Keiko Nakamura, Director General of JT Biohistory Research Hall
Five years have passed since the Great East Japan Earthquake. Disaster-hit regions are struggling to restore food and agriculture, the basic forms of life, which were destroyed by earthquake and tsunami. The biggest concern lies over the effects of the nuclear accident at the Fukushima No. 1 power plant. Most of the radiation contaminated areas are agricultural fields, forests and mountains. All the people who consume farm items produced in the region, as well as the owners of the lands, want to know the actual conditions of contamination.
Radioactive substances absorbed in soil
The University of Tokyo’s Graduate School of Agricultural and Life Sciences has been conducting research on soil contamination from right after the radioactive fallout. Since the disaster took place in March, when there were hardly any crops and all the leaves have fallen off from trees, radioactive materials that fell on farmlands were absorbed in soil. The research team observed the downward movement of cesium in the ground and found that cesium is strongly retained in the soil, while a very small amount is absorbed by plants. Cesium was weakly fixed in soil immediately after it was absorbed, but the fixture has become stronger as the time passed. After the summer of 2011, the movement of cesium to crops has been less than the detectable level. Their data also shows that the strongly fixed cesium does not flow into rivers or groundwater.
As a matter of fact, few agricultural items inspected by the Fukushima Prefectural Government are found to contain more than the standard limit of 100 becquerels per kilogram of cesium, except for some soybeans and rice plants once in a while. A close examination revealed that low potassium and excess nitrogen lead to more absorption of cesium. It is something we can handle.
Meanwhile, it is more difficult to decontaminate forests. According to the research, most of radioactive cesium that fell into forests remain there, with less than 1 percent having flown downstream even a year after. Mushrooms grown on logs has been one of the major agricultural produce in the Tohoku region, but the amount of cesium in the logs still marks several hundred becquerels per kilogram in 2015, above the standard limit, which means they cannot be used. The team continues to conduct thorough investigation on other products, including fruits, livestock and livestock products, and also on wildlife and the region as a whole from the perspectives of agriculture.
The interesting thing about their work is that the team has cooperated with local residents to come up with an easy and effective way to decontaminate farmlands. They first let water flow into the paddy fields and then stir the surface layer where cesium remains. The particles of the clay layer where the cesium is absorbed are small and don’t sink easily. By raking out the floating clay particles and draining them into a pit they dug besides the fields, they can remove 90 percent of the cesium contained in the paddy fields. After a while, the water seeps underground, but the clay solidifies and stays at the bottom of the pit. If the pit is covered 50 cm deep with new soil, the amount of radioactive substances will be reduced to one thousandth of what they were before and the fields become usable as farmland.
Revitalizing the agriculture industry of Tohoku
This is a good example of local residents and researchers joining hands to come up with reliable data and taking actions to create a community for the future by making use of that data. I sincerely hope that the agriculture industry of Tohoku will come alive again.
<Profile> Born in Tokyo in 1936, Keiko Nakamura has served such posts as professor of Waseda University. She advocates biohistory, a field of study to view human beings within 3.8 billion years of history of life on Earth.
(March 14, 2016)