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Ya know. I dont know either, thats a great question. I am not at a computer, but go to scholar.google.com and do a search you may find some studies on it. I like using it for science type stuff like that instead of regular google, because the studies are from far more reputable searches than good ol ad driven google search.

I wasnt even aware google had such a thing. Thanks for the reply

From what I have read, this is a concern where larger cities are considering doing this. Heavy metals along with pharmaceuticals being present in waste are a huge concern in areas with a very dense or large population. For the individuals doing this on their own private land there is little risk. We know when we are taking meds that should not end up in our food... and if you are eating your own produce and animals produced in healthful way there is little if any risk of heavy metals being present.