Acid mine drainage, also called acid and metalliferous drainage (AMD) or acid rock drainage (ARD), is the flow of acidic water from metal mines and coal mines.

Acid rock drainage can happen naturally in some areas as part of the rock breaking down over time. However, large-scale activities like mining or construction, especially in areas with many sulfur-containing minerals, make this process worse. Places where the ground has been disturbed, such as construction sites or highways, can produce acid rock drainage. In some areas, water that flows from coal storage areas, coal processing plants, and coal waste piles is very acidic and is considered acid rock drainage. This type of water, along with lower pH levels, harms the water life in nearby streams.

Similar chemical reactions can also occur when acid sulfate soils, which form in coastal or estuary areas after the last major rise in sea levels, are disturbed. These soils create a similar environmental risk.

Nomenclature

Historically, the acidic water from active or abandoned mines was called acid mine drainage, or AMD. In the 1980s and 1990s, scientists introduced a new term, acid rock drainage, or ARD, to show that acidic water can come from sources other than mines. For example, a paper shared in 1991 at a large international meeting discussed predicting acid rock drainage and lessons from a database. Both AMD and ARD describe water with low pH caused by the oxidation of sulfide minerals. However, ARD is the more general term.

If water from a mine is not acidic but contains dissolved metals or metalloids, or if it was originally acidic but became neutral as it flowed, it is sometimes called "neutral mine drainage" or "mining-influenced water." These terms are not widely used or accepted.

Occurrence

Sub-surface mining often takes place below the water table, so water must be pumped out of the mine to avoid flooding. When a mine is no longer used, the pumping stops, and water flows into the mine. This water entering the mine is the first step in most acid rock drainage situations. Tailings piles or ponds, mine waste rock dumps, and coal spoils are also major sources of acid mine drainage.

When exposed to air and water, metal sulfides (often pyrite, which is iron-sulfide) in the surrounding rock and overburden break down and produce acidity. Bacteria and archaea speed up the breakdown of metal ions, though these reactions can also occur without living organisms. These microbes, called extremophiles because they survive in harsh conditions, live naturally in the rock. However, limited water and oxygen usually keep their numbers low. Extremophiles known as acidophiles prefer the low pH levels found in abandoned mines. In particular, a type of microbe called Acidithiobacillus ferrooxidans plays a major role in breaking down pyrite.

Metal mines can create highly acidic water when the ore contains sulfide minerals or is linked to pyrite. In these cases, the main metal ion may not be iron but could be zinc, copper, or nickel. The most commonly mined copper ore, chalcopyrite, is a copper-iron-sulfide and is often found with other sulfides. Because of this, copper mines are frequently major sources of acid mine drainage.

At some mines, acidic drainage is found within 2–5 years after mining starts, while at others, it may take many decades to appear. Also, acidic drainage can continue for decades or even centuries after it is first detected. Because of this, acid mine drainage is seen as a serious long-term environmental issue linked to mining.

Chemistry

The chemical process of pyrite oxidation, which creates ferrous ions and then ferric ions, is very difficult to understand. This difficulty has made it hard to create good ways to treat the problem.

Although many chemical reactions cause acid mine drainage, pyrite oxidation is the most important cause. A simple equation for this process is:

When sulfide turns into sulfate, it makes ferrous iron (iron(II)) soluble. This iron(II) then changes into ferric iron (iron(III)):

Either of these reactions can happen on its own or can be helped by tiny living things called microorganisms. These microorganisms use the reaction to get energy. The ferric ions made in this process can also help break down more pyrite and turn back into iron(II):

The overall result of these reactions is the release of hydrogen ions (H+). This makes the water more acidic and keeps ferric ions dissolved.

Effects

Underground at the Iron Mountain Mine, water temperatures can reach as high as 47 °C (117 °F). The pH of the water can be as low as −3.6.

Organisms that cause acid mine drainage can survive in water with pH very close to zero. Negative pH happens when water evaporates from acidic pools, which increases the amount of hydrogen ions.

