Environmental Consequences of Open Pit Mining

An open pit mine is a type of excavation where surface and subsurface materials (soil and rock) are removed, typically through use of explosives and mechanical mining and hauling equipment, to gain access to commercially valuable ores or other buried natural resources, including coal and gemstones, that are relatively close to the surface. The mined material may be defused throughout the rock, contained in veins, or concentrated in layers. The pit (also known as borrow) is “open” to the atmosphere for as long as the mine continues in operation. Waste overburden may be piled in a dump at the surface, near the edge of the open pit, or deposited in adjacent valleys. Processed rock from which the ore has been removed is known as tailings and is normally pumped as a slurry to a nearby tailings or settling pond, where the water eventually evaporates, leaving behind hazardous materials. 

Author’s Note: A quarry is an open-pit mine that produces building materials and dimension stone, such as granite, quartzite, marble, slate, bluestone, limestone, or sandstone, etc.

Historically, the far greater majority of open pit mines have been constructed as an exercise in economics constrained by geologic and mining engineering principles with little or no consideration given to the environment in which the mining occurs. Even today, many American mining operations pay little more than lip service to environmental protection or restoration or mitigation.

By its very nature, open pit mining (also known as strip mining and opencast mining) is environmentally destructive. Surface mining operations, especially those that are large-scale, always alter and disturb the Earth’s surface, even if what are usually euphemistically known as “effective” mitigation measures are applied and the site is restored to a condition said to “approach” or “resemble” its natural state. That disturbance, in turn, has numerous direct, indirect, short- and long-term potentially adverse effects on the landscape and on nearby human communities, including the following and other factors too numerous to detail in this modest definition. Readers who desire a more detailed examination of the adverse effects of mining in general should consult three previous posts on this blog on the effects of underground mining (see posts on March 5, 6, and 7).

Topographic modifications: including the large-scale removal of soil, vegetation, and overburden to access ore or other mineral deposits and create nearby sites for tailings storage, water storage dams and reservoirs, berms, and waste disposal pits. Those alterations can be caused by the construction of exploration or access roads and mining-smelting infrastructure—such as cranes, hoists, conveyor systems, buildings, electrical substations and distribution lines, power generating equipment and facilities, workshops, showers and decontamination facilities, testing labs, wastewater treatment facilities, offices, parking-vehicle storage areas, material storage, and smelters, etc.—the use of mechanized equipment such as off-road vehicles, drill rigs, or seismic exploration vehicles and construction-operation of mining activities in previously remote, roadless, mountainous regions, high latitudes, or wetlands where human activity had resulted in little alteration of relatively pristine ecosystems. Perhaps the most egregious and horrific example of topographic modifications in strip mining is the mountaintop removal/valley filling coal mining process currently ongoing in parts of Appalachia that leaves behind a brutally scarred, environmentally sterile moonscape that destroys human settlement patterns as well as habitats and ecosystems.


Author’s Note: A common topographic alteration to the landscape in open pit mines is an engineered dam/dyke system known as a tailing pond/pit, which is used to store mining refuse/gangue produced by the separation of economically valuable materials from the uneconomic part of an ore. In a great many instances, the containment structures comprising these ponds are not meant to be permanent and may be constructed of compacted earth, forming what are embankment dams. In areas of seasonally high rainfall, those structures may experience greater volumes of water than was anticipated and fail, often catastrophically, flooding areas downstream with a slurry of mud-like materials and water, resulting in loss of life and property. Here's an example. The Mount Polley open pit copper/gold mine, covering 18,892 hectares or nearly 46,700 acres in south-central British Columbia is operated by Vancouver-based Imperial Metals. The mill processes 20,000 tons of rock per day. In early August, 2014, a tailings pond dam at the mine failed, releasing at least 10 million cubic meters of contaminated water and waste rock into a local creek, expanding its width from four feet to 150 feet before entering nearby Quesnel Lake and its connected waterways, which are important habitats for Chinook and Sockeye Salmon as well as Rainbow Trout and White Sturgeon. The pond contained tailings from the mining process and were contaminated with arsenic, cadmium, copper, lead, and mercury, among other toxins and heavy metals that are hazardous to life. Another very recent example is the Samarco iron ore mine in Minas Gerais, Brazil, a joint venture between the Anglo-Australian mining giant, BHP Billiton, and Brazil’s biggest iron ore miner, Vale SA, which failed in early November 2015, releasing 81 million cubic yards of mining waste, killing perhaps 20 people, destroying nearby villages housing more than 600 people, polluting drinking water over an enormous area, and adversely affecting the Rio Doce, which is one of Brazil's most important rivers, and the adjacent landscape for more than 300 miles downstream. Not to mention the issue that mining companies love to gloss over in their press releases meant to reassure the affected public: residues from the iron mining operations and the consequent flooding are toxic and may be very hazardous to human and animal health. According to Brazil’s environmental agency, IBAMA, 50 million metric tons of toxic sludge from the mine dam failure are, at the end of December 2015, spreading along 30 miles of coastal beaches between the states of Rio de Janeiro and Bahia, turning the pristine blue waters brown and sliming the beaches themselves. When tailings impoundments are abandoned, improperly remediated, and gradually dry up, the resulting dust, which can and often does consist of toxic materials rather than simple mud/dirt particulates, can cover the adjacent environment and nearby settlements, increasing risks to life and health, including respiratory, skin, gastrointestinal (from inadvertent ingestion) and other adverse effects that can include different types of cancer.

