What Is Acid Mine Drainage? Definition, Causes, and Examples

Acid mine drainage and toxic water in abandoned pyrite mine

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Acid mine drainage happens when the iron sulfides unearthed by mining activity interact with water and air and oxidize. The process creates sulfuric acid, a highly corrosive acid capable of breaking down surrounding rocks, which can cause toxic metals to enter and eventually dissolve into the water.

While other issues stem from abandoned mine drainage (like alkaline drainage, which occurs when calcite or dolomite is present, or metal drainage, which happens when high levels of lead or other metals drain from abandoned mines), acid mine drainage is believed to be the most prevalent.

Over 12,000 miles of stream have been affected by acid mine drainage in the United States. These streams are mostly located in areas with extensive mining activity, such as Appalachia. In Pennsylvania alone, for example, over 5,500 miles of stream are impacted by acid mine drainage.

Acid Mine Drainage Definition

Bright orange polluted stream
Wendy Van / Getty Images

Acid mine drainage refers to the acidic water that forms when surface water (which can mean rainwater, snowmelt, pond water, etc.) and air are exposed to iron sulfide, typically pyrite, a solid waste byproduct of coal mining.

A water sample that reads below a pH of 7 is considered “acidic,” and the more acidic it is, the quicker it erodes rocks and other materials. The U.S. Geological Survey (USGS) has recorded some acid mine drainage with a pH as low as between 2.5 and 4.

The chemical reaction forms sulfuric acid and dissolved iron, some or all of which can separate to form the rust-colored sediments that become visible on the bottoms of polluted streams. The acid runoff dissolves heavy metals like copper and mercury into the ground or surface water.

There are places where acidic streams can occur naturally, such as those in Yellowstone National Park, but most result from mining activity.

Where Acid Mine Drainage Happens

Acid mine drainage mostly occurs where mining is done to extract coal or metals from sulfur-bearing rocks. Silver, gold, copper, zinc, and lead are commonly found in association with metal sulfates, so their extraction can cause acid mine drainage.

Environmental Impacts

The heavy metals produced by acid mine drainage don’t biodegrade, meaning they’ll continue to accumulate inside living things as they move up the food chain. Environmental issues linked to this water pollution have the potential to affect humans, wildlife, and everything in between. 

The low water pH doesn’t just contaminate drinking water; it can also be strong enough to corrode infrastructure (such as bridges) and pollute the soil.

This kind of extreme acidity is also toxic to most aquatic life, creating a domino effect on the ecosystem. Even worse, outflows of acid mine drainage into the sea can cause coral mortality and deplete bottom-dwelling organisms in the ocean.

Acid Mine Drainage Disasters

One of the more famous accidents related to acid mine drainage occurred at the Los Frailes mine in Aznalcóllar, Spain, in 1998, when a broken dam released 4-5 million cubic meters of pyrite sludge—containing excessive levels of iron and sulfur with copper, zinc, lead, and silver—into the Agrio River Valley.

Thousands of hectares of farmland were flooded in toxic water and slurry as the Agrio River rose 3 meters (9.8 feet), narrowly missing one of Europe’s most important wetlands at Doñana National Park. The cleanup operation took a total of three years and cost nearly $269 million.

Solutions to Acid Mine Drainage

There are a couple of ways to address acid mine drainage, such as adding lime or other alkaline materials to the contaminated land to neutralize the acidity. Passive treatment systems, where water flows in naturally, require little to no operation or maintenance; in the more complex cases, active systems that need regular operation and maintenance may be utilized. 

Traditionally, acid mine drainage is treated by collecting the spillage in retention ponds or treatment plants and adding chemicals that neutralize the acidity, causing the dissolved metals to precipitate out of the contaminated water and form into solids.

In 2020, scientists from Penn State developed a two-stage treatment process that allowed them to recover higher concentrations of critical, rare earth elements from acid mine drainage. By adding carbon dioxide to the drainage, it created a chemical reaction that formed into solid minerals called carbonates. These extra carbonates then bond with the earth elements and precipitate out of the water at low pH levels. Using this new process, the researchers were able to recover over 90% of aluminum and 85% of rare earth elements, compared to traditional methods, which only precipitate 70% of the same elements.

Originally written by
Frederic Beaudry

Dr. Frederic Beaudry is an associate professor of environmental science at Alfred University in New York.

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