Environmental advocates routinely oppose mining and hydrocarbon-development projects because they believe that remote areas of the Amazon should remain pristine. Typically, they organize their opposition during the environmental review process with campaigns that focus on issues that are local (water quality), regional (biodiversity) and global (climate change). Occasionally, they succeed in stopping a project, but more often they must settle for environmental and social action plans that ameliorate only the worst aspects of what are, essentially, irreversible changes to habitats and ecosystems.

Indigenous peoples are outspoken in their opposition to mineral development because their communities can be transformed by immigrants or disrupted by catastrophic events that threaten their livelihoods. They have been particularly successful in resisting projects that impinge upon their territories due to a legal principle incorporated into international law, which obligates governments and developers to obtain their ‘free prior and informed consent’ (FPIC). Not surprisingly, their successes in halting or delaying several highly lucrative development projects have led to a political backlash.

Industrial mines

A modern industrial mine is a highly engineered hole in the ground. Depending upon the target minerals and the geological setting, the mine can be a complex underground structure or a surface excavation covering thousands of hectares. All mines are accompanied by infrastructure to transport massive amounts of rock and ore and a mill to grind the ore into fine particles that are (usually) mixed with water to create a slurry that is processed to concentrate the target mineral into an industrial commodity.

An industrial mine is physically unsafe due to the heavy machinery designed to excavate, transport and crush millions of tonnes of rock; consequently, mining companies spend significant financial resources to make them less hazardous. They also present a long-term health threat to employees and the surrounding communities, particularly if the milling technology generates large volumes of tailings. Tailings are dangerous because they tend to be toxic, especially if they have been treated with chemical reagents to free and concentrate the target minerals. They are extraordinarily hazardous if they are stored in poorly engineered ponds that can leak – or fail – and release their toxic contents into the environment.

Accidents have provoked mistrust among inhabitants living on mining landscapes, and environmental advocates question the integrity of the waste-management strategies in an inadequately regulated mining industry. Mining companies are aware of the environmental risks associated with their industry; this does not mean, however, that they take the necessary measures to ensure they are fail-safe.

Mine types and their impacts

Underground mines were the predominant mining technology for centuries because they are efficient at extracting valuable minerals from high-grade veins and lodes. They are used in the modern mining industry only when the ore body is both deep and rich. Underground mining is a mature technology and miners have access to a variety of technological options that reduce risk and maximize productivity. In modern mines, waste rock from the construction of shafts and logistical areas is used to backfill previously mined areas, a practice that provides structural support against cave-ins, while minimizing the need to manage waste rock on the surface.

An open-pit mine is a large-scale, hard-rock operation that exploits a mineral resource close to the earth’s surface. The use of open-pit mines is the consequence of a decrease in the availability of high-grade mineral resources that can be profitably exploited by an underground mine. A moderate- or low-grade ore body demands an operation that is based on the economies of scale, which is why open-pit mines tend to be extraordinarily large. Individual pits are abandoned after about ten years, and mines that operate over longer periods do so by opening additional pits above adjacent ore bodies. The most obvious impact, a giant hole in the ground, is accompanied by two others: a massive amount of waste rock and waste tailings, both of which must be accommodated in areas adjacent to the mining pits.

‘Overburden’, is the term used to describe the waste rock removed from the surface to expose the ore body; unlike an underground mine, it cannot be used to backfill the pit, which becomes wider and deeper as the mine matures.

Fortunately, waste rock is usually chemically inert and is simply consigned to a nearby area and planted with grass or trees to control erosion. Managing the risk associated with tailings is more challenging because chemical processing renders them toxic, while their physical state (pulverized rock sludge) makes them difficult to store.

A strip mine is similar to an open-pit mine but is associated with a mineral resource that occurs as a subsurface layer extending over a relatively large and homogeneous landscape (bauxite, phosphate or cassiterite). Strip mines are more amenable to reclamation because the overburden is used to backfill previously mined areas. Before the development and implementation of environmental laws, operators made no serious effort to reclaim strip mines (see Guyana and Suriname below). Currently, multinational mining companies are spending significant financial resources to restore a semblance of natural vegetation.

The chemistry of mineral concentration

Tailing storage facilities are the most pernicious of the environmental liabilities that confound the mining sector. Not all tailings are equally bad or equally voluminous. The amount depends upon the mineral concentration of the ore body, while the degree of toxicity depends on the chemistry used to extract the target mineral from the pulverized ore.

