Just to add a little twist, a lot of people talk about bio-filtrations systems for urban living (apartment blocks, office buildings).
In fact, I saw a TV show a couple of years ago about a major office hi-rise in Toronto, Canada that used bio-filtration systems
to filter all of its water. Some kind of "green" project just when "green" was coming into vogue.
Well, the best bio-filtration system that I can think of would be a swamp. I'm willing to bet that the water exiting a swampy area
will probably be good to drink.
Not too sure about water seeping up through rocks as you never know what it might be passing through, lead or uranium deposits
perhaps, or as stated previously, buried contaminants.