Abstract
More than 10 years have passed since the Fukushima nuclear accident. We, researchers in the agriculture department at the University of Tokyo, have begun to work right after the accident to determine the effect of radioactive materials on agricultural production, and still now, we are continuing the research, how radioactive cesium has been moving or what kind of problems are left in the environment, etc. Radioactive contamination was found everywhere in the environment after the accident. However, during these years, the radioactivity of the soil, rivers, or mountains gradually decreased. One of the reasons is decontamination of the place. Since radioactive cesium hardly moves from the place where it was adsorbed, the removal of the surface soil could reduce the radioactivity. Mixing the surface soil with deeper soil could also reduce the radioactivity in the field. The radioactivity of the agricultural products grown in the field was hardly detected a few years after the accident. Now, the only place where decontamination activities are difficult to perform is the mountain area, because the area is large, covering approximately 70% of Fukushima prefecture. Therefore, the radioactivity in the mountain is one of the issues that has grabbed the attention of the people now, and more research results related to the mountains are collected in this book compared to those of the former 3 books of this series already published.
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1.1 Introduction
In these 10 years, the radioactive cesium distribution and movement were surveyed. The results we found at Fukushima were presented in the paper and the former 3 books of this series (Nakanishi 2018; Nakanishi and Tanoi 2013, 2016; Nakanishi et al. 2019). The most important result was that the fallout was adsorbed firmly on the surface of any materials exposed to the air at the time of the accident and then hardly moved or even washed. This kind of nature is different from the well-known chemical behavior of the cesium we experience in the laboratory. This element behavior shown by a very small number of elements is known as the radio-colloidal nature, which is mainly discussed in radiochemistry. We could detect radioactive cesium, because it emits radiation, and in principle, we can detect only one element by radiation measurement. Such a high sensitivity to detect one atom could not be accomplished by the other analytical tools. Therefore, the method of decontamination was rather simple, just removing the surface of anything contaminated, since the radioactive cesium does not move. In the case of soil, radioactive cesium adsorption was featured on a small soil component, clay. The adsorption of the fallout was very strong so that most of the plants grown in soil absorbed a very limited amount of radioactive cesium from the soil, which dramatically reduced the radio-contamination of the agricultural products grown in the field.
We have been continuing the research to determine the behavior of radioactive cesium at contaminated sites for more than 10Â years, and now, through radioactive analysis, the data allowed us to estimate the future behavior of radioactive cesium. To get an overall picture of the degree of contamination, we studied the radioactive cesium effects on soil, water, crops, animals, mountain areas, etc., and a comprehensive explanation on radio-contaminations is described here; this comprises an extension of our research we already presented in the former books of this series.
1.1.1 Soil
Since the accident was in late winter, there were hardly any plants growing in the field, except for wheat. In the agricultural field, everything exposed to the air was contaminated, including the leaves of wheat, and the main contaminated place was the soil surface. It was very difficult to remove radioactive cesium from the soil, since it was firmly adsorbed on the soil and was hardly moved even under hard rain.
It was estimated that radioactive cesium will go downward with time, since global fallout, which occurred during the 1960s due to the atomic bomb test, and is still able to be measured in many places throughout Japan. When the radioactivity of this fallout was measured, it was found that they were found at a certain depth of the soil, indicating that they moved downward and could not be detected at the surface. In the case of the Fukushima accident, to measure the downward speed of radioactive cesium, pipes were prepared and buried in a straight from the surface of the soil. Periodically, the counter was moved in the pipe from the upper part to the bottom to measure the radioactivity profile in the soil with depth. The movement of radioactive cesium during the first few weeks was relatively fast, but the movement slowed gradually with time, and in these years, the movement of radioactive cesium was approximately 1–2 mm/year.
In the case of global fallout, the measurement was performed in a place remote from the Fukushima area, where fallout by the Fukushima Nuclear Accident was not found. The moving distance of radioactive cesium from the surface, after approximately 60Â years, was approximately 8Â cm, indicating that the moving speed of radioactive cesium from the fallout of Fukushima Nuclear Accident was roughly the same.
Then, the soil particle adsorbing a high amount of radiocesium was identified. Only in the fine clay, called weathered biotite, was highly accumulated the radiocesium. Other components of the soil, especially larger than the fine clay, radioactive cesium was not found. Then, the method of adsorption within the clay was studied. The absorption in the fine clay was very tight, and the distribution of radioactive cesium was found to be uniform within the clay. Sometimes, the edge of the clay has been discussed to confine the radiocesium; however, such an edge was difficult to observe.
