The past fifty years of coal mining have left the surface of the land scarred, and have polluted 13,000 miles of streams (Hadley, Snow 1974). Many laws and regulations have been passed to help treat and control the problem of acid drainage. The EPA has helped establish new U.S. effluent limits including reduced drainage acidity in which the pH must fall between 6 and 9, and the average iron content must not exceed 3 mg/L (Mehrotra, Singhal 1992). Mines in operation since 1978 must chemically treat their effluent water, at an estimated cost of $1 million per day (Perry 1992). Processes preventing acid discharge include use of filtration equipment and drainage ponds. Chemical and biological processes are the most common methods to treat acid drainage.
The EPA cites acid drainage from abandoned coal mines as the primary water quality problem in Appalachia (Appalachian Regional Commission 1973). Acid mine drainage is polluted water that normally contains high levels of iron, aluminum, and acid (Hadley, Snow 1974). Drainage acidity arises from oxidation of pyrite, the crystalline form of iron sulfide (Hadley, Snow 1974). The contaminated water is often reddish-brown in color, indicating high levels of oxidized iron (Hadley, Snow 1974). Mining disturbs pyrite and, as a result, pyrite weathers and reacts with oxygen and water in the environment.
Pyrite oxidation creates sulfuric acid and ferrous and ferric sulfates. The method by which pyrite oxidizes to form sulfuric acid and ferric hydroxide proceeds as follows:
Equation 1 describes the oxidation of pyrite that occurs abiotically with catalysis by Thiobacillus ferrooxidans (Forstner, Salomons 1988). Equations 2 and 3 represent the oxidation of ferrous iron to ferric iron and the consequent precipitation of ferric iron as ferric hydroxide. The two reactions account for the characteristic reddish-brown color of sediments contaminated by acid drainage. The three reactions combine to form the fourth stoichiometric equation, describing the complete reaction of pyrite and the formation of sulfuric acid (Hadley, Snow 1974).
Primary, secondary, tertiary,and downstream factors influence the quality and quantity of mine drainage.
The physical characteristics of importance are particle size, physical weathering tendency, and permeabilty. The particle size relates to the amount of surface area that is exposed to oxidation. The smaller the particle size, the more total surface area is exposed, and the greater potential for oxidation. Weathering increases the surface area of the material. The increase in surface area and the physical weathering and fracturing of strata increase the permeability of the waste material.
The sequence of stacking different wastes may affect the water quality of mine drainage. By contributing to the alkalinity of the percolating water, calcareous material placed atop pyritic material will reduce both the potential for oxidation and the acidity generated (Forstner, Salomons 1988).
The hydologic regime of a mine can influence the quality of mine drainage. Because waste inundation limits the transfer of oxygen, significant acid generation in a saturated zone may not occur. A fluctuating water table induces pyrite oxidation during declines in the water table (Forstner, Salomons 1988).
Downstream factors may impact the quality and quantity of acid drainage. Physical processes such as dilution and precipitation and chemical processes such as neutralization will permit a stream to assimilate acid drainage, but not without incurring a great deal of acid damage to the preceding stream area (Forstner, Salomons 1988).
Acid drainage problems exist in Pennsylvania, West Virginia, Ohio, Kentucky, Maryland, Indiana, Illinois, Oklahoma, Iowa, Missouri, Kansas, Tennessee, Virginia, Alabama, and Georgia. Pennsylvania and Northern West Virginia, the two most extensively mined states in Appalachia, lack limestone formations, and as a result experience severe acid drainage pollution (Appalachian Regional Commission 1973).
Remediation of acid drainage is difficult and expensive. Many companies use hydrated lime, sodium hydroxide, sodium carbonate, or ammonia to treat acid mine water, with each chemical offering the advantage of neutralizing acidity. Bactericides including antibiotics, heavy metals, detergents, and food preservatives have also been found to reduce acid mine drainage, however, antibiotics and heavy metals are too costly and also too dangerous to the surrounding aquatic life to be effectively used. Alconox, an inexpensive commercial detergent, and sodium lauryl sulfate both are found to reduce acid production in mine drainage (Hedin, et al 1994).
Crushed limestone is a common material used to neutralize acid drainage. The limestone reacts with acidic water in the following manner:
The natural bicarbonate in limestone neutralizes the hydrogen ions, but the metals in solution may not be simultaneously removed by the process. The resulting, neutralized water is typically still high in iron and sulfate content (Hadley, Snow 1974).
Because chemical treatment is so expensive, biological alternatives are currently being researched and implemented. The use of built wetlands, a method developed by the Department of Interior's Bureau of Mines, precipitates metals and neutralizes acidity through biological activity (Perry 1992). The first operational constructed wetland system to treat acid mine drainage was built in 1982. Now over 300 wetland water treatment systems have been built on mine lands, with over 200 systems in Appalachia alone (Bastian, Crook 1992).
The first wetlands were planted with a plant called Sphagnum in an attempt to simulate natural bog-type wetlands (Frostman 1995). The large surface areas of aquatic plants and algae serve as substrate to support bacteria. The filtering and settling mechanisms effectively remove suspended solids that do not normally settle. The decaying biomass of plants and algae provide anaerobic conditions and nutrients to the sulfate reducing bacteria. The large surface area of leaves enhance evapotranspiration and help dispose of excess water (Forstner, Salomons 1988).
While some wetlands handle large volumes of water, over 100 gallons per minute, most systems handle smaller flows of 5-20 gallons per minute (Perry 1992). The most effective wetlands have 15 square meters of wetland for every liter per minute of flow (Frostman 1993). Wetlands are commonly constructed with a layer of limestone 8-16 cm thick, which produces the necessary neutralization of acids. A 15-45 cm thick layer of organic substrate, in which plants root, is the significant component of the wetland system (Frostman 1995).
Open limestone channels are an important innovation in acid mine drainage treatment. The channels are created by filling drains or lining stream beds with high quality limestone. Results from field sites show that acid and metals in acid drainage were reduced by 25 to 40% even when the limestone became coated by iron and aluminum (Mehrotra, Singhal 1992). Most sites still require chemical treatment to meet effluent standards, however, the costs of the chemical treatment decrease with the initial biological treatment (Kleinmann, Perry 1991). Unfortunately, many of the long term benefits of the cleaning activity of wetlands will not be seen for hundreds of years. The construction of more conventional treatment plants at each mine site would not be economical, nor would it add to the already degraded aesthetics of the site.
In preventing acid drainage, water and air contact with the acidic material must be eliminated. Prevention effectiveness depends on the nature of the mine and the strata's geological characteristics.
Preventing water from reaching underground mines involves the use of diversion ditches and pipes to divert water from acidic areas. Another method to prevent acid drainage is to prevent the material from oxidizing. By burying mine waste, or covering the waste with an impermeable liner, pyrite cannot oxidize and sulfuric acid cannot form. Some mines use a method in which an asphalt emulsion of polyurethane sealant encapsulates the mine waste (Mehrotra, Singhal 1992).
For economic reasons, treatment is the most practical solution to acid drainage pollution.
The face of acid mine drainage treatment is changing. With research and new technology, mines are moving away from the more conventional forms of treatment and towards the use of ecologically engineered wetlands. Unfortunately, most acid pollution stems from abandoned mines, where the responsible parties are difficult to locate. As a result, local and federal government agencies assume the financial responsibility of remediation. New effluent limits have helped curb the onslaught of future damage, but much work remains to be done. The costs of abatement of acid mine drainage are high, but the benefits to society and the environment are far greater in the long run.
Photo credits to Dr. Carl Zipper and the Powell Creek Project
Student authors: Judi Todd and Kelli Reddick
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
