9.5 Environmental Impacts of Metallic Mineral Mining
Carolina Londono Michel
The true cost of mining
Mining comes with a price. We are not talking about the economic investment that companies do, or the price that consumers along the chain pay for the metals. We are talking about the cost to ecosystems, Earth systems, and even social systems that are paid in the places where mining is developed. The impacts can be so large and last for so long that we cannot calculate them in terms of currency.
Environmental impacts caused by mining include soil destruction, erosion, formation of sinkholes, loss of biodiversity, and contamination of soil, groundwater and/or surface water by chemicals released during the mining processes. In some cases, miners log the forests near mines to create space to store the created debris and soil. Often, miners need to use adjacent water sources to process the ore. Contamination from leakage of chemicals can also affect the health of the local population, if not properly controlled. Extreme examples of pollution from mining activities include breaking of dams containing toxic water that flood villages living downstream or contaminate waterways killing the fish and lending the water poisonous, or coal fires, which can last for years or even decades, producing massive amounts of environmental damage. [4]
Mining companies in most countries are required to follow stringent environmental and rehabilitation codes in order to minimize environmental impact and avoid impacting human health. These codes and regulations all require the common steps of environmental impact assessment, development of environmental management plans, mine closure planning (which must be done before the start of mining operations), and environmental monitoring during operation and after closure. However, in some areas, particularly in the developing world, government regulations may not be enforced or it may be hard to hold large, multinational companies accountable. [4]
Mine waste: Tailings
To extract the ore from rock, ore mills need to crush large volumes of rock. This generates piles of non-economic material, a form of “waste” called tailings. For example, for each ton of copper, 99 tons of waste are generated, and the amount of waste is larger for gold and silver. Tailings can be toxic. Tailings are usually produced as a slurry (mixed with water) and are most commonly dumped into ponds made from naturally existing valleys. These tailing ponds are secured by impoundments (dams or embankment dams). In 2000, it was estimated that 3,500 tailings impoundments existed and that every year, 2 to 5 major failures and 35 minor failures occurred; for example, in the Marcopper mining disaster, at least 2 million tons of tailings were released into a local river. The tailings and the waste rock at most mines are an environmental liability because they contain pyrite, FeS2, plus small amounts of ore minerals. Thus, besides dam failure, tailings can produce acid drainage. Tailings ponds and waste-rock storage piles must be carefully maintained to ensure their integrity and monitored to ensure that acidic and metal-rich water is not leaking out.
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Acid drainage
The primary impact of metallic mineral mining comes from the mining itself, including disturbance of the land surface, covering landscapes with tailings impoundments, and increased mass wasting by sped up erosion. In addition, many metal deposits contain pyrite, an uneconomic sulfide mineral placed on waste dumps, which may generate acid rock drainage during weathering. In the presence of oxygenated water, sulfides undergo complex reactions to release metal ions and hydrogen ions, lowering pH to highly acidic levels. Mining and processing of mined materials typically speeds up reactions. If not managed properly, these reactions may lead to acidification of streams and groundwater plumes that can carry dissolved toxic metals.
In mines where limestone is a waste rock or carbonate minerals like calcite or dolomite are present, their acid-neutralizing potential helps reduce the likelihood of generating acid drainage. This happens because the carbonate ion in calcite and dolomite can capture the hydrogens (acidity) generated by the sulfides. Thus, the pH can be close to neutral. Although acid drainage and the neutralization by lime are natural processes, it is very important to isolate mine dumps and tailings from water, both to prevent the dissolution of pyrite and the subsequent percolation of the sulfate-rich water into waterways. The industry has taken great strides in preventing contamination in recent decades, but earlier mining projects are still causing problems with local ecosystems. [3]
What is the chemistry of Acid Mine Drainage?
| pyrite(solid) + oxygen + water ↔︎ iron hydroxide(solid) + sulfate ion + acid ions
FeS2 (s) + 15/4 O2 + 7/2 H2O ↔︎ Fe (OH)3(s) + 2 SO4– + 4 H+
|
Notice that each pyrite mineral would produce four hydrogens, which are the ions that decrease the pH, creating acid conditions.
Note: we use pyrite as an example because it is very common and one of the main contributors to acidity in waste rocks of metal mining. But a host of other chemical processes can produce acid mine drainage.
Now let’s look at what happens when we add calcite to counter the acid ions:
| Calcite(solid) + acid ions ↔︎ calcium + water + carbon dioxide
2 CaCO3(s) + 4 H+ ↔︎ 2Ca2+ + 2 H2O + 2 CO2
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What happened to the hydrogens?
What happened to the calcite?
