How is erosion affecting the recovery of the Fukushima area?

On March 11, 2011, Japan experienced the unprecedented Tohoku earthquake. It was the largest in Japan’s history, and created a massive tsunami that impacted Japan’s northeastern coast. A result of the earthquake and tsunami was the meltdown of the Fukushima Daiichi Nuclear Power Plant. This meltdown had immediate and long-term impacts on the area.

On February 13, 2021, just shy of a decade after the disaster, Japan was hit with a 7.3 magnitude quake—an aftershock of the same earthquake that caused the disaster at Fukushima Daiichi. At the time of publication of this article, Japanese officials have not identified any irregularities in their nuclear power plants, but residents along the coastline are wary.

The Soil Science Society of America and American Society of Agronomy are holding an information campaign to increase awareness on the 10th anniversary of this disaster.

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Meltdown of the reactors caused diffusion of radioactive material around the power plant, and residents to evacuate. Decontamination efforts were carried out from 2013 till 2017. The goal was to allow evacuated citizens to return to their homes and resume their lives. Decontamination was applied mostly on residential areas, arable lands, and roads.

Fukushima has a large, forested area and radioactive Cesium was deposited on forests in Fukushima after the accident of the nuclear power plant. However only the edge of the forests that affects residents was decontaminated. These forests can be a source of eroded soil if not managed properly – and that soil could contaminate other areas of Fukushima.

map of Fukushima
A map of Fukushima showing regions of coast and mountains, indicating the flow of sediments and rivers.

Japan is in an Asian monsoon region, so annual rainfall in Fukushima is high. Under natural conditions vegetation in Fukushima tends to shift to broad leaved forest. In Fukushima, forests cover 71% of the area. Thirty-five percent of the forest is natural and hardwood, and the rest is planted with coniferous trees. The forest floor is generally covered with leaf cover, and some weeds. Only a small fraction of these forests received decontamination work.

Forests have important roles as catchment for the heavy rains. Agriculture, an important industry of the region, relies on water resources from surface water from the forests. One area of my research was to study how Cesium in the forest behaves. Does the deposited Cesium migrate to surface water bodies?

In chemical terms, Cesium is monovalent cation – it has a positive charge. This means it can make bondsand “stick to” some types of soil particles, especially clay and organic particles like decomposed leaf litter. Analysis of river water in Fukushima showed most of the radioactive Cesium in river water was adsorbed to suspended solids, and the soluble form was minor. This may suggest radioactive Cesium moves with the sediments when erosion occurs – out from the forest and into other areas of Fukushima.

Geographically, Fukushima is divided as three strips of areas from east to west. The east most area is coastal area and called Hama-dori. Heading west, one approaches Naka-dori and then Aizu, which are divided by forested mountains. Sediments from the east slope of the Abukuma mountains that separates Naka-dori and Hama-dori migrate east toward coastal area, while those from the west side of the mountains flow into Abukuma river which flows to north at the center of Naka-dori region.

Edge of forest with trees, soil and grass
Decontamination work at a forest edge in Fukushima. Photo courtesy of Taku Nishimura

There is good news from water analysis of the Abukuma river. The research suggested discharge of Cesium deposited by the accident was small. Each year approximately 0.1% or less radioactive Cesium deposited by the 2011 event is discharged through Abukuma river to Pacific Ocean. It can be said most of the radioactive Cesium deposited in Abukuma mountains remains in the forest, and a small part of it discharged.

There are two types of radioactive Cesium. And they give off radioactivity at different rates – one’s half-life is two years, and the other is 30 years. That means that even 90 years after the nuclear event, 1/16th of the radioactive Cesium will still be contaminating the Fukushima region. The expected reduction of radioactive Cesium due to erosion is far smaller than the natural decay of radioactive Cesium.

Why is sediment and Cesium discharge from forests in Fukushima so small and where has deposited Cesium gone?

Simulation of soil loss with GIS in the forests of Iitate village suggested sediment sources in the forest were limited. Sediment sources near the water stream could contribute to the discharge of sediments and Cesium from the forest. Some eroded sediments settled near the source, too. Well-established forest canopy, weeds and litter layer at the forest floor may have contributed to a limited source of sediment in the forest.

Stream with small trees nearby
Small stream of water from the forest is an important source of water for the pioneer farmers of Fukushima. Photo courtesy of Taku Nishimura 

Radioactive Cesium in the forest is thought to circulate in the forest. Tree roots absorb radioactive Cesium in the soil, which then migrates to the leaves. Decomposition of fallen leaves at the forest floor and/or litter layer supplies the Cesium to the soil, and the roots extract some of them. There may be a kind of competition between root Cesium extraction and Cesium adsorption by clay minerals in the soil. It can be thought of a closed loop with a sink of Cesium. This loop is expected to reach steady state in the future.

As long as the forest conditions are the same, radioactive Cesium is expected to be kept in the forest and will not affect the activities of people in the region. To renew the forest, many landowners manage the land by cutting trees to renew the forest. However, the harvest of trees needs to be managed well, to reduce the effects of erosion out of the forest.

Another remaining concern is forest management. Forests, especially planted forests, need management. In Fukushima, 80% of planted forests needed management before the Tohoku earthquake, and most of the management has been suspended since 2011 due to the radioactive pollution. Lack of proper management could cause an incomplete canopy as well as degraded forest floor conditions and an enlarged risk of soil erosion.

Answered by Taku Nishimura, University of Tokyo

To read more about Fukushima, read these blogs:

What are the long-term effects of the Fukushima disaster on local agronomy?

Dealing with the fallout in Fukushima, Part 1

Dealing with the fallout in Fukushima, Part 2

Read Farming in Fukushima: One Decade After Nuclear Disaster

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