About 4 years ago, on the Cross Vermont trail, I stopped at the Onion River Campground. On the back of the bathroom door was a 'boil water' notice for the town! I survived.
Filters and iodine or chlorine tablets will take care of bacteria, and some viruses, I don't think they will do much for dissolved toxins like lead, pesticides, or what ever plastic acronym is currently in vogue.