Acid mine drainage (AMD), or acid rock drainage (ARD), refers to the outflow of acidic water from (usually) abandoned metal mines or coal mines. However, other areas where the earth has been disturbed (e.g. construction sites, subdivisions, transportation corridors, etc.) may also contribute acid rock drainage to the environment. In many localities the liquid that drains from coal stocks, coal handling facilities, coal washeries, and even coal waste tips can be highly acidic, and in such cases it is treated as acid rock drainage. Acid rock drainage occurs naturally within some environments as part of the rock weathering process but is exacerbated by large-scale earth disturbances characteristic of mining and other large construction activities, usually within rocks containing an abundance of sulfide minerals.
Sub-surface mining often progresses below the water table, so water must be constantly pumped out of the mine in order to prevent flooding. When a mine is abandoned, the pumping ceases, and water floods the mine. This introduction of water is the initial step in most acid rock drainage situations. Tailings piles or ponds may also be a source of acid rock drainage.
After being exposed to air and water, oxidation of metal sulfides (often pyrite, which is iron-sulfide) within the surrounding rock and overburden generates acidity. Colonies of bacteria and archaea greatly accelerate the decomposition of metal ions, although the reactions also occur in an abiotic environment. These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and oxygen supplies usually keep their numbers low. Special extremophiles known as acidophiles especially favor the low pH levels of abandoned mines. In particular, Acidithiobacillus ferrooxidans is a key contributor to pyrite oxidation.
Metal mines may generate highly acidic discharges where the ore is a sulfide or is associated with pyrites. In these cases the predominant metal ion may not be iron but rather zinc, copper, or nickel. The most commonly-mined ore of copper, chalcopyrite, is itself a copper-iron-sulfide and occurs with a range of other sulfides. Thus, copper mines are often major culprits of ARD.
The chemistry of oxidation of pyrites, the production of ferrous ions and subsequently ferric ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options.
Although a host of chemical processes contribute to ARD, pyrite oxidation is by far the greatest contributor. A general equation for this process is:
2FeS2(s) + 7O2(g) + 2H2O(l) → 2Fe2+(aq) + 4SO42-(aq) + 4H+(aq)
The oxidation of the sulfide to sulfate solubilizes the ferrous iron (iron(II)), which is subsequently oxidized to ferric iron (iron(III)):
4Fe2+(aq) + O2(g) + 4H+(aq) → 4Fe3+(aq) + 2H2O(l)
Either of these reactions can occur spontaneously or can be catalyzed by microorganisms that derive energy from the oxidation reaction. The ferric irons produced can also oxidize additional pyrite:
FeS2(s) + 14Fe3+(aq) + 8H2O(l) → 15Fe2+(aq) + 2SO42-(aq) + 16H+(aq)
The net effect of these reactions is to release H+, which lowers the pH and maintains the solubility of the ferric ion.
Effects on pH
Photograph of Tinto River. (Credit - Carol Stoker)
In some ARD systems temperatures reach 117 degrees Fahrenheit (47 °C), and the pH can be as low as -3.6.
ARD-causing organisms can thrive in waters with pH very close to zero. Negative pH occurs when water evaporates from already acidic pools thereby increasing the concentration of hydrogen ions.
About half of the coal mine discharges in Pennsylvania have pH under 5 standard units. However, a significant portion of mine drainage in both the bituminous and anthracite regions of Pennsylvania is alkaline, because limestone in the overburden neutralizes acid before the drainage emanates.
ARD has recently been a hindrance to the completion of the construction of Interstate 99 near State College, but this ARD didn't come from a mine: pyritic rock was unearthed during a road cut and then used as filler material in the I-99 construction.
When the pH of ARD is raised past 3, either through contact with fresh water or neutralizing minerals, previously soluble Iron(III) ions precipitate as Iron(III) hydroxide, a yellow-orange solid colloquially known as Yellow boy. Yellow boy discolors water and smothers plant and animal life on the streambed, disrupting stream ecosystems. The process also produces additional hydrogen ions, which can further decrease pH. Research is currently being conducted as to the feasibility of using Yellow boy as a commercial pigment.
Trace metal and semi-metal contamination
Many acid rock discharges also contain elevated levels of potentially toxic metals, especially nickel and copper with lower levels of a range of trace and semi-metal ions such as lead, arsenic, aluminium, and manganese. In the coal belt around the south Wales valleys in the UK highly acidic nickel-rich discharges from coal stocking sites have proved to be particularly troublesome.
In the United Kingdom, many discharges from abandoned mines are exempt from regulatory control. In such cases the Environment Agency working with partners has provided some innovative solutions, including constructed wetland solutions such as on the River Pelenna in the valley of the River Afan near Port Talbot and the constructed wetland next to the River Neath at Ynysarwed.
Although abandoned underground mines produce most of the ARD, some recently mined and reclaimed surface mines have produced ARD and have degraded local ground-water and surface-water resources. Acidic water produced at active mines must be neutralized to achieve pH 6-9 before discharge from a mine site to a stream is permitted.
In Canada, work to reduce the effects of ARD is concentrated under the Mine Environment Neutral Drainage (MEND) program. Total liability from acid rock drainage is estimated to be between $2 billion and $5 billion CAD. Over a period of eight years, MEND claims to have reduced ARD liability by up to $400 million CAD, from an investment of $17.5 million CAD.
