Keywords

7.1 Key Points on Radioactive Contamination of Forests

To begin with, let’s summarize some of the important points when dealing with radioactive contamination of forests. The general behavior of radiocesium, not only in forests, requires an understanding of the effects of radioactive decay.

  • Radiation from the major radionuclides, radiocesium, decreases according to its physical half-life due to radioactive decay.

  • Ten years after the accident, the ratio of cesium-134 has become very small, and cesium-137 has become more important.

  • The physical half-life of cesium-137 is as long as 30 years, but the lifespan of trees and the cutting cycle of planted forests is longer than this.

We have understood many things about the behavior of radiocesium deposited on forests by this accident. We also need to take into account the condition of the forests in Japan when considering countermeasures.

  • In Fukushima, forests cover a large area, accounting for about 70% of the total area.

  • Radiocesium was largely trapped in trees immediately after the accident, but now most of it has migrated to mineral soil, especially the surface layer.

  • Radiocesium is expected to remain in the soil surface layer for a long time.

  • Radiocesium does not run off much from forests.

  • Absorption of radiocesium from soil to trees has been occurring. It is not reported to be particularly high, as was observed in Europe during the Chernobyl nuclear accident, probably due to differences in the soil environment.

  • The radiocesium concentration in trees varies depending on the species, site, and environment.

  • Some non-wood forest products and wildlife, such as mushrooms and wild plants, continue to have high levels of radiocesium.

Many countermeasures were considered and tested based on the various findings from the surveys and studies.

  • If decontamination is to be carried out, it would be more efficient to remove trees and the soil surface organic layer immediately after the accident, or to remove the soil surface organic layer at a time when most of the radiocesium has been transferred to the organic layer.

  • Decontamination generates a huge amount of waste.

  • Decontamination is more effective when it is conducted close to the place where air dose rates are to be reduced. Even if the range of forest decontamination is extended beyond 20 m from the forest edge, the effect of decontamination will reach a ceiling, and the reduction effect of air dose rates outside of the forest will be small.

  • Given the large area and complex topography, there is a limit to the amount of radiocesium that can be reduced in the environment through treatments and practices such as decontamination and potassium fertilization in forests.

  • Forests have the property of retaining radiocesium inside while letting almost no radiocesium flow out of the forest, and the decontamination area is limited to 20 m from the forest edge. As a result, air dose rates in forests are higher than those in residential areas and farmland in the same area.

It is also important to understand the following points from the perspective of radiation protection.

  • There are internal exposure and external exposure, and we need to prevent both.

  • The radiation dose is determined by multiplying the intensity of the radiation received by the time it is received.

  • Therefore, reducing either or both of them can reduce exposure dose.

In consideration of the above, as stated in the ICRP publication 111, it is necessary for residents and the government to communicate sufficiently while being aware of the balance between the advantages and disadvantages of radiation protection measures. As more time passes, not only the situation of radioactive materials and radiation, but also the situation of forests and society will change. It is also important to understand that the best way to deal with the situation is to constantly review and change the evaluation accordingly over time.

7.2 A Guide to Understanding and Dealing with the Contamination

There are two important points to consider when dealing with radioactive contamination. The first one is, of course, that the stronger the contamination (higher air dose rate and higher radiocesium concentration), the more serious the problem becomes, and the second one is that the severity of the problem differs depending on the target of use (e.g. wood, mushrooms, wild plants, forestry, recreation, etc.) even in areas with the same level of contamination. The first point is that, from the perspective of countermeasures against radioactive contamination, as we have seen in Chaps. 2 and 5, it is necessary to take different measures for each level of contamination to reduce exposure to radiation. The second point can be attributed to the fact that the uptake, distribution, and temporal changes of radiocesium vary depending on the target, as seen in Chaps. 3 and 6, as well as the fact that the reference values related to the effects on humans may also differ. In this way, dealing with the contamination tends to be complicated due to the complex involvement of two different factors, the contamination level and the target, but from the perspective of radiation protection, it is necessary to understand the current situation for each contamination level and target and determine how to deal with it.

Based on these two points and the characteristics of each target that have been clarified in previous studies, we would like to propose the following classification of the relationship between the degree of contamination, and the activities in forests and the use of forest products in the forest 10 years after the accident.

