The Rocky Mountains, stretching more than 3,000 miles from British Columbia to New Mexico, are famous for their concentrated deposits of metals. Rich veins of gold, silver, lead, zinc, and copper, formed over millions of years, have drawn miners to the region since the 1850s. Prospectors, searching for these precious ores, dug thousands of mines deep into the Rockies. Now abandoned, these mines are causing serious environmental problems.

Because prospectors dug many mines deep below the water table, they had to pump the water out constantly. When they abandoned a mine, water flooded the empty shafts. This water reacts with sulfide minerals and oxygen to release sulfuric acid. The acidic water dissolves metals in the walls of the mine, creating highly acidic, metal-rich water that flows out of the mine to pollute rivers and streams. Abandoned mines can continue to produce this metal-rich water indefinitely.

Not all metals in water are bad. For example, some metals, like iron, cobalt, copper, and zinc, are necessary for optimal growth, development, and reproduction—but only in very small quantities. When these metals are present in greater quantities in water, they can have harmful effects on aquatic life and on animals, including humans, that drink the water. In addition, acidic mine drainage can contain toxic metals such as arsenic, cadmium, lead, and mercury, which are known to be hazardous to human health. For example, arsenic has been associated with increased risk of some cancers and mercury can damage the brain and nervous system.

What happens in Rocky Mountain streams can affect distant cities and states. Since metals are not broken down in nature, they can be carried long distances by the moving water. More than 25 million people in the Southwest depend on the Colorado River for drinking water and water for irrigation, and the majority of the Colorado River's water originates in the mountains of Colorado.

The best way to keep metals out of the water supply is to stop them at the source. Today, there are many ways to treat acid mine drainage. The most common process uses lime, a strong base made from burnt limestone or chalk. Lime is mixed into a tank containing acid mine drainage, neutralizing some of the acid in the water. As the water becomes less acidic, most toxic metals separate out of the water. The resulting mixture flows to another tank, where the toxic metal sludge settles to the bottom. The treated water can be recycled and the toxic sludge removed as a solid waste.

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