, The Asahi Shimbun learned on Dec. 6.

The ministry said the tests will take place at three government-related facilities in Tokyo, Saitama and Ibaraki prefectures.

But authorities said they have yet to gain the understanding of residents at all three candidate sites on the reuse of the soil, which still contains low-level radioactive substances.

Decontamination work was carried out on soil exposed to radioactive materials after the triple meltdown at the Fukushima No. 1 nuclear power plant operated by Tokyo Electric Power Co.

The decontaminated soil has been kept at an interim storage facility in Fukushima Prefecture, but a law requires final disposal of the soil outside the prefecture by 2045.

The volume of decontaminated soil in Fukushima Prefecture, excluding the difficult-to-return zones where radiation levels remain high, is about 14 million cubic meters, enough to fill 11 Tokyo Domes.

Reusing the soil is part of the government’s efforts to reduce that volume before disposal.

The ministry is considering conducting the tests at the Shinjuku Imperial Garden in Tokyo, the National Institute for Environmental Studies in Tsukuba, Ibaraki Prefecture, and the National Environmental Research and Training Institute in Tokorozawa, Saitama Prefecture.

Tokorozawa city will hold a briefing on the plan for about 50 residents on Dec. 16.

Under the experiment in Tokorozawa, decontaminated soil will be reused for lawns, and tests will be conducted to verify changes in radiation doses in the air.

For the trial runs in Tokyo and Ibaraki Prefecture, the soil will be used for parking lots and flower beds.

“We would like to use the experiments to gain public understanding regarding the reuse of the soil,” Environmental Minister Akihiro Nishimura said at a news conference on Dec. 6.

Only soil that measures below 8,000 becquerels per kilogram, the threshold set by the government, will be used in the trial runs.

The ministry has been conducting experiments on reusing the decontaminated soil for farmland in Iitate, Fukushima Prefecture.

But plans for similar tests in Minami-Soma and Nihonmatsu cities, also in the prefecture, fell through after residents opposed.

https://www.asahi.com/ajw/articles/14786753

December 11, 2022 Posted by | Fuk 2022 | , , | Leave a comment

Ministry of the Environment to Reuse Decontaminated Soil in Shinjuku Gyoen and Other Places

December 6, 2022

The Ministry of the Environment is considering conducting demonstration tests to reuse soil removed from decontamination sites in Fukushima prefecture outside the prefecture, including the ministry’s Environmental Research and Training Institute (Tokorozawa City, Saitama Prefecture) and Shinjuku Imperial Garden (Shinjuku Ward, Tokyo).

The other is the National Institute for Environmental Studies (Tsukuba City, Ibaraki Prefecture). 

At a press conference after the Cabinet meeting on the same day, Environment Minister Akihiro Nishimura explained that there were several candidate sites and that he was coordinating with related local governments.

This is an important project for Fukushima Prefecture. We want to confirm the safety of the project and help build understanding. The Environmental Research and Training Center plans to reuse the land to create a lawn square, and will hold a briefing session for local residents on March 16.

https://www.hokkaido-np.co.jp/article/770796/

December 11, 2022 Posted by | Fuk 2022 | , , | Leave a comment

Reuse of decontaminated soil to be tested outside Fukushima

Dec. 6, 2022

Japan’s Environment Ministry is planning its first trial outside Fukushima Prefecture on the reuse of soil that was decontaminated after the 2011 nuclear accident.

The ministry says the demonstration will take place at the National Environmental Research and Training Institute in Saitama Prefecture, near Tokyo.

Officials will use the soil in a courtyard to grow a lawn. They plan to brief nearby residents in mid-December to seek their understanding, and begin the trial in January at the earliest.

Soil exposed to radioactive fallout from the accident at the Fukushima Daiichi nuclear plant has been cleansed and kept in intermediary storage in the prefecture.

The government plans to reuse the soil for public works projects as long as the concentration of radioactive substances falls below a certain threshold.

Trials to reuse the soil to grow vegetables and create earth mounds have so far only taken place in Fukushima.

Environment Minister Nishimura Akihiro told reporters on Tuesday that his ministry hopes to use the experiment in Saitama to confirm safety and gain public understanding for the recycling of the soil.

He said the ministry will coordinate with other candidate sites where similar trials could be held.

A law mandates the final disposal of the decontaminated soil outside Fukushima by 2045. But it remains unclear how this will be achieved.

https://www3.nhk.or.jp/nhkworld/en/news/20221206_17/

December 11, 2022 Posted by | Fuk 2022 | , , | Leave a comment

A Survey on the Concentration of Radioactive Cesium in Japanese Milk Samples (2021)

by Citizens’ Nuclear Information Center · Published December 4, 2022 · Updated December 4, 2022

Investigating the origin of the contamination and assessing the risks of cases where the radioactive cesium level is below the Standard Limits for Radionuclides in Foods

By Tanimura Nobuko (NPO Citizens’ Nuclear Information Center) and Fuseya Yumiko (NPO Shinjuku Yoyogi Citizen Monitoring Center)

  1. Introduction

Radioactive contamination of food due to the Fukushima Daiichi Nuclear Power Station accident

The Fukushima Daiichi Nuclear Power Station accident occurred in March 2011, resulting in the contamination of the environment and food by radioactive materials. At that time, no standard limit of radiation levels of food distributed in Japan existed, and a temporary standard was therefore set by the government. In April 2012, about one year after the accident, new standard limits were established, in which radiation exposure due to the consumption of food was limited to 1mSv per year, thus making the standard value of radioactive cesium 100Bq/kg for general food, 50Bq/kg for milk and baby/infant food and 10Bq/kg for drinking water. While this was supposed to regulate contaminated food, there were many reports of the discovery of food that was over the standard limit being distributed. In fiscal year 2020, twenty-two cases of food over the standard limit were recognized.

Due to inadequate food contamination checks and distribution management systems, some people who refused to be unnecessarily exposed to radiation decided to select food according to the area in which it was produced in order to avoid risks of radiation exposure. The Japanese government terms this action—avoiding the food produced in the affected area of the Great East Japan Earthquake—‘reputational damage,’ something which must be eliminated if reconstruction is to proceed.

Risks of radiation and risks of chemical materials

However, is making a choice of avoiding a health risk an unfair act that prevents the reconstruction of the affected areas?

The cancer risk of chemicals in tap water and other substances is regulated to a level of 1 in 100,000 per substance, which is usually the risk for a lifetime of ingestion of such substances. But those who do not want to take the risk of developing cancer from chemicals in food are able to buy organic products at some additional cost. This consumer action is not criticized by the government as ‘reputational damage.’

The standard for radiation exposure from consuming food is set at 1mSv/year. According to the ICRP (International Commission on Radiological Protection) the fatality risk is about 5% per 1Sv and ‘the standard is based on the hypothesis that the probability of radiation-induced cancer or hereditary effects increases in direct proportion to the increase in dose.’ In addition, it estimates that the fatality risk is 0.4% if a person continues to be exposed to radiation at 1 mSv per year throughout his/her lifetime.

Adding five hundred-thousandths per year (0.005% for 1mSv/year) means that if we take 80 years as the average lifetime, four hundred people’s deaths are added per hundred thousand people, a risk that is two digits higher than that for chemical materials.

It is sometimes claimed that as the risk of radiation exposure is the total of the risks of various radionuclides, comparing the regulation for one chemical material to the regulation for radiation exposure is improper, since there are thousands of chemical materials that we may come into contact with in daily life. However, the radiation exposure standard of 1mSv per year is not the ‘total’ amount of exposure but an ‘additional’ exposure. The following are all conveniently used to explain that exposure of 1 mSv or less is safe: a) the radiation dose limit for the public from nuclear facilities under pre-accident conditions, b) the new standard for radioactive cesium in food, c) the standard of 8,000 Bq/kg for “designated waste” introduced to handle the large amount of radioactive waste generated by the Fukushima nuclear accident (exposure of workers disposing of the waste), and d) the standard for the disposal of radioactively contaminated water, which has been the focus of attention at the Fukushima nuclear power plant, now undergoing decommissioning. The fact that these exposure risks add up, as well as the effects of this addition, have not been explained to civil society by the regulators, and of course have not been discussed.

In addition, the risk assessment of carcinogenic chemical materials is based on ‘causing cancer,’ but the risk assessment of radiation exposure is based on ‘deaths from cancer’; it is impossible to compare the two risks. ICRP estimates that the risk of ‘developing cancer’ is twice as high as that of ‘death from cancer.’

Further, it is the user who decides whether or not to use chemical materials, weighing the advantages and disadvantages of use, but in the case of exposure due to nuclear power plant accidents, there are no direct advantages to anyone.

Research and objective

We would like to support people’s right of choice to avoid the risk of radiation exposure by measuring even low levels of radioactive cesium contained in milk and disclosing the areas of production and levels of contamination.

Even though many people avoided milk produced in the Tohoku district and chose milk produced in Hokkaido to avoid risks of radiation exposure. In FY2020 our research revealed the unanticipated fact that the milk produced in Hokkaido is also contaminated due to the Fukushima accident. The survey was therefore expanded to include western Japan products in order to compare contamination on a national scale.

Each measurement sample was 22kg of commercial milk which had an identifiable production location. In FY2021, the milk produced in 11 areas was included in the survey: Iwate (K), Miyagi (L), Ibaraki (M), Tokyo (N), Shizuoka (O), Ehime and Kochi (P), Miyazaki and Kagoshima (Q), Nagasaki (R), Oita (S), Shimane (T), and Ishikawa (U). (See Table 1. Areas A to J indicate areas surveyed in FY2020).

In measuring the concentration of radioactive cesium, 2kg of the sample was used as a direct measurement sample and the remainder (20kg) was used as a concentrated measurement sample. In order to detect small amounts of radioactive cesium, we performed the ammonium phosphomolybdate (AMP) method on whey after separating the whey from the milk.

 The germanium semiconductor detector (BSI Co. GCD70-200) was used for gamma ray detection in these radioactivity measurements.

The results of the measurements are shown in Table 1. In FY 2021, the production areas in which cesium 137 was detected in the direct measurement were Iwate (K) and Miyagi (L). No cesium 134 was detected in any areas in the direct measurement. In the concentrated measurement, cesium 137 was detected in all areas including the Kyushu district. The most contaminated area was Miyagi (L) with 152mBq/kg. This figure was higher than the 135mBq/kg found in Fukushima (H). The second most contaminated was the milk produced in Iwate (K), 79mBq/kg. These were followed by Shizuoka (O) (16mBq/kg), Ibaraki (M) (11mBq/kg), Tokyo (N) (7.3mBq/kg), Miyazaki and Kagoshima (Q)(7.0Bq/kg), Oita (S) (5.7mBq/kg), Shimane (T) (5.4mBq/kg), Nagasaki (R) (5.2mBq), Ehime and Kochi (P) (5.1mBq.kg) and Ishikawa (U) (3.9mBq/kg). Cesium 134 was detected only in Miyagi (L) (4.4mBq.kg) and Iwate (K) (2.0mBq/kg).

Please note that about 90% of cesium in milk exists in the whey, and therefore adopting this cesium concentration method indicates figures that are 10% lower in the concentrated measurement than in the direct measurement.

  1. Discussion

Origin of cesium 137

At the time the Fukushima Daiichi Nuclear Power Station accident occurred, cesium 134 and cesium 137 were emitted into the environment at the ratio of about 1:1. The half-life of cesium 137 is about 30 years, and that of cesium 134 is about 2.1 years. By taking into account the half-life of cesium 134 and 137, and the length of time from the disaster to the measurement, it is possible to calculate a cesium ratio (cesium 134/cesium 137) of cesium that was emitted as a result of the accident at the time of the measurement. Five years after the accident (March 2016), the cesium ratio had fallen to 0.21, Ten years after (March 2021), it had decreased to 0.046.