Base metals, such as copper, zinc and nickel, are characterized by ores with extremely low grades of mineralization, typically between 0.5 % and 2.5 %; consequently, these mines produce vast amounts of rock that must be pulverized and processed using acidic solutions and flotation ponds that demand large volumes of water and milling technology known as ‘flotation’, where chemical reagents are added to an aerated slurry permeated with bubbles. Mineral particles liberated by the reagents adhere to the bubbles and float to the surface; the residue sinks to the bottom and is removed as a thick slurry that is dumped into a pond or impoundment referred to as a tailings storage facility.

The tailings are noxious because the reagent (sulphuric acid) is toxic, but also because the process liberates other heavy metals, such as arsenic, cadmium, lead and antimony. The volume of tailings produced by an open-pit exploiting a low-grade (< 2%) copper deposit is massive, because ~95 % of the original ore is converted to tailings. The technology is widely used in Peru and Brazil.

A less detrimental technology is the ‘heap-leaching’ process where crushed ore is placed in a ‘heap’ on a pad that is irrigated with reagents leaching the minerals from the pulverized rock. The target mineral is chemically extracted from the solvent, which is recycled and stored in small pools, while residues are immobilized in the heap.

Superficially, a tailings heap may resemble waste rock from the overburden; but tailing heaps are toxic and must be isolated indefinitely by a geomembrane. Mines must build and maintain a containment pond below a tailings heap to collect and recycle the reagent (over the short term) and guard against the release of toxic residues into regional watersheds (over the long term). This technology is used by many of the large-scale gold mines in Peru and will be used increasingly as the formal gold sector expands activities in Brazil, Guyana and Suriname.

The degree of toxicity is a function of the chemical procedures used to free the target metals from the mineral ore. Iron ore and bauxite have mineral grades that range from 40 to 65 %, which significantly decreases the relative volume of tailings when compared to the original volume of the mined ore. Nonetheless, the gigantic size of these mines, which are among the largest on the planet, generates massive volumes of tailings. Because of its high grade, the concentration process is largely mechanical, and, with very few exceptions, tailings are less toxic than at mines that use chemical processes to free the target metals from the mineral ore.

The legacy of tailings storage facilities

On 19 August 1995, a tailings containment dam at the Omai gold mine in Guyana collapsed and spilled three million cubic meters of cyanide-laced effluent and heavy metals into the Essequibo River. The cause of the disaster was the object of considerable speculation, but eventually independent engineers attributed the failure to a combination of poor design, substandard maintenance and extreme rainfall, which overwhelmed the mine’s tailings storage facility. The containment dam was rebuilt, and the mine continued to operate until 2007 when it was closed due to the low price of gold and declining productivity. Legal action filed on the behalf of Guyanese citizens was disallowed in Canada based on jurisdictional grounds, while the Supreme Court of Guyana dismissed the case for lack of evidence. The operating company, Cambior Inc, was never held responsible for the incident.

Mining engineers have long been aware of the need to improve the design and management of tailings storage facilities (TSF) and investment in new technologies reflects the increasing demands by regulatory agencies and investors for improved environmental risk management. Despite numerous initiatives and significant investment, the number of incidents classified as ‘serious’ or ‘very serious’ has increased over the last thirty years.

Ironically, the incidence of containment structure failures has decreased; however, the severity of impacts has increased because: (1) the expansion in the scale of mining operations driven by the reduction in ore grades is generating ever larger quantities of mine tailings; and (2) the financial pressure to control costs motivates operators to stretch the capacity of waste-management facilities.

The key infrastructure asset of a conventional tailings storage facility is a dam. Dams are seldom built to their final dimensions, but are raised incrementally as the mine matures. Engineers use one of three major approaches.

Historically, upstream designs were most prevalent because they are the most economical to build; they are unstable, however, because the dam foundation is built on top of the tailings. Downstream designs are considered to be best, but many modern facilities, and all TSFs that have been retrofitted, use the center-line design because it minimizes the amount of rock required to reinforce the containment dam and avoids rebuilding the entire structure.

Regardless of the design, any engineering solution can fail when conditions cause the dam to breach or slide downhill due to a liquefaction event. There are three, overlapping and synergistic, causes of failure: (1) structural, which occurs when there is a physical rupture of the embankment material; (2) hydraulic, which describes the erosive impact of surface water and rain; and (3) infiltration, which is the consequence of the internal erosion of fine particles via a process referred to as ‘piping’. The collapse of a dam can occur suddenly if monitoring has been inadequate or maintenance delayed; typically, it occurs during a decadal-scale rainfall event or earthquake, which will quickly reveal the weaknesses of a TSF.

The financial shock caused by recent dam failures in Minas Gerais has finally forced mining companies to proactively review and reinforce all the containment dams in their active and inactive portfolio. It also motivated governments to conduct inventories of the tailing dumps at closed, abandoned and derelict mines. In Amazonian Brazil this encompasses more than one hundred tailings storage facilities in Amazonian jurisdictions with a clearly identified corporate owner. These include the ever expanding tailing ponds at the bauxite mines and refineries in Pará and the Pitinga mine in Amazonas, as well as the copper mines in the Carajás mining district. This inventory does not, however, take in the thousands of unorganized ponds on the tributaries to the Tapajós River, nor the decades-old legacies of cassiterite mining in Rondônia.

The situation is more complicated in Peru, where a recent nation-wide inventory identified more than 8,500 sites with some form of environmental liability; about 75 % of those are located on an Amazonian tributary. More than 800 are derelict mines lacking a legal entity that can be held responsible for management and remediation. The most prominent (and largest) are associated with the polymetallic mines that use sulphuric acid as the reagent for extracting the target minerals from pulverized ore. Perhaps the most salient risk, and one that is seldom discussed, is catastrophic failure due to periodic weather events linked to the El Niño and climate change. Earthquakes occur with decadal-scale regularity and represent a long-term liability for all containment structures.

Across the entire Pan Amazon, there has been no serious effort to mitigate the environmental impacts, nor to restore the riparian landscapes that have been transformed by wildcat placer mines. No attempt has been made to remediate their impacts by investments in reforestation, soil reclamation or wetland restoration. Regulation of informal mining is ineffective because governmental agencies do not have the resources to impose effective control. Elected officials do not have the political will to confront large populations who have grown accustomed to operating outside the confines of the law.

Remediation and restoration ecology

The mining industry manages the environmental risk inherent in its operations by implementing the so-called ‘mitigation hierarchy’: avoid, minimize, remediate and offset. Mainstreaming these concepts onto their business operations has allowed companies to improve their profitability, while limiting their exposure to long-term liabilities, particularly those linked to derelict mines and tailing storage facilities. Not all impacts can be avoided or completely mitigated, however. Consequently, long-term liabilities must be remediated or offset. For example, habitat loss caused by a massive open-pit could be offset by creating a protected area with similar biodiversity. In contrast, tailing storage facilities cannot (and should not) be offset; the risk from catastrophic failure must be eliminated by effective remediation.

The simple and least expensive option is to ‘decommission’ a tailings pond by reinforcing the dam, while removing excess water to ensure it is secure. Nonetheless, even a decommissioned TSF will require routine maintenance. A definitive solution, known as ‘de-characterization’, requires the detoxification of the tailings, the removal of the dam and the transformation of the site so that it merges into the surrounding landscape. De-characterization seems absurd when the TSF is adjacent to a massive abandoned open-pit mine but might be a logical solution for smaller tailings storage facilities associated with underground mines, or the floodplains impacted by cassiterite mines in Rondônia and Amazonas.

Restoring a landscape to its original state is, perhaps, an idealistic and unattainable goal in most instances. But there are exceptions. Brazil’s largest bauxite miner (Mineração Rio Norte) has spent decades, and tens of millions of dollars, working to restore the ecological functionality and some of the biodiversity that characterized pre-mining landscapes at the Trombetas mine in Oriximiná. Measures include saving and reusing topsoil and spreading woody debris and detritus on the soil surface in order to reintroduce soil fungi. Native tree communities are reassembled using seedlings germinated in nurseries from seed collected locally; epiphytes and lianas are salvaged when the mine is opened and maintained in living collections for reintroduction.

According to the MRN website, more than 5,750 hectares have been incorporated into the restoration process out of a total of 8,600 hectares that have been mined since initiating operations in 1979. Similar efforts are underway at the Alcoa mine, which initiated mining activities in 2010 on the opposite side of the Amazon River in Juruti municipality. Presumably, the mine in Paragominas in Eastern Pará operated by Norsk Hydro also has a forest restoration program. In contrast, the (currently inactive) bauxite mine at Los Pijigüaos mine in Venezuela is being remediated with cultivated grasses and commercial tree species.

“A Perfect Storm in the Amazon” is a book by Timothy Killeen and contains the author’s viewpoints and analysis. The second edition was published by The White Horse in 2021, under the terms of a Creative Commons license (CC BY 4.0 license).

To read earlier chapters of the book, find Chapter One here, Chapter Two here, Chapter Three here and Chapter Four here.

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Mayra Editor

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Amazon MiningBiodiversityEnvironmentGold MiningIllegal MiningIllegal TradeIndigenous PeoplesMiningPollutionBrazilLatin AmericaPeruSouth AmericaVenezuela

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