Margarete Kalin and William N. Wheeler
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Environmental Science. Please check back later for the full article.
The first treatise on mining and extractive metallurgy, published in 1556, mentioned the side effects of mining, namely dead fish and poisoned water. These same side effects are still with us today, even though our knowledge of extractive techniques and chemical processes has grown tremendously. The dead fish and poisoned water, we now know, are caused by oxidative weathering of minerals, resulting in acidic and metal-laden water. The weathering is exacerbated by microbes that break chemical bonds in pyrite to derive their energy.
To compound the problem, our insatiable appetite for metals and energy, combined with our development of industrial tools, has allowed us to dig mines vastly larger than those envisioned in 1556. This exponentially increases the weathering area available in waste rock and finely ground rock (tailings). Through infiltration of atmospheric precipitation, severely polluted seepages emerge from these mining wastes to surface and ground water.
Since metals are essential products needed in society, cost-effective remediation measures need to be developed. New sustainable approaches to mining need to be established. Currently, engineered covers and dams contain and reduce the infiltration of atmospheric precipitation, slowing the weathering process. However, weathering will continue for millennia. With this much time, covers will break down and dams will leak. Currently accepted practice is to integrate basic neutralizing agents (lime) to wastes or seeps in perpetuity. These and other stop-gap measures do not show any resemblance to sustainable mine development and reclamation.
What is needed is a paradigm shift in thinking about mine waste management. Waste rock and tailings need to be thought of as primitive ecosystems, characterized by harsh physical and chemical conditions. These harsh environments are similar to those encountered in the vicinity of hot springs characterized by highly acidic, or alkaline and saline conditions. These ecosystems are populated by thermophilic, acidophilic, and halophilic microbes (as a group called extremophiles), all of which can modify their surroundings. If managed properly, based on ecological principles, mines and these ecosystems will provide the resources of the future.
Ecological engineering utilizes ecological, geo-microbiological, and physical processes to change the conditions within the wastes to favor microbial remediation. To counter oxidative conditions, reductive environments and their microbes are supported with the ecological measures introduced. Reducing conditions can be generated in sediments and on the water-rock or water-sediment interphases through microbial growth. Gradually, contaminated acidic or alkaline water is cleansed by indigenous biota. These organisms sequester metal ions on or inside their cells and neutralize aquatic waste streams. Eventually, biomass (and metals) are relegated to the sediment, where they are bio-mineralized - forming new biogenic ore bodies. Re-oxidation of bio-mineralized metals is prevented by the introduction of underwater and emerging vegetation, which reduce mixing and consume oxygen above and at the sediment-water interphase. Natural cycles of oxidation and reduction have been operating on the planet for millennia, producing biogenic ore bodies, and are ecologically sound, sustainable approaches.