The cesium ratio differs slightly according to each reactor. Thus the cesium ratios due to the accident in the fallout in each location are different. The initial cesium ratios in the atmospheric fallouts in each area from March to May in 2011 were calculated using data from the environmental radiation database.

The proportions of cesium-137 (derived from the Fukushima nuclear reactor / total in the milk sample) were derived by calculating the measured value of cesium 134 concentration and calculated cesium 134/137 ratio at the time of measurement.

In the survey conducted last fiscal year (2021), the production areas where cesium 134 was detected were Hokkaido, Fukushima, Gunma, Tochigi, Iwate and Miyagi. The cesium 134/137 ratios in the atmospheric fallouts soon after the Fukushima nuclear disaster were as follows: Hokkaido 1.05, Fukushima 0.94, Gunma 1.00, Tochigi 1.01, Iwate 1.00 and Miyagi 1.00. The cesium 134/137 ratios (at the time of the measurement) were calculated by using the figures for the half-life of each cesium radionuclide and the number of years from the disaster to when the measurements were taken. The following figures were obtained: Hokkaido 0.053, Fukushima 0.044, Gunma 0.045, Tochigi 0.045, Iwate 0.035 and Miyagi 0.036. Dividing the cesium 134 concentration of the measurement by the cesium 134/137 ratio (at the time of the measurement) of the sample production area, the results of cesium 137 concentrations that derived from the Fukushima nuclear disaster were obtained and are shown in Table 2.

The following is one calculation example for Iwate. The cesium 134/137 ratio measured on November 19 in 2021 which was derived from the Fukushima nuclear disaster was calculated as 0.035 in Iwate. As the result of the measurement, 2.0±0.1mBq/kg of cesium 134 was detected and therefore cesium 137 derived from Fukushima reactor should be 56±3.9mBq/kg on the basis of the cesium 134/137 ratio. However, the measurement result of the concentration of cesium 137 in the sample was 79±0.8mBq/kg. The reason why it was higher than expected is because it contained cesium 137 that traces back to nuclear weapon tests and other sources. Thus, out of the total cesium 137 contained in the milk produced in Iwate, it was concluded that the ratio of 0.71±0.05 was derived from the Fukushima nuclear accident.

The origins and concentrations of cesium 137 in the samples were compared based on the production areas (Figure 2). In western Japan (P-U), which was supposed to have been mostly uninfluenced by the Fukushima nuclear disaster, the concentrations of cesium 137 were below 7mBq/kg. This cesium 137 was thought to be derived from various nuclear weapon tests and the Chernobyl disaster.

By contrast, in eastern Japan (except Hokkaido) H-O, the cesium concentrations derived from nuclear weapon tests and the Chernobyl accident were 7-19mBq/kg, and in Hokkaido the cesium concentration tends to be higher (15-66mBq/kg) than those in eastern Japan.

Consideration of health risk

Assuming that the radioactive contamination level of milk is 50Bq/kg of and also assuming that radiation exposure from all food is up to 1mSv per year, what is the additional risk of cancer death caused by radiation exposure through food based on the concentration of Cs137 in the milk measured in this study?

 The concentration of cesium in milk in this survey was found to be 4-150mBq/kg, and when milk is at the contamination level of 150mBq/kg, this is equivalent to 0.003mSv per year. Under these conditions the lifetime risk of dying from cancer increases by 1.2 people per 100,000.

As noted at the beginning of the article, in general, carcinogenic chemical materials are regulated so that their concentration causes one person per 100,000 to develop cancer in his/her lifetime. If people are to face an equivalent risk of cancer death from consuming radioactively contaminated food, the detection limit should be lowered to 0.1Bq/kg (100mBq/kg), and this information should be published to allow citizens who wish to avoid exposure to make choices on what products to buy. A sufficient number of detections are also required to support this choice of citizens to avoid exposure.

  1. Summary

 The Fukushima Daiichi nuclear accident has caused serious environmental radiation contamination to Fukushima and surrounding areas. Since then, some people have selected and purchased food that is produced in western Japan and Hokkaido in preference to food produced in the affected area in order to avoid radiation exposure through food.

 The concentrations of radioactive cesium in milk produced in specific areas across Japan were measured using the AMP method and the germanium semiconductor detector. This procedure made it possible to compare contamination in each area.

In all measurements, figures were considerably lower than the new standard limit of radioactive cesium contained in food (50Bq/kg), but the commercially available milk produced in Miyagi measured in 2021 was more highly contaminated than that produced in Fukushima measured in 2020, which suggests that Fukushima products are not necessarily the most contaminated. The milk produced in Hokkaido tended to contain more cesium than that produced in western Japan.

The risk of dying from cancer caused through food intake 10 years after the Fukushima Daiichi nuclear accident in the contaminated area was calculated based on the measurement values obtained this time, and was found to be of around the same order as the management standards for carcinogenic chemical substances, even though we are talking about a risk of suffering from cancer with chemicals and a risk of dying from cancer with radiation exposure.

Every person’s sense of values, what he/she thinks is the highest priority and what risk he/she wants to avoid, should be respected. The option of avoiding the risk of exposure to radiation should be thought of as important as the option of avoiding the risk of chemical substances and a mechanism must be established to allow this.

Western Japan, which was not so much affected by the Fukushima nuclear disaster, has also been contaminated by radiation from a historical angle; the harsh fact is that the past contamination caused by atmospheric nuclear weapons tests is still contained in food. We must become more aware that the mistake which the current generation has made by causing serious environmental radiation contamination from the Fukushima Daiichi nuclear accident will continue to affect generations in the future.

Source: https://cnic.jp/english/?p=6377

December 11, 2022 Posted by | Fuk 2022 | , , | Leave a comment

Environmentalists devoted to reveal nuclear contamination in Fukushima

A team of elderly Japanese environmentalists has been devoted to revealing the real environmental conditions of Fukushima after the nuclear incident in 2011. The team, with all members over 60 years old, volunteered to have a routine check for nuclear radiation in Fukushima. Masami Aoki, 77 and a former media worker, is one of the persons in charge of the team founded in 2012. Over the past 10 years, Aoki has worked with his team to examine nuclear radiation levels near the Fukushima Daiichi Nuclear Power Plant, including Futaba Machi and Minamisoma.

https://news.cgtn.com/news/2022-11-26/Environmentalists-devoted-to-reveal-nuclear-contamination-in-Fukushima-1fhsmcE7PZC/index.html

November 27, 2022 Posted by | Fuk 2022 | | Leave a comment

Plaintiff woman gives statement: “I can’t even think about what’s going to happen in the future.”

Here’s how it looked at the third trial yesterday.

Typically, one or two in 1 million people have pediatric thyroid cancer

Over 300 cases have been tested so far with approximately 38,000 people. “The plaintiff also argues that in epidemiology survey by experts using data from the town’s rural health survey, etc., the plaintiff’s thyroid cancer to be seen as a cancer causing outbreak (outburst) was an extremely high value of 94,9~99/3%. In the past, the causal relationship between the events and the disease that cause this 50-70% probability, has been recognized, and the causal relationship of damage and thyroid cancer is “with a high level of coincidence (and sometimes it’s good to treat it as proven.”

Supporters’ meeting for the thyroid cancer lawsuit. The plaintiffs’ lawyers explained their claims in the trial.

November 10, 2022

On November 9, the third oral argument was held at the Tokyo District Court in a lawsuit filed by seven men and women, aged 17 to 28, who were living in Fukushima Prefecture at the time of the accident, claiming that they developed thyroid cancer as a result of the accident at TEPCO’s Fukushima Daiichi Nuclear Power Plant. In their statements, the plaintiffs expressed their anxiety about the future, saying, “We can’t even think about the future.

 The plaintiff, a woman in her 20s who was in the first year of junior high school and living in Nakadori at the time of the accident, made a statement of opinion. After the second surgery, the wound, which extended down to her ear, did not close easily, and after she was discharged from the hospital, she said, “I was very upset when fluid started flowing from my neck.

 Recently, her cancer recurred, and there is talk of a third surgery. While she was frankly worried about her future and said, “The present, the future, in fact, it’s not good,” she added, “I am glad that it was me who got sick and not my relatives or friends.

Supporters’ meeting for the thyroid cancer lawsuit. Plaintiffs’ lawyers explained their claims in the trial November 9, 2022, Kasumigaseki, Tokyo; photo by Tetsuya Kasai.

Since the nuclear accident, more than 300 people in the prefecture have been diagnosed with thyroid cancer or suspected thyroid cancer. The woman told the judge, “I want to tell the judge that there are more than 300 people who are worried and their families are also worried. I hope that the current situation will change, even if only a little.

 On the day of the hearing, the plaintiffs mainly presented rebuttals and statements of opinion in response to TEPCO’s claims. In response to TEPCO’s claim that the plaintiffs were exposed to low levels of radiation (less than 100 millisieverts) and that the risk of developing thyroid cancer did not increase, the plaintiffs pointed out that “there is a risk even at much lower levels than 100 millisieverts,” citing overseas papers.

Plaintiffs’ lawyers hold a press conference on the thyroid cancer lawsuit.

The plaintiffs also claimed that an epidemiological survey conducted by experts using data from the prefectural health survey showed that the “probability of cause” of the plaintiffs’ thyroid cancer being attributable to radiation exposure was extremely high, ranging from 94.9% to 99.3%. In past pollution lawsuits, a causal relationship between the causative event and the disease was recognized even when the probability was 50-70%, and the causal relationship between radiation exposure and thyroid cancer “can be treated as proven with a high degree of probability,” he said.

 The two plaintiffs are scheduled to present their arguments on January 25 and March 15 next year, respectively. (The two plaintiffs are scheduled to present their opinions on January 25 and March 15.)
https://www.asahi.com/articles/ASQC97SZSQC9UGTB001.html?iref=pc_photo_gallery_bottom

November 20, 2022 Posted by | Fuk 2022 | , | Leave a comment

Long-lived radionuclides from the Fukushima nuclear power plant in Japan, and consequences for Pacific ecosystems and seafood consumers

October 28, 2022

Nicholas Fisher

Distinguished Professor

School of Marine and Atmospheric Sciences

Stony Brook University, Stony Brook, New York

Abstract: After the Fukushima accident in March 2011, marine organisms, seawater and sediment were contaminated with both 134Cs and 137Cs that was released into coastal waters. We analyzed radionuclides in Pacific biota, including plankton, diverse invertebrates, and pelagic and benthic fish. Field data (~41,000 data points) showed temporal declines of 137Cs levels were >10x lower in benthic than pelagic fish, reflecting 137Cs declines in sediments and seawater, consistent with lab studies showing benthic fish acquiring 137Cs from benthic invertebrate diets. Bluefin tuna that spawn near Japan and migrate to waters off California were contaminated with Fukushima-derived radiocesium that they obtained from Japanese waters. The consequent risk to seafood consumers was assessed and compared to that from naturally occurring radionuclides.

Bio:  I am a marine biogeochemist who has focused on the bioaccumulation of diverse contaminants in marine organisms. This research has considered the impacts of this bioaccumulation on organisms and public health, and has also considered the influence of organisms on the cycling and fate of the contaminants. Most of this work has involved metals and long-lived radionuclides. I received a BA from Brandeis University, and a PhD from Stony Brook, I was a postdoctoral investigator at the Woods Hole Oceanographic Institution, after which I worked for a government lab in Melbourne Australia, the IAEA Lab in Monaco, the Brookhaven National Lab, and Stony Brook University (since 1988).