Since contaminated soil was only at the surface, within approximately 5Â cm from the surface, removing the soil surface was the effective method of decontamination. The soil surface was removed from many farming lands using heavy machinery. However, machines used on the fields pressed the soil so that the air layer in the soil was lost. To grow the plants again in contaminated fields, first, air must be introduced, and the uniform growth of the plants must be resumed. However, these activities are difficult to perform in many areas, because the farmers are getting too old for these works.
The features of the soil contamination found were as follows
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The soil surface, less than 5Â cm from the surface, was highly contaminated
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Hardly moved downward with hard rain
The moving speed was not related to the amount of rain
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The speed of the downward movement of radioactive cesium was approximately 1–2 mm/year
This speed was found to be similar to that of the global fallout during the 1960s, which can still be measured in Japan
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Most of the radioactive cesium was adsorbed tightly in fine clay particles
The distribution of the radioactive cesium in the clay was homogeneous
Particles larger than fine clay were not contaminated
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After the removal of the soil surface, there are other problems to restart farming, such as the introduction of the air layer in the soil and the resumption of homogenous plant growth
1.1.2 Water
There are two forms of radioactive cesium in water, dissolved or suspended. The water in the rivers investigated contained only suspended radioactive cesium. The radioactive cesium was contained firmly in the fine clay and moved with water in a suspended form. Therefore, the suspended cesium was easily separated by simple filtration.
When river water, as well as soils under water, was sampled periodically along the river, it was found that when the water moving speed was fast, most of the suspended cesium was moved with water in the river. However, when the moving speed slowed, for example, at the inside curve of the meandering river, suspended clay was sedimented.
To determine whether biological accumulation occurs by any plants, insects, or animals, many samples along the food chain were collected and measured. The result was that there was no specific living thing found that accumulated radioactive cesium.
Then, the next investigation was to determine how much radioactive cesium moves with water or how much radioactive clay comes out from the mountain. Several devices were placed throughout the mountain so that the water flowing speed and the actual water amount moving on the soil as well as on the trees were analyzed. When there was a large amount of rain, the amount of flowing water increased, and the radioactivity of the water collected was high at the beginning of the rain. However, soon after the first flow, there is a high amount of water coming out from the soil, since mountains store a high amount of water in the soil. Therefore, the radioactivity found in the initial water was soon diluted with the reserved water flowing out from the mountain.
In the case of water reservoirs, unexpected results were found. When the soil surface under the water, at the bottom of the pond, was measured, highly contaminated soil was found. To determine the origin of the radioactive cesium, tracing the route of the river and using the land were investigated. Only in the upper part of the land of this water reservoir containing highly radioactive soils was decontamination activity a vigorously conducted area. This means that the radioactive cesium removed by hard washing flowed out from the residential part and flowed into the pond. Additionally, it was found that the cesium adsorbed on concrete or paved surfaces could be removed to some extent by hard washing, causing secondary contamination of the pond. Once it flew into the pond, the radioactive cesium was adsorbed firmly on the soil surface at the bottom of the pond.
The feature of water contamination found can be summarized as follows
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Dissolved radioactive cesium was rarely found.
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Radioactive cesium adsorbed on fine clay was suspended in water.
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Clay with radioactive cesium was separated from water by simple filtration.
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With a decrease in water flow, some of the suspended radioactive cesium was sedimented.
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Radioactive soil that comes out from mountains by water flow was approximately 1/1000 of the fallout.
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There was no living thing found that specifically accumulated radioactive cesium.
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Vigorous decontamination activities of concrete or paved roads partially remove radiocesium, and the decontamination water flows into the water reservoir.
1.1.3 Mountain
Since the priority of decontamination was focused on human activity-related areas, including agricultural lands, most of the mountain area was left behind. The other reason was that the mountain area is large, approximately 70% of Fukushima prefecture, so it was difficult to cover the whole area of the mountain to decontaminate.
What had happened to the mountain was like the farming land mentioned above, everything exposed to the air was contaminated. Most of the mountains were contaminated by coniferous forest; therefore, the needle-like leaves and the bark of the trees were highly contaminated. The degree of contamination was higher with respect to height. In the case of branches, the upper sphere accepted more radiocesium compared to that of the lower part, facing the soil.
