Koichi Oyama noticed something strange when he was measuring radiation levels in Minami-Soma, Fukushima Prefecture. In many places where the readings jumped, the municipal assembly member found patches of dried dark soil.
Further studies found similar patches of soil--along with high radiation readings--in parts of Tokyo. In fact, the radioactive soil has been discovered as far away as Miyagi, Yamagata and Niigata prefectures.
Researchers are now referring to "black soil" to describe these patches of dirt with unusually high levels of radiation. It is a sort of play on the “black rain" term used by victims of the atomic bombings in Hiroshima and Nagasaki to describe the mysterious precipitation that seemed to bring strange illnesses and untold suffering.
Yet black soil, as ominous as it may seem, could end up actually helping in the decontamination efforts following last year's accident at the Fukushima No. 1 nuclear power plant.
But for now, nothing is being done about the black soil with high levels of radiation.
"Because it normally is found on the ground, we believe it is not something that will have immediate effects on human health," a Minami-Soma municipal government official said.
Minami-Soma was the first area in which attention was focused on the black soil. In autumn last year, a number of places in the city had limited but high levels of airborne radiation, often close to 10 times higher than levels in surrounding areas. Invariably, the black powdery material was found on the ground or road immediately below the spots of high radiation.
Oyama and his group collected the soil and had it measured by Tomoya Yamauchi, a professor specializing in radiation measurement at Kobe University. The results showed the soil contained radioactive cesium at levels of 1.08 million becquerels per kilogram.
According to Oyama, black soil with similarly high radiation levels was subsequently found in other parts of Minami-Soma--and some samples contained plutonium and strontium.
Similar samples of black soil were collected from more than 100 locations around the nation, mainly in eastern Japan. Analyses are continuing on the samples with the cooperation of Iwaki Meisei University and Tohoku University.
In late May, a group of about 90 people organized by parents concerned about the effects of radiation on their children met in Mizumoto Park in Tokyo's Katsushika Ward.
Those who brought their own dosimeters began measuring soil in ditches and under trees, and several people immediately recorded radiation levels exceeding 1 microsievert per hour. The highest level recorded was 1.117 microsieverts per hour on the surface of black soil along an asphalt road running through the park.
The radiation level at a nearby lawn was 0.25 microsievert. The rain and wind is believed to have left almost intact the radioactive cesium that had accumulated on the lawn and grass after last year's Fukushima nuclear accident.
However, the radiation level of the black soil was more than four times as high as that on top of the lawn.
Preliminary findings showed that areas with high readings were in the path of the radioactive plume from the Fukushima No. 1 nuclear plant soon after the crisis started. The main areas affected were in northwestern Fukushima Prefecture and a wide part of the northern Kanto region. The eastern part of Tokyo also recorded higher levels of radiation.
The highest level of radioactivity detected--about 5.57 million becquerels per kilogram--came from black soil collected in the Kanaya neighborhood of the Odaka district of southern Minami-Soma. In 36 out of 41 locations in Fukushima Prefecture where black soil was collected, the radioactivity level exceeded 100,000 becquerels per kilogram. If that level was found in incinerator ash, it would have to be handled very carefully and buried in a facility that had a concrete exterior separating it from its surroundings.
There have also been locations in the greater Tokyo area with high radioactivity levels. For example, Kawajima in Saitama Prefecture had a level of about 420,000 becquerels per kilogram. The area in front of the Crafts Gallery of the National Museum of Modern Art, Tokyo, in the Kitanomaru Park of Tokyo's Chiyoda Ward had a reading of about 90,000 becquerels per kilogram while Shinbashi in Minato Ward had a reading of about 70,000 becquerels per kilogram.
In 23 of 29 areas in the greater Tokyo area, the radioactivity level was of a degree that would require waterproof sheets covering the material if it had been incinerator ash at similar levels.
One of the instructors who took part in the May gathering at Mizumoto Park was Yukio Hayakawa, a professor of volcanology at Gunma University. He has conducted his own study of airborne radiation levels at about 60 locations, from Hokkaido in the north to Kagoshima Prefecture in the south, and has released the results on his blog.
The highest level found by Hayakawa was 9.1 microsieverts per hour in Koriyama, Fukushima Prefecture. In the greater Tokyo area, two cities in Chiba Prefecture had high levels, with 3.8 microsieverts per hour in Abiko and 2.5 microsieverts per hour in Kashiwa. Both cities attracted attention after the Fukushima nuclear accident as "hot spots" close to Tokyo.
"We have to surmise that the radiation may have spread at least as far away as the Tokai region, and it may have even reached the Kansai region," Hayakawa said.
The Tokai region includes Shizuoka Prefecture, where levels of radioactive cesium exceeding provisional government standards were detected in tea leaves.
While Hayakawa has not personally conducted a study in the Kansai region, there has been a reported recording of 0.14 microsievert per hour, or double the airborne radiation level at 1 meter above ground, on fallen leaves that had accumulated in a ditch of a park in Mino, Osaka Prefecture.