Watch live here

Source: https://nuc.berkeley.edu/event/long-lived-radionuclides-from-the-fukushima-nuclear-power-plant-in-japan-and-consequences-for-pacific-ecosystems-and-seafood-consumers/

October 31, 2022 Posted by | Fuk 2022 | , | Leave a comment

The mishandling of scientifically flawed articles about radiation exposure, retracted for ethical reasons, impedes understanding of the scientific issues pointed out by Letters to the Editor

October 23, 2022

JoSPI

Tanimoto Y, Hamaoka Y, Kageura K, Kurokawa S, Makino J, Oshikawa M. The mishandling of scientifically flawed articles about radiation exposure, retracted for ethical reasons, impedes understanding of the scientific issues pointed out by Letters to the Editor. JoSPI. Published online October 23, 2022. doi:10.35122/001c.38474

Abstract

We discuss the editorial handling of two papers that were published in and then retracted from the Journal of Radiological Protection (JRP).1,2 The papers, which dealt with radiation exposure in Date City, were retracted because “ethically inappropriate data were used.”3,4 Before retraction, four Letters to the Editor pointing out scientific issues in the papers had been submitted to JRP. The Letters were all accepted or provisionally accepted through peer review. Nevertheless, JRP later refused to publish them. We examine the handling by JRP of the Letters, and show that it left the reader unapprised of a) the extent of the issues in the papers, which went far beyond the use of unconsented data, and b) the problems in the way the journal handled the matter. By its actions in this case, JRP has enabled unscientific, unfounded and erroneous claims to remain unacknowledged. We propose some countermeasures to prevent such inappropriate actions by academic journals in future.

https://www.jospi.org/article/38474-the-mishandling-of-scientifically-flawed-articles-about-radiation-exposure-retracted-for-ethical-reasons-impedes-understanding-of-the-scientific-iss?fbclid=IwAR3U0HFOpC0YWX6bMCR0bGtkf9FeRYfnzR011SBoLN2TKKZlf0G-VWCy114

October 26, 2022 Posted by | Fuk 2022 | , | Leave a comment

104th protest in front of TEPCO’s head office, “Net for the Realization of No Exposure to Radiation” issued a full-length letter of offer summarizing the actual damage caused by exposure to radiation.

August 1, 2022
Children suffering from radiation exposure: “Acknowledge the relationship between the nuclear accident and childhood thyroid cancer
The Network for the Realization of No Exposure to Radiation Takae Miyaguchi

The Fukushima nuclear power plant accident spread enormous amounts of radiation.
 After the accident, many people gathered in front of the TEPCO headquarters in protest. 14 years ago, a joint protest began on the first Wednesday night of every month, and this May marked the 104th such event.
 About 100 people gather each time. Before the protest, drums are beaten and a microphone relay is used to protest and make a request to TEPCO. Each time, we hand the TEPCO a written request, and if there is any doubt about the response, we submit it again.
 This is the first time that the “Network for the Realization of Exposure Free Japan,” which I am involved with, has submitted a written request to the TEPCO. The content of the letter is a reflection of the situation and thoughts I have been experiencing through my support for the “Children’s Lawsuit for Exposure to Radiation” and other activities.
The following is a summary of the letter.
 Eleven years have passed since the accident at TEPCO’s Fukushima nuclear power plant, but the declaration of a nuclear emergency remains in effect. At the Fukushima nuclear power plant, exposure work with no foreseeable future and convergence work continue amidst the remaining debris with high concentrations of radioactive materials that are inaccessible to humans. Radioactive materials have been released into the air and are flying into the Tokyo metropolitan area on the wind. The Fukushima nuclear accident has not ended.
 TEPCO understands the despair, grief, and anger of the victims of the nuclear accident, and in an effort to hold TEPCO accountable, victims have filed lawsuits against TEPCO in various regions, and the courts have confirmed TEPCO’s responsibility for the nuclear accident, but the amount of compensation awarded by the courts to each person is shockingly small compared to the extent of the damage.
 What the victims truly desire is the return of their hometowns as they were before the nuclear accident, where people made their living, families lived, children cheered, and people laughed, and where life was normal, rooted in the local climate, and connected to history! This is what we have been trying to achieve for the past 11 years.
 Eleven years later, the evacuation designation has been lifted except for some areas that are difficult to return to.
The policy of forcing people to return to their hometowns because their annual exposure level is below 20 mSv, 20 times the allowable annual exposure level of 1 mSv for the public, is unacceptable. We denounce the depth of TEPCO’s crimes of spreading massive amounts of radioactive materials, polluting the mountains, rivers, and land of Fukushima, destroying our hometowns, and depriving people of their livelihoods.

In the face of radiation taboos and discrimination
Young People Who Courageously Stood Up Against Radiation Taboo and Discrimination

He continued.
 In January of this year, six young people who had developed childhood thyroid cancer rose to their feet. Three or four years later, many of them were found to have pediatric thyroid cancer in a Fukushima health survey, and all of them underwent surgery.
 Thyroid cancer is a slow-growing cancer, and the prognosis for surgery is good, according to the committee’s experts. However, some of the plaintiffs had recurrence after surgery, reoperation, RAI isotope treatment, and some were found to have distant metastasis in the lungs.
 Their health did not recover even after the surgery, they dropped out of college, resigned from the company where they worked, were not hired when they were told they had cancer, etc. They have thought about, worried about, and suffered from the despair of having the door closed to them at the starting line of their lives, anxiety about the recurrence of cancer in the future, treatment costs, work, and whether or not they will be able to make a living independently.
 Why is it that nearly 30 out of 380,000 children in Fukushima have developed thyroid cancer, compared to only one or two out of a million children in Japan? Why have nearly 300 cases been reported among 380,000 children in Fukushima? The Prefectural Health Study Review Committee acknowledges the high incidence of childhood thyroid cancer, but denies any causal relationship with the nuclear accident, saying that it is overdiagnosis.
 Last July, the Hiroshima A-bomb “black rain” victims’ lawsuit recognized that internal exposure is not a matter of quantity, but that if even a small amount of radiation enters the body’s tissues and is deposited, it damages cells and causes cancer. In the case of the Chernobyl nuclear power plant accident, a causal relationship between childhood thyroid cancer and the nuclear power plant accident was recognized. The plaintiffs want to clarify why they developed pediatric thyroid cancer.
 The relationship between childhood thyroid cancer and the nuclear power plant accident” is a taboo subject, and the plaintiffs have been hiding their illness for a long time for fear of being discriminated against, but they want to make their illness public and have the court find a “causal relationship between childhood thyroid cancer and exposure to radiation. They have stood up courageously to make TEPCO pay compensation for their illness. We demand the following
TEPCO must admit that it is the perpetrator of the Fukushima nuclear power plant accident and the spreading of radioactive materials.
Please admit the causal relationship between radioactive iodine released from the Fukushima nuclear power plant and childhood thyroid cancer as soon as possible.
Please take responsibility for the future of these six young people.
We ask that you take responsibility for the future of these six young people.
 The fight to leave a world without radiation exposure to children continues.
https://note.com/jinminshinbun/n/n137ff335aaa0?fbclid=IwAR2FJl-KiOLqm4GjfTdOT6uBxZ_xlAzh7BOiwkkUyeOXHsyAUXBc88jeFck

August 4, 2022 Posted by | Fuk 2022 | , , , | Leave a comment

Indonesia lifts restrictions on post-Fukushima food imports at Japan summit

TOKYO, July 27 (Xinhua) — Japanese Prime Minister Fumio Kishida and visiting Indonesian President Joko Widodo held talks in Tokyo on Wednesday ahead of this year’s Group of 20 major economies’ summit in Bali in November which Widodo will host.

Following a summit meeting between the leaders, Kishida told a joint press conference that Indonesia has lifted all restrictions on imports of Japanese food products that were imposed in the wake of the Fukushima nuclear crisis in 2011.

Kishida said he was thankful for the move and that the lifting of import restrictions on food products from seven previously affected prefectures here would “encourages people in the disaster-hit areas.”

Widodo, for his part, said he asked Japan to ease or scrap tariffs it imposes on Indonesian tuna, pineapples and bananas.

He also passed on his condolences over the fatal shooting of former Prime Minister Shinzo Abe, who was gunned down during a stump speech earlier this month.

Widodo will conclude his visit to Japan with a meeting with Emperor Naruhito later in the day and will then depart for South Korea, government officials here said.

http://www.china.org.cn/world/Off_the_Wire/2022-07/27/content_78344112.htm

July 31, 2022 Posted by | Fuk 2022 | , , | Leave a comment

Persistent impact of Fukushima decontamination on soil erosion and suspended sediment.

Published: 14 July 2022

Abstract

In Fukushima, government-led decontamination reduced radiation risk and recovered 137Cs-contaminated soil, yet its long-term downstream impacts remain unclear. Here we provide the comprehensive decontamination impact assessment from 2013 to 2018 using governmental decontamination data, high-resolution satellite images and concurrent river monitoring results. We find that regional erosion potential intensified during decontamination (2013–2016) but decreased in the subsequent revegetation stage. Compared with 2013, suspended sediment at the 1-year-flood discharge increased by 237.1% in 2016. A mixing model suggests that the gradually increasing sediment from decontaminated regions caused a rapid particulate 137Cs decline, whereas no significant changes in downstream discharge-normalized 137Cs flux were observed after decontamination. Our findings demonstrate that upstream decontamination caused persistently excessive suspended sediment loads downstream, though with reduced 137Cs concentration, and that rapid vegetation recovery can shorten the duration of such unsustainable impacts. Future upstream remediation should thus consider pre-assessing local natural restoration and preparing appropriate revegetation measures in remediated regions for downstream sustainability.

Main

Radioactive material leakage from nuclear industry activities or nuclear accidents poses a major threat to the environment and the economy1,2. Historically, widespread radioactive contamination has been observed several times, such as the Kyshtym accident (Soviet Union) in the 1940s–50s3, Windscale accident (England) in 1957 (ref. 4) and Chernobyl accident (Soviet Union) in 1986 (ref. 5). Long-term radiation exposure and radiophobia have increased the health risk and psychological pressure on the people of these regions, resulting in the abandonment of large areas rich in environmental resources and consequently in constrained sustainable human development6,7. As a key means for recovering contaminated regions, mechanical decontamination has been implemented in many legacy sites, including Hanford (United States)8 and Chernobyl9. However, almost all attention has been directed to understanding in situ decontamination effects10,11 and atmospheric particle resuspension issues12 and little is known about if these perturbations would have secondary environmental impacts on their downstream catchments for the long-term.

On 11 March 2011, the most recent large-scale nuclear accident happened at the Fukushima Daiichi Nuclear Power Plant (FDNPP), Japan13. Over 2.7 PBq of fallout 137Cs (half-life T1/2 = 30.1 yr) from the FDNPP was deposited in the terrestrial environment, causing long-term radioactive contamination on large-scale neighbouring catchments13,14. To recover contaminated soil and decrease the exposure dose in Fukushima, the Japanese government evacuated the residents in 2011 and large-scale decontamination was implemented in contaminated villages15,16 in 2012. Within a few years, dramatic land-cover changes occurred in the agricultural regions where 5 cm of the surface soil and vegetation were removed and replaced with uncontaminated soil (Fig. 1), with the subsequent natural restoration promoting revegetation in these decontaminated regions11,13,16.