To observe the inner contamination of the tree, cedar and pine trees were downed, and the wood disks were prepared from upper part to bottom. Then, it was found that the radiocesium adsorbed on the bark was partially moved into the inner part of the trunk. The amount of radiocesium moved inside was higher at higher positions of the trunk and higher in cedar trees than in pine trees. There was a freshly downed cedar tree, and the radioactivity inside the tree was also measured. Interestingly, even in the downed tree, the radiocesium was moving inside the tree.
After several years, contaminated conifer leaves or part of the bark of small branches fell to the ground and increased the radioactivity on the ground. Then, the radioactive cesium accumulated in these tissues was transferred to the soil surface during the process of decomposition by microorganisms. Therefore, in most forests, radioactive cesium is now moving toward the soil surface.
Mushrooms have a problem, since they collect radiocesium during growth, and the radiocesium circulates around the small area where mushrooms grow. It was so interesting that many mushrooms growing in the area where fallout from Fukushima cannot have any influence are still collecting or maintaining the radiocesium they collected at the time of global fallout, approximately 60Â years ago.
The features of the mountains found were as follows
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Leaves and bark of the tree was highly contaminated.
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The degree of contamination was higher with respect to the height.
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Part of the radioactive cesium was moved inside the tree.
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The amount of radiocesium inside the tree was higher in cedar trees than in pine trees.
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Even the cut-down trees contaminated with fallout, radiocesium moved inside the tree.
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After several years, the contaminated needle-like leaves in coniferous trees or part of the bark or branches fell down to the ground.
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The tissue that fell from the trees was decomposed by microorganisms, and the radiocesium adsorbed on the tissue was transferred to the soil.
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Gradually, the soil surface in the mountains has been accumulating high amounts of radiocesium.
1.1.4 Others
Animals grown outside are also contaminated. However, when noncontaminated food was supplied, most of the radioactive cesium was metabolized to the outside of the body after several months. Although the half-life of cesium-137 is 30Â years, its biological half-life is estimated to be within 100Â days. In the case of the meadow, a trial to propose a new agricultural system has been introduced. For example, how to establish the circulation of metabolites using contaminated compost. Wild birds were found to be contaminated; however, the birds were found to be decontaminated the following year through the renewal of feathers. The contamination of each organ in wild boar was also investigated.
The most effective way to reduce the uptake of radiocesium to the plant is to supply potassium to the field. Although potassium is an essential element for plants to grow, the amount of potassium for pasture should be kept low, since more potassium causes disease in animals.
1.2 Conclusion
The essential point is that the fallout does not move, which makes a large difference from that of the contamination caused by heavy metals we experienced before. Heavy metals are dissolved in water and accumulate in plants, fish, or animals, which causes serious health effects on people who eat them. However, in the case of fallout, most of them did not dissolve in water, therefore, was not accumulated highly in living things; therefore, when supplied as foods, they did not cause any serious health effect in human beings.
As we pointed out, we could separate the contaminated soil from that of the matrix soil, showing a clue to decontaminate the soil. One of the methods is, as we have shown, the introduction of water to the field to separate the fine particles and leave most of the matrix there. What we must consider is the reservation of the soil itself, which is an indispensable resource to grow serials or vegetables in the field. Since it takes so long a time to create soil in nature, we always must pay close attention to it.
References
Nakanishi TM (2018) Agricultural aspects of radiocontamination induced by the Fukushima nuclear accident—a survey of studies of the University of Tokyo Agricultural Department (2011–2016). Proc Jpn Acad Ser B 94:20–34
Nakanishi TM, Tanoi K (eds) (2013) Agricultural implications of the Fukushima nuclear accident. Springer, Tokyo
Nakanishi TM, Tanoi K (eds) (2016) Agricultural implications of the Fukushima nuclear accident. The first 3 years. Springer, Tokyo
Nakanishi TM, O’Brien M, Tanoi K (eds) (2019) Agricultural implications of the Fukushima nuclear accident (III). After 7 years. Springer, Tokyo
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Nakanishi, T.M. (2023). An Overview of Our Research. In: Nakanishi, T.M., Tanoi, K. (eds) Agricultural Implications of Fukushima Nuclear Accident (IV). Springer, Singapore. https://doi.org/10.1007/978-981-19-9361-9_1
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