About half of the coal mine discharges in Pennsylvania have pH levels below 5. However, some mine drainage in the bituminous and anthracite regions of Pennsylvania is alkaline because limestone in the overlying rock neutralizes acid before the drainage flows out.

When the pH of acid mine drainage increases past 3, either through contact with fresh water or neutralizing minerals, previously soluble iron(III) ions form a yellow-orange solid called yellow boy. Other types of iron precipitates, such as iron oxides, oxyhydroxides, and sulfates like jarosite, can also form. These precipitates can discolor water and cover plant and animal life on the streambed, harming stream ecosystems (a specific offense under the Fisheries Act in Canada). This process also creates more hydrogen ions, which can lower the pH further. In some cases, the high concentration of iron hydroxides in yellow boy can be used for commercial purposes, such as pigments.

Many acid rock discharges also contain high levels of toxic metals, especially nickel and copper, along with smaller amounts of other metals such as lead, arsenic, aluminum, and manganese. These metals dissolve only in water with very low pH, like the acidic water produced by pyrite oxidation. In the coal belt around the south Wales valleys in the UK, very acidic, nickel-rich water from coal storage areas has caused significant environmental issues.

Acid mine drainage harms wildlife in affected water bodies. Aquatic macroinvertebrates in streams or parts of streams affected by acid mine drainage show fewer numbers, less variety, and lower total mass. Many fish species cannot survive in this polluted water. Some macroinvertebrate species are only found at certain pollution levels, while others can live in a wider range of conditions.

Identification and prediction

In a mining project, it is best practice to perform a geochemical assessment of mining materials during the early planning stages to identify the risk of acid mine drainage (AMD). This assessment helps to understand how chemical properties are spread and change across the materials, as well as how likely they are to produce acid or release harmful elements.

The assessment may involve:

• Collecting samples;
• Conducting static geochemical tests, such as measuring acid levels and sulfur types;
• Performing kinetic geochemical tests, like oxygen consumption tests (e.g., OxCon), to measure how quickly acid is produced;
• Using models to predict oxidation processes and the spread of pollutants; and
• Using models to analyze the makeup of the materials.

Treatment

In the United Kingdom, many water discharges from old mines are not controlled by rules. To help, the Environment Agency and Natural Resources Wales work with groups like the Coal Authority to create wetland systems. Examples include a wetland on the River Pelenna near Port Talbot and another next to the River Neath at Ynysarwed.

Although old underground mines are the main source of acid mine drainage (ARD), some recently closed surface mines have also caused ARD, harming groundwater and surface water. At active mines, acidic water must be treated to reach a safe pH level between 6 and 9 before it can be released into streams.

In Canada, efforts to reduce ARD are managed by the Mine Environment Neutral Drainage (MEND) program. The cost of dealing with ARD is estimated to be between $2 billion and $5 billion. Over eight years, MEND claims to have reduced this cost by up to $400 million with an investment of $17.5 million.

At some ARD sites, limestone or other rocks that can neutralize acid are not available or not easily accessible. In these cases, crushed limestone is added to the site to help neutralize the acid.

Limestone is a common and low-cost material for neutralizing acid, but it has some problems. Small pieces of limestone can form a protective layer of gypsum and other materials, which stops the limestone from working properly. This issue was observed at Cwm Rheidol in Wales, where limestone use had less success due to this layer forming on the limestone.

For large amounts of acid water, a more expensive process called high-density sludge (HDS) is often used. In this method, lime is mixed into a tank with acid mine drainage and recycled sludge to raise the pH to about 9. At this pH, toxic metals become solid and settle out of the water. Air may be added to help remove iron and manganese. The settled material is reused, and clean water is released. This process also produces gypsum and leftover lime, which help prevent the water from becoming acidic again.

A simpler and cheaper method is lime neutralization, which uses a lime storage area, mixing tank, and settling pond. This works well for small amounts of acid water but is less effective and may leave more metals in the water.