The effectiveness of such intensive decontamination at reducing radiation exposure in situ is apparent, with air dose rates decreasing by 20–70% after decontamination13. However, as 137Cs can firmly bind to clay minerals, it is transported along with suspended sediments (particulate 137Cs) in the river system to the Pacific Ocean17,18. Comprehensively assessing the impacts of land-use changes in decontaminated regions on the downstream ecosystem is also necessary from the perspective of environmental sustainability. Moreover, recent studies have shown notable differences in the reduction in the particulate 137Cs concentration over time across 30 rivers in Fukushima19,20, further underscoring the need to study land-use impacts on downstream particulate 137Cs discharge into the ocean.

In addition to contaminant migration, land-use changes induced by strong perturbations (for example, decontamination) also alter land–ocean sediment transfer patterns21,22, which in turn affect elemental cycles23, biodiversity24 and global climate25. Systematically assessing the perturbations’ impacts on downstream river suspended sediments (SS) has thus become a joint goal of many related disciplines and a key part of developing science-based catchment management strategies26,27,28,29. However, owing to the limited availability of reliable and concurrent river monitoring data, how downstream river SS variations and land-use changes are linked remains unclear21.

With approximately 11.9% of the watershed area subjected to government-led decontamination between 2013 and early 2017, the Niida River Basin in Fukushima (Fig. 2a)11,30 provides an excellent opportunity to examine the long-term impact on the dynamics of SS and particulate 137Cs in river systems. Previous short-term river studies19,20,31,32,33,34 have suggested an increased river SS load and decreased particulate 137Cs concentrations during decontamination. Several studies that analysed geochemical fingerprints in deposited sediments have implied a significant contribution of sediment sources from the decontaminated regions to the river35,36,37. Yet, given that threats to sustainable catchment management from anthropogenic perturbations are often long-term28,29, more comprehensive and reliable data on quantified land cover and continuous river records are required to explore the effect of decontamination on river SS and particulate 137Cs discharge.

Here we provide a comprehensive assessment of the impacts of land-use changes in decontaminated regions on river SS and particulate 137Cs dynamics, as well as the downstream discharge. We mapped the evolution of decontaminated region boundaries using governmental decontamination documents (Fig. 2b), photographed the land cover in the decontaminated regions using drones and quantified the land-use changes using the normalized difference vegetation index (NDVI) at 10 m spatial resolution. Meanwhile, we conducted a long-term field investigation (Methods and Supplementary Table 1) spanning the decontamination (2013–2016) and natural restoration (2017–2018) stages to continuously record the fluctuations in water discharge and turbidity (10 min temporal resolution) and particulate 137Cs concentrations, both upstream and downstream. Combining the above quantitative data, we systematically reveal that long-term land-use changes in upstream decontaminated regions greatly affect sediment and 137Cs discharge from downstream river systems into the Pacific Ocean.

Land-cover changes in decontaminated regions

Regional decontamination was accomplished in March 2017, spanning over 22.9% and 11.9% of the upstream (Notegami) and downstream (Haramachi) watershed areas, respectively (Fig. 2c). In 2014, agricultural land (18.02 km2) was one of the major land uses in the regions where decontamination was ordered, with a significant increase of over 720% compared with that in 2012. Conversely, the changes in the ordered grassland (1.93 km2) were minimal, with an approximately 26% increase between 2012 and 2014. Given that overall land-cover changes were more pronounced in agricultural lands than in grasslands or residential lands, large-scale agricultural land decontamination may severely alter landscape erodibility and consequently the sediment supply. Moreover, the proximity of the decontaminated agricultural land to rivers increases sediment transport from terrestrial environments.

Drone photographs (Fig. 3a and Supplementary Fig. 1) showed significant land-cover changes in the upstream decontaminated regions. For instance, the agricultural land at the Hiso site (D3, Fig. 3a) was almost bare in August 2016 when the decontamination was completed. However, natural restoration caused the recovery of vegetation during the post-decontamination stage (August 2018). Considering the decontamination sequence and seasonal dependence of the plant growth cycle38, drastic spatiotemporal land-cover changes in the decontamination regions are conceivable.

To quantitatively estimate and compare land-cover changes in the decontaminated regions, we generated NDVI maps in these regions based on the available satellite images from Sentinel 2 and the Moderate Resolution Imaging Spectroradiometer (MODIS) between 2011 and 2018. The comparison between realistic scenarios and Sentinel 2-based NDVI maps (spatial resolution: 10 m) from drone observation sites (Fig. 3a and Supplementary Fig. 1) confirmed the feasibility of NDVI images in distinguishing bare land and agricultural land with vegetation cover. To improve the temporal–spatial resolution of the NDVI dataset, we used the enhanced spatial and temporal adaptive reflectance fusion model (ESTARFM)39 to fuse Sentinel 2 and MODIS maps. Subsequently, we used interpolation to link these newly generated NDVI data (spatial resolution: 10 m) and plotted daily NDVI variations for all decontaminated regions between 2011 and 2018 (Fig. 3b).

In the daily NDVI variation curve, the peak NDVI during 2013–2014 was similar to the pre-decontamination stage (2012) but decreased by approximately 10% in 2016. After decontamination, the peak value presented an increasing trend under the influence of vegetation recovery. However, as the government lifted the evacuation zone after 2017 and allowed the residents to return, vegetation was again removed from some areas planned for agricultural activities in 2018 (Supplementary Fig. 1b). Further analyses of the NDVI variations in the decontaminated regions scheduled in 2012 (Fig. 3c), 2013 (Fig. 3d) and 2014 (Fig. 3e) showed that the NDVI peaks were decreased by approximately 12%, 11% and 15%, respectively, within 2–3 years after decontamination was ordered, thereby providing unambiguous evidence for decreasing vegetation land cover caused by decontamination.

We converted all NDVI maps derived from ESTARFM-images to C × P (cover management and support practice factors, respectively) maps using empirical models40,41. We then estimated erosion potential (that is, K × LS × C × P in the revised Universal Soil Loss Equation (RUSLE); Methods) maps using the LS (slope length and slope steepness factors, respectively) map (Supplementary Fig. 2) and K (soil erodibility) factor in the decontaminated regions. We found that the slopes of decontaminated regions were generally similar for each decontamination-ordered year (Supplementary Fig. 2), suggesting that the erosion potential was consistent with the NDVI in the decontaminated regions. Therefore, we also estimated the daily variation curve of the erosion potential using the mean CP, LS and K factors in the decontaminated regions.

Here we show the ESTARFM-based NDVI (Fig. 3f) and erosion potential (Fig. 3g) during the summer season for each year (specific periods in Supplementary Table 2). NDVI showed a decreasing trend from 2013 to 2016, while the erosion potential peaked in 2016, representing approximately 98% and 52% increases over the pre-decontamination (2011) and natural restoration (2018) stages, respectively. Combining the corresponding NDVI (Supplementary Fig. 3) and erosion potential maps (Supplementary Fig. 4), significant changes in the spatial differences between the land cover and erosion potential during decontamination were also observed.

Response of river SS to land-cover changes

The downstream river SS load (L; Fig. 4a) exhibited a strong correlation with water discharge (Q) during the monitoring periods (Supplementary Fig. 5 and Supplementary Table 3). Under the range of water discharges from 0.1 to 100 m3 s−1, river SS carrying capacity exhibited a considerable increase from 2013 to 2016 and a slight decrease after decontamination (2017 to 2018). Contrastingly, the range of water discharges was relatively narrow upstream, and a steady decrease in SS loads has been observed since 2015. Although the above result suggests an increase in SS supply during the decontamination stage, the high SS carrying capacity in 2015 is not consistent with the actual decontamination progress. Since decontaminated regions tend to be bare land, sediment loads are prone to increasing during rainstorms due to soil erosion. Governmental decontamination plans showed that over 50% of agricultural land decontamination was planned to be implemented in 2016, implying that the erosion potential of the decontaminated regions should have been higher in 2016, rather than in 2015.

The variations in downstream river SS loads over the 6 years (Supplementary Table 4) exhibited a similar trend to peak river SS load in 2015 (126.7 ± 0.3 Gg yr−1). This was an approximately 1,776%, 140% and 215% increase relative to that of 2013, 2016 and 2018, respectively. The historical rainfall records (Fig. 4b) show that the rainfall in September 2015 (551 mm) was more than two-fold greater than that during the same period in 2016 (274 mm), implying that the SS peak may be related to strong runoff. Here we estimate the SS loads at 1-year-flood discharge (Q = 95 m3 s−1) using established LQ curves, which allow for the comparison of dynamic variations in SS loads under the same flood conditions. In Fig. 4b, a significant increasing trend during the decontamination period is shown, with a 237.1% increase in 2016 compared with 2013. After decontamination, the SS loads drastically decreased by approximately 41% from 2016 to 2017, implying changes in sediment yield and transfer patterns due to natural restoration. These results reveal that river SS loads responded closely to land-cover changes during the study period.

To better explore the response of river SS load to land-cover changes, we extracted river monitoring data during each rainfall event and quantitatively linked the river SS to the corresponding soil loss from the decontaminated regions. We found that SS loads during rainstorms were highly correlated with water discharges in both upstream and downstream areas (Supplementary Fig. 6). Comparing similar rainfall events, significantly greater SS concentrations are observed in 2015–2016 than in other years (Supplementary Figs. 7 and 8). Considering that the land-cover changes induced by decontamination were more pronounced in the summer season, the regression was performed for SS loads between May and October and soil loss during the corresponding period. A more significant correlation was observed (Fig. 4c) between estimated soil loss by RUSLE and SS load upstream (R2 = 0.55, P < 0.01, N = 34) than downstream (R2 = 0.27, P < 0.01, N = 52). Eliminating the effect of rainfall and normalizing by discharge (Fig. 4d) results in a more evident relationship between the erosion potential and SS loads downstream (R2 = 0.35, P < 0.01, N = 52). Overall, these results demonstrate the connection between river SS dynamics and land-cover changes in the decontaminated regions. The short distance between the upstream catchment and decontaminated regions makes soil erosion a critical driver for upstream river SS transport, whereas the downstream river is dependent on long-distance SS transport, making water discharge an important driver for the downstream catchment.

Long-term impact on river SS and 137Cs discharge

From August 2014 to March 2017, the particulate 137Cs concentration in Haramachi exhibited a steep decrease, contrasting remarkably with the limited 137Cs variation observed in the early decontamination stages (January 2013 to August 2014; Fig. 5a). The effective half-life of the particulate 137Cs (eliminated by the natural attenuation factor) during this decontamination period (1.87 yr) was considerably faster than that of physical decay of 137Cs (30.1 yr), the early decontamination period (16.9 yr) and the surrounding contaminated catchments (mean of 4.92 yr)20. Such a sharp decrease in the 137Cs concentration was also observed at the other three monitoring sites (Supplementary Fig. 9a). Because the 137Cs concentration in decontaminated soil was considerably lower than that in the contaminated soil37,42, these results suggest the contribution of sediment from decontaminated regions to the river system. Moreover, strong negative correlations were observed between measured 137Cs levels and decontamination progress at all monitoring sites (Fig. 5b and Supplementary Fig. 9b), which further supports our interpretation. The observed 137Cs concentration increased by approximately 150% in 2018 compared with that at the end of 2016, which may be caused by the weakened sediment supply from decontaminated regions owing to natural restoration and resulting in a relatively increased contribution of sediments from contaminated forest regions13.

