Environment/Biological Production

Tomoko Ota

Research on Environmental Radioactivity toward Sustainable Resource Exploitation and Conservation

Tomoko Ota , Assistant Professor

Faculty of Engineering, Graduate School of Engineering (Resource Circulation System, Department of Socio-Environmental Engineering, School of Engineering)

High school : Tokyo Kodaira Minami High School

Academic background : Graduate School of Science and Technology, Meiji University

Research areas
Environmental Dynamics & Radioactivity
Research keywords
Environmental Radioactivity, Sustainable Recycling Society, Environmental Conservation

What is Research on Environmental Radioactivity?

I conduct research on the dynamics of the materials found in water or energy resources and living creatures using materials with environmental radioactivity as tracers. Environmental radioactivity means a radioactivity that occurs in nature or natural materials which have existed on earth since ancient times. On the other hand, the radioactivity which is artificially generated by nuclear experiments or at nuclear power plants is called artificial radioactivity, which includes cesium-137 that you most likely heard after the accident at Fukushima Daiichi nuclear power plant in 2011.   

Radioactivity has a half-life, which depends on the species of nucleus. With this figure in mind, tracking the concentrations and isotope ratios for environmental radioactivity contained in water, atmosphere, soil, animals, plants and similar environments, provides origin data or the residence time of the water or energy resource, or the deposition speed of deposited materials. I am trying to make a contribution to sustainable conservation and the development of resources using environmental radioactivity.


Investigating the Iodine Accumulation Mechanism in Japan

Is Japan a nation poor in resources? Yes, in terms of energy resources such as oil, but not necessarily for other resources. Japan is a leading producer of iodine in the world. Iodine is widely used for electronic products, antibacterial and antifungal agents and pharmaceutical products, and Japan is proud of its status as the second largest producer in the world. Iodine, which is contained in brine (groundwater rich in salt concentration), exists in Japan only in limited areas such as Chiba, Miyazaki and Okinawa prefectures. It is not known yet when and how the iodine was accumulated, or why it exists only in limited areas. Recently, more people believe that the iodine accumulated on the ocean floor was subducted under the Japanese islands moving with the oceanic plate, and cold spring water containing the iodine was trapped in the stratum.

However, we assume that the iodine did not move with the oceanic plate, rather that the deposits accumulated and existed on the ocean floor since ancient times, and these deposits were trapped in the stratum. We developed a method to reproduce the iodine isotope ratio without contamination caused by artificial 129I (which is significantly larger than naturally occurring iodine), and we are proposing the possibility of the initial value in oceanic iodine having an isotope ratio one-order lower than the conventionally accepted one. In addition, a dating method combining multiple environmental radioactivity as tracers could verify that the estimated age of the groundwater is almost the same as the age of the layer with the iodine accumulation zone. We also showed that the iodine might be generated in-situ because the water flow in the layer pore spaces was diffusion-controlled.

Fig. 1 Chemical Separation

Fig. 2 Seaweed Sample before World War II (Used as a biotracer)



Investigating the Route of Radioactive Cesium Absorbed by Trees – Using Forest Resources in Fukushima-

Japan contains steep mountainous ranges and holds many forests. The nuclear disaster in Fukushima in 2011 deposited a lot of radioactive cesium in the forests, at and around Fukushima. As a result, Fukushima, in particular, its forest industry has fallen victim to a great number of harmful rumors, and it is important to investigate and clarify the route of the radioactive cesium that was absorbed into the trees and estimate the future distribution of the cesium in the trees.

Fig. 3 Forest Investigation Place in Fukushima

In the nuclear power plant accident at Chernobyl in 1986, it was recognized on an international level that cesium was mainly absorbed into trees through the roots, after cesium had penetrated into soil. Therefore, in Fukushima, the concentration in trees is expected to increase in the same way via the root absorption. However, we believed that this idea, while accepted on an international level, cannot be applied to Fukushima because the geological conditions are different from that of Chernobyl.

 My colleagues and I identified the absorption route and found that cesium was absorbed not through the roots but through the bark and leaves, based on the distribution data of radioactive cesium in the rings of broad-leaf and needle-leaf trees and based on the analysis using environmental dynamics of the cesium in the forest.

 The radioactive cesium in trees in Fukushima was originally attached to the bark and leaves from the radioactive plume emitted during the accident and then absorbed. It would never become enriched any further but decay as time passes. The forest in Fukushima does not require intensive overcutting, rather standard cutting based on conventional forest industry operation should be sufficient.

Establishing an evaluation model that takes into account the local forestry ecosystem and soil characteristics as well as clarifying the absorption mechanism of radioactive cesium in the Fukushima forest, regardless of the accepted theory on the international level, will be critical in creating a forest decontamination and radioactivity control plan and proper forest conservation in Fukushima.


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