Not all, but most pollution comes from nearby agriculture-
Studies Confirm Presence, Severity of Pollution in US National Parks. Science Daily
The worst of the problems is pesticides.
Ralph Maughan
Dr. Ralph Maughan is professor emeritus of political science at Idaho State University with specialties in natural resource politics, public opinion, interest groups, political parties, voting and elections. Aside from academic publications, he is author or co-author of three hiking/backpacking guides, and he is past President of the Western Watersheds Project and the creator of The Wildlife News.
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I’m not surprised, given the proclivity of so many humans to traveling by mechanical means, not the two legs nature gave them. I’d be happiest to be able to put our cars behind us for just long enough to backpack a bunch of miles from the nearest road.
It’s absolutely no surprises at all that pesticides are the biggest culprit.
What other creature on the planet *poisons* its own food on purpose?
We really are insane. That is the cold hard truth.
Invasion of non-native species also tends to come locally from the outside if the Park. An exception to this can come to Parks that attract many vehicles from distant places.
Slightly off-topic but a good source of invasives in the southwest:
http://www.desertmuseum.org/invaders/