If you have safe to drink water from a spring or well then you are a lucky ......
NOTE TO THE GENERAL READER: Situations like the above are very rare. 99% of the fresh water on this planet is most definatly not safe to drink. That is why to have to purify it by one means or another. In a survival situation it is simple prudence to assume that the water supply is contaminated. And act accordingly.
Imagine the worst case of the two-step that you have ever had, and then think about what that would mean in a survival situation.