I watched a kids show with my grand daughter years ago.
I did the same as you. Ask questions that no one could answer.
Where does this mercury come from? Is there a plant up stream that has dumped the stuff into the water?

Just a kids show about earth and how we have torn it apart.
Mercury comes from all the wood that is in the water. We, I too, add a few trees and other woods to the water every year to attact fish where we want them to be.
Captain Planet is the name of the show. One of those eco programs.