Believe it or don’t, but our drinking water is pumped from the aquifer. It is minimally treated for nitrates (farming and ranching country), examined and treated for waterborne pathogens, and pumped to the city. Very little treatment overall. The mountains act as a natural filter as the stuff soaks to the aquifer.

On the other hand, slurping down some from a creek up in the hills will almost certainly guarantee you a case of giardiasis or worse.