Soils: changes in characteristics through accelerated wind and water erosion, sharply increased acidity and salt content, development of nutrient deficiencies or imbalances, compaction, surface crustiness, or desiccation. Soils can be removed partially or entirely, altered, indurated, contaminated with toxins, or otherwise adversely affected by road building or mining construction to certain depths below the surface such that short-term and even mid-term recovery following reclamation is problematic. The fairly intense disturbance of soil surfaces by mining activities may make soils susceptible to water and wind erosion, thus contributing to sediment loading in local or regional stream systems that reduce water quality and aquatic habitat. Chemical particulates and metals from smelter emissions and airborne tailings can settle on soil surfaces near or some distance from mineral processing facilities, although typically contamination of soils decreases with distance from the contaminant source. In many cases, toxic substances have been deposited along roads leading from the mine and have adversely affected not only soils in residential areas but also the health of people and animals living in proximity to the roads.

Surface water: changes in quality (especially in terms of drinking water and such uses as agriculture and other water-intensive industrial/commercial operations), discharge quantities such as stream flow regime fluctuations with sharper flow peaks and reduced dry season flows, stream channel alterations from erosion and slumping, and runoff including wash-off or hazardous chemical leachates (such as sulfates, sulfides, and salts) from unrehabilitated and poorly revegetated mine dumps and discard areas and the interrelated problems involving the release of heavy metals (including lead, copper, mercury, aluminium, selenium, zinc, uranium, nickel, chromium, and others) and acids from tailing piles, especially sulfuric, into nearby water bodies where excess acid generation overwhelms the natural buffering capabilities present in adjacent land and water resources. In the 1990s, the U.S. Forest Service estimated that between 20,000 to 50,000 mines on federal lands were generating toxic acid discharges that adversely effected between 5,000 to 10,000 miles of streams. 

Author’s Note: Another point to consider in the above materials about the nature of leachates is the following example: fresh water that is poured over ground coffee beans leaches far more essence of coffee than if the same water percolated over whole coffee beans since more surface area is exposed to the chemical effect. So it is with mine tailings versus intact igneous rock structures. That specific environmental problem is extremely important and complex since different metal species have different bio-availabilities and some of those species are much more toxic and hazardous than others. Speciation (the term of interest used above is metal species) is the proportion of the metal in different forms, such as ions, ion pairs and combinations, complex molecules, colloids, and precipitates. Incidentally, if you do not believe that acid mine drainage is an extraordinarily serious real world problem with working and abandoned mines, you should wake up and read about the horrific Animas River spill in Colorado (USA) in early August 2015 that released three million gallons of lead, arsenic, cadmium, and other highly toxic chemicals into the stream from the abandoned Gold King mine, closed one hundred years ago, that had never been properly remediated. As of mid-2016, the Gold King Mine continues to discharge acid mine drainage at a rate of about 600 gallons per minute. The really depressing news is that abandoned mine is just one of hundreds of thousands like it in the American West, at least 20,000 in Colorado alone, most of which are environmental disasters waiting to happen. All because the General Mining Law of 1872, which is still the governing legislation, did not and does not require mining companies to clean up their messes and instead allows them to turn their backs on the problem and simply walk away with their profits intact. For more information on that law and its effects on American mining, especially on gold mining, see my blog post of 4-2-12.

Groundwater: adverse effects on quality and quantity especially with regard to dewatering activities, drawdown, and techniques used to control dust and other particulates as well as acid mine drainage that infiltrates the groundwater system and adversely affects the natural pH levels, causing cascading effects on flora and fauna when the groundwater and surface water systems interact. This topic is far more complex and critical than can be described in a few short paragraphs. Serious students of the topic must investigate the various sub-topics independently.