Low Contaminated Areas (0.5 μSv/h or Less at the Time of the Accident, 0.1 μSv/h or Less in 2020)

  • No problems with entry, recreation, or wood use as building materials.

  • With regard to the use of broadleaf trees as mushroom logs and firewood, wildlife, wild mushrooms, and wild plants, the radiocesium concentration may exceed the criteria that restrict their use, or the standard limit for food, depending on the environment and type.

Moderately Contaminated Areas (2 μSv/h or Less at the Time of the Accident, About 0.5 μSv/h or Less in 2020)

  • No problems with entry, recreation, or wood use as building materials.

  • Broadleaf trees, wild mushrooms, and wild plants are likely to exceed the criteria in many areas.

Highly Contaminated Areas (10 μSv/h or Less at the Time of the Accident, About 2.5 μSv/h or Less in 2020)

  • Temporary entry into forests is not a problem, but since external exposure above the criteria for the general public is received even in the vicinity of decontaminated residential areas, it is necessary to control the accumulated exposure dose when continuously entering forests for work or daily use.

  • When carrying out forest maintenance and decontamination work, it is necessary to control exposure dose by using the radiocesium concentration in the soil as a simple indicator.

  • Wood can be used for building materials, but the bark may exceed the criterion for designated waste.

  • Broadleaf trees, wild mushrooms, and wild plants are expected to exceed the criteria for the long term, regardless of the type.

Difficult-to-Return Areas as of December 2020 (10 μSv/h or More at the Time of the Accident, About 2.5 μSv/h or More as of 2020)

  • Entry into the forests is restricted except for special operations such as decontamination.

  • It is recommended to continue to restrict access to the site until the air dose rate drops to around 2.5 μSv/h or less.

  • Since the main radioactive material remaining even in 2020 is cesium-137 with a half-life of 30 years, the decrease in air dose rates and radiocesium concentration in forest products in the future will be very slow.

  • Particularly in most-highly contaminated areas, it is expected to take several decades to more than 100 years for forests and forest products to become available.

In the above classification, for the sake of clarity and simplification, the air dose rate is used as indicators for classifying responses to contamination. The basis for above classification is the criteria set by governmental and international organizations, but some criteria are set for radiocesium concentrations and not for air dose rates. The classification based on air dose rates alone is therefore not strict; however, there are some reasons why we dared to propose rough guidelines for action based on air dose rate. One reason is that it is difficult to optimize the adaptation for the various status of contaminations in areas because the criteria are uniformly determined. Therefore, we believe that it is useful to use the air dose rate, which is an easy-to-understand indicator of the level of contamination in the region, to have an overview and understanding of the whole picture to prevent excessive exposure. What is important in using the air dose rate as a guide is that the measures to be taken according to the level of contamination will change with the passage of time on a yearly basis. Especially in the first few years after the accident, the distribution of radioactive materials in the forest changes significantly. In addition, the effect of cesium-134, which emits more intense radiation at the same concentration than cesium-137, rapidly decreases by a factor of ten in 7 years. Now that knowledge has been accumulated through 10 years of research and monitoring, this book proposes to review the criteria set immediately after the accident based on the distribution and dynamics of radiocesium, and to help to reorganize them from the viewpoint of how to deal with radioactive contamination of forests. In addition, we would like you to keep in mind that this kind of guideline should be checked and reviewed at each milestone in time, for example, 10 or 20 years.

7.3 Future Measures

So, now that we are in the tenth year of the Fukushima nuclear accident, and the rate of reduction in radioactive contamination is slowing down, what further measures can be taken? Let us first consider the two axes that are important for considering countermeasures (Fig. 7.1). The first axis is “countermeasures through the application of technology (technology based countermeasures)” and “countermeasures through management (management based countermeasures)”. The former can be also called mainly dynamic measures or hardware measures, while the latter can be called static measures or software measures. However, measures cannot be clearly divided into two, and all measures have both aspects. The second axis is countermeasures against “air dose rate” and countermeasures against “radiocesium concentration”. The former are measures to reduce the air dose rate or to keep enough distance from radioactive materials, which are mainly measures for external exposure. The latter are measures to reduce the radiocesium concentration in trees and forest products, for example, and to prevent people from eating contaminated food , which are mainly measures to prevent internal exposure. It is important to be aware that there are advantages (benefits) and disadvantages (risks) to each measure, and it is desirable to maximize the advantages while being aware of both. Let’s consider the following specific measures.

