Here’s a report from Jonathan Thompson writing for the High Country News. Click through and read the whole thing and check out Thompson’s drawings explaining acid mind drainage and the geology of the area. Here’s an excerpt:
While there are a variety of ways that mining can pollute watersheds, the most insidious and persistent is acid mine drainage, which is really a natural phenomenon exacerbated by mining. Acid mine drainage was the root cause of the Gold King blowout, and it plagues tens of thousands of abandoned mines across the West. It’s almost impossible to fix, and it lasts forever…
…the early settlers also were struck by the reddish orange color (like the Animas River after the “spill”) of some of the mountains. They were also struck by the same orange in some streams during times of high runoff, streams that were lifeless even then. Indeed, an observer in 1874 noted that Cement Creek was “so strongly impregnated with mineral ingredients as to be quite unfit for drinking.”[…]
Mining begins. The tunnels follow veins of gold or silver deep underground. The adits (horizontal tunnels) and shafts (vertical tunnels) intersect the cracks and faults through which groundwater had run toward springs. The groundwater follows the path of least resistance: The new mine adit. Whereas the cracks and faults are mostly anaerobic, or free of oxygen, the mine is relatively rich in oxygen. Meanwhile, the water as it flows through the mine runs over deposits of pyrite, or iron sulfide. Water (H2O) meets up with oxygen (O2) and pyrite (FeS2). A chain of reactions occurs, one of the products being H2SO4, otherwise known as sulfuric acid. The result is acid mine drainage, water that tends to have a pH level between 2 (lemon juice) and 5 (black coffee).
So now there is acidic water running through the mine. And since the mine follows the metals, so does the water, picking up the likes of zinc, cadmium, silver, copper, manganese, lead, aluminum, nickel and arsenic on the way. The acidic water dissolves these metals, adding them to the solution. After the water pours from the portal (mine opening), it percolates through metal-rich waste rock piled up outside the portal, picking up yet more metals. Next, the water may run through old tailings or leftovers from milling ore and pick up yet more nasty stuff. The soup that eventually reaches the stream is heavily laden with metals and highly acidic. It is acutely and chronically toxic to fish and the bugs they eat.