During the various stages involved in mining, there are a number of byproducts created. Some of these materials can be relatively nontoxic, while others must be treated or disposed of in a manner that will prevent damage to human health or the environment. These materials are generally referred to as waste products and fall into two major categories: mine water and solid wastes.
Mine water refers to the groundwater or surface water that accumulates in mines. When mines are abandoned, they often flood with groundwater and discharge at a mine adit or seep into groundwater sources. The discharges can contain metals and are often very acidic; this is called acid mine drainage. Contemporary mining operations use pumps to deal with the large quantities of water that infiltrate the mine cavities and need a permit to discharge the water.
- Overburden – Overburden is the topsoil and surface rock that is directly above a mineral deposit. This material is removed to get at the ore body and is usually discarded or, in modern times, stored until it can be reused during the reclamation process.
- Waste rock – Waste rock is the low grade ore and other rock that surrounds a mineral deposit. The size and type of waste rock depends on the geology of the area. In the past, it was usually discarded on hillsides or in creek drainages. Waste rock piles may have traces of minerals and can be acid generating.
- Tailings – Tailings are fine grained materials that result from crushing and processing raw ore. Tailings often contain low concentrations of minerals, as well as residue from chemical agents. Because of their high moisture content and lack of vegetation, they can also be instable and erode easily.
Traditional mining operations simply did not consider the long term impact that their actions would have on the land. There were no regulations or standards to guide their activities and no concept of reclamation or restoration. In their struggle to make a living, they denuded forests, dammed streams, and unearthed rock, metals, and other materials that had been buried for millennia. This disturbance of surface and underground resources has resulted in a number of environmental problems, including the following:
- Acid Mine Drainage (AMD) - Acid mine drainage refers to the yellowish-orange water that flows out of old mines. This toxic discharge results from a chemical reaction that occurs when metal sulfides, such as iron pyrite (fool’s gold), are exposed to air and water. The most common solution formed is sulfuric acid, which dissolves other metals like arsenic, cadmium, chromium, and lead and carries them into streams or groundwater. The acid discharges can have a PH level comparable to battery acid and kill vegetation, as well as fish and other aquatic organisms. This same process can occur when rain or surface water seeps through waste rock or tailings piles near the mine. This latter process is often called Acid Rock Drainage (ARD).
- Erosion/sedimentation - Erosion occurs when the earth’s surface is worn away by water or wind. Abandoned mine lands are often susceptible to erosion and sedimentation because of waste rock piles, roads, leach pads, and other unvegetated areas. Of particular concern are tailings piles which expose mounds of find grained sediment to the elements. As soil particles are washed into a stream, sedimentation occurs as they drop to the streambed and cover rocks and vegetation. Sediment changes the light, temperature, and oxygen conditions of a waterway and can bury trout eggs or young, smother vegetation, and impair the growth of other aquatic organisms. Sediment can also fill pools, destroy stream channels, and lead to greater risks of flooding.
- Contamination groundwater and surface water - The formation of acid mine drainage is not necessary for metals to contaminate groundwater or surface water. Metals can be dissolved in water sources regardless of the water’s PH level. High metal concentrations in water can be toxic to plants and wildlife. They can also bioaccumulate in fish tissues and can be passed on to humans and other animals through the food chain. Open-pit mines, tunnels, and other mine workings can also be a direct threat to groundwater contamination when they extend below the water table. When these areas fill up with water, they can lower the water table and contribute to dewatered streams, springs, and wells.
- Chemical agents - Cyanide, mercury, and sulfuric acid have all been used in the processing stage to separate valuable minerals from raw ore. Although these chemical agents are more of a problem in active operations, they can be found in tailings piles and waste rock piles, where they can leach into the ground or surface water. Containers holding these chemical agents, or even petroleum products, may have been buried underground and can leak into water sources. Containment ponds at cyanide mines have also been known to kill visiting waterfowl.
- Air pollution - Dust and sediment from crushing and loading operations, roads, or explosives can carry heavy metals and contaminate nearby topsoil. Asbestos, arsenic, lead, and other toxic metals are also exposed in waste piles and can be carried by wind or disturbed by off road vehicles. At old smelters and refining sites, the surrounding topsoil may be contaminated with several inches of toxic material.
Public Health and Safety
Beyond their damaging impact on the environment, abandoned mine sites create a number of serious threats to public health and safety. The following problems may be present at an abandoned mine site near your community:
- Accidents - Abandoned mine shafts and other structures are a proven public safety hazard. Hikers, hunters, and four wheelers sometime fall into these pits, causing death or serious injury. Highwalls, or cliffs that are left over from strip mining activity, are a similar threat. Water often accumulates at the base of highwalls and can present a drowning risk.
- Water – Drinking water supplies can be contaminated due to contact with heavy metals or chemical agents used in processing. Furthermore, when mine workings are abandoned, they can fill up with groundwater, thereby lowering the local water table, causing problems for irrigation systems and livestock wells.
- Subsidence - Subsidence is caused by large cavities that are left by underground mining operations. Over time, these large rooms or tunnels can weaken and collapse. They are a threat to homes, roads, or buildings that are unknowingly built above the mine workings.
- Fires - Trash and debris can accumulates in open pits or mine portals and catch on fire. Especially problematic at coal mines, coal veins can then catch on fire and burn through the vein underground.
- Air pollution - Tailings piles and topsoil at abandoned mine sites sometimes contain lead, arsenic, and other minerals which can become airborne pollutants due to wind or other disturbances.
Impacts on Fish
There are numerous impacts to fish species downstream of abandoned mine sites, including:
Acid Mine Drainage - Acid mine drainage harms their respiratory function, and low PH can impact reproduction rates and rearing success. Low PH can also kill aquatic plants and macroinvertebrates, thereby diminishing important food sources and disrupting the natural food chain.
Heavy metals contamination - Dissolve heavy metals can bioaccumulate in trout and salmon and collect on their gills, causing respiratory problems. Some metals have been shown to severely impact the juvenile salmon life cycle and limit growth rates. Furthermore, cutthroat trout show poor feeding activity when exposed to metal laden macroinvertebrates, and brown trout have avoided metal contaminated water in laboratory experiments.
Sedimentation - Sediment from waste rock and tailings piles can cover spawning beds and smother juvenile trout. Sediment can also raise the water temperature, decrease oxygen supplies, and block the sun from reaching underwater vegetation.
- In Colorado, there are 20,299 abandoned mine sites and 1,300 miles of adversely affected streams! As one example, mining waste has killed 20 miles of the Animas River fishery in southwest Colorado.with the nearby molybdenum mine.
- In New Mexico, at least eight miles of the Red River's aquatic life (including its trout) have been decimated by heavy metal waste associated with the nearby molybdenum mine.
- In Montana, a tailings dam in the headwaters of the Blackfoot River breached in 1975 and sent sever thousand tons of mine waste into the river. The toxic material has been traced as far as 16 miles downstream and killed all aquatic life in the first ten miles of the river.