Mine Drainage | U.S. Geological Survey

BACKGROUND

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Mine drainage refers to any surface water or groundwater that drains from an active or abandoned mining operation. Mine drainage can be high-quality similar to natural waters or contaminated by leftover materials. Polluted mine drainage can be extremely acidic and is often laden with high concentrations of toxic, heavy metals. In general, the more acidic the water, the more likely it is to be harmful to living organisms.

Mine drainage forms from a chemical reaction between water and rocks containing sulfur-bearing minerals. The resulting waters become rich in sulfuric acid and dissolved iron. As the iron settles out of the water, it can form red, orange, or yellow sediments in the bottom of streams. The acidic 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.

THE CHEMISTRY OF MINE DRAINAGE

Because the chemistry of water samples can rapidly change if removed from the mine, many mine drainage measurements are made in the field. The first measurement typically looks at acidity, which is reported as pH. A neutral pH has a value of 7. Any sample that reads below a pH of 7 is characterized as being acidic. Anything greater than 7 is described as being basic. The more acidic the water is, the better it is at eroding mining slag, rocks, and other materials. The water then transports the contaminated mine materials to nearby rivers before eventually depositing the materials downstream. Some mine drainage has been seen to have pH in the 2.5-4 range1.

Another water-quality parameter that is useful for characterizing water quality of acid mine drainage is specific conductance. Conductance is a measurement of the electrical conductivity in a water sample and is a indicator of what's dissolved in a liquid. It is also very inexpensive to measure, unlike testing for metals and other pollutants. Different substances will affect the conductance of water, allowing scientists to use changes in conductance as an indicator of changes of whatever is in the water - in this case the amount of contaminants coming from the mine.

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RELATED USGS RESEARCH

• USGS Mine Drainage Activities
• Contamination associated with active and abandoned hard-rock mining
• USGS Central Mineral Resources team

ADDITIONAL RESOURCES

U.S. Environmental Protection Agency

• Abandoned mines' role in nonpoint source pollution