1.37 Million Bq/kg Radioactive Cesium in Earthworm Castings in Fukushima

There was a piece of news about 20,000 becquerels/kg of radioactive cesium found in earthworms collected in Kawauchi-mura, Fukushima Prefecture (20 kilometers from Fukushima I Nuclear Power Plant) in Mainichi Shinbun (2/6/2012).

The article says the researchers at the Forestry and Forest Products Research Institute, a government institution, found radioactive cesium in earthworms collected at 3 locations in Fukushima. The amounts varied significantly (from 20,000 becquerels/kg to 290 becquerels/kg), and the researchers (or Mainichi Shinbun reporter) attributed to the varying air radiation levels in these 3 locations.

Ummm, earthworms live in the soil, not in the air, I thought. Still, 20,000 becquerels/kg was high, until I read Professor Bin Mori's blog about his own experiment using earthworms.

Professor Mori found over 1.37 million becquerels/kg of radioactive cesium in excrement of earthworms he picked up in Watari District of Fukushima City, where radioactive cesium exceeding the national provisional safety standard (500 becquerels/kg) has been found in rice.

From Professor Mori's blog (2/6/2012):

生きとし生けるものはすべて放射能染してしまったのである（ミミズの銀とセシウムについて）

All God's creatures, great and small, have been contaminated with radiation (About radioactive silver and cesium in earthworms)

福島県を中心に、我々が採取してきた生物を厳密に測定すると、これまで放射性セシウム汚染していない生物はいない。だから、このブログの最後に示す本日の毎日新聞の報道のようにミミズも当然放射性セシウム汚染している。

Our study on the living organisms collected in and around Fukushima Prefecture shows that there is no organism that is not contaminated with radioactive cesium. As the Mainichi Shinbun reported today, earthworms are also contaminated.

小生は、東電福島第一原発から約６０キロ離れた福島市渡利地区で、ミミズはもっとも土壌と接触している（土を丸ごと食べる！）から、強度にセシウム汚染しているに違いないと思って、長さ５センチぐらいのミミズを多数採取した。

I collected a lot of earthworms that are about 5 centimeters in length in Watari District of Fukushima City, about 60 kilometers away from the Fukushima I Nuclear Power Plant. Earthworms were in direct contact with soil, and so I assumed they would be highly contaminated with radioactive cesium.
　　　　
　　それを予備的にＮａＩ検出器で一匹一匹調べた。多いものではCs-137が数万ベクレル／Ｋｇあった。すべてのミミズが強く汚染していたが個体毎に非常にばらつきがあったのが不思議であった。

As a preliminary test, I used the NaI scintillation detector over each worm. On the high end, there were several tens of thousands of becquerels/kg of cesium-137. All worms were highly contaminated, but I wondered at why there were wide variations between the worms.
　　
　　よく観察すると、その理由として、ミミズに呑み込まれた土が体内にまだ残留しているミミズは放射性セシウム値が明らかに高いと思われた。

On closer observation, it seemed that the worm that still had the soil it had eaten remaining in the body tested high in radioactive cesium.

そこで、現場で偶然長さ３０センチにわたる大きなミミズを捕まえていたので、これを、水で十分に洗って室温で放置して２週間にわたって糞を全部吐き出させて、乾燥させた。それをゲルマニウム半導体検出器で測定した。乾燥させた理由は、通常放射性セシウムの測定値を乾物重あたりで表示するためでもある。（写真１）

One of my catch was a large earthworm, 30 centimeters in length. So I washed it carefully with water, and left at room temperature for 2 weeks until it excreted all the castings and was dry. I measured [the worm and the castings] using the germanium semiconductor detector.
　　
そうすると、このミミズの放射性セシウム値は顕著に少なかった（表１）。これは意外であった。

To my surprise, radioactive cesium in this worm was significantly less (Table 1). I didn't expect it to be this low.

Table 1: Radioactivity of earthworm in Watari District in Fukushima City
　　

Radionuclides	Radioactivity （Ｂｑ／ｋｇ）	Ratio
Ｃｓ１３４	360	42
Ｃｓ１３７	480
Ag110m	20	1

さらに意外であったのは、ミミズの糞の放射能が驚くほど高かったことである（表２）。

Even bigger surprise was that the radioactivity in the castings was extremely high (Table 2).

