I suspect you may not find a lot of data except in city watersheds where the water is sampled regularly by the city. I'd like to see some data, too, but I don't think that any governmental agency takes samples in the backcountry unless a major problem is reported (see next paragraph). I know that the Forest Service has eliminated the expense of testing the water for most of their front-country car campgrounds by removing the water system altogether and posting signs that the water is unsafe for drinking. In the wilderness, you're definitely on your own!
In the Rockies, you'll find a lot of livestock--lots of horse use, of course, as well as still-existing grazing allotments (a legal use in wilderness areas, BTW). You have to remember that without horses (still the primary trail users) and livestock grazing, very few trails in the Rockies would exist, and that nearly all trail maintenance is done by horse outfitters (at their own expense). In addition to the livestock issue, in popular areas not all humans dispose of their excrement properly. I understand that Lonesome Lake in Wyoming's Cirque of the Towers was one of the earliest places affected by that problem, which is why they've banned camping within 1/4 mile of the lake.
At least that will give you an idea of what might get into the water! There's also the silt in glacier-fed streams to consider, which may or may not be a problem where you're going.
I had a friend who carefully filtered his water until the last two nights of a 9-day trip, where the spring was coming right out of the hillside. Guess what he came down with (confirmed by lab tests) about 10 days later? It turns out that that's an extremely popular area for hunters to camp in the fall, and evidently the ground water was polluted.