The USGS Water Science School
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Mining and Water Quality
Sampling a seep draining mine tailings along Silver Creek, near Park City, Utah. Contaminants from hardrock mining can come from adits, waste rock piles, and from tailings that were stored along streams. Credit: USGS View full size
Mine drainage is metal-rich water formed from a chemical reaction between water and rocks containing sulfur-bearing minerals. The resulting chemicals in the water are sulfuric acid and dissolved iron. Some or all of this iron can come out as solids to form the red, orange, or yellow sediments in the bottom of streams containing mine drainage. The acid runoff further dissolves heavy metals such as copper, lead, mercury into groundwater or surface water. The rate and degree by which acid-mine drainage proceeds can be increased by the action of certain bacteria.
Acidic, metal-laden drainage from abandoned coal mines can have substantial effects on aquatic resources. Problems that can be associated with mine drainage include contaminated drinking water, disrupted growth and reproduction of aquatic plants and animals, and the corroding effects of the acid on parts of infrastructures such as bridges. As with any environmental damage, there are costs associated with trying to come up with a solution to the problem. In the Appalachian region of West Virginia, for example, the cost of correcting acidic mine drainage-related problems with currently available technology is estimated at $5-$15 billion.
The world's most acidic water is found in a mine
In a study performed by the U.S. Geological Survey, the most acidic waters ever measured are percolating through an underground mine near Redding, Calif. Hot acid solutions, more concentrated than battery acid, are dripping off colorful mineral stalactites in the abandoned copper and zinc mine at Iron Mountain, a northern California "Superfund" site that is undergoing remediation by the US Environmental Protection Agency (EPA). Although a water-treatment plant at Iron Mountain has reduced the amount of copper and zinc leaching from Iron Mountain by 80 to 90 percent since 1994, some acid waters from the mine site still make their way to Spring Creek, a tributary to the Sacramento River, a few miles upstream from Redding.
pH values are given as being in a ranging of 0 to 14, with 7 being neutral. Values of pH near 0 are highly acidic, becoming less acidic and more alkaline at the higher numbers. Because pH is measured on a logarithmic scale, each declining unit represents 10 times more acidity. USGS scientists said several of the drip-water samples at Iron Mountain had pH values below zero, indicating hydrogen ion activities greater than one. The lowest pH found at California mine site was -3.6.
View the USGS California Water Science Center page on this topic.
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