I would also like more background on the heavy metal issue.
I know there are many places in the world where natural (geological) arsenic is a major health issue. Exacerbated, unfortunately, by well-intentioned well drilling programs.
I also recall Colin Fletcher mentioning desert springs that might be poisonous due to heavy metals.
I suppose that for me (and many of us) the primary concern is methyl-mercury (fat-soluble) in aquatic animals (i.e., tasty fish). But that's a more complicated situation.
Anyway, details would be most welcome.