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Peachs, cesium, fruit and fukuashima

Dear Reader,

I have read with interest the blog of a lady called Dr Susan Burton who teaches English at a Japanese University. She is quite reasonable to be concerned about the levels of radioactivity in her diet. In her blog she questioned the wisdom of selling fruit from the Fukuashima area in the supermarket, right now I can not say if they are safe or not to eat. This question is one which can be better answered by the radiation protection authority in Japan.

Susan lamented that the cesium has a half life which is greater than a decade, she was worried that the cesium would spoil her enjoyment of Japan for a very long time. This comment about cesium and peaches prompted me to check the literature on radioactivity and fruit.

Back in 2001 a person in Italy published a review paper on the transfer of radioactivity from soil to fruit (F. Carini, Journal of Environmental Radioactivity, 2001, volume 52, pages 237 to 279). I went through this review and I found some data for peach trees.

One of the most important things to know about farming in a radioactive area is the transfer factor. The transfer factor is a measure of how easy it is for radioactivity to get from the soil into the part of the plant which you eat. It is defined as the ratio of the radioactivity (Bq kg-1) of the food to the radioactivity level of the soil (Bq kg-1). It is important to bear in mind that the transfer factor depends on the species of the plant, the soil type and the element.

Element Soil type Transfer factor
Cs Not recorded 0.0131
Cs Sandy loam 0.009
Sr Not recorded 0.0218
Sr Sandy loam 0.07
Pu Not recorded 0.000163
Am Not recorded 0.000436
I Not recorded 0.0109
Ru Not recorded 0.00109
Ce Not recorded 0.000436
Cm Not recorded 0.000436

As the main radioisotopes released by the accident were noble gases, iodines and cesiums it should be clear from the table above that in future years the humble peach tree will have a filtering effect. While most people do not like eating mud, if you were to eat mud then you would get a greater intake of cesium than if you ate the same mass of fruit from a tree grown on the radioactive soil.

I predict that in future years that the radioactivity level in the fruit will be dictated by the absorption of radioactivity by the roots of the tree and the transport of the radioisotopes through the tree into the fruit. But this year due to the direct deposition of radioactivity onto the leaves of the plant we need to consider a different route.

Sadly I could not find any results for peach trees but for bean plants I could find some results. In an experiment a leaf of a bean plant was soaked in a solution of a radioisotope to simulate radioactive rain. Then the plant was grown further before the radioactivity levels in different parts of it was measured. It was found that most of the radioactivity absorbed remained in the leaves.

Only 13 % of the cesium, 0.06 % of the strontium, 0.2 % of the americium and 0.002 % of the plutonium was found in the bean pods.(P. Henner, C. Colle and M. Morello, Journal of Environmental Radioactivity, 2005, volume 83, pages 213 to 229) This suggests that while cesium might be mobile inside plants the other elements are not very mobile inside the plants. While this effect may protect the consumer from plutonium, strontium and americium (which have not been released from Fukuashima in large amounts) due to the fact that the cesium is mobile it is possible that fruit grown this year on trees may be contaminated by cesium which was absorbed directly into leaves and then transferred through the plant into the fruit. So this year great care is needed to check the contamination level of the fruit, in future years it is likely that the cesium contamination level in the fruit could be much lower than this years contamination level.

Now one of the wicked lies which some parts of society like to either spread about deliberately, imply or assume is that humans are powerless in the face of the evil radioactive atoms. This idea is clearly wrong in several ways.

  1. Atoms and radiation knows no morality, no matter how good or evil you are atoms / radiation will treat you the same way.
  2. Humans can take action to alter their exposure to radiation and radioactivity.

For example by changing farming methods the level of cesium in the crop can be lowered. I saw one paper (W.L. Robison, P.H. Brown, E.L. Stone, T.F. Hamilton, C.L. Conrado and S. Kehl, Journal of Environmental Radioactivity, 2009, volume 100, pages 76 to 83) which explained that by using potassium fertiliser on coconut trees which were growing on Bikini island (Where the Americans used to test H-bombs) that the cesium level in the edible parts of the coconuts can be greatly lowered.

My advice to any Japanese farmers who might be reading this blog is to do the following.

  1. Find out what sort of soil your farm has.
    1. Clay soil tends to bind cesium more than sandy soil
  2. Find out from a cesium map how contaminated your farm is likely to be
  3. Ask the farmer’s union, TEPCO and the state radiation protection authority for advice on how to lower the contamination level of your crops. I would suggest that you ask about the following
    1. Deep ploughing to prevent the transfer of cesium via grass to livestock
    2. Prussian blue to decontaminate livestock
    3. Potassium fertilizers to prevent plants taking up cesium
    4. Changing to a different crop which is less able to take up contamination. Oilseed rape might be a good plant. The oil pressed from the seeds is normally has very little contamination in it even if the rest of the plant is contaminated. Also onions may be a good crop to plant. Below is some data from 1989 in Finland ( A. Paasikallio, A. Rantavaara and J. Sippola, The Science of the Total Environment, 1994, volume 155, pages 109-124) which shows that some crops are better able to avoid taking up cesium from the soil.

Transfer factors for different crops in three different soil types


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