Given that the variation in 137Cs concentrations reflects a change in sediment source, the deviations between the measured 137Cs and the natural decrease in 137Cs derived from surrounding contaminated catchments provide a way to quantitatively estimate the contribution of sediment from decontaminated regions (Fig. 5c). In the early decontamination period, our results showed slight variations in the 137Cs concentration (Fig. 5a), which could be attributed to the contribution of sediment from upstream regions with different degrees of contamination. During the main decontamination period, the erosion potential in the summer of 2015 was approximately 22% higher than that in 2014 and the heavy rainfall caused the largest flooding event during the study period (26-year flood). This may result in the sediment from decontaminated regions not being the dominant source for downstream. In 2016, decontamination caused an increase by approximately 59% in the erosion potential compared with 2013, with the contribution percentage steadily increasing over this period to a maximum of 75.7% ± 3.2% (value ± 95% uncertainty). After decontamination, the decreased contribution of sediment from decontaminated regions and the increased 137Cs concentrations can both be attributed to the reduction of soil loss from upstream due to natural restoration.

The 137Cs discharge from contaminated catchments around the Fukushima region into the Pacific Ocean is another ecological issue of global concern. Our data show that the export flux of particulate 137Cs from the downstream of the Niida river (that is, Haramachi) peaked in 2015 (1.24 TBq yr−1, equalling 0.65% 137Cs loss), which is an approximately 667%, 233% and 429% increase relative to that in 2013, 2016 and 2018, respectively (Fig. 5d). Although such 137Cs loss is negligible compared with the terrestrial inventory, it is approximately 105 times greater than that in the pre-Fukushima stage43,44. Accordingly, the dynamic variations in 137Cs discharge from terrestrial environments into the Pacific Ocean, and its drivers, require more attention in the future.

Here we used SS loads at one-year-flood discharge to normalize the 137Cs flux and found its peak occurred in 2015 (Fig. 5e). Additionally, the reduction of the normalized 137Cs flux from 2013 to 2016 (~32%) was similar to natural attenuation in the non-contaminated catchment (~34%) over same period, which may be due to the increased SS load during decontamination offsetting the role of declining 137Cs concentrations in reducing 137Cs emission. During the subsequent natural restoration period, the rapid NDVI increase (Fig. 3f) suggested vegetation recovery in decontaminated regions and a decrease in regional erosion potential (Fig. 3g). This resulted in an approximately 24% decrease in sediment yield from the catchment and an approximately 31% decrease in the contribution of sediment from decontaminated regions (Fig. 5c) from 2016 to 2018. Due to the mutual balance of these effects, there were no significant changes in normalized particulate 137Cs flux in 2018 compared with 2016.

Discussion

Our work highlights the great potential of interdisciplinary analyses for understanding river SS variation and quantifying the contribution of sediment from specific regions. Fukushima decontamination practices, like a controllable validation experiment, justified the reliability of using long-term 137Cs monitoring data for tracing sediment source dynamics due to specific perturbation. Combining the long-term dataset of 137Cs (or other fallout radionuclides) in SS with remote sensing images would provide additional evidence to determine if the changes in the downstream SS transport pattern are linked to the upstream perturbation.

With these interdisciplinary analyses, we systematically reveal how changes in land use in the decontaminated regions significantly influences downstream river SS and 137Cs discharge into the ocean. Indeed, the secondary environmental impacts of surface remediation are being increasingly considered in the broader field concerning remediation of regions contaminated with hazardous materials (for example, heavy metals and organic contaminants)45. The concept of environmental sustainability-centred green remediation has also been brought up in many scenarios46,47. The Fukushima decontamination practice provides evidence showing that mechanical remediation can cause persistently excessive SS load downstream, though it also reduced river 137Cs concentrations. Since persistently excessive turbidity in rivers affects not only surrounding residents’ water use but also trophic level structure in aquatic environments48, the unsustainable downstream impacts caused by upstream decontamination should be highly regarded. The vegetation recovery after land development is highly dependent on local conditions49, and the soil used for decontamination and local high rainfall amount in Fukushima promoted rapid vegetation recovery11,13, which shortened the duration of such unsustainable impacts. Therefore, future upstream contaminated lands that await mechanical remediation need to consider the pre-assessment of local natural restoration conditions or the preparation of appropriate revegetation measures in the catchments’ regulatory frameworks, which would minimize the impact of long-term decontamination on downstream sustainability.

Methods

Study region

The Niida River Basin (265 km2) is located about 40 km northwest of the damaged FDNPP. The topography of its upstream is almost mountainous and its soil types are mainly cambisols and andosols, while fluvisols are the dominant soil type in the downstream plain50. The monitoring data from the Japan Meteorological Agency show that the average rainfall in the Niida River Basin is greater than 1,300 mm, with more than 75% of the rainfall occurring between May and October. According to the third airborne monitoring survey by the Japanese government, the 137Cs inventory in the Niida River Basin was over 700 kBq m−2 (ref. 14). Because of particularly high contamination in its upstream watershed (over 1,000 kBq m−2)51, the government-led decontamination was implemented in the upstream basin from 2013 to 2016 (~1% of the area was extended to March 2017).

Land-cover observation

We constructed the vector decontamination maps based on the paper maps from the Ministry of the Environment, Japan. The boundaries of the decontaminated regions with different land-use types were first outlined by creating polygons using Google Earth. Subsequently, the projections of these polygons were imported to ArcMap v.10.3 to quantitatively evaluate their area.

During the decontamination (2016) and post-decontamination stages (2018), drone photography was utilized (Fig. 2a, triangle) to compare land-cover changes. A commercially available drone (Phantom 4, DJI product) was employed at 100 m above the ground in Matsuzuka (D1; 37.689° N, 140.720° E), Iitoi (D2; 37.663° N, 140.723° E) and Hiso (D3; 37.613° N, 140.711° E) to take photographs.

Quantification of land-cover changes in decontaminated regions

We calculated NDVI within the boundary of the decontaminated regions to quantify the land-cover changes. Through the spectral reflectance dataset in the red (R, nm) and near-infrared (NIR, nm) regions, the NDVI was calculated as52:

NDVI=NIR−RNIR+R.

(1)

The available satellite images from 2011 to 2018 from Sentinel 2 were downloaded from the United States Geological Survey53, while the concurrent MODIS images were derived from the National Aeronautics and Space Administration’s Reverb54. The wavelength bands and spatiotemporal resolutions of the satellite images used here are summarized in Supplementary Tables 5 and 6.

To confirm the reliability of the newly generated NDVI variation curve, NDVIs for the same date as the Sentinel 2 images were estimated using the interpolation and compared with the Sentinel 2-based NDVI. The linear regression analysis showed that the fitting R2 was 0.99 (N = 16, P < 0.01). We also calculated the NDVI in the decontaminated region based on available satellite images of Landsat 5/7/8 (ref. 53) and established a daily NDVI variation curve. The linear regression analysis also showed a high R2 between two daily NDVI curves (R2 = 0.97, N = 2868, P < 0.01). Therefore, these NDVIs calculated by different satellite images confirmed the reliability of the NDVI variation curve based on ESTARFM.

Estimation of erosion potential in decontaminated regions

To link the land-cover changes in decontaminated regions with the soil erosion dynamics, we defined an erosion potential (K × LS × C × P) based on the RUSLE.

The soil loss (A, t ha−1 yr−1) of a specific region can be estimated as55:

A=R×K×LS×C×P,

(2)

where R is the precipitation erosivity factor (MJ mm ha−1 h−1 yr−1), K represents the soil erodibility factor (t h MJ−1 mm−1), L and S are slope length factor (dimensionless) and slope steepness factor (dimensionless), respectively, and C and P are the cover management factor (dimensionless) and support practice factor (dimensionless), respectively. Because these parameters are often set as fixed values, it is difficult to assess the soil loss dynamics during anthropogenic disturbances. To address this problem, we used daily NDVI data to estimate C × P and then considered these dynamic factors in RUSLE.

Wakiyama et al.40 reported a correlation between vegetation cover in Fukushima and the sediment discharges from the standard USLE plot (that is, soil loss, A) that have been normalized by R, K, S and L factors40,56. Therefore, this empirical equation reflects the quantitative relationship between vegetation fractions (VF) and C × P.

To quantify daily C × P changes in decontaminated regions, we first converted the interpolated daily NDVI into the VF by a semi-empirical equation41:

VF=1−(NDVI−NDVI∞NDVIs−NDVI∞)0.6175,

(3)

where NDVIs and NDVI represent the NDVI value for land cover corresponding to no plants and 100% green vegetation cover, respectively. Since these values mainly depend on plant species and soil types, we followed previous methods applied to agricultural land and set NDVIs and NDVI as 0.05 and 0.88, respectively41.

Subsequently, the C × P was estimated by the empirical equation derived from uncultivated farmlands and grasslands (R2 = 0.47, N = 145)40:

C×P=0.083×e−5.666×VF.

(4)

Since the soil type used for decontamination is generally the same, the K factor was set as a constant (0.039; ref. 40). For the LS factor, we downloaded a digital elevation model from the Geospatial Information Authority of Japan (spatial resolution: 10 m) to build an LS map using55:

LS=[QM22.13]y×(0.065+0.045×Sg+0.0065×S2g),

(5)

where Qa is the flow accumulation grid, Sg represents the grid slope as a percentage, M is the grid size and y is a parameter depended on slope steepness. We here used the y values recommended by a published study, ref. 55.

The calculated LS-factor map (Supplementary Fig. 2) showed a relatively consistent LS distribution in space. Based on the ESTARFM-generated satellite images, we compared C×P and erosion potential (K × LS × C × P) and found a significant correlation (R2 = 0.99, P < 0.01, N = 174). Since these results suggest that LS factors in decontaminated regions have a negligible effect on the erosion potential, the mean LS factor and interpolated NDVI based on the daily variation curve (Fig. 3b) were used to estimate the daily erosion potential.

Monitoring of river discharge and turbidity

The water-level gauges (in situ Rugged TROLL100 Data Logger) and a turbidimeter (ANALITE turbidity NEP9530, McVan Instruments) were installed in each monitoring site to continuously recording the water level and turbidity with a temporal resolution of 10 min. As ocean tides may influence the accuracy of water-level monitoring, the Sakekawa site (M4 in Fig. 2) was excluded from the river monitoring programme.

The recorded water level (H, m) was converted to the water discharge (Q, m3 h−1) based on the annual HQ curves for each monitoring site. These curves were calibrated using a synchronous monitoring dataset of 10-min-resolution water level and discharge provided by the Fukushima prefecture’s official monitoring network57. Because of occasional damage to the water-level gauge at the Haramachi site, the available monitoring data with a temporal resolution of 10 min recorded by the Fukushima prefecture’s official monitoring network57 were used to fill the gaps. The percentages of filling data from official monitoring network were all less than 34% except for 2015 (56.6%). Although similar situations occurred in Notegami, we were unable to fill in gaps with other data due to the lack of a concurrent monitoring network.

The hourly SS concentration (Css, g m−3) at each monitoring site was calculated from the measured turbidity (T, mV) using a calibrated curve20. As the turbidimeter was susceptible to the moss and debris flowing in the river, the dataset was verified with an automated check by HEC-DSSVue (The U.S. Army Corps of Engineers’ Hydrologic Engineering Center Data Storage System) before transforming the data.

The SS load was estimated as the product of the corresponding datasets of discharge and SS concentration, after which we can obtain the annual SS load (L, ton yr−1) by taking the sum:

L=∑(Q×Css).

(6)

We estimated values for gaps including missing and abnormal data through a linear model established by 10-min-resolution monitoring data at the same site. The reliability of the gap-filling strategies used in this study has been documented by Taniguchi et al.19,20 These procedures vastly enhance the possibility of reconstructing the complete dataset. In this study, only the error in converting from water discharge to SS load was considered in the uncertainty assessment, and all estimates were within 0.5% (95% confidential interval) in this case. To reduce the uncertainty of LQ fitting, the 10-min monitoring dataset (discharge and SS load) was transformed to a 1-hour dataset.

