Minute' amounts can actually give you immunity to the poison, by your getting teeny amounts over a period of time...it would take a larger dose of that poison to kill someone that has been drinking their water over a period of time, than someone who has not been exposed to it..
Not saying its a good thing, but studies have been done of the amount of natural arsenic in water in rural area's...
I can’t see anyone adding it, but who knows? (Not me!) As for natural sources, I’d rather have a little arsenic in my well than, say, lead. That’ll make ya crazy!