Acid mine drainage occurs when iron sulphides released by mining activity react with water and oxygen to form sulfuric acid. This acid is highly corrosive and can dissolve surrounding rocks, allowing toxic metals to seep in and out.
While other problems are associated with abandoned mine drainage (e.g. alkaline drainage due to calcium and dolomite or metal drainage due to high levels of lead and other metals), acid mine drainage is considered the most common.
Acid mine drainage affects more than 12,000 miles (19,000 km) of streams across the United States. Most of these streams are in mining regions like Appalachia. For instance, more than 5,500 miles (8,100 km) of streams in Pennsylvania are affected.
Acid Mine Drainage Definition
Acid mine drainage is the acidic water formed when surface water (e.g., rainwater, snow melt, pond water) and air come into contact with iron sulfide, usually pyrite (a solid waste byproduct from coal mining). A water sample with a pH of less than seven is considered acidic. The higher the pH, the faster the acid mine drainage can erode rocks and materials. According to the USGS, some acid mine drainage has a pH of 2.5 to 4.
The chemical reaction combines sulfuric acid with dissolved iron, which can be partially or entirely dissolved to form rust-coloured sediment visible on the bottom of polluted streams. Acid runoff releases heavy metals such as copper and mercury into groundwater or surface water.
While some naturally occurring acid streams exist, such as in Yellowstone National Park, the majority are the result of mining operations.
Environmental Impacts
Heavy metals released by acid mine drainage are non-biodegradable, which means they will continue to build up in living organisms as they move through the food chain. The environmental problems associated with this water pollution can affect people, wildlife and everything in between.
Low water pH pollutes drinking water, corrodes infrastructure (e.g., bridges), and contaminates soil. Extreme acidity levels can also be toxic to aquatic life, causing a ripple effect throughout the ecosystem. Acid mine drainage into the ocean can also cause coral mortality and reduce the number of organisms that live on the ocean’s bottom.
Disasters Related to Acid Mine Drainage
One of the most well-known acid mine drainage incidents occurred in 1998 at the Los Frailes mine in Agrio River Valley, Spain. A broken dam discharged 4 to 5 million cubic meters of acid sludge—high concentrations of iron and sulfur combined with copper, zinc, lead, and silver.
Tens of thousands of farmland were inundated by contaminated water and slurry after the Agrio River overflowed its banks and rose by 3 meters, or 9.8 feet, almost touching down on one of Europe’s most significant wetlands in Doñana National Park. The cleanup process lasted three years and cost nearly $269 million.
How to Reduce Acid Mine Drainage?
There are two types of acid mine drainage systems: passive and active. Passive systems, where the water enters naturally, require minimal operation or maintenance. On the other hand, if the mine is more complex, active systems may be used that require regular operation or maintenance.
Acid mine drainage has traditionally been treated by capturing the spillage in a retention pond or treatment plant and neutralizing the acidity with chemicals. This results in the dissolved metals precipitating out of the polluted water and forming solids.
In 2020, Penn State researchers developed a two-step treatment process for acid mine drainage. This process uses carbon dioxide to create a chemical reaction that forms solid minerals known as carbonates. The extra carbonates bond to the earth’s elements, precipitating them out of the drainage at low pH. By using this new process, Penn State scientists could recover more than 90 per cent of aluminium and 85 per cent of rare earth elements from acidic mine drainage compared to traditional methods that only precipitate 70 per cent of the same elements.