Table 2: Radioactivity of the worm castings

Radionuclides	Radioactivity （Ｂｑ／ｋｇ）
Cs-134	569,032
Cs-137	804,340
Ag-110m	0

　
念のために、この場所の土壌の放射能も測定した（表３）。

I measured the radioactivity of the soil where I got the worm (Table 3).

Table 3: Radioactivity of the soil

Radionuclides	Radioactivity （Bq/kg）	Ratio
Cs-134	37,629	345
Cs-137	44,031
Ag-110m	237	1

　

　　ちなみにこのミミズがいたどぶのヘドロから１センチ離れた表面の放射線線量値は１３．３１マイクロシーベルト／時間という非常に高い値を示した。

The worm was found in the sludge of a ditch. The radiation level at 1 centimeters off the surface of the sludge was extremely high, at 13.31 microsieverts/hour.

計算の結果ミミズは土壌の放射性セシウムを０．０１倍（８４０Bq／８１６６０Bq）しか濃縮していないが、一方では放射性銀を０．０８５倍（２０Bq／２３７Bq）に濃縮していた。

According to my calculation, the worm concentrated radioactive cesium in soil at a ratio of 0.01 (840 Bq/81660 Bq). On the other hand, it concentrated radioactive silver at a ratio of 0.085 (20 Bq/237 Bq).

ミミズの放射性セシウムに対する放射性銀の比は、土壌の放射性セシウムに対する放射性銀の比に対して、８．２倍（３４５／４２）の濃縮率を示していた。つまり、セシウムよりは銀がはるかにミミズでも生物濃縮されやすいことがわかる。

The ratio of radioactive cesium to radioactive silver in the worm was 8.2 times the ratio in soil (345/42). In other words, in earthworms [as well as other living organisms we tested], radioactive silver tends to get concentrated more than radioactive cesium.　　　　　　

また、ミミズの糞は土壌の放射性セシウムを16.8倍（１３７３3７２Bq／８１６６０Bq）に濃縮していることがわかった。糞の放射性銀は検出限界以下であった。

Also, I found that radioactive cesium in the worm castings was 16.8 times as much as that in soil (1,373,372 Bq/81660 Bq). Radioactive silver in the worm castings was below detection level. 　　　　　

ここで詳しくは述べないが、すべての生物において、放射性銀のほうが放射性セシウムよりも生物濃縮されやすい傾向にある。

I won't elaborate here, but there is a tendency in all living organisms [that I tested] that radioactive silver gets more concentrated in the organisms than radioactive cesium.
　　　　　　　　　　　　　　　　
　　私見では、下記の記事の森林総研の報道発表は、ミミズを体内に含まれている糞ごと測定している可能性が高い。　　　　　

In my personal opinion, the Forestry and Forest Products Research Institute, as reported by Mainichi Shinbun, is likely to have measured the radioactivity of earthworms with the castings that are still inside the worms.

Earthworms are near the bottom of the food chain in the forest. They eat dirt , or rather, eat organic materials already digested by bacteria and other organisms in the dirt. I am very curious to see if anyone has studied the effect of radiation in organisms such as bacteria in the soil.

What we need here is the ecologists who can evaluate the whole forest ecosystem, from the lowest in the food chain such as bacteria that break down the raw materials (such as fallen leaves) up the chain - worms, insects, frogs, lizards, fish, birds, and mammals. There is also another chain of concentration that may go like - bacteria that break down the raw materials, bacteria that feed nutrients from the breakdown of the raw materials to the plant root, worms, insects/lizards/birds that eats part of the plant.

Why study these, instead of human beings? Alternation of generations is much faster in them, so the effect of radiation may be easier to observe, if any.

But alas, only a handful of researchers in Japan seem even interested in studying. Most of those who did study seem to be waiting for their peer-review magazine debut.

Dr. Bin Mori is professor emeritus at the University of Tokyo Graduate School of Agricultural and Life Sciences. His specialty is plant nutrition.

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