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Trying to trace cesium spread in forests and soil

December 22, 2015

Researchers trying to unravel spread of cesium and its impact on ecosystem after Fukushima disaster


Trying to trace cesium spread in forests and soil


More than 90 percent of the fir trees in forests close to the site of Japan's 2011 nuclear disaster are showing signs of abnormality, and plant lice specimens collected in a town more than 30 kilometers from the crippled facility are missing legs or crooked.

But it remains unclear whether the mutations in plants and animals are definitively connected to the disaster at the Fukushima No. 1 nuclear power plant.

All that scientists in Japan are prepared to say is they are trying to figure out the effects of radioactive cesium caused by the release of huge amounts of radioactive materials from the triple meltdown at the Fukushima plant triggered by the Great East Japan Earthquake and tsunami.

Scientists are seeking answers to how radioactive cesium spread in forests and the soil after the accident, along with signs of mutation in plants and animals in areas close to the stricken nuclear plant.

Understanding how cesium and other radioactive particles spread after the disaster is key to putting the consequences of the nation’s worst nuclear accident into perspective.

The research has major ramifications in terms of what authorities and residents living near a nuclear power plant can expect if a similar accident occurs again. It also offers valuable insight for evacuees weighing their options about rebuilding their lives near the stricken plant.

Among radioactive substances, cesium-137 is of primary concern as it has a half-life of 30 or so years. As forests were excluded from decontamination work, an undetermined amount of cesium is bound to remain in forests and lie buried in the ground for many years to come.

Mountainous forests cover 70 percent of the Fukushima Prefecture’s land space.

The government-affiliated Japan Atomic Energy Agency (JAEA) is among research organizations studying the effects of radioactivity and the way cesium spreads in forested areas.

During a recent field trip in Kawauchi, radiation levels in the air showed 1.2 to 1.3 microsieverts per hour at a survey point.

Cesium in the soil registered between 300,000 and 400,000 becquerels of radioactivity per square meter.

The survey point used to be in a “No Entry Zone,” a designation covering a 20-km radius from the plant, which was evacuated soon after the nuclear accident triggered by the magnitude-9.0 earthquake and the towering tsunami it generated.

Now the survey site is designated as a “zone in preparation for the lifting of the evacuation order” in line with a government reassessment of the situation facing affected communities.

Rotting twigs and branches, along with leaf cover, blanket the steep slopes of the cedar forest. During the survey, researchers marked a 66-square-meter rectangular tract as a benchmark and collected rainwater and fallen leaves from the plot.

They also measured the radioactivity of rainwater. The researchers did this by wrapping tree trunks with a cover and collecting rainwater flowing down on to it.

Before the Fukushima disaster, the only data available to JAEA researchers on the long-term transfer of cesium in the soil was limited to work done in laboratories.

“We had to fumble our way to find out in what form cesium existed in the forest and housing areas after it was dispersed from the nuclear plant,” said Kazuki Iijima, who is attached to the agency’s Fukushima Environmental Safety Center.

Scientists had to gingerly examine a proposed method to decontaminate residential areas before cleanup operations got under way.

Cesium in leaves finds its way into the soil through defoliation, according to researchers.

In the case of cedar trees, for example, leaves are replaced every three to four years.

Fallen cedar leaves from the time of the nuclear accident were riddled with cesium, which then seeped into the soil. Each new bed of fallen leaves creates more weight on the topsoil and pushes the cesium down further.

This way, radiation levels in the air in the affected area drop faster than the natural decay of cesium over time.

Researchers' past studies of the forest showed that only 0.1 percent of the total amount of cesium in the surveyed sites spread from the area over a one-year period.

“Most of it remains on the topsoil up to 5 to 10 centimeters from the surface,” Iijima said.

Because cesium attaches itself to dirt and dissolves in water, it is easily spread. It is also deposited in riverbeds and at the bottom of lakes.

At the Ogaki dam, almost 20 km northwest of the nuclear plant, researchers took cesium measurements of 800,000 becquerels per kilogram at a site 20 cm below the lake bed close to where the Ukedogawa river empties out.

But a reading close to the surface of the lake bed showed below 200,000 becquerels.

The difference, researchers say, is easy to explain: Dirt containing high levels of cesium flowed into the dam in the immediate aftermath of the accident, while dirt with lower radiation levels accumulated on top of it.

Researchers are still trying to figure out whether the release of radioactive materials affected the growth of plants and animals.

Scientists have reported on mutations and abnormalities among species varying from fir trees and plant lice to Japanese monkeys, carp and frogs.

The National Institute of Radiological Sciences (NIRS), a government-affiliated entity, said in late August that the trunks of fir trees are not growing vertically.

Fir trees are among the 44 species that the Environment Ministry asked the NIRS and other research organizations to study in trying to determine the effects of radiation on living creatures.

The NIRS reported that the frequency of these mutations corresponds to a rise in natural background radiation.

More than 90 percent of fir trees in the town of Okuma, just 3.5 kilometers from the crippled plant, showed signs of abnormal growth.

“We need to figure out cumulative radiation doses in fir trees when doing additional research,” said an NIRS researcher.

Among other changes reported: the legs of plant lice collected in Kawamata, a town more than 30 km from the plant, were found to be missing or crooked and the white blood cell count of Japanese monkeys was lower in Fukushima, the prefectural capital, which is about 60 km from the plant.

Other studies by scientists who research living creatures in their field work monitored earthworms, carp, frogs, flies and gold beetles.

After the nuclear disaster, the researchers began looking at problems from a new perspective: flora and fauna affected by radiation.

Manabu Fukumoto, a professor of pathology at Tohoku University’s Institute of Development, Aging and Cancer, cautioned not to jump to conclusions that nuclear fallout is the culprit behind all these findings.

“We cannot conclude definitively that they have been caused by radiation until (reliable estimates for) cumulative doses are calculated,” said Fukumoto, who also serves as the chief of the Japanese Radiation Research Society.

But assessing the effect on animals in the wild is proving a challenge for scientists.

Before the Fukushima disaster, most experiments designed to evaluate the impact of radiation on animals had been conducted in laboratories.

In these experiments, animals were exposed to varying intensities of radiation under the supervision of researchers.

In the natural environment, however, estimating their external exposure is difficult as creatures roam rather than stay in one spot.

In addition, doses of their internal exposure can vary significantly, depending on what they preyed on when and how much.

There is also a possibility that some animals, even if they exhibited signs of radiation's effect, may no longer be alive for analysis. They may have been killed by their natural enemies.

In addition, scientists cannot rule out factors such as fluctuations in temperature, the presence of farm chemicals and heavy-metal contamination behind the abnormalities.

Experts say they need to produce similar results in lab tests based on their monitoring.

“We need to continue to monitor the environment for at least five or six more years,” Fukumoto said. “And at the same time, we should start analyzing the reported phenomena.”


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