Material adapted from: Hudson, T.L, Fox, F.D., and Plumlee, G.S. 1999. Metal Mining and the Environment, p. 11,41-46. Published by the American Geosciences Institute Environmental Awareness Series. Click here to download the full handbook.
The major potential environmental impacts associated with mining and associated mineral processing operations are related to erosion-prone landscapes, soil and water quality, and air quality. These potential impacts are recognized and addressed in current mining operations as well as in some former mining operations by reclaiming areas of physical disturbance to prevent erosion, stabilizing soils containing metals or chemicals to prevent unwanted metal releases into the environment, preventing and/or treating water contamination, and controlling air emissions.
At many sites, the key reclamation, soil treatment, and water quality concerns owe their origin to the same process — the oxidation of sulfide minerals, especially the iron sulfide, pyrite. Oxidation of sulfide minerals can produce acidic conditions that release metals in both waste materials and water.
Mining in the early days took place at a time when environmental impacts were not as well understood and, most importantly, not a matter of significant concern. As a result, historical mine sites may still have areas that are not reclaimed, remnants of facilities, and untreated water. This inherited legacy of environmental damage from mining is not indicative of the mining cycle today.
Now, mine closure and a number of activities to mitigate the impacts of mining are an integral part of all metal mine planning and mineral development from the discovery phase through to closure:
Reclamation entails the re-establishing of viable soils and vegetation at a mine site. Although regulatory agencies may require complex reclamation designs, simple approaches can be very effective. One simple approach depends on adding lime or other materials that will neutralize acidity plus a cover of top soil or suitable growth medium to promote vegetation growth. Modifying slopes and other surfaces and planting vegetation as part of the process stabilizes the soil material and prevents erosion and surface water infiltration. Even this simple approach is likely to cost a few thousand dollars per acre to implement. Where soils have a sustained high acidity, the costs of using this approach can increase, sometimes to tens of thousands of dollars per acre. The challenge to find cost-effective reclamation approaches continues.
Promising reclamation options in the future may include using sludge, “biosolids,” from municipal waste water treatment processes as an organic soil amendment, and growing plant species that are more tolerant of acidic conditions.
High levels of metals in soils, not just acidity, can be harmful to plants, animals, and, in some cases, people. A common approach used in dealing with contaminated soil is to move it to specially designed repositories. This approach can be very expensive and controversial, but it is sometimes required. With this approach, the volume and toxicity of the soil is not reduced, the soil is just relocated. Effective soil treatment approaches in the future depend upon better understanding of the risks associated with metals in mine wastes. These “natural” metals in minerals may not be as readily available in the biosphere, and therefore, they may not be as toxic as the metals in processed forms, such as lead in gasoline.
Future approaches may include:
- Using chemical methods to stabilize metals in soils, making them less mobile and biologically available.
- Using bacteriacides that stop the bacterial growth that promotes the oxidation of pyrite and the accompanying formation of sulfuric acid.
- Using bioliners, such as low permeability and compacted manure, as barriers at the base of waste piles.
- Permanently flooding waste materials containing pyrite to cut off the source of oxygen, stop the development of acidic conditions, and prevent mobilization of metals.
The most common treatment for acidic and metal-bearing waters is the addition of a neutralizing material, such as lime, to reduce the acidity. This “active” treatment process, which causes the dissolved metals to precipitate from the water, usually requires the construction of a treatment facility. The ongoing maintenance that such a plant requires makes this treatment technique very expensive.
Aside from the expense, some active treatment plants generate large amounts of sludge. Disposal of the sludge is a major problem. Because of the cost and the physical challenges of dealing with sludge, alternatives to active treatment facilities are needed. Some possible alternatives include:
- Using “passive” wetland systems to treat metal-bearing water. This approach has been successfully used where the volumes and acidity of the water are not too great. Passive wetland systems have the added advantage of creating desirable wildlife habitat.
- Using in-situ treatment zones where reactive materials or electric currents are placed in the subsurface so that water passing through them would be treated.
- Combining treatment with the recovery of useful materials from contaminated water.
Although the discharge of acidic drainage presents several challenges to protecting water quality, the significance and widespread occurrence of acid rock drainage warrant special efforts to prevent or minimize its occurrence. Prevention must be addressed during exploration activities, before the beginning of newly-planned mining operations. In some cases, it may even be possible to prevent or reduce acid rock drainage in old or abandoned mining areas. Current and potential treatment approaches for acid rock drainage are similar to those already described. Possible measures to prevent or significantly reduce acid rock drainage include:
- Flooding of old underground mine workings to cut off the oxygen supply necessary to the sustained generation of acidic waters.
- Sealing exposed surfaces in underground workings with a coating of material that is non-reactive or impermeable to inhibit the oxidation process.
- Backfilling mine workings with reactive materials that can neutralize and treat waters that pass through them.
- Adding chemicals to the water in flooded surface and underground mine workings that can inhibit acid-generating chemical reactions and precipitate coatings that will seal off groundwater migration routes.
- Isolating contaminated waters at depth by stratification, allowing viable habitat to develop near the surface in the water that fills large open pits.
Smelter emissions, especially sulfur dioxide and particulate materials, have historically presented significant environmental problems. Modern smelting technology has met this challenge by drastically reducing the amount of emissions. An example is the modernized smelter built by Kennecott Utah Copper that processes ore concentrates from the Bingham Canyon Mine near Salt Lake City. Using technology developed by the Finnish company Outokumpu, this smelter has reduced sulfur dioxide emissions to 95 percent of previous permitted levels. This smelter, which came online in 1995, is the cleanest in the world. It captures 99.9 percent of the emitted sulfur.
- Metal Mining and the Environment (Booklet), American Geosciences Institute
Provides basic information about the mining cycle, from exploration for economic mineral deposits to mine closure. The booklet discusses the environmental aspects of metal mining and illustrates the ways science and technology assist in preventing or reducing environmental impacts.
- No Silver Bullet, But a Silver Lining (Webpage), U.S. Geological Survey
Short description of how USGS mineral science shows promise for reducing mining impacts
- Regulatory Information by Sector: Mining (Except Oil and Gas) (Webpage), Environmental Protection Agency
Links to information on laws and regulations that regulate mining activities related to air quality, asbestos, waste, and water.
- Contact your state mining agency: Links to State Mining Agencies, Mine Safety and Health Administration