Back in their mining era (1860 CO rush started to 1921 for this mine) that stuff probably went down the same streams untreated. Little or no regulation for most of that time. So its already been there. Passage of time had fixed it.
Oh..so its OK that the EPA breached a settling pond hey?
Back in the day...was there several dams supplying water to huge areas like now?
They leached those contaminations slow back then, not that it was acceptable. They had nearly no idea and most likely did not care either.