While I'm basically with you on coal, this was apparently an old gold mine, unused for decades. http://www.denverpost.com/environment/ci_28595759/animas-river-contaminated-by-1-million-gallons-contaminated
Most detailed explanation I've yet read on what happened http://www.kunc.org/post/why-was-environmental-protection-agency-messing-mine-above-silverton
Mine waste of many types is typically one of the most difficult types of contamination to deal with, of course, because there's no way that a toxic element (i.e., lead, cadmium, mercury) can break down into something less poisonous. The acid wastewater associated with old mines or tailings ponds is unfortunately quite efficient at dissolving toxic metals in soil, which concentrates the problem- and then mobilizes it, unless it's held in containment. Since water typically proves to be difficult to stop from running downhill, incidents like these are a constant risk for any containment impoundment.
The acid wastewater is a problem in itself, of course- although because it's a chemical in solution and not an element, at least that problem can be neutralized to some extent through balancing it out with alkaline materials like limestone. Fortunately, the Animas River basin is apparently quite alkaline- so much so that even this spill only manages to lower the pH of the river to 5.8, from 7.8. That's acidic, but only slightly so.
The spill began on Cement Creek, a tributary so impaired that it's fishless. But there are fish- trout, in fact- in the Animas River. Many of them, apparently- it's a Colorado Gold Medal stream.
The reason the EPA was there in the first place was to try to control the impairment of the larger watershed by continuing leakage from pooled water in the old mines- much the same as they've been involved with the MDE in remediating the North Branch of the Potomac and the Savage River in Maryland. Obviously, sometimes this can get tricky.
Although the Animas River is a Gold Medal trout stream, popular river rafting destination, and even a supplemental drinking water source for the region around Durango, parts of the riverbed retain high concentrations of toxic metals like lead. http://pubs.usgs.gov/of/1997/ofr-97-0151/images/fig20.gif http://pubs.usgs.gov/of/1997/ofr-97-0151/html/stpmw08.shtml That explains the interest of the EPA in addressing the problems of pollution sources like Cement Creek- especially the drinking water thing. Don't let anyone snow you; the US EPA was trying to keep a real problem from getting worse. The crew just made a mistake. (Would people try to snow you about this? Read some of the comments on this well-reported article in the Durango (CO.) Herald http://www.durangoherald.com/article/20150806/NEWS01/150809765/River-users-warned-of-mine-waste- )
The spill was estimated as 1 million gallons (for the sake of the EPA, whose aims I support, I hope that isn't an underestimate.) One million gallons sounds like a lot of water, but it's a relative thing. It's about 1 1/3 Olympic swimming pools worth- 133680 cubic feet. The Animas River pushes that volume of water in about 10 minutes in Silverton, near the top of its course where the spill occurred. http://waterdata.usgs.gov/nwis/uv?09359020
Apparently, the technical term for a sudden discharge like this is a "pulse"- the water will be flowing through in a concentrated slug, getting more diluted as the rivercourse gets larger along its 120-odd mile length below the spill, to where the Animas eventually meets the San Juan River main stem. The SJ is in turn a tributary of the Colorado.
The real question is how high the concentration of toxics is. Most of the material is sediment, which is plenty poisonous to fish gills, and as riverbottom smothering silt. That's bad, but most of that habitat damage will be cleared up in a few seasons. Hopefully the more long-lasting problems with toxic metals are less severe. Too early to tell.