Air quality through discharge from nearby refinery smelter stacks: pollutants include a variety of particulates (especially dust derived from many stages in the mining operation), gases that contribute to acid rain, metal vapors, noxious odors, etc., including lead and copper toxicity affecting livestock, wildlife, and human populations from air-borne particulates. Air quality issues generated by mining-milling-smelting operations not only include emissions of particulates but also fugitive dust, odors, and sulfuric acid that can produce extensive regional air pollution through sulfuric deposition and acidification of streams and lakes that in turn can cause adverse changes in aquatic biotic composition and chemical processes. Those adverse atmospheric effects can be felt many hundreds of miles from the mining source in rural areas, cities, and even other countries.

Other Air Quality Issues: Extracting metals and other ores, whether in open-pit or underground mines, using modern industrial processing typically results in large surface mounds of commercially worthless or waste rock, variously known as tailings, gangue (pronounced "gang"), or chat. At many mine sites, millions of tons of waste rock remain, the fines/dust of many of those mines are subject to airborne dispersal onto adjacent properties and steams as well as into geographically removed areas. Those airborne materials can contain heavy metals and other toxic/hazardous chemicals that can be inhaled and ingested by the residents/visitors, causing various health-related problems including lung and other diseases. Those hazardous materials can be especially deleterious to small children under six years of age who are at high risk owing to stage of physiological development.

Chemical residues: especially those from acids, hazardous and toxic chemicals, explosives, and radionuclides. Tailings ponds may frequently be toxic due to the presence of unextracted sulfide minerals and acids in the tailings or gangue (commercially valueless material in which ore is found). Those hazardous residues may wind up contaminating surface streams or groundwater or both.

Noise and vibration associated with mining activities: including the use of explosives, operation of enormous trucks and other heavy mechanical-electrical equipment, and various milling-smelting operations adversely affect nearby settlements as well as areas under human use such as parks, churches, schools, streams used for fishing and recreation, and lands used for hunting. Since most people have never experienced large-scale industrial noise and vibration it is difficult to adequately portray the critical disruptive effects they have on all human activities, including speech, sleep, thought itself, concentration, and mental health.

Flora and fauna: indigenous species are adversely affected by alterations to and loss of native habitats, vegetation cover, invasion by alien plant/animal species, altered plant community species composition, contamination and destruction of entire food webs. Disturbances of natural, quasi-natural, or cultural landscapes inevitably result in changes in composition and structure of plant species, disrupt soil strata, and stimulate invasion by disturbed-site plant species that in turn can alter composition of local invertebrate and other associated species and habitat. Those disturbances may be associated with development of roads or use of off-road vehicles during exploration and development activities. Emissions from various onsite mineral processing facilities may also act to fumigate plant communities adjacent to  through releases of toxic materials, such as sulfur dioxide (SO2), which is often lethal to foliage and consequently to the plant itself. Increasing distance from the pollution source generally results in the plant community composition gradually returning to pre-disturbance characteristics. However, long-term particulate and smelter emissions have historically adversely affected surrounding ecosystems, habitats, plant/animal biodiversity, and soils.In many cases, surface disturbance is so significant and persistent, the consequent loss of predator and other animal species can alter the local and regional ecology for many decades, even centuries.

Author's Note: The Berkeley Pit in Butte, Montana, is an inactive open pit copper mine owned and managed by Atlantic Richfield and Montana Resources. The Pit is one mile long by half a mile wide with an approximate depth of 1,780 feet holding about 45 billion gallons of water that is highly acidic (sulfuric acid) and is contaminated by high concentrations of dissolved toxic heavy metals. The Berkeley Pit was designated as a federal Superfund site in 1983. This mine popped into the news in late November 2016 when several thousand migrating snow geese died when they landed in the Berkeley Pit’s toxic water. For more information on the Berkeley Pit, see: http://www.pitwatch.org/berkeley-pit-history/

Land Subsidence after strip mine rehabilitation. Subsidence can occur after an open pit mine has been reclaimed due to changes in underground water supply, differential settling, and subsurface mechanical erosion that forms cavities or pipes that may collapse, causing the land surface to subside.

Cultural factors, including:

Aesthetic, noise, and visual effects. Although many supporters of open pit mining play down this type of adverse effect, imagine the stress caused by living with nearly constant noise and vibration from mining activities. And if the mine is located in an area that was previously forested, the visual effects, meaning the loss of vegetation and the implementation of an industrial landscape, can be significant. Loss of a landscape that has meaning for people as a result of open pit mining and the resulting environmental degradation causes mental anguish and suffering that is largely or even entirely discounted or denied by mine operators but is nevertheless real.

