Naturally-occurring? Bananas have radioactive potassium for instance. Somehow the biological process (banana plant/animal digestion) must concentrate it.
The banana plant extracts potassium from the soil and deposits it in the banana, but it doesn't do anything to change the isotopic ratio, the makeup of the potassium in the banana will match the makeup of the potassium in the soil.
The plant might put a higher percentage of potassium in the fruit than exists in the soil, or it might not. So it isn't clear if it concentrates it (it certainly makes it more appealing to consume it).
The plant might put a higher percentage of potassium in the fruit than exists in the soil, or it might not. So it isn't clear if it concentrates it (it certainly makes it more appealing to consume it).
I'm not sure if that matches your meaning or not.