Acid Mine Drainage
Definitions of "acid mine drainage" from reliable sources
Bright orange-colored water and stained rocks are usually tell-tale signs of acid mine drainage. The orange color is caused by ferric hydroxide ( Fe(OH)3 ) precipitating out of the water. The precipitate forms as the acid mine drainage becomes neutralized. At low pH values, the metal ions remain soluble. When the pH rises, the iron oxidizes and precipitates out. Depending on the conditions, the orange-colored precipitates may form inside the mine or several miles downstream. The precipitates can be harmful to aquatic life. The clumps reduce the amount of light that can penetrate the water, affecting photosynthesis and visibility for animal life. Futhermore, when the precipitate settles, it blankets the stream bed, smothering the bottom-dwellers and their food resources.
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.
Drainage of water from areas that have been mined for coal of other mineral ores; the water has low pH, sometimes less than 2.0 (is acid), because of its contact with sulfur-bearing material; acid drainage is harmful because it often kills aquatic organisms.
This refers to water pollution that results when sulfur-bearing minerals associated with coal are exposed to air and water and form sulfuric acid and ferrous sulfate. The ferrous sulfate can further react to form ferric hydroxide, or yellowboy, a yellow-orange iron precipitate found in streams and rivers polluted by acid mine drainage.
AMD can be characterized by low pH and increased acidity, elevated heavy metals, sulfate, and total dissolved solids (TDS). The low pH water that results from acid generation is capable of solubilizing heavy metals contained within the waste rock. Most harmful to the environment is the high metals loading in the water emanating from the waste material. As AMD flows away from the acid-generating source and moves into the receiving environment where the pH is buffered, discoloration of the streambed or the material that the AMD is passing over often is caused due to precipitation of solid metal hydroxides.
AMD is defined as drainage that occurs as a result of sulfide oxidation in rock exposed to air and water. In the case of iron sulfide (pyrite/marcasite), the chemical reaction in the acid-generating process can be simplified to: FeS2 + 15/4 O2 + 7/2 H2O >>> Fe(OH)3 + 2SO4 + 4H. In the presence of oxygen and water, pyrite oxidizes to form iron hydroxide (commonly called "yellowboy"), sulfate, and hydrogen ions. The liberation of hydrogen ions causes acidity in water passing over the rock. Every mole of pyrite yields four moles of acidity.
When reactive sulfide rock is initially exposed to flowing water and oxygen, sulfide oxidation and acid generation begins. Any calcium-based carbonate in the rock immediately neutralizes this small amount of acidity and maintains neutral to alkaline conditions in water passing over the rock (3). As acid generation continues and the neutralizing agent is consumed or is rendered ineffective in further neutralization, the pH of the water decreases, which in turn enhances the conditions for further acid generation. As the rate of acid generation accelerates, the pH progressively decreases in a step-like manner. Each plateau of relatively steady pH represents the dissolution of a neutralizing mineral that becomes soluble at that pH (3). If the rate of acid generation remains high enough to remove all of the neutralization potential in the rock, the pH values will drop below 3 and AMD will become severe. These various stages can last for weeks, months, or centuries until the sulfide minerals completely oxidize and the rock becomes inert, or until special waste management and AMD control actions are taken.
"Orange precipitate" being discharged into the brook by the "Acid Water Treatment Facility" next to the "Devco waste rock pile" at Pioneer Coal's Prince Mine site
NSEL's definition of the "orange precipitate" at the Prince Mine
I am responding to your questions related to the "orange precipitate" raised during our recent meeting with Minister Parent in Sydney. I am responding as Executive Director responsible for the Environmental Monitoring and Compliance Division.
The Department has reviewed a considerable volume of information with respect to the concern of an orange precipitate at the surface coal mine and reclamation project of Pioneer Coal Limited, Point Aconi, Cape Breton County. This precipitate has been observed in a portion of MacDonald Brook adjacent to the existing settling pond infrastructure from the former Cape Breton Development Corporation's (Devco) underground coal mine, namely the Prince Mine.
A review of historic photographs provided by Nova Scotia Department of Natural Resources shows a history of coal extraction in the area of the former Prince Mine. These photos date back over a span of 60 years, and it is conclusive that both bootleg and commercial mining have been very active.
Pioneer Coal Limited (PCL) is the present owner of the former Prince Mine site. In addition to an approval to operate a surface coal mine, the site is also a reclamation project. PCL is actively preparing the site for surface mining of coal; and, at the same time, they are carrying out reclamation of old workings, buildings, spoil piles, etc., left by bootleg operations and previous landowners.
The orange precipitate viewed is caused by water which has an elevated iron load and becomes oxygenated when discharged over the ground surface. This exposure to oxygen causes the dissolved iron in the water to precipitate out of solution, which results in staining and iridescence on the rocks and vegetation at the discharge area.
In this situation, the water being discharged is characterized by a slightly basic pH (>7) and therefore is not acidic. Although the look is not aesthetically pleasing, a bioassay test has shown that mortality or stressed behaviour did not occur in fish exposed to this water.
NSEL and the federal Department of Fisheries and Oceans and Environment Canada are collaborating and assisting each other with reviewing and monitoring the situation. Information gathered has been referred to Department of Fisheries and Oceans and Environment Canada regarding the discharges to MacDonald Brook as these agencies may be the more appropriate regulatory agencies to deal with this matter from an "adverse effect" perspective.
Presently, NSEL is developing a weekly pH monitoring schedule. In addition, the Department is reviewing several possible areas where this discharge water may be receiving iron. The possible areas include drainage originating at nearby bootleg workings, Devco waste rock pile, and the settlement/surge pond.
It is important to keep in mind that, throughout the life of the mining project, PCL will be addressing a great number of environmental and safety issues that have existed prior to its interest in the property. Addressing these liabilities will foster the surrounding area to return to a rehabilitated state that will be beneficial for all.
If you wish to obtain a copy of lab test results, please contact Mrs. Carla Heggie, Information Access & Privacy Manger, at (902) 424-8472. Other inquires regarding the approval can be directed to Mr. Brad Langille, Inspector Specialist at (902) 563-2100.
"This water is highly acidic and cannot be discharged into the environment untreated"
- “Acidic drainage is recognized as the largest environmental liability facing the mining industry and, to a lesser extent, the public through abandoned mines.” - Mine Environment Neutral Drainage (MEND) Program, Natural Resources Canada