The EPA has 48 pages of water quality criteria - and that's just Chapter 3.

https://www.epa.gov/sites/default/fi...k-chapter3.pdf

In the "old" days, campgrounds had a water pump and iron pipes,
so the water sometimes has an irony (ha-ha) taste, but was cold and perfectly safe.
However, the EPA is very down on untested and unregulated water sources,
so many public campgrounds got rid of the pump given the additional costs.

I've drunk the water from every campground and filtered water in the backcountry.
I also feel 100% confident drinking from natural springs and snowmelt.
Remember, the water source is wilderness on the continental divide.

I'm of the school that having a few - not a lot - of little things in the water is a good thing.
If we do not encounter some minor pathogens, we lose the ability to fight off the big ones.
After you have been drinking Great Plains water, I'm sure you'll appreciate the mountain water.
Enjoy Glacier. And start super early on Going to the Sun to get the early reflections and least traffic.