Acid Mine Drainage and Tailings

Mining digs into rock that has never touched air or water before, and that rock is often full of pyrite, iron sulfide. Once exposed, pyrite reacts with water and oxygen and oxidizes. The sulfur ends up as sulfuric acid, and the iron goes into solution. That acid runs off through the mine shaft or waste piles as drainage. Further downstream, the dissolved iron reacts with more oxygen and precipitates out as a rusty orange solid, which is the telltale color coating the streambed below an old mine. The reaction keeps running on its own for decades after a mine closes, as long as leftover pyrite, water, and air keep meeting.

Tailings are the ground-up waste rock left after the valuable ore is removed, and they still carry sulfide minerals and heavy metals like arsenic, lead, and cadmium. Mining companies pile this slurry behind earthen dams in tailings ponds, letting solids settle and water evaporate or get treated before release. The dam is the only thing separating that metal-laden sludge from the watershed. When a dam fails, the pond empties fast, sending a wave of acidic water and heavy metals downstream, burying streambeds and poisoning water supplies far beyond the mine site.

This topic extends the acid-base and water-pollution ideas from earlier in the course into mining specifically. Low pH and dissolved metals stress aquatic life the same way other point-source pollutants do: they lower species diversity, kill off acid-sensitive insects and fish, and leave only tolerant species behind. It also pairs with reclamation practices, like sealing mine shafts, adding limestone to neutralize acid, and capping tailings ponds, which are the engineered fixes for a problem rock chemistry creates on its own.