Generally, limestone or other calcareous strata that could neutralize acid are lacking or deficient at sites that produce acidic rock drainage. Limestone chips may be introduced into sites to create a neutralizing effect. Where limestone has been used, such as at Cwm Rheidol in mid Wales, the positive impact has been much less than anticipated because of the creation of an insoluble calcium sulfate layer on the limestone chips, binding the material and preventing further neutralization.
Cation exchange processes were investigated as a potential treatment for ARD. Not only would ion exchangers remove potentially toxic heavy metals from mine runoff, there was also the possibility of turning a profit off of the recovered metals. However, the cost of ion exchange materials compared to the relatively small returns, as well as the inability of current technology to efficiently deal with the vast amounts of mine discharge, renders this solution unrealistic at present.
Constructed wetlands systems have shown promise as a more cost-effective treatment alternative to artificial treatment plants. A spectrum of bacteria and archaea, in consortium with wetland plants, may be used to filter out heavy metals and raise pH. Anaerobic bacteria in particular are known to be capable of reverting sulfate ions into sulfide ions. These sulfide ions can then bind with heavy metal ions, precipitating heavy metals out of solution and effectively reversing the entire process.
Interestingly enough, T. ferrooxidans - the very bacteria which appears to be the problem - has also been shown to be effective in treating heavy metals in constructed wetland treatment systems.
The attractiveness of a constructed wetlands solution lies in its passivity - building an artificial wetlands is a relatively cheap one-time investment which continuously works to reduce acidity and heavy metal concentration. Although promising, constructed wetlands take much time to completely cleanse an area, and are simply not enough to deal with extensively polluted discharge. Constructed wetland effluent often requires additional treatment to completely stabilize pH. Also, the products of bacterial processes are unstable when exposed to oxygen, and require special disposal to ensure no further contamination. Other issues include seasonal variation in the activity of cleansing organisms, as well as the lack of a practical passive means of moving mine discharge through the most efficient regions of purification.
Active Treatment with Aeration
In some discharges, HCO3-, a base, enters into the runoff from the breakdown of organic matter in the mine, such as mine timbers, or from the groundwater interaction with limestone. The base then neutralizes the acid in the runoff, forming carbonic acid.
H+ + HCO3- = H2CO3. (1)
When this solution reaches the ground surface, the water is exposed to the air and the dissolved CO2 will degas into the atmosphere. This lowers the concentration of CO2, allowing more H2CO3 to decompose, which in turn allows the neutralization of more acid.
H2CO3 = H2O + CO2. (2)
The rise in pH promotes the oxidation of the iron and the formation of iron hydroxide, which will precipitate out of the solution, leaving little iron left in the water. Large aeration systems can be used to allow more CO2 to outgas, and thus precipitate more iron out of the solution. This explained method can only work, however, for runoff which is naturally alkaline (basic).
Precipitation of metal sulfides
Most base metals in acidic solution precipitate in contact with free sulfide, e.g. from H2S or NaHS. Solid-liquid separation after reaction would produce a base metal-free effluent that can be discharged or further treated to reduce sulfate, and a metal sulfide concentrate with possible economic value.
List of selected acid mine drainage sites worldwide
This list includes both mines producing ARD and river systems significantly affected by such drainage. It is by no way complete, as worldwide, several thousands of such sites exist.
* Berkeley Pit superfund site, covering the Clark Fork River and 50,000 acres (200 km²) in and around Butte, Montana, USA
* Britannia Beach, British Columbia, Canada
* Clinch-Powell River system, Virginia and Tennessee, USA
* Iron Mountain Mine, Shasta County, California, USA
* Monday Creek, Ohio, USA
* Various Coal Mines in the anthracite and bituminous coal regions of Pennsylvania, USA
* The Irwin Syncline in Southwestern Pennsylvania
* Pronto mine tailings site, Elliot Lake area, Ontario, Canada
* North Fork of Kentucky River, Kentucky, USA
* Cheat River Watershed, West Virginia, USA
* Copperas Brook Watershed, from the Elizabeth Mine in S. Strafford, Vermont, impacting the Ompompanoosuc River
* Davis Pyrite Mine in NW Massachusetts
* Wiconisco Creek Central Pennsylvania has Limestone diversion well to moderate pH
* Hughes bore hole, Pennsylvania
* Cwm Rheidol, Wales
* Afon Pelenna, Wales
* Ynysarwed, Wales
* Whittle Colliery, Northumberland, England
* Wooley Colliery, Yorkshire, England
* Black Clough, Lancashire,England
* Quaking Houses, Durham,England
* Avoca, Wicklow, Ireland
* Aznalcollar mine on the Agrio River, Spain
* Wheal Jane, Cornwall, England
* Tinto River, Spain
* Buller coalfield in the north-west of the South Island, New Zealand
* Queen River, Tasmania, Australia,
* King River, Tasmania, Australia,
* Potosi, Bolivia, metal mines in and around Cerro Rico
* Cerro de Pasco, Peru, metal mine in the Central plain of the Peruvian Andes