When I went to Durham University on an open day, they showed us a little experiment they were doing on soils in forested areas around Sellafield.

It was suggested at the time that the radioactive material would be washed out (leached) of the soil, and so there would be practically no radioactive material remaining after ten years.

They were wrong.

By measuring the amount of radioactivity of samples over time, the amount is reducing only fractionally faster the half-life of the material... it's decaying where it rests, not being washed out.

That's not to say that soil is dangerous, just that there's too much we don't know about radioactivity...