Fig. 7.1
figure 1

Two axes for considering countermeasures

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Technology Based Measures Against High Air Dose Rate

Decontamination of forests by removal of the organic layer is currently the only measure that can be taken (Sect. 6.1). However, 10 years have passed since the accident, and most of the radiocesium has migrated from the organic layer to the surface mineral soil below it, so removal of the organic layer is not expected to substancially reduce air dose rates. In addition, removal of the organic layer not only reduces the air dose rate, but also changes the soil environment, which may affect the multi functions of forests. The secondary effects of forest decontamination, other than reduction of radiocesium, need to be verified in the future. Since the forest area in Japan is as large as 70% of the land area, the possibility that the effect will not be worth the cost should always be considered. If decontamination is to be carried out in the future, priority should be given to satoyama forests (managed forests in mountain villages, which are close to farmlands and residential areas), which are frequently used and have a high public need for exposure dose reduction.

Management Based Measures Against High Air Dose Rate

Mapping of air dose rates for each forest is necessary for people in areas where forests are used to avoid exposure to radiation as much as possible (Sects. 5.2 and 6.1). A map that clearly shows the approximate possible exposure doses received from the environment for each type of people’s activities will help people make decisions when they resume various activities in contaminated areas. In forests, it is expected that air dose rates will generally continue to decrease in accordance with radioactive decay (Sects. 3.6 and 6.1). Now that cesium-137 is the major radioactive material, it may take 100–200 years for the dose to be sufficiently reduced in some places.

Technology Based Measures Against High Radiocesium Concentration

Potassium fertilization, which has been implemented as an all-round measure in croplands, is also effective in reducing the radiocesium concentration in forest trees. Nonetheless, there are still some issues that need to be considered. Potassium fertilization in forests is more costly than in farmland, and it is not yet known how long the fertilization effect will last in forests; these problems should be solved before it can be put to practical use. However, even in cases where it is difficult to actively apply potassium, it has been shown that the radiocesium concentration is low in broadleaf trees used for mushroom logs that grow on land with high soil potassium concentration (Sect. 6.5). As a countermeasure using this property, the use of abandoned farmland , which has been increasing in Japanese mountain villages since before the accident, can be considered. Abandoned land that was used as farmland is expected to have high potassium concentration due to fertilizer application over the years. Although the area of such abandoned land may be limited, the potassium concentration in the soil varies depending on the soil type and topography, even in forest land that was originally used as forest. If we can efficiently find forest areas with high soil potassium concentrations, we may be able to partially restart producing logs for mushroom cultivation.

Management Based Measures Against High Radiocesium Concentration

One option would be delaying the harvest of trees. It takes a long time to grow trees. If we take advantage of this, it is possible that the concentration of cesium-137 will be sufficiently low due to radioactive decay by the time the trees are harvested. For this reason, it is necessary to improve the accuracy of prediction so that reliable information about future cesium-137 concentration in trees can be provided. It is not only trees that need to be predicted for future concentrations. It is also necessary to predict the concentrations of forest products such as mushrooms and wild plants. The question of “when?” is one of the biggest concerns we have when talking to people who work and live in forestry and forest products industries in the affected areas. It would also be useful to learn about the differences in the radiocesium concentration in different species of mushrooms and wild plants, as well as how to cook them in a way that reduces the radiocesium concentration in wild plants. In addition, among the criterion for general foods that have been uniformly applied since April 2012, it is possible to consider a new criterion for local consumption of minor foods such as mushrooms and wild plants (Sect. 6.4). After estimating the exposure dose through food intake based on the data, it may be possible to examine the benefits of mountain bounties from the standpoint of radiation protection and reconsider the optimization of preventing exposure and protecting the food culture of mountain villages. If people who choose to live in contaminated areas are able to think about the risks of exposure and make decisions for themselves, they will be able to live with peace of mind.

