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How soil removes cesium from rain water

I wrote this lunch time that I thought that well water should remain free of radioactive cesium because the water has to flow through lots of soil before it reaches the part of the well where you draw water from. Now here is an explanation.

It has been well known for many years that the clay minerals in soil are able to bind to cesium. Recently C. Gil-Garcia A. Rigol and M. Vidal published a paper (New best estimates for radionuclide solid-liquid distribution coefficients in soils, Part 1: radiostrontium and radiocaesium, Journal of Environmental Radioactivity, 2009, volume 100, pages 690-696) in which they explain how well cesium and strontium stick to soil. 

For cesium the Kd value is very high. Now what is a Kd value, the Kd is a constant to describe how well a solid removes a metal from solution. The Kd value is normally expressed in L Kg-1.

Kd is defined as

“the ratio between the amount of a radioisotope (Bq Kg-1) absorbed on a specific solid to the amount of radioisotope present in solution (Bq L-1) at equilibrium”

Kd = Radioisotope on the solid (Bq Kg-1) / radioisotope in solution (Bq L-1)

If we use dimensional analysis we can confirm that the units of Kd will be L Kg-1.

According to this paper for a typical soil the Kd for cesium is 1700 L Kg-1. Now lets do a calculation with this.

Imagine we have one litre of water which contains Cs-137, if we were to add ten grams of soil and stir it up then some of the cesium will bind to the soil particles. Once the system reachs equilibrium we will have a lower cesium concentration in solution. We can use GCSE level maths and A-level chemistry to work out this new concentration.

Imagine we have a bucket of water which contains some Cs-137 dissolved in 1 litre of water, if we tip in 10 grams of soil and stir it up.

When it has reached equilibrium the specific activity of the soil in Bq Kg-1 will be 1700 times that of the water in Bq L-1.

So if we assume that the soil has an activity of 1700 Bq per kilo, then the ten grams of soil will have an total activity of 17 Bq on the soil, as the water has a specific activity in Bq L-1 1700 times lower than that of the soil then the water will have an activity of 1 Bq L-1.

If we now consider how much activity is absorbed onto the soil, it is 94 %.

If we add some more soil to the bucket to make the bucket contain 100 grams of soil and 1 L of water (this is still a muddy water rather than a wet soil). Then we can repeat the maths.

If we assume that the soil has an activity of 1700 Bq per kilo, if 0.1 kilo of soil is present then the soil will have 170 Bq of cesium on it. The activity of the water is still going to be 1700 times lower in Bq L-1 than the activity of the soil in Bq Kg-1. So the water will have 1 Bq of Cs per litre

This gives us a much better decontamination of the water. In a real system the solid to liquid ratio is likely to be different.

PS. If you are going to drink from a well check that it is a safe well in terms of disease causing microbes (bugs)

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