Superfund sites
Source: Environmental Protection Agency.
In the US, the improper, or lack there of, management of hazardous waste has left thousands of contaminated sites accross the nation. Mining byproducts have been dumped, left out in the open, or poorly managed. These contaminated sites include manufacturing facilities, processing plants, landfills, and mining sites. Common contaminants include lead, asbestos, dioxin, and radiation wast.
In the late 1970s, toxic waste dumps such as Love Canal and Valley of the Drums received national attention when the public learned about the risks to human health and the environment posed by contaminated sites.
- Protect human health and the environment by cleaning up contaminated sites;
- Make responsible parties pay for cleanup work;
- Involve communities in the Superfund process; and
- Return Superfund sites to productive use.
Environmental regulation of copper mining in Arizona
The main environmental protection agencies which govern a mine’s potential to contaminate the local environment include the Arizona Department of Environmental Quality (ADEQ) and the United States Environmental Protection Agency (US EPA). These two agencies, as well as other county or local agencies, ensure that operating and closed mines do not release contaminated or hazardous materials outside of the mine site. Hazardous materials might leave a mine site through wind, which can carry dust; rain, which can flow in washes and streams; and in the groundwater flow, which can contaminate local sources of drinking water. If hazardous materials or contaminated water were to leave a mine site, mine owners could face very large fines daily, be rejected for future permits, and even face time in jail.
Mines on reservations must meet environmental quality standards set out by the respective reservations. For instance, the Navajo Nation Environmental Protection Agency (NNEPA) has well-defined water and air quality standards, which the mines must comply with. Many of the laws in NNEPA are modeled after the US EPA; companies working in such areas often follow the governing body with the strictest policies to ensure adequate environmental compliance. If there is no formal tribal environmental protection agency, the mines will be governed by the US EPA. Typically, mining companies will have environmental engineers on staff at the site or use environmental consulting firms to interact with the regulatory agencies.
The Concerns of the Apache Tribe over Mining
Throughout history, tribes have faced displacement, discrimination, and marginalization due to mining on their lands (Ballard, 2003). Environmental health is an important concern for communities living near mine sites. The impacts of mining on sacred and ancestral lands are of concern for tribal communities. Although U.S. laws mostly protect sacred lands on and off tribal reservations, there are still potential risks for loss. For example, traditional livelihoods may be limited due to lack of access to land and/or destruction of important resources (e.g., mountains, vegetation, wildlife). Tribal communities often rely on natural resources found on sacred lands for cultural, medicinal, and spiritual purposes. For example, in the Navajo Nation in northeastern Arizona and southeastern Utah, Navajo people living in and near uranium mining areas used mill tailings, a sandy waste containing heavy metals and radium, which is radioactive, to build their traditional earthen homes (hogan), many of which remain in use today (DOE, 2013). Another example is the nearly 100 sacred and cultural sites of the Tohono O’odham Nation, which may be impacted by the proposed development of the Rosemont Copper Mine in southern Arizona (Tohono O’odham, 2009). A last example is the Oak Flat area east of Superior, AZ. Lands sacred to the San Carlos Apache tribe, where Resolution Copper is proposing to mine (Allen, 2015). Innovative mining companies implementing responsible mining have recognized the need for more respectful relationships with tribal nations to ensure that when mining is undertaken, the rights and interests of the People are considered.
Rehabilitation of mined areas
After the mining finishes, the mine area must undergo rehabilitation. Waste dumps are contoured to flatten them out, to further stabilize them. If the ore contains sulfides (e.g., pyrite), engineers usually cover it with a layer of clay to prevent access of rain and oxygen from the air, which can oxidize the sulfides to produce sulfuric acid, acid mine drainage. Then they cover this layer with soil and plant vegetation to help consolidate the material. Eventually, the protective layer will erode. But engineers hope that the rate of acid leaching will be slowed down such that the environment can handle the load of acid and associated heavy metals. There are no long-term studies on the success of these covers due to the relatively short time in which large-scale open pit mining has existed. It may take hundreds to thousands of years for some waste dumps to become “acid neutral” and stop leaching into the environment. They usually fence the dumps off to prevent livestock from denuding them of vegetation. The open-pit is then fenced, to prevent access, and it eventually fills up with groundwater. In arid areas, it may not fill due to deep groundwater levels.
CaCO3
A carbonate mineral that strongly reacts with dilute acid. Calcite composes the sedimentary rock limestone and composes the skeletons of some ocean life.
A carbonate mineral, CaMg(CO3)2, that is more resistant to weathering by acid at room temperatures. Dolomite often composes the material of fossilized shells.