In the above, we have presented several specific measures to deal with radioactive materials related to the use of forests and forest products. It does not mean that any one of these measures is definitively effective. What is important is to understand the dynamics of radiocesium in forests and the characteristics of air dose rates, and to use the characteristic that “radiation from radiocesium decreases according to its half-life”, and to gradually restore our relationship with forests while avoiding unnecessary exposure by taking a good mixture of various measures. This is what we think we should do. In the early days of the accident, there was limited information on the actual situation of radioactive contamination and the exposure to radiation. By using the data accumulated during the past 10 years, it has become possible to more accurately estimate the exposure doses from activities in the forest and from eating forest products. The basic idea is to maximize the benefits by accepting the situation and weighing the advantages of using forests and forest products against the disadvantages of exposure. To determine whether the newly selected measures are effective, it is necessary to continue monitoring the contamination status and exposure dose through repeated trial and error, and to take action based on the data. To do this, it is important for researchers to have more dialogues with local residents and governments to deepen their understanding of the essence of what the local people and society want.

7.4 Challenges Remaining for Researchers

As we sort out what we have learned from the past 10 years of research and study, and the actual efforts being made in the forest, it has become clear that while the results of our research to date have been useful, there are also things that need to be explored further in terms of research in the future.

Continue Monitoring

The half-life of cesium-137 is 30 years, and it is necessary to continue monitoring in the forest. Continuing surveys at the same sites where we have been conducting surveys for the past 10 years will maintain the continuity of data and provide better data. We also need multi-point data such as surveys and monitoring by government agencies because there is variation by location and tree species. Research in the forests of Europe contaminated by the Chernobyl nuclear accident was active for about 10 years after the accident, but was eventually halted and then restarted after the Fukushima accident. It is important to continue the monitoring that has been conducted since the immediate aftermath of the accident to avoid such interruptions in Fukushima.

Predict the Future with Accuracy

The behavior of radiocesium in the forest is gradually shifting from the quasi-equilibrium to the equilibrium phase. Even in equilibrium, some of the radiocesium will continue to move in the forest. If we can accurately predict the future of radiocesium, it will be easier to take countermeasures against contamination and provide residents, businesses, and governments with a highly accurate forecast. To improve the accuracy of predictions, we need to (1) understand the concentration characteristics of each species and the factors that cause differences among the species, (2) understand the factors that cause variations in concentration among and within individuals, and (3) determine the sustainability of the effects of artificial measures such as potassium fertilization.

Select Trees Suitable for Conversion (Replacement of Trees)

If it is clear that the current forest cannot be used for its original purpose for a long time, it will be necessary to consider conversion. To do so, it is necessary to select trees that are useful alternatives and do not absorb radiocesium easily, or trees whose use is not affected by radiocesium contamination. Researchers from not only environmental sciences but also breeding and economic perspectives will be required to collaborate.

Develop More Efficient Decontamination and Radiation Dose Reduction Methods

Forest decontamination is one of the few methods to reduce air dose rates. Although it is known that there are limitations in terms of cost and effectiveness, as described in the previous sections (Sect. 6.1), it is important to continue working on the development of more efficient decontamination methods and methods of radiation dose reduction. In addition, we can now effectively organize and prepare for the best measures to be taken in forests in the event of another large-scale nuclear accident in Japan or elsewhere in the world in the future. Preparedness is essential if we are to continue using nuclear power plants.

Deepen Communication Among Government, Residents, and Researchers

It is important for researchers to return the results of their research to the local residents and businesses. Radioactive contamination is a complex issue, and there is no single answer, as it depends not only on the level of contamination in the area, but also on the living and economic conditions of the community. Now that we have a basic understanding of the situation and the future course of radioactive contamination, it will be more important than ever for researchers to deepen the dialogue and collaboration with the government and local residents, to consider various measures and options together, and to take a continuous and combined response.

How to Deal with Forests in Difficult-to-Return Areas

Since the occurrence of the Fukushima Daiichi Nuclear Power Plant accident, people in various organizations have been making efforts to lift the evacuation order for all areas. However, it is inevitable that some areas of forests that cannot be fully decontaminated will remain difficult to use in the long term, and we need to consider how to use such forests in the future. Forests in difficult-to-return areas have higher air dose rates than the residential areas, depending on the location, and it will be necessary to plan for a period of 100 years until the air dose rates are reduced to the target level for exposure protection. For example, if it is not possible to enter the area for a long period of time, it could be used as a mega-plant for renewable energy, or as a base for long-term observation of environmental changes caused by the nuclear power plant accident by conducting periodic ecological surveys while maintaining the natural state with no alteration, as is the case around the Chernobyl Nuclear Power Plant. It is also necessary to conduct research to maintain the forest health ecosystems that have been and will continue to be abandoned for a long time.

