Mining and Its Environmental Impacts

The choice of mining method is dictated by geology, not preference. Surface mining (open-pit, strip, placer) is used when a deposit is shallow and broad. Underground mining (room-and-pillar, longwall, block caving) is used when a deposit is deep and narrow. The deeper the ore, the more energy, water, and waste rock must be handled per tonne of product.

• Open-pit mining — a large conical excavation used for low-grade, near-surface deposits (most copper, iron, and diamond mines). The pit grows outward and downward in benches.
• Strip mining — removing long strips of overburden to reach shallow coal or lignite seams, common in the western United States and Australia.
• Placer mining — dredging or sluicing river gravels for gold, tin, or diamonds. Often artisanal in scale but can disrupt entire river systems.

Surface mining disturbs the largest land area per tonne of ore but is generally safer for workers and cheaper per unit of output.

• Room-and-pillar — tunnels (rooms) are excavated, leaving pillars of ore to support the roof. Used for flat-lying coal or salt beds.
• Longwall mining — a mechanical shearer moves along a coal face, and the roof is allowed to collapse behind it. Highly efficient but causes surface subsidence.
• Block caving — ore is undercut until it collapses under its own weight and is collected from below. Used for massive, low-grade deposits like porphyry copper.

Underground mining disturbs less surface land but carries higher worker-safety risks and energy costs.

When sulfide minerals (especially pyrite, FeS₂) are exposed to air and water by mