Acid Rock Drainage

ARD includes drainage of water originating in underground mines and runoff from open-pit workings, waste rock dumps, mill tailings, and ore stockpiles, sometimes in flows measured in millions of gallons per day. Although normally mines extracting coal or nonferrous metals suffer the greatest problems, similar problems may arise during construction or other engineering activity on certain rock types.

ARD results from the exposure of pyrite (FeS₂), marcasite (FeS₂), and pyrrhotite (FeSi__x) to oxygen and water, resulting in the formation of sulfuric acid, ferrous iron (Fe²⁺), and ferric iron (Fe³⁺). Ferric iron is a powerful oxidizing agent and oxidizes insoluble metal sulfides to water-soluble sulfates. These may then be dissolved and transported in the acid medium.

The rate of ferric iron and acid generation is increased dramatically by the presence of certain bacteria (e.g., Thiobacillus ferrooxidans, Thiobacillus thiooxidans). Metals commonly found in ARD include copper, zinc, nickel, cadmium, arsenic, aluminum, and manganese, although the concentrations are largely dependent on the mineralogy of the ore body, the surrounding host rock, and attenuation reactions.

The nature of ARD originating from waste products is also influenced by process efficiency and economics because these in turn partially determine the concentration and type of residual metal contaminants (present as sulfides or other species) in waste rock and tailings and in the unexploited host rock.

The dominance of waste management today is perhaps a reflection of the prevailing attitude toward closure, namely that a civil engineering-style remedial approach is sufficient to properly decommission a site and avoid possible liability in the future. Although this approach may have been valid twenty years ago, technology, legislation, and the expectations of stakeholders have moved on.

If there is a generic "thread" that ties current waste management approaches it is the possibility of future litigation and liability associated with what might be considered short-term (but relatively cheap) solutions to long-term environmental problems. The threat of incurring financial penalties in the future has helped to catalyze the industry into exploring alternatives to current practices, to establish "best practice" standards, and to move toward preventive action, particularly within the mineral processing arena.

This is not to say that waste management has no role in optimizing future environmental protection. However, in a legislative framework increasingly leaning toward preventive rather than remedial measures, any environmental benefits of remedial approaches relative to alternative approaches need to be quantified rather than relying on empirical and historical precedent.

At local and regional levels stakeholder groups may balance the environmental and social impacts with the socioeconomic benefits that often accrue (at these levels and at the national level) from mining, for example: (1) contribution to local economies. Mining may form a vital part of rural economies in particular. (2) Contributions to the macroeconomy: Mining forms a vital part of the gross domestic product (GDP) of some countries. (3) Provision of consumer products: Many products that are not directly associated with mining but that are essential to a modern lifestyle depend on tin products (e.g., food containers).

Consequently, social and environmental impacts may be mitigated by a willingness to accept a certain degree of environmental impact in return for other benefits. Therefore, a social-impact analysis may be required at the site level to fully understand an operation's contribution to social effects and the level of significance that stakeholders attach to them.

The legacy of mining that took place before environmental and social issues were considered is a universal problem that the mining industry faces, and the industry is often criticized in terms of today's regulation for practices that were legal at the time. Dealing with this legacy is problematic—companies are loath to address the legacy for fear of being seen to admit or accept liability and responsibility.

Some experts suggest that the mining industry should explore regulatory, voluntary, and fiscal approaches to the gradual reclamation of abandoned operations that are likely to impact its ability to operate in the future. This approach has been examined in the United States to assess the potential to rework abandoned sites, with the liability for environmental impacts remaining with the state or federal government, removing the liability risk as a major obstacle to restoration through reworking (this approach applies to sites where the reworking is undertaken by a company other than that originally responsible for the environmental impact).

The remediation of existing mine sites represents a significant opportunity to develop innovative technologies for reprocessing mine wastes and to develop policy and technical procedures to promote the utilization of secondary wastes rather than primary resources where possible. Increased constraints relating to the exploitation of primary resources have elevated wastes into potential resources themselves.

Regulatory standards and quality standards may impede the use of innovative technologies, and there may also be technical barriers to the reprocessing of certain complex waste materials. Innovative technologies may also have cost and performance drawbacks that in turn generate a lack of incentive to invest in such technologies.