7.5 What Should Researchers Do in the Event of a Similar Accident?

The Fukushima nuclear accident has been compared to the Chernobyl nuclear accident, but in the Fukushima accident, data collection began earlier than in the Chernobyl accident, and the data collected are more comprehensive, and the results of the investigation were published in a more open manner. This was probably due to the fact that the Chernobyl accident took place in a country in the former Eastern bloc, the current emphasis on transparency, and the widespread use of the Internet. In addition, very early data were obtained in the forest, since observation systems for the dynamics of water and trace elements in the forest had been set up before the accident, and some of them could be used directly for radiocesium research. In addition, we were able to predict the approximate dynamics of radiocesium in the forest based on the results of research on the Chernobyl accident published in papers and reports, so we were able to quickly set up surveys and observations in preparation for this. Nevertheless, as a researcher, we still have some points to reflect on. To prepare for the future, we have compiled a list of lessons learned on how to conduct effective surveys, including our own reflections as researchers.

Use of existing observation systems

  • In the immediate aftermath of the accident, the distribution of radiocesium in the forest changed in a matter of hours to days. To capture such fast changes, it is effective to use existing systems for sampling, such as the observation system used to observe the movement of water in the forest.

How to capture data over time and space

  • The dynamics of radiocesium in the forest change from time to time. Contamination is widespread, and the extent of contamination varies. The types of forests and soils are also diverse. Therefore, it is necessary to have a survey method and a system that enables wide and long observation in time and space.

How to collect data to capture cycling

  • In forests, unlike farmland, trees are perennially growing (there for decades while growing), and radiocesium circulates in the forest ecosystem. It will be necessary to analyze various parts of the forest, including trees and soil, and to acquire data with an awareness of cycling.

The season in which the accident occurs may have a significant impact on the process from deposition to transfer to soil and subsequent internal cycling through tree phenology (biological seasonality), precipitation patterns and timing, etc.

  • Both the Chernobyl and Fukushima nuclear accidents occurred in early spring, before the deciduous broadleaf trees started to leaf out. In Japan, it is also the time of spring rains. What would have happened if the accident had occurred when the deciduous broadleaf trees already had leaves (e.g., summer)? What if it had happened during a dry season when there is no rain? What if it had happened during the rainy season? The mode of deposition and the initial dynamics of radionuclides in the forest would have been very different.

The possibility that, although the phenomena are generally similar, the speed of change over time may not necessarily be the same as in the past

  • The behavior of radiocesium in the forest that was observed in the Fukushima accident was approximately similar to that in Chernobyl. However, the speed at which radiocesium transfers from the tree canopy and organic layer to the mineral soil was not necessarily the same. Also, due to the different topography and precipitation, there were different points to watch such as runoff. While referring to past results, it is necessary to conduct surveys according to the location and season in which the accident occurred.

How to get data for various components of the forest (e.g. wood, mushroom, soil) and species

  • In addition to the basic components of trees (leaves, branches, and other parts) and soil (organic layer, and mineral soil layers at different depths), forests are ecosystems that contain many species of living things, such as mushrooms, wild plants, insects, and large animals. It is difficult to cover all of them, but it is necessary to conduct as wide a sampling as possible.

Use of geographic information systems (GIS) and models to understand and forecast large areas

  • In general, radioactive contamination covers a wide area. It is essential to visualize the spatial extent of contamination by using geographic information systems.

  • Rapid and regular air dose rate surveys from aircraft and basin-scale surveys using unmanned aerial vehicles (UAVs) are also effective.

  • The behavior of radiocesium in forests is dynamic and shifts over time. In such a case, model-based analysis is more powerful. For more accurate model analysis, it is essential to adjust and verify the model with data.

Observations based on periods of rapid movement and slower movement of radioactive materials

  • The key transfer processes are different during the period of faster and larger transfer just after and in the early phases of radioactive deposition, and during the period of entering a quasi-equilibrium phase after several years. It will be necessary to adjust the focus of observations and their frequency according to the situation.