Calcium silicate, made from steel slag, can also neutralize acid. It removes hydrogen ions from water, raising the pH. Unlike limestone, calcium silicate does not form a protective layer on its surface, so it remains effective even in the presence of heavy metals.

Cation exchange methods have been studied to remove metals from mine water. These methods use special resins to trap metals, but the resins must be cleaned with chemicals, which can create concentrated waste. A South African company has developed a patented process using this method to treat mine water efficiently.

In the 1980s, wetland systems were proposed to treat ARD from abandoned coal mines in Eastern Appalachia. These wetlands usually receive water that has already been treated with limestone to reach a neutral pH. Metals in the water can then settle out through chemical reactions, oxidation, or the action of bacteria that change sulfate into sulfide, which binds with heavy metals to remove them from the water.

Wetland systems are attractive because they are relatively low-cost. However, they are limited by the amount of metal they can handle, especially if the water flow or metal concentration is too high.

Metagenomic study

With the development of advanced sequencing methods, scientists can directly sequence the genomes of microorganisms living in acid mine drainage environments. Nearly complete genome sequences help researchers better understand these communities and map out how the microbes process nutrients. Our understanding of acid-loving microbes in acid mine drainage is still limited: we know about many more species linked to acid mine drainage than we can determine their roles and functions.

Microbes and drug discovery

Scientists have started to study acid mine drainage and mine reclamation sites to find special types of soil bacteria that might lead to new medicines. Soil microbes have been a source of effective medicines for a long time. New research, such as that done at the Center for Pharmaceutical Research and Innovation, shows that these extreme environments could be untapped sources of new discoveries.

List of selected acid mine drainage sites worldwide

This list shows mines that create acid mine drainage and rivers that are greatly affected by this drainage. The list is not complete because there are thousands of similar sites around the world.

• West Rand Goldfield, Witwatersrand, South Africa
• Avoca, County Wicklow, Ireland
• Aznalcollar mine on the Guadiamar, Spain
• Wheal Jane, Cornwall, England
• Tinto River, Spain
• Odiel River, Spain
• Libiola's mine, Italy
• Spree River, Germany
• The Lusatian Lake District and the Central German Lake District, which were created by open-pit lignite mining, face issues with acid mine drainage

• Argo Tunnel, Idaho Springs, Colorado, US
• Berkeley Pit superfund site, covering the Clark Fork River and 50,000 acres (200 km²) in and around Butte, Montana, US
• The Summitville Mine in Rio Grande County, Colorado. The area has both natural and mining-exacerbated acid drainage flowing into the Wrightman Fork, then into the Alamosa River, which flows into the San Luis Valley
• Britannia Beach, British Columbia, Canada
• Clinch-Powell River system, Virginia and Tennessee, US
• Iron Mountain Mine, Shasta County, California, United States
• Monday Creek, Ohio, US
• The Irwin Syncline in Southwestern Pennsylvania
• Pronto mine tailings site, Elliot Lake area, Ontario, Canada
• North Fork of Kentucky River, Kentucky, US
• Old Forge borehole, Lackawanna River, Pennsylvania. Discharges 40–100 million gallons of acid mine drainage per day.
• Cheat River Watershed, archived 18 February 2020 at the Wayback Machine, West Virginia, US
• Copperas Brook Watershed, from the Elizabeth Mine in S. Strafford, Vermont, impacting the Ompompanoosuc River
• Davis Pyrite Mine in NW Massachusetts
• Hughes bore hole, Pennsylvania
• Gold King Mine, Colorado, US
• Acid mine drainage in Ontario, Canada

• Brukunga, South Australia
• Grasberg mine, Papua province, Indonesia
• McArthur River zinc mine, Northern Territory, Australia
• Mount Morgan Mine, Queensland, Australia
• Ok Tedi environmental disaster caused by Ok Tedi Mine, Ok Tedi River, Papua New Guinea
• Tui mine, an abandoned mine on the western slopes of Mount Te Aroha in the Kaimai Range of New Zealand, considered to be the most contaminated site in the country
• West Coast mineral fields, Tasmania, Australia