Considering the river SS is often transported by discharge, we used downstream LQ curves to estimate river SS loads at 1-year-flood discharge (Q = 95 m3 s−1), which eliminates the influence caused by different annual water discharges. The 1-year-flood discharge was calculated from the daily maximum discharge data from 1 January 2013 to 30 September 2020 at the Haramachi site.

To compare river SS dynamics during rainfall events, we here defined a rainfall event as the increase in water discharge exceeding 1.4 and 1.6 times the baseflow before precipitation for the upstream and downstream catchments, respectively. As a result, a total of 64 and 72 rainfall events from the Notegami and Haramachi sites were identified.

To study the dynamic relationship between soil loss from decontaminated regions and river SS load, we estimated eroded soil amount during each rainstorm using RUSLE. Specifically, the NDVI during a specific rainfall was determined by interpolation. Subsequently, the corresponding C × P can be estimated using equations (3) and (4). With the mean values of the K and LS factors, the erosion potential can then be calculated. Finally, precipitation erosivity factor (Supplementary Table 7) for each rainfall event can be calculated as58:

R=1nj=1nk=1mj(EI30)k,

(7)

where n is the number of years used, mj is the number of precipitation events in each given year j and E and I30 represent each event’s kinetic energy (MJ) and maximum 30 min precipitation intensity (mm h−1), respectively, for each event k. The event’s erosivity, EI30, can be calculated as58:

EI30=(∑r=10ervr)I30,

(8)

where er denotes the unit rainfall energy (MJ ha−1 mm−1) and vr provides the rainfall volume during a set period (r) (mm). For this calculation, the criterion for the identification of a precipitation event is consistent with previous work, that is, the cumulative rainfall of an event is greater than 12.7 mm (ref. 58). If another rainfall event occurs within 6 h of the end of a rainfall event, they are counted as one event. Therefore, the unit rainfall energy (er) can be derived for each time interval based on rainfall intensity (ir, mm h−1)58:

er=0.29[1−0.72e(−0.05×ir)].

(9)

The calculation’s required parameters were derived from the historical precipitation record from the Japan Meteorological Agency59. For the Notegami catchment, the precipitation monitoring data were derived from Iitate. For the Haramachi catchment, the precipitation was obtained from three adjacent monitoring sites (that is, Haramachi, Iitate and Tsushima) with the specific weights of 0.143, 0.545 and 0.312, respectively. These weights were determined by the Voronoi diagram method in a Geographic Information System60.

River monitoring of particulate 137Cs

At each monitoring site, the suspended sediment sampler proposed by Phillips et al.61 was installed at 20–30 cm above the riverbed for the time‐integrated sampling of river suspended sediment. The reliability of this sampler has been widely proven in past studies19,20. After sampling, the trapped turbid water and SS samples were transferred into a clean polyethylene container and stored until laboratory analysis.

The SS samples were separated from the collected water mixture via natural precipitation and physical filtration, dried at 105 °C for 24 h and subsequently packed into a plastic container. The activities of 137Cs in the SS samples (C, Bq kg−1) were determined via the measurement system, which consists of a high-purity germanium γ-ray spectrometer (GCW2022S, Canberra−Eurisys, Meriden) coupled to an amplifier (PSC822, Canberra, Meriden) and multichannel analyser (DSA1000, Canberra, Meriden). The measurement system was calibrated with the standard soil sample from the International Atomic Energy Agency. Under the 662 keV energy channel, each measurement batch would take approximately 1–24 h to make the analytical precision of the measurements within 10% (95% confidential interval). All measured 137Cs concentrations were decay-corrected to their sampling date. Moreover, the results obtained in this study were also normalized by their initial average 137Cs inventory in the catchment (D, Bq m−2) to eliminate the effect caused by spatial differences.

As 137Cs concentration in the sediment sample depends on particle size19,20, we conducted a particle size correction for all measured data in Takase, Ukedo and Haramachi to eliminate this effect. The particle size distributions for dried SS samples were analysed using the laser diffraction particle size analyser (SALD-3100, Shimadzu Co., Ltd.). With the parameterized particle size distributions, the particle size correction coefficient (Pc) can be calculated by19:

Pc=(SsSr)v,

(10)

where Sr and Ss represent the reference and collected samples’ specific surface areas (m2 g−1). The exponent coefficient, v, is a fitting parameter associated with chemical and mineral compositions. In this study, the same parameters measured in the Abukuma River, the major river in the Fukushima area, were applied for Sr (0.202 m2 g−1) and v (0.65). The specific surface area for collected SS samples was estimated by the following equation under a spherical approximation20:

Ss=∑(6×ρ−1×d−1i×p−1i),

(11)

where ρ is the particle density and di and pi denote the ratio and diameter of the particle size fraction for particle i. Therefore, the 137Cs concentration corrected for particle size can be obtained by dividing the measured 137Cs concentration by Pc.

Considering that the decrease in particulate 137Cs concentration in a catchment was also affected by natural attenuation, there is a need to eliminate this effect from the declining trend of our observed 137Cs dataset to highlight the impacts of decontamination. The Ukedo and Takase are rivers surrounding the Niida River with similar contaminated situations. Our long-term 137Cs monitoring data from downstream of these two catchments showed that their 137Cs decline trends were relatively steady. Although there is a dam reservoir upstream of Ukedo, the 137Cs concentration observed both upstream and downstream showed a similar declining trend62. Therefore, the above evidence suggests that natural attenuation was the dominant factor controlling the 137Cs decrease in these two rivers. Here we assumed that the natural attenuation trend of 137Cs in the surrounding catchments (Ukedo and Takase) with little effect by decontamination was similar to that of the Haramachi catchment. Thus, we fitted their time change curves of 137Cs concentration (normalized by average 137Cs inventory of the corresponding catchment) using an exponential model. We then estimated the 137Cs concentration at the same sampling time as Haramachi in the two catchments by using the fitting models. Finally, we calculated the mean value of the two datasets and recalculated the effective half-life (Teff = ln(2)/λ; λ is the fitting exponential term) of the natural attenuation by the exponential model.

The 137Cs flux (LCs, Bq) for each monitoring site was estimated by the product of the SS flux and the 137Cs concentration in the suspended sediment sample. We then took the sum over that year:

LCs=∑(Q×Css×C).

(12)

According to the law of error propagation, we considered the error from SS load and 137Cs measurement in the combined uncertainty assessment for 137Cs fluxes and found their values are all within 1.1% (95% confidential interval).

Using 137Cs as a tracer in estimating SS source contribution

Although 137Cs has been widely used in tracing sediment source, the spatial variability of the 137Cs deposition inventory in the Fukushima catchment hinders the estimation of the source contribution from a specific region. However, for the decontaminated catchments, as the 137Cs concentration in decontaminated soil was much lower than that in contaminated regions (for example, forested regions and the riverbank)11,13,63, the fluctuations in the particulate 137Cs concentration can help to identify the sediment from the decontamination regions. Specifically, we assumed that the particulate 137Cs concentrations in surrounding contaminated watersheds (that is, having similar land-use composition) follow a similar decline trend driven by natural reasons, while the decontamination-induced land-cover changes cause other sediment sources to mix with the original river SS and thus result in a deviation in observed 137Cs concentrations from this natural trend. Therefore, the relative contribution (RC) of the specific sediment source can be expressed as:

RC=(Cm−Cn)(Cs−Cn),

(13)

where the Cm is the measured 137Cs concentration and Cs and Cn represent the 137Cs concentration in a specific sediment source and the naturally varied 137Cs concentration at the same time as the measured 137Cs. For data comparability, all 137Cs concentrations presented here were corrected by their particle size and 137Cs inventory. We also excluded the samples with collection weights below 0.5 g from the calculations due to their high uncertainty in Pc measurement.

In this study, the specific sediment source is the decontaminated soil in the Niida River Basin where the 137Cs concentration was approximately 53.99 ± 40.90 Bq kg−1 (refs. 37,42; mean ± standard deviation, N = 8). The natural decline of the 137Cs concentration (that is, λ) was established using temporal variation in 137Cs data originating from the Ukedo and Takase rivers, which were scarcely influenced by decontamination. The first measured 137Cs data in Haramachi were set as the starting point of its natural decline curve. The total uncertainty for the contribution percentage of SS from the decontaminated regions was calculated by the propagation of error from each part with the uncertainties originating from the measured 137Cs concentration, 137Cs concentration in a specific source and the natural 137Cs concentration. For the uncertainty in the 137Cs concentration in decontaminated soil (Cs), we set the standard deviation as its error source, while the natural 137Cs concentrations were calculated by the propagation of the 95% confidential interval of the fitting curves.

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Data availability

Ordered decontamination process data are available from http://josen.env.go.jp/plaza/info/weekly/weekly_190607.html. Particulate 137Cs monitoring data in Haramachi, Takase and Ukedo during 2012–2017 are available from: https://doi.org/10.34355/Fukushima.Pref.CEC.00014, https://doi.org/10.34355/CRiED.U.Tsukuba.00020 and https://doi.org/10.34355/Fukushima.Pref.CEC.00021. The rest of data presented in this study are available from the corresponding author upon request.

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Acknowledgements

We appreciate B. Matsushita for his valuable suggestions on satellite image processing and NDVI calculation, J. Chen for sharing the code for ESTARFM, J. Takahashi for sharing 137Cs in decontaminated soil data, H. Kato for his suggestions on presenting the results, S. Fujiwara for his assistance on calculating the precipitation erosivity factor, Y. Yamanaka and T. Kubo for their work on decontamination map development, field investigation and preliminary data analysis and Y. He and F. Yoshimura for their constructive suggestions on improving figure quality. We also acknowledge funding support from the commissioned study from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) FY2011–2012, Nuclear Regulation Authority FY2013–2014, Japan Atomic Energy Agency-funded FY2015–2021, Grant-in-Aid for Scientific Research on Innovative Areas grant number 24110005, Grant-in-Aid for Scientific Research (A) 22H00556, Agence Nationale de la Recherche ANR-11-RSNR-0002 and the Japan Science and Technology Agency as part of the Belmont Forum.

Author information

Authors and Affiliations

  1. Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, Tsukuba, Japan Bin Feng, Yuichi Onda, Asahi Hashimoto & Yupan Zhang
  2. Institute of Environmental Radioactivity, Fukushima University, Fukushima, JapanYoshifumi Wakiyama
  3. National Institute of Technology, Tsuyama College, Tsuyama, JapanKeisuke Taniguchi

Contributions

B.F. and Y.O. conceived the study; B.F. performed the data evaluation and all analyses, interpreted the data, wrote the manuscript and prepared all figures and tables in close discussion with Y.O.; Y.O. provided funding support for the field monitoring and all needed resources; K.T. and Y.Z outlined the boundary of the decontamination regions and implemented the drone observations of the sites; and Y.W. and K.T. performed the field river monitoring and determined the particulate 137Cs concentration. B.F., A.H. and Y.Z. prepared all satellite images, ran the NDVI calculation and processed ESTARFM. All listed authors contributed to the editing of the manuscript and approved the final version.

Source: https://www.nature.com/articles/s41893-022-00924-6

July 16, 2022 Posted by | Fuk 2022 | , , , | Leave a comment

Decadal trends in 137Cs concentrations in the bark and wood of trees contaminated by the Fukushima nuclear accident.

