Sunday, February 05, 2017

Acid Mine Drainage (click here)

How Can It Be Treated?
Here in the United States, the Bureau of Land Management estimates that between 80,000 and 100,000 abandoned mines exist within the US. Fortunately, most of these mines are relatively small in size, as are the acidic streams coming out of them. As such, these mines can be treated by smaller, more passive-type systems. Some of these passive-type systems are shown in the diagram below – limestone drains or open limestone channels:

Water. Water-pollution problems caused by mining include acid mine drainage, metal contamination, and increased sediment levels in streams. Sources can include active or abandoned surface and underground mines, processing plants, waste-disposal areas, haulage roads, or tailings ponds. Sediments, typically from increased soil erosion, cause siltation or the smothering of streambeds. This siltation affects fisheries, swimming, domestic water supply, irrigation, and other uses of streams.

The map (click here) is from a study by Michigan State University. It maps rivers and streams that have been effected by increased sediment levels and chemicals in the water from mining.

Fish have to swim in water. When sediment builds up the fish cannot navigate the stream and their ability to spawn is ultimately eliminated. Additionally, chemicals alter the DNA when fish do spawn. As climate problems continue to compound the importance of clean water and good habitat are vital.

Acid mine drainage (AMD) is a potentially severe pollution hazard that can contaminate surrounding soil, groundwater, and surface water. The formation of acid mine drainage is a function of the geology, hydrology, and mining technology employed at a mine site. The primary sources for acid generation are sulfide minerals, such as pyrite (iron sulfide), which decompose in air and water. Many of these sulfide minerals originate from waste rock removed from the mine or from tailings. If water infiltrates pyrite-laden rock in the presence of air, it can become acidified, often at a pH level of two or three. This increased acidity in the water can destroy living organisms, and corrode culverts, piers, boat hulls, pumps, and other metal equipment in contact with the acid waters and render the water unacceptable for drinking or recreational use. A summary chemical reaction that represents the chemistry of pyrite weathering to form AMD is as follows:

Pyrite + Oxygen + Water = "Yellow boy" + Sulfuric Acid

"Yellowboy" is the name for iron and aluminum compounds that stain streambeds. AMD can enter the environment in a number of ways, such as free-draining piles of waste rock that are exposed to intense rainstorms, transporting large amounts of acid into nearby rivers; groundwaters that enter underground workings which become acidic and exit via surface openings or are pumped to the surface; and acidic tailings containment ponds that may leach into surrounding land.