Land use modifications including reduced agricultural or grazing capacity; introduction of non-native species as part of re-planting plans; increased infrastructure, including power, water, and gas corridors; transport and service corridors (railway lines, roads, pipelines, conveyors, airstrips, port facilities); social infrastructure costs, including rapidly increased demand for mining-related housing, commercial-retail services, food and entertainment, education, social services, emergency services, medical services, etc.; loss of natural and semi-natural streams and associated valleys when those areas are buried by overburden and turned into wastelands; loss of residential uses and communities in valleys buried by overburden; and addition of new surface uses and facilities (e.g. offices, laboratories, workshops, vehicle parking, repair/service areas, fuel storage and dispensing depots, and warehouses, etc.). A land use cost seldom anticipated or adequately planned for is what happens to built socioeconomic uses-infrastructure (residences, schools, commercial-retail, roads, water and sewage facilities, etc.) after the mine is depleted and abandoned and the workers depart for places unknown.

Economic Costs: including short-, mid-, and long-term costs to the general public (meaning through actions of state and federal governments) of clean-up not paid for by the mining firm or the bond, which is an all too common occurrence, especially in the American Southwest. Those costs can be both significant and long-term.

The set of environmental consequences that any specific open pit mining operation will have on the natural and cultural environments depends on a number of factors: type of rock and ore being mined, scale and longevity of mining operations, efficiency and effectiveness of environmental management systems and mitigation measures that are employed by mine management, the nature and level of enforcement of government environmental regulations, and the sensitivity of the receiving environment (especially the abundance/scarcity of both surface and ground waters). Although a number of analytic methods have been proposed to measure the adverse environmental effects of surface mining, most of those techniques compartmentalize adverse effects and seldom treat those adverse effects as inextricably interrelated or systemic (for example, see Folchi, Roberto. 2003. "Environmental impact statement for mining with explosives: A quantitative method." I.S.E.E. 29th Annual Conference on Explosives and Blasting Technique. Nashville, Tennessee). Consequently, little attention is often paid to the importance of habitats, plant and animal biodiversity, watersheds, etc. as the adverse effects are reduced to quantitative scores that can be interpreted and manipulated in various ways that can justify mining operations.

Real World Examples: A mind-numbing example of the horrific multiple adverse effects on biogeophysical and human systems is Arch Coal’s Hobet 21 mountaintop removal coal mine in Lincoln County, West Virginia, where hundreds of millions of tons of former mountain top wastes have been dumped into a nearby miles-long valley with horrific adverse environmental effects.[1] For those few readers who may not be familiar with the process, mountaintop removal is a form of open pit coal mining that uses heavy explosives to remove hundreds of vertical feet of mountainous terrain to access coal seams at varying depths beneath the surface. That “overburden,” meaning millions of tons of rock fragments, is then dumped directly into adjacent valleys, destroying existing habitat, burying streams, wetlands, and forests, and causing mud and debris slides, dislodged boulders, flash floods, and the deposition of heavy metals, carcinogens, and other toxic-hazardous materials in streams and lakes.

According to the Mountain Justice website, “Mountaintop removal/valley fill coal mining (MTR) has been called strip mining on steroids.” The process is one of leveling rugged mountainous topography and leaving behind a sterile moonscape. In the past several decades, mountaintop removal mining has transformed some of the most biologically diverse temperate forests in the world in West Virginia, Kentucky, and Ohio into enormous, biologically barren open wounds on the landscape. Many sources estimate that over 500 mountains have been leveled, and many thousands of miles of Appalachian streams have been buried and polluted by that mining process. In the State of Kentucky, almost 4,000 miles of streams have been polluted, physically degraded, or destroyed by mountaintop mining.

Open pit/open cast miining techniques are also frequently used to produce precious and semi-precious metals, especially, copper, silver, and gold, in developed as well as developing nations. Examples include the Bingham Canyon Copper Mine (also known as the Kennecott Mine) southwest of Salt Lake City, Utah; Chile’s Chuquicamata Mine, the world’s largest open pit copper mine and the second deepest open-pit mine; and the Grasberg Mine in the province of Papua in Indonesia, the world’s largest gold mine and the second largest copper mine; according to the non-profit Earthworks, that mining operation dumps about 80 million tons of mining debris into the Ajkwa River system every year.

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[1] For a book full of ugly examples of how mining can kill a mountain ecosystem and the surrounding valleys, see: Erik Reece, Lost Mountain: A Year in the Vanishing Wilderness. New York: Riverhead Books/Penguin Group, 2006.