Published: 04 July 2022

Abstract

Understanding the actual situation of radiocesium (137Cs) contamination of trees caused by the Fukushima nuclear accident is essential for predicting the future contamination of wood. Particularly important is determining whether the 137Cs dynamics within forests and trees have reached apparent steady state. We conducted a monitoring survey of four major tree species (Japanese cedar, Japanese cypress, konara oak, and Japanese red pine) at multiple sites. Using a dynamic linear model, we analyzed the temporal trends in 137Cs activity concentrations in the bark (whole), outer bark, inner bark, wood (whole), sapwood, and heartwood during the 2011–2020 period. The activity concentrations were decay-corrected to September 1, 2020, to exclude the decrease due to the radioactive decay. The 137Cs concentrations in the whole and outer bark samples showed an exponential decrease in most plots but a flat trend in one plot, where 137Cs root uptake is considered to be high. The 137Cs concentration ratio (CR) of inner bark/sapwood showed a flat trend but the CR of heartwood/sapwood increased in many plots, indicating that the 137Cs dynamics reached apparent steady state within one year in the biologically active parts (inner bark and sapwood) and after several to more than 10 years in the inactive part (heartwood). The 137Cs concentration in the whole wood showed an increasing trend in six plots. In four of these plots, the increasing trend shifted to a flat or decreasing trend. Overall, the results show that the 137Cs dynamics within forests and trees have reached apparent steady state in many plots, although the amount of 137Cs root uptake in some plots is possibly still increasing 10 years after the accident. Clarifying the mechanisms and key factors determining the amount of 137Cs root uptake will be crucial for predicting wood contamination.

Introduction

After the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March of 2011, a wide area of forests in eastern Japan was contaminated with radionuclides. In particular, radiocesium (137Cs) has the potential to threaten the forestry and wood production in the contaminated area for many decades because it was released in large amounts (10 PBq)1 and has a relatively long half-life (30 years). Radiocesium levels for some wood uses are strictly regulated in Japan (e.g., 40 Bq kg−1 for firewood2 and 50 Bq kg−1 for mushroom bed logs3), meaning that multipurpose uses of wood from even moderately contaminated areas are restricted. Although a guidance level of radiocesium in construction wood has not been declared in Japan, the permissible levels in some European countries (370–740 Bq kg−1)4,5,6 suggest that logging should be precautionary within several tens of kilometers from the FDNPP, where the 137Cs activity concentration in wood potentially exceeds 1,000 Bq kg−1 [refs. 7,8]. To determine whether logging should proceed, the long-term variation in wood 137Cs concentration must be predicted as accurately as possible. Many simulation models successfully reproduce the temporal variations in the early phase after the FDNPP accident, but produce large uncertainties in long-term predictions9. To understand the 137Cs dynamics in forests and trees and hence refine the prediction models, it is essential to provide and analyze the observational data of 137Cs activity concentrations in tree stem parts.

Accident-derived 137Cs causes two types of tree contamination: direct contamination by 137Cs fallout shortly after the accident, and indirect contamination caused by surface uptake from directly contaminated foliage/bark10,11 and root uptake from contaminated soil12. The 137Cs concentration in bark that pre-exists the accident was affected by both 137Cs drop/wash off from bark surfaces and 137Cs uptake because the bark consists of a directly contaminated outer bark (rhytidome) and an indirectly contaminated inner bark (phloem). Given that the 137Cs content was 10 times higher in the outer bark than in the inner bark in 201213 and the 137Cs concentration in the whole bark decreased during the 2011–2016 period at many study sites8, the temporal variation in the whole bark 137Cs concentration during the early post-accident phase must be mainly contributed by drop/wash off of 137Cs on the outer bark surface.

In contrast, stem wood (xylem) covered by bark was contaminated only indirectly. Although 137Cs distribution in sapwood (outer part of stem wood; containing living cells) and heartwood (inner part of stem wood; containing no living cells) is non-uniform and species-specific8,13,14,15, the 137Cs concentration in whole wood depends on the amount of 137Cs uptake. Because the dissolvable 137Cs on the foliar/bark surface decreased significantly within 201116, the main route of 137Cs uptake since 2012 is likely root uptake rather than surface uptake. A monitoring survey during 2011–2016 showed that the temporal trend in the whole wood 137Cs concentration can be increasing, decreasing, or flat8, suggesting that 137Cs root uptake widely differs among sites and species.

Meanwhile, many simulation models have predicted an initial increase in the whole wood 137Cs concentration after the accident, followed by a gradual decline9. The initial increase is attributable to the increase in soil 137Cs inventory, and the following decline is mainly attributed to radioactive decay, dilution by wood biomass increment, and immobilization in the soil. Therefore, the trend shift from increasing to decreasing is a good indicator that shows the 137Cs dynamics within the forest have reached apparent steady state, which is characterized by slower changes in 137Cs concentration, bioavailability, and partitioning in the forest12,17,18. However, the timing of the trend shift predicted by the models have large uncertainty, varying from several years to a few decades from the accident9. Moreover, the trend shift has not been confirmed by observational data after the FDNPP accident. Although our monitoring survey cannot easily identify the key driving factors of the temporal trends, it can directly discern the trend shift from increasing to decreasing, and the timeframe of the increasing trend. The confirmation of the trend shift will accelerate the understanding of key factors of 137Cs root uptake, because important parameters such as transfer factor and CR are originally defined for a steady state condition18.

The present study aims to clarify the temporal trends of 137Cs concentrations in bark and wood of four major tree species (Japanese cedar, Japanese cypress, konara oak, and Japanese red pine) at multiple sites during the 10 years following the FDNPP accident. Detecting a trend shift from increasing to decreasing in the wood 137Cs concentration was especially important to infer whether the 137Cs dynamics within the forest have reached apparent steady state. We update Ohashi et al.8, who analyzed the monotonous increasing or decreasing trends during 2011–2016, with observational data of 2017–2020 and a more flexible time-series analysis using a dynamic linear model (DLM). The DLM is suitable for analyzing data including observational errors and autocorrelation, and has the advantage of being applicable to time-series data with missing years. For a more detailed understanding of bark contamination and the 137Cs dynamics in tree stems, we also newly provide data on the 137Cs concentrations in the outer and inner barks. The temporal trends in the 137Cs CRs of outer bark/inner bark, heartwood/sapwood, and inner bark/sapwood were analyzed to confirm whether the 137Cs dynamics within the trees have reached apparent steady state.

Materials and methods

Monitoring sites and species

The monitoring survey was conducted at five sites in Fukushima Prefecture (sites 1–4 and A1) and at one site in Ibaraki Prefecture (site 5), Japan (Fig. 1). Sites 1, 2, and A1 are located in Kawauchi Village, site 3 in Otama Village, site 4 in Tadami Town, and site 5 in Ishioka City. Monitoring at sites 1–5 was started in 2011 or 2012, and site A1 was additionally monitored since 2017. The tree species, age, mean diameter at breast height, initial deposition density of 137Cs, and sampling year of each sample at each site are listed in Table 1. The dominant tree species in the contaminated area, namely, Japanese cedar (Cryptomeria japonica [L.f.] D.Don), Japanese cypress (Chamaecyparis obtusa [Siebold et Zucc.] Endl.), konara oak (Quercus serrata Murray), and Japanese red pine (Pinus densiflora Siebold et Zucc.) were selected for monitoring. Japanese chestnut (Castanea crenata Siebold et Zucc.) was supplementally added in 2017. The cedar, cypress, and pine are evergreen coniferous species, and the oak and chestnut are deciduous broad-leaved species. Sites 1 and 3 each have three plots, and each plot contains a different monitoring species. Site A1 has one plot containing two different monitoring species, and the remaining sites each have one plot with one monitoring species, giving ten plots in total.

Locations of the monitoring sites and initial deposition densities of 137Cs (decay-corrected to July 2, 2011) following the Fukushima nuclear accident in Fukushima and Ibaraki Prefectures. Open circles indicate the monitoring sites and the cross mark indicates the Fukushima Dai-ichi Nuclear Power Plant. Data on the deposition density were provided by MEXT19,20 and refined by Kato et al.21. The map was created using R (version 4.1.0)22 with ggplot2 (version 3.3.5)23 and sf (version 1.0–0)24 packages.

Sample collection and preparation

Bulk sampling of bark and wood disks was conducted by felling three trees per year at all sites during 2011–20168,25 and at sites 3–5 and A1 during 2017–2020. Partial sampling from six trees per year was conducted at sites 1 and 2 during 2017–2020 (from seven trees at site 2 in 2017) to sustain the monitoring trees. All the samples were obtained from the stems around breast height. During the partial sampling, bark pieces sized approximately 3 cm × 3 cm (axial length × tangential length) were collected from four directions of the tree stem using a chisel, and 12-mm-diameter wood cores were collected from two directions of the tree stem using an automatic increment borer (Smartborer, Seiwa Works, Tsukuba, Japan) equipped with a borer bit (10–101-1046, Haglöf Sweden, Långsele, Sweden). Such partial sampling increases the observational errors in the bark and wood 137Cs concentrations in individual trees26. To mitigate this error and maintain an accurate mean value of the 137Cs concentration, we increased the number of sampled trees from three to six. The sampling was conducted mainly in July–September of each year; the exceptions were site-5 samples in 2011 and 2012, which were collected irregularly during January–February of the following year. The collected bark pieces were separated into outer and inner barks, and the wood disks and cores were split into sapwood and heartwood. The outer and inner bark samples during 2012–2016 were obtained by partial sampling of barks sized approximately 10 cm × 10 cm from 2–3 directions on 2–3 trees per year.

The bulk samples of bark, sapwood, and heartwood were air-dried and then chipped into flakes using a cutting mill with a 6-mm mesh sieve (UPC-140, HORAI, Higashiosaka, Japan). The pieces of the outer and inner bark were chipped into approximately 5 mm × 5 mm pieces using pruning shears, and the cores of the sapwood and heartwood were chipped into semicircles of thickness 1–2 mm. Each sample was packed into a container for radioactivity measurements and its mass was measured after oven-drying at 75 °C for at least 48 h. Multiplying this mass by the conversion factor (0.98 for bark and 0.99 for wood)8 yielded the dry mass at 105 °C.

Radioactivity measurements

The radioactivity of 137Cs in the samples was determined by γ-ray spectrometry with a high-purity Ge semiconductor detector (GEM20, GEM40, or GWL-120, ORTEC, Oak Ridge, TN). For measurements, the bulk and partial samples were placed into Marinelli containers (2.0 L or 0.7 L) and cylindrical containers (100 mL or 5 mL), respectively. The peak efficiencies of the Marinelli containers, the 100-mL container, and the 5-mL container were calibrated using standard sources of MX033MR, MX033U8PP (Japan Radioisotope Association, Tokyo, Japan), and EG-ML (Eckert & Ziegler Isotope Products, Valencia, CA), respectively. For the measurement of the 5-mL container, a well-type Ge detector (GWL-120) was used under the empirical assumption that the difference in γ-ray self-absorption between the standard source and the samples is negligible27. The measurement was continued until the counting error became less than 5% (higher counting errors were allowed for small or weakly radioactive samples). The activity concentration of 137Cs in the bark (whole) collected by partial sampling was calculated as the mass-weighted mean of the concentrations in the outer and inner barks; meanwhile, the concentration in the wood (whole) was calculated as the cross-sectional-area-weighted mean of sapwood and heartwood concentrations. The activity concentrations were decay-corrected to September 1, 2020, to exclude the decrease due to the radioactive decay.