One reason for the wide prevalence of the black soil is that it contains a micro-organism known as cyanobacteria that is common around Japan.
Although there are various types of cyanobacteria, large amounts of oscillatoriales and Nostoc commune have been found in the black soil samples.
The bacteria is of a blue-green color, but it turns black upon drying.
There are two major theories on why the cyanobacteria in the black soil has such high levels of radiation, but nothing has been confirmed.
One theory posits that the radiation becomes concentrated in the cyanobacteria. There is scientific evidence that cyanobacteria absorbs cesium, but confirmation has yet to be made on whether such radiation concentration has occurred in the cyanobacteria found in the black soil samples.
Hayakawa’s theory is that the high radiation levels are due to radioactive cesium from the Fukushima nuclear accident that was pushed by rain and wind and accumulated along roads. He believes that the phenomenon occurs even in regular soil containing no cyanobacteria.
According to Hayakawa's theory, all dirt along roads is capable of recording high radiation levels, regardless of the color of the soil.
While researchers study the black soil phenomenon, cleanup efforts have been slow.
"Under the law to prevent radiation illnesses, when the concentration of radioactive cesium exceeds 10,000 becquerels per kilogram and the total volume reaches 10,000 becquerels, it must be handled as a radioisotope,” Yamauchi said. “Radioisotopes have to be handled with care by storing them in metal barrels."
In light of such figures, only about 1.8 grams of the black soil with radioactivity levels of 5.57 million becquerels found in Minami-Soma would require handling as a radioisotope. About 111 grams of the soil in Tokyo with radioactivity levels of 90,000 becquerels would require similar handling.
"In areas around the nuclear plant, it is not unusual to have radiation levels of 4 million becquerels per kilogram,” said Hideaki Sasaki, an associate professor of microbial physiology at Iwaki Meisei University who has been involved in measuring the radioactivity in the soil. “For that reason, it is possible that soil in Minami-Soma would have radioactivity levels that exceed 5 million becquerels per kilogram."
Sasaki also pointed to the cause of the radioactive cesium detected in Niigata Prefecture along the Sea of Japan.
"While the United States and the Soviet Union conducted nuclear testing in the atmosphere in the 1960s, there are almost no effects from that testing in the soil today,” Sasaki said. “The only possible explanation is the accident at the Fukushima No. 1 nuclear power plant."
Ayako Ishikawa, 34, heads the group of parents who organized the May gathering in Mizumoto Park. According to Ishikawa, black soil is often found on top of asphalt and in areas where rainwater can easily accumulate or where snow drifts occur.
In particular, the areas where black soil was commonly found were along roads, sidewalks, squares or in parking lots without barriers.
In March, Ishikawa collected soil near a fence of a concrete square in Edogawa Ward and along a sidewalk of a national road in Koto Ward. She had the soil samples measured by Yamauchi.
The results showed radioactive cesium of about 243,000 becquerels per kilogram in the Edogawa Ward soil and about 90,000 becquerels per kilogram in the Koto Ward soil.
Ishikawa has subsequently found other areas within the Tokyo metropolitan area with high radiation levels.
"Despite that, no efforts have been made to prevent the spread of the radiation," Ishikawa said. "I have found signs that baby carriages have passed close by such locations and of children touching the soil."
Yamauchi said: "The black soil could become airborne, and that could lead to internal exposure if the soil is inhaled through the mouth. Decontamination efforts should be implemented immediately."
Minami-Soma has been aggressive about decontamination efforts, but it and other local governments do not apparently share the same sense of urgency concerning the black soil.
"While we are aware of the existence (of the black soil), it does not meet the guidelines established by the central government," an official with the Environment Bureau of the Tokyo metropolitan government said.
Currently, there are two major standards to begin decontamination work.
One is if airborne radiation levels exceed the 0.23-microsievert-per-hour standard set by the central government for a height of 1 meter above ground. The other is if the airborne radiation level at a height of 1 meter is more than 1 microsievert per hour higher than recordings in surrounding areas.
However, airborne radiation levels 1 meter above the black soil do not reach either of these standards.
Although that is the reasoning government officials give for not decontaminating those areas, the radiation standards for incinerator ash show that the black soil has radiation levels that require careful handling.
The discovery of the black soil is not all bad news.
Experiments are continuing on using the absorbent qualities of cyanobacteria to remove radioactive cesium from the soil.
In August 2011, Micro Algae Corp. of Gifu city, a research and development company handling micro-organisms, began a joint research study with Iwaki Meisei University.
Soil from Iwaki contaminated with radioactive materials was placed in trays. One form of cyanobacteria known as Nostoc commune was scattered in the soil.
In one month, about 8 percent of the radioactive cesium in the soil had moved. Calculations showed that in one year, about 69 percent of the cesium could be removed.
That would mean that if black soil is removed and buried, it could contribute to decontaminating those areas since the cyanobacteria could absorb high levels of radioactive materials from the surrounding environment.
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