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Hardrock Mining: Environmental
Hardrock mining is a large-scale industrial activity that takes place in the natural environment, potentially disturbing large amounts of material and land area. Hardrock mining generates large volumes of mining waste because of the high waste-to-product ratios associated with producing most ores. This page describes some of the potential environmental effects of hardrock mining.
EPA recognizes that some of the discussion on this page may not accurately reflect the environmental conditions at modern hardrock mining operations that are well-designed, well-operated, and well-regulated. The intent of this discussion is to highlight environmental problems at (predominantly historic) mining sites and to indicate the potential problems that could occur at existing and future sites.
Acid Mine Drainage
Acid mine drainage is the drainage that results from sulfide oxidation in rocks exposed to air and water. Metal sulfide minerals are common constituents in the rocks associated with metal mining activity. Before mining, oxidation of these minerals and the formation of sulfuric acid is a slow function of natual weathering processes. Natural discharges from such deposits pose little threat to aquatic ecosystems except in rare instances. Mining and benefication operations greatly increase the rate of these same chemical reactions by removing sulfide rock material and exposing the material to air and water.
Acid generation primarily results from the oxidation of metallic sulfides. The major metallic sulfide of concern is iron sulfide (FeS2) or pyrite. Other metal sulfides that contribute to acid generation include lead sulfide (galena), zinc sulfide (sphalerite), and iron copper sulfide (chalcopyrite).
Both water and oxygen are necessary to generate acid drainage. Water serves as both a reactant and a medium for the bacteria to catalyze the oxidation process and transports the oxidation products. Oxygen is particularly important to maintain bacterially-catalyzed oxidation at pH values below 3.5
During acid generation, the pH values of the associated waters typically decrease to values near 2.5. These conditions result in the dissolution of the minerals associated with the metallic sulfides and release of toxic metal cations (e.g., lead, copper, silver, manganese, cadmium, iron, and zinc). In addition, the concentration of dissolved anions (e.g., sulfate) also increases.
Acid generation and drainage affect both surface water and groundwater. The sources of surface water contamination are leachate from mine openings, seepage and discharges from waste rock, tailings, ground water seepage, and surface water runoff from waste rock and tailings piles. Mined materials such as waste rock or tailings used for construction or other purposes (e.g., road beds, rock drains, and fill material) can also develop acid mine drainage.
The receptors of contaminated surface water include birds, fish, and other aquatic organisims. Humans can also be affected by direct ingestion of contaminated surface water or direct contact through outdoor activities such as swimming.
There are no easy or inexpensive solutions to the problem of acid mine drainage. Two primary approaches to addressing acid generation are:
1. Avoiding mining deposits with high acid generating potential
2. Isolating or otherwise special-handling wastes with acid generating potential
In practice, avoiding mining in areas with acid generating potential may be difficult because of widespread distribution of sulfide minerals. Isolation of materials with the potential to generate acids is now being tried as a means of reducing the perpetual effects of mining waste on surface water and groundwater. Control of the material can be implemented by preventing or minimizing its contact with oxygen, preventng its contact with water, and/or ensuring that an adequate amount of natural or introduced material is available to neutralize any acid produced. Techniques used to isolate acid generating materials include subaqueous disposal, covers, waste blending, hydrologic controls, bacterial controls, and treatment.
Erosion and Sedimentation
Erosion is the process by which soil particles are detached, suspended, and transported from their source of origin. Erosion can be caused by water, primarily through direct impact with raindrops and precipitation run-off, or by wind in arid environments. Sedimentation occurs when eroded particles are deposited at a different location than the source of origin.
The extent of erosion and sedimentation depends on various factors, including the degree at which the surface has been disturbed, the prevalence of a vegetative cover, the type of soil, the slope length, and the degree of slope. Disturbed areas with little or no vegetative cover, soil high in silt, or a steep slope are the areas most likely to erode.
Erosion and sedimentation affect surface water and wetlands more than any other media. Erosion can also adversely affect soil organisms, vegetation, and revegetation efforts because it results in the movement of soil, including topsoil and nutrients, from one location to another.
Cyanide and Other Chemical Releases
The mining industry has a long history of cyanide use. For decades, cyanide has been used as a pyrite depressant in base metal flotation. It has also been used for over a century for gold extraction. After cyanide leaching of gold heaps proved feasible in the 1970s, the relatively high price of gold has made cyanide leaching of relatively low-grade ores economically feasible. Figure 2 illustrates cyanide.
Cyanide exists in many forms, depending on the starting compound and environmental conditions. The most common cyanide compound used in mining is sodium cyanide (NaCN).
Cyanide released into the environment can adversely impact water, soil, aquatic organisms, wildlife, waterfowl, and humans. Cyanide-contaminated solution in tailing ponds and solution retention basins has proven to be attractive to unsuspecting waterfowl and wildlife. These organisms have suffered both acute and chronic poisoning as a result of direct contact with and ingestion of cyanide-contaminated solution. Leakage from linear failure at heaps can allow the release of cyanide and other toxic constituents directly into the environment.
Other chemicals used during the beneficiation process, stored on-site for use, or used in vehicles and equipment can impact human health and the environment if released. These chemicals include oil, petroleum products, solvents, acids, and reagents.
Control and Remediation