Discussion

Causes of temporal trends in bark 137Cs concentration

The 137Cs concentration in the whole bark decreased in many plots, clearly because the outer bark 137Cs concentration decreased. However, the whole bark 137Cs concentration showed a relatively small decrease or even a flat trend in some plots (site-2 cedar and site-1 cypress and oak). In the site-1 cypress plot, where the whole bark 137Cs concentration decreased relatively slowly, the inner bark 137Cs concentration notably increased. Similarly, although we lack early phase monitoring data in the site-2 cedar and site-1 oak plots, the inner bark 137Cs concentration in both plots is considered to have increased prior to monitoring because the sapwood 137Cs concentration increased in both plots and the CR of inner bark/sapwood was constant in all other plots. Therefore, the low-rate decrease or flat trend in the whole bark 137Cs concentration in some plots was probably caused by an increase in the inner bark 137Cs concentration, itself likely caused by high 137Cs root uptake (as discussed later).

The 137Cs concentration in the outer bark decreased in all four plots monitored since 2012 (site-1 and site-3 cedar, site-1 cypress, and site-3 pine), confirming the 137Cs drop/wash off from the bark surface. The constant (exponential) decrease in three of these plots indicates that the 137Cs drop/wash off was still continuing in 2020 but with smaller effect on the outer bark 137Cs concentration. In contrast, the decrease in the site-1 cypress plot seemed to slow down since around 2017. Furthermore, Kato et al.32 reported no decrease in 137Cs concentration in the outer bark of Japanese cedar during the 2012–2016 period. Such cases cannot be fitted by a simple decrease of the outer bark 137Cs concentration. As a longer-term perspective, in the outer bark of Norway spruces (Picea abies) affected by the Chernobyl nuclear accident, the biological half-life of 137Cs concentration was extended in areas with higher precipitation, suggesting that high root uptake of 137Cs hinders the decreasing trend33. The present study showed that 70–80% or more of the 137Cs deposited on the bark surface (outer bark) was removed by drop/wash off after 10 years from the accident and that the 137Cs CR of outer bark/inner bark became constant in some plots. These facts suggest that the longer-term variations in outer bark 137Cs concentration will be more influenced by 137Cs root uptake, although it is uncertain whether root uptake caused the slowing down of the decrease rate seen in the site-1 cypress plot. Further studies are needed to understand the 137Cs concentration in newly formed outer bark and to determine the 137Cs CR of outer bark/inner bark at steady state.

Causes of temporal trends in wood 137Cs concentration

The temporal trends of the 137Cs concentration in the whole wood basically corresponded to those in the sapwood. The exceptions were the site-3 and site-4 cedar plots, where the sapwood 137Cs concentration did not increase but the whole wood 137Cs concentration was raised by the notable increase in the heartwood 137Cs concentration. This behavior can be attributed to a species-specific characteristic of Japanese cedar, which facilitates Cs transfer from sapwood to heartwood8,15,34. The present study newly found that the increase in the 137Cs CR of heartwood/sapwood in the cedar plots became smaller or shifted to a flat trend around 2015–2016, indicating that 137Cs transfer between the sapwood and heartwood has reached apparent steady state at many sites 10 years after the accident. Therefore, after 2020, the whole wood 137Cs concentration in cedar is unlikely to increase without a concomitant increase in the sapwood 137Cs concentration.

The increasing trends in the 137Cs concentrations in whole wood and sapwood (site-2 cedar, site-1 cypress, and site-1 and site-3 oak plots) are seemingly caused by the yearly increase in 137Cs root uptake; however, the wood 137Cs concentration can also increase when the 137Cs root uptake is constant or even slightly decreases each year. This behavior can be shown in a simple simulation of the temporal variation in the wood 137Cs content (the amount of 137Cs in stem wood of a tree). If the 137Cs dynamics within a tree have reached steady state and the proportion of 137Cs allocated to stem wood become apparently constant, the wood 137Cs content in a given year can be considered to be determined by the amount of 137Cs root uptake and the amount of 137Cs emission via litterfall. The flat 137Cs CR trend of inner bark/sapwood during 2012–2020 (see Fig. 5) indicates that the 137Cs dynamics, at least those between the inner bark and sapwood, reached apparent steady state within 2011. Here we assume that (1) the annual amount of 137Cs root uptake is constant, (2) the proportion of 137Cs allocated to stem wood is apparently constant, and as assumed in many forest Cs dynamics models17,35,36,37, (3) a certain proportion of 137Cs in the stem wood is lost via litterfall each year. Under these conditions, the simulated amount of 137Cs emission balanced the amount of 137Cs root uptake after sufficient time, and the wood 137Cs content approached an asymptotic value calculated as [root uptake amount × allocation proportion × (1/emission proportion − 1)]. Note that the asymptotic value increases with increasing root uptake amount and decreasing emission proportion and does not depend on the amount of 137Cs foliar/bark surface uptake in the early post-accident phase. Nevertheless, the amount of 137Cs surface uptake in the early phase critically determines the trend of the wood 137Cs content. More specifically, the trend in the early phase will be increasing (decreasing) if the surface uptake is smaller (larger) than the asymptotic value. Finally, the temporal variation of the 137Cs concentration in wood is thought to be the sum of the dilution effect of the increasing wood biomass and the above-simulated variation in the wood 137Cs content. Therefore, in the early post-accident phase, the wood 137Cs concentration will increase when the wood 137Cs content increases at a higher rate than the wood biomass. As the wood 137Cs content approaches its asymptotic value (i.e., steady state), its increase rate slows and the dilution effect proportionally increases. Then, the wood 137Cs concentration shifts from an increasing trend to a decreasing trend. The trends of the 137Cs concentrations in whole wood and sapwood in the site-3 oak plot follow this basic temporal trend, which is similarly predicted by many simulation models9.

In other plots with the increasing trend (site-2 cedar and site-1 cypress and oak), the increase in the 137Cs concentrations in whole wood and sapwood became smaller or shifted to a flat trend around six years after the accident; however, it did not shift to a decreasing trend. This lack of any clear shift to a decreasing trend, which was similarly seen at sites with hydromorphic soils after the Chernobyl nuclear accident38,39, cannot be well explained by the above simulation. A core assumption of the simulation that the yearly amount of 137Cs root uptake is constant is probably violated in these plots, leading to underestimations of the root uptake amount. Although the inventory of exchangeable 137Cs in the organic soil layer has decreased yearly since the accident, that in the mineral soil layer at 0–5 cm depth has remained constant40. In addition, the downward migration of 137Cs has increased the 137Cs inventory in the mineral soil layer below 5-cm depth41,42. If the steady state 137Cs inventory of the root uptake source can be regarded as sufficient for trees, any increase in the 137Cs root uptake is likely explained by expansion of the root distribution and the increase in transpiration (water uptake) with tree growth. When the wood 137Cs content increases at a similar rate to the wood biomass, the increasing trend will not obviously shift to a decreasing trend. Therefore, assuming the 137Cs allocation and emission proportions in the mature trees do not change considerably with time, the amount of 137Cs root uptake is considered to be increasing yearly in these four plots.

In the remaining plots with the decreasing or flat trend (site-1 cedar, site-4 cedar without outliers, site-5 cypress, and site-3 pine), according to the above simulation, the amount of initial 137Cs surface uptake was larger than or similar to the asymptotic value, i.e. the amount of 137Cs root uptake is relatively small and/or the proportion of 137Cs emission via litterfall is relatively high. However, the amount of 137Cs root uptake in the plots with the flat trend is possibly increasing because the flat trend has not shifted to a decreasing trend. In these plots, although it is difficult to confirm apparent steady state of the soil–tree 137Cs cycling because of the lack of an initial increasing trend, the recent flat trends in the 137Cs CRs of heartwood/sapwood and inner bark/sapwood indicate that the 137Cs dynamics, at least within the trees, have reached apparent steady state.

Various factors were found to increase the 137Cs root uptake after the Chernobyl nuclear accident; for example, high soil water content, high soil organic and low clay content (i.e., low radiocesium interception potential [RIP]), low soil exchangeable K concentration, and high soil exchangeable NH4 concentration12,43. After the FDNPP accident, the 137Cs transfer from soil to Japanese cypress and konara oak was found to be negatively correlated with the soil exchangeable K concentration44,45 and the 137Cs mobility is reportedly high in soils with low RIP46. However, neither the soil exchangeable K and Cs concentrations nor the RIP have explained the different 137Cs aggregated transfer factors (defined as [137Cs activity concentration in a specified component/137Cs activity inventory in the soil]) of Japanese cedars at sites 1–446,47. Because the 137Cs dynamics within the forest and trees in many plots reached apparent steady state at 10 years after the FDNPP accident, the 137Cs aggregated transfer factor is now considered to be an informative indicator of the 137Cs root uptake. Therefore, a comprehensive analysis of the 137Cs aggregated transfer factor and the soil properties at more sites than in the present study will be important to understand key factors determining the amount of 137Cs root uptake by each tree species at each site.

Validity and limitation of the trend analyses

Although the application of the smooth local linear trend model failed in plots monitored for less than five years, it was deemed suitable for analyzing the decadal trend because it removes annual noises, which are probably caused by relatively large observational errors (including individual variability)26. Moreover, the algorithm that determines the trend and its shift between 2 and 4 delimiting years was apparently reasonable, because the detected trends well matched our intuition. However, when judging a trend, the algorithm simply assesses whether the true state values significantly differ between the delimiting years. Therefore, it cannot detect changes in the increase/decrease rate (i.e., whether an increasing/decreasing trend is approaching a flat trend). For example, the whole bark 137Cs concentration in the site-1 cypress plot was determined to decrease throughout the monitoring period. In fact, the decrease rate slowed around 2014 and the decreases were slight between 2014 and 2020 (see Fig. 2). Similarly, the sapwood 137Cs concentration in the site-1 cypress and oak plots was determined to increase throughout the monitoring period, but the increase rate has clearly slowed since around 2017. To more sensitively detect the shift from an increasing/decreasing trend to a flat trend, other algorithms are required. Nevertheless, this algorithm is acceptable for the chief aim of the present study; that is, to detect a trend shift from increasing to decreasing.

Conclusions

In many plots monitored at Fukushima and Ibaraki Prefectures, the 137Cs concentrations in the whole and outer bark decreased at almost the same yearly rate for 10 years after the FDNPP accident, indicating that the direct contamination of the outer bark was mostly but not completely removed during this period. Moreover, the 137Cs concentration in the whole bark decreased at relatively low rates or was stable in plots where the 137Cs root uptake was considered to be high. This fact suggests that indirect contamination through continuous root uptake can reach the same magnitude as direct contamination by the accident.

In all of our analyzed plots, the 137Cs CR of inner bark/sapwood has not changed since 2012, indicating that 137Cs transfer among the biologically active parts of the tree stem had already reached apparent steady state in 2011. In contrast, the 137Cs CR of heartwood/sapwood in six out of nine plots increased after the accident. In four of these plots, the 137Cs CR of heartwood/sapwood plateaued after 3–6 years; in the other two plots, the plateau was not reached even after 10 years. Therefore, saturation of 137Cs in heartwood (an inactive part of the tree stem) requires several years to more than one decade.

The 137Cs concentration in the whole wood showed an increasing trend in six out of nine plots. In four of these plots, the increasing trend shifted to a flat or decreasing trend, indicating that the 137Cs dynamics in many forests reached apparent steady state at 10 years after the accident. However, the lack of the clear shift to a decreasing trend indicates that the 137Cs root uptake is probably still increasing in some plots. Continuous monitoring surveys and further studies clarifying the complex mechanisms of 137Cs root uptake in forests are needed in order to refine the simulation models and improve their prediction accuracy.

https://www.nature.com/articles/s41598-022-14576-1

July 10, 2022 Posted by | Fuk 2022, Fukushima continuing, Reference | , , , | Leave a comment