Fixed-point monitoring during normal times, use of archived soil and wood samples from before the accident

  • We do not know when an accident will occur. Information about the situation before the accident, such as what the radiocesium concentration was in normal times and how the effects of global fallout were reduced, will help us to better analyze the situation after the accident.

Long-term measures on a multi-decade scale after initial emergency measures

  • Because of the half-life of cesium-137 and the perennial nature of forests, it is necessary to consider research and countermeasures over the long term.

  • In the long term, major environmental changes, such as climate change, will also occur.

  • Poorly managed forests are susceptible to disease, insects, and fires, and trees may expand their growing range, causing the forest area itself to expand.

Interdisciplinary collaboration among research institutes and universities, collaboration among experts in diverse fields, and sharing of accurate analytical techniques

  • Researchers from many institutions and universities were involved in the survey. In addition, the fields of study varied. It is important to cooperate with each other, for example, by sharing roles in areas of expertise.

  • Techniques for accurately measuring air dose rates and radiocesium concentrations are essential, and they need to be deployed quickly. It is necessary to share the techniques among experts and government research organizations as soon as possible after the accident.

  • In addition to analysis, sampling techniques are important. Proper sampling of various parts of the forest (e.g. wood, stream water) requires sound knowledge and skills in forest research. It is necessary to collaborate and share these techniques as well.

Collaboration between government and researchers

  • In the Fukushima nuclear accident, a huge amount of data was obtained by the government. If anything, researchers were good at taking detailed data at a limited number of points, while the government was good at taking data from wide areas.

  • By sharing data, more comprehensive and useful data can be obtained, which can be used for countermeasures.

  • For this purpose, it is necessary for the government and researchers to cooperate technically and exchange information.

Unified data format, data sharing, and rapid data release

  • Information on radioactive contamination is of great interest to residents, businesses, and governments. The usual process in the scientific community of “observation, analysis, and publishing a paper” is not enough to provide information quickly, on time as required. It is necessary to release information and data quickly and carefully.

  • With the spread of the Internet and the promotion of open data in the scientific community, data release and compilation has become easier and faster than it was during the Chernobyl accident.

  • Formatting the data will make it easier to consolidate the data.

  • However, there is a question of permanence, for example, will materials published on the Web be accessible in 10 years? It is necessary to have a system to ensure the permanence of materials on the Web.

  • The whole world has been watching this accident closely. Overseas researchers are also studying Fukushima. It is necessary to disseminate information and provide data for Japan and overseas.

Information that balances overview, detailed process, and variability

  • Because of the interest of residents and the government, researchers are required to communicate with people outside the scientific community. Therefore, it is necessary to communicate overview, details, scientific limitations, and uncertainties in a balanced and easy-to-understand manner.

7.6 Toward the Future

In the past 10 years, most of the radiocesium that fell on the forests due to the Fukushima nuclear accident, cesium-134, has physically decayed to negligible levels. However, cesium-137 will remain in the forest ecosystem, mostly in the surface layer of the soil , but some of it will circulate in the forest. Cesium-137 will also decrease slowly but surely due to radioactive decay, and air dose rates will decrease accordingly. Trees have a long life span, and forests have been nurtured in a slower time than other ecosystems. We, Japanese, have been enjoying various blessings from these forests. Forests not only function as a place for timber production and water source, but also have a strong cultural and psychological connection with us. Fortunately, the radiation level caused by the accident was not strong enough to destroy the functioning of the forest ecosystem itself. Although contaminated, the forests are still alive and strong as ever.

However, as we have seen in this book, the radioactive contamination has changed the lives of people working in forests. Some effective measures have been proposed to deal with radioactive forests, but there is no definitive solution. While we can successfully incorporate various countermeasures, fundamentally we can live with the forest by avoiding radiation exposure through the natural reduction of radiation levels and by making good use of the long time scale of the forest and its ability to retain radiocesium. In recent years, we have received encouraging news that the forests around Chernobyl have become like a nature reserve with living creatures running around. The creatures of the forest are not defeated. We believe that we, too, can continue to live together with the forests, based on accurate scientific information and taking measures to avoid radiation exposure, without giving up. We hope this book will help people to do so.