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Where does polonium come from in tobacco fields? If it’s in the topsoil then why wouldn’t it be depleted after 3 or 4 crops?

Was it released into some Appalachian watershed by Oak Ridge and is therefore in the rivers/groundwater?

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Phosphate fertilizer, I think. It contains some polonium IIRC. Minerals have impurities.

Polonium comes from uranium. It's on the decay chain, via radium and then radon. Soil contains uranium at about the same concentration as the underlying bedrock (typically a few ppm by mass.)

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