Water Pollution: Mining's Toxic Legacy

Water is essential to life on Earth. However, human activities such as mining threaten the water sources that we all depend on. Mining consumes, diverts, and pollutes water resources. It affects freshwater through the heavy use of water in processing ore, and through water pollution from discharged mine effluent and seepage from tailings and waste rock impoundments. Mining can also deplete surface and groundwater supplies. Water pollution from mine waste rock and tailings may need to be managed for decades, if not centuries, after a mine's closure.

Acid Mine Drainage

AMD is often released from waste rock piles, tailings, open pits, underground tunnels, and leach pads. It can be identified by "yellow boy", an orange-yellow substance that occurs when the pH of AMD rises above 3, causing previously dissolved iron to precipitate out. AMD can have a pH of 4 or lower, similar to battery acid, and can kill aquatic life, making water virtually unusable.

AMD can persist indefinitely, continuing to cause harm long after mining has ended. This is due to the potential for AMD to occur as long as its source rock is exposed to air and water, which can last for hundreds or even thousands of years. As a result, water treatment may be required in perpetuity, posing a significant economic burden on the public if the mining company is unwilling or unable to cover the costs.

The chemistry of AMD is complex, and the oxidation of pyrites in particular plays a significant role in its formation. Colonies of bacteria and archaea, known as extremophiles, can greatly accelerate the decomposition of metal ions, though AMD can also occur in an abiotic environment. These microbes thrive in the low pH conditions of abandoned mines.

Several technologies exist to treat AMD, including neutralization with calcium carbonate, lime, or calcium silicate, and the use of constructed wetlands. However, preventing AMD through improved environmental risk assessment and adopting a liability prevention approach is key to mitigating its impacts on water resources.

Heavy Metal Contamination

The contamination of water by heavy metals is of particular concern as it can have serious ecological and human health impacts. Even in minor quantities or at low concentrations, heavy metals can be highly toxic. For example, arsenic, lead, cadmium, nickel, mercury, chromium, cobalt, zinc, and selenium are highly toxic to humans and other organisms. The toxicity of heavy metals in water is further exacerbated by low pH conditions, such as those created by acid mine drainage.

The impact of heavy metal contamination from mining activities has been observed in various parts of the world. For instance, in South Morocco, studies have shown high concentrations of zinc, lead, copper, and cadmium in soil and plant samples collected from mining sites. Similarly, in South Africa, coal and gold mining activities have led to the presence of toxic levels of iron, aluminium, manganese, and sulphates in water resources, rendering them unfit to support life.

The effects of heavy metal contamination on human health can be severe and include blood disorders, kidney damage, miscarriages, reproductive disorders, and an increased risk of various cancers. Additionally, heavy metals can accumulate in the food chain, further magnifying their toxic effects.

To address the issue of heavy metal contamination from mining, various remediation technologies have been developed, including precipitation, adsorption, biosorption, ion exchange, and membrane filtration. However, these technologies often come with their own challenges, such as high costs and the production of additional waste.

Processing Chemicals Pollution

In modern mine planning and permitting, regulatory agencies often require considerable analysis and planning during the project design phase to minimize potential impacts on water resources. However, despite improvements in mining practices, significant environmental risks remain. For instance, the chemicals used in the extraction and processing of ores can present water quality issues. While large-scale modern mines carefully manage on-site chemical usage, historic and artisanal mines can release compounds such as mercury and cyanide from gold processing.

Water discharged from mining operations must meet stringent water quality standards. Federal, state, and local regulations govern mine waste and water management, as well as closure planning and post-closure mitigation and monitoring. Financial assurance requirements have evolved to better accommodate the known costs of environmental cleanup and mine closure, including any long-term post-closure water treatment requirements.

Water quality control options can consist of methods to limit reactions between water and mine wastes, manage the flow of potentially impacted waters, and treat impacted waters before discharge. Treatment can include active measures such as wastewater treatment plants with reverse osmosis, aeration, or clarification systems, as well as passive treatments like engineered wetlands and reactive barriers or drains.

Erosion and Sedimentation

The process of erosion involves the weathering and dislodging of particles from rock material by water, wind, glaciers, and even plant and animal activities. In the context of mining, erosion is often accelerated due to human activities, leading to increased rates of sediment transport. The sediment cycle includes erosion, transportation, and deposition. Water acts as a key agent in fluvial sedimentation, carrying sediment through river systems until it is deposited in a lake or the sea.

Excessive sedimentation caused by mining activities can have several detrimental effects on water bodies and aquatic life. Firstly, it can clog riverbeds and smother watershed vegetation, wildlife habitats, and aquatic organisms. This sedimentation can directly impact fish populations by reducing light penetration, irritating gills, disrupting protective mucus coverings, increasing water temperatures, and burying fish eggs. Additionally, sediment particles can carry toxic compounds, such as agricultural or industrial chemicals, further endangering aquatic life and ecosystems.

The impacts of erosion and sedimentation in mining areas can be mitigated through the implementation of best practices and regulations. This includes proper land reclamation and restoration, erosion control measures, adherence to environmental impact assessments, and community engagement to foster responsible mining practices. Sediment control structures, such as sediment ponds and silt fences, can capture and settle sediments before they enter water bodies, reducing their environmental impact.

Overall, erosion and sedimentation are significant factors in the pollution of water caused by mining activities. By understanding the sediment cycle and implementing sustainable practices, it is possible to minimize the negative impacts on water quality and aquatic ecosystems.

Depletion of Surface and Groundwater Supplies

Mining can deplete surface and groundwater supplies in several ways. Firstly, groundwater withdrawals may damage or destroy streamside habitats many miles from the actual mine site. For example, in Nevada, the driest state in the United States, the Humboldt River is being drained to benefit gold mining operations, with mines in the northeastern Nevada desert pumping out more than 580 billion gallons of water between 1986 and 2001. This has had a significant impact on the local ecosystem, as it has lowered the water table and dried up the river.

Mining operations also require large amounts of water for various processes, such as drainage, cooling, and aqueous extraction. This heavy use of water can reduce the availability of surface and groundwater supplies, particularly in regions that are already water-stressed. The Canadian mineral industry, for instance, generates one million tonnes of waste rock and 950,000 tonnes of tailings per day, requiring substantial water resources to manage.

The construction and maintenance of roads, open pits, and waste impoundments during the mining process can also lead to soil erosion, which can decrease water availability for plant growth. This, in turn, can result in a population decline in the plant ecosystem. Additionally, altered drainage patterns can redirect water flow, intensifying erosion and sedimentation of nearby water bodies.

Furthermore, the disposal of wastewater from mining processes can contaminate surface and groundwater supplies. If proper precautions are not taken, unnaturally high concentrations of chemicals, such as arsenic, cyanide, sulphuric acid, and mercury, can spread over a significant area of surface or subsurface water. Submarine tailings disposal, where waste is pumped to great depths, is considered a better option than dumping it into surface waters or forests. However, land storage and refilling of the mine after depletion is the best option, provided that no forests need to be cleared for debris storage.

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