JAMES A. JACOBS AND STEPHEN M. TESTA
Introduction
The presence of acid mine drainage has the potential, and under certain conditions has actually devastated rivers, streams, and aquatic life for a very long time. Mineral resources such as coal, and metal ores such as gold, silver, and copper, are often rich in sulfide minerals, reflecting rock or sediment environments generally high in sulfur content and low or devoid of free molecular oxygen. Once exposed to water and air during mining, pyrite and other iron sulfide rocks release sulfuric acid in the presence of extremely acidophilic microorganisms. These complex interactions occur in microbial communities of autotrophic and heterotrophic bacteria and archaea which catalyze iron and sulfur oxidation, determining the release rates of metals and sulfur to the environment as acid mine drainage (Baker and Banfield, 2003). Even eukaryotic life forms (fungi and yeasts, protozoans, microalgae, and rotifera) may be part of microbial communities present in low-pH environments. Although the primary aerobic iron- and sulfur-oxidizing bacteria have been studied for decades, more recently, DNA analysis and genetic studies have identified some archaea, and even a few eukaryotes, to be present in the microbial community in the extremely low-pH acid mine drainage environments studied (Baker and Banfield, 2003). The ecology and biodiversity of acid mine and rock drainage microbial communities have been well documented by Baker et al. (2004, 2009) and Rawlings and Johnson (2002, 2007).
Once the sulfuric acid is created, the pyrite dissolves in drainage water, releasing associated metals and metalloids such as aluminum or arsenic into the surrounding environment. Wherever iron sulfides are exposed, such conditions can occur: open pits, underground excavations, leach pads, and tailing and waste rock piles.
Contaminated water flowing from abandoned coal mines is one of the most significant contributors to water pollution in former and current coal-producing areas. Acid mine drainage can have severe impacts on aquatic resources, can stunt terrestrial plant growth and harm wetlands, can contaminate groundwater, can raise water treatment costs, and can damage concrete and metal structures. In the Appalachian Mountains of the eastern United States alone, more than 7500 miles of streams are affected. The Pennsylvania Fish and Boat Commission estimates that the economic losses on fisheries and recreational uses are approximately $67 million annually. While most modern coal-mining operations must meet strict environmental regulations concerning mining techniques and treatment practices, there are thousands of abandoned mine sites in the United States.
Treatment of a single site can result in the restoration of several miles of affected streams. Acid runoff from the Summitville mine in Colorado, a designated federal Superfund site, killed all biological life in a 17-mile stretch of the Alamosa River. Acid and metals in runoff from the mining of molybdenum at the Questa mine in New Mexico adversely affected biological life along 8 miles of the Red River. The effect on the environment can be severe. Streams and surface water bodies with a pH of 4.0 or lower can be devastating to fish, animals, and plant life. Once started, the process becomes very difficult to stop and can occur indefinitely, requiring mitigation and water treatment long after mining ends—in perpetuity. Along with countless other mines throughout the world featuring serious long-term environmental impacts, acid drainage at the Golden Sunlight mine is estimated to continue for thousands of years.
Common iron sulfide minerals, primarily pyrite (FeS2), but also marcasite (FeS2), arsenopyrite (FeAsS), and chalcopyrite (CuFeS2), are exposed to the oxygen in the atmosphere during mining, excavation, or through natural erosion processes, and the compounds react with oxygen and water to form sulfate, resulting in acid drainage. This acidity results from the action of extremely acidophilic bacteria, which generate their energy by oxidizing ferrous iron [Fe(II) or Fe2+] to ferric iron [Fe(III) or Fe3+] using oxygen for cellular respiration. The ferric iron, in turn, dissolves the pyrite to produce soluble ferrous iron and sulfate. The ferrous iron is then available for oxidation by the aerobic acidophilic microbes, which scavenge dissolved oxygen in the pore space or water column. This biogeochemical cycle continues until the iron sulfide mineral (e.g., pyrite) is dissolved.
The oxidation of pyritic sulfur is a heat-generating reaction. In coal seams the pyrite oxidation reaction is sufficiently exothermic that mined-out areas in underground coal mines in high-sulfur-content coal seams have been documented to have had spontaneous combustion. The heat generated from the oxidation of pyritic sulfur can increase the temperature of the surrounding coal, increasing the rate of oxidation and causing coal degradation to occur (Smith et al., 1996). Excluding oxygen from the air in mined-out areas is the theoretical solution to spontaneous combustion related to pyrite oxidation, but in practice it is difficult to hermetically seal mined-out areas to exclude oxygen. Heat from the reaction can occur not only in subsurface mines but also in mine waste rock piles, where the heat is dissipated by thermal conduction or convection. Stability analysis of mine waste rock indicates that convective flow can occur because of the high porosity of the material. Convection cells formed in waste rock would draw in atmospheric air with oxygen and continue to drive the oxidation reaction. Convection gas flow due to the oxidation of sulfide minerals depends on the maximum temperature in the waste rock. The maximum temperature depends on the ambient atmospheric temperature, the strength of the heat source, and the nature of the upper boundary. If the sulfide waste is concentrated in one area, as is the case with encapsulation, the heat source may be very strong (U.S. Environmental Protection Agency, 1994). Due to the exothermic nature of the oxidation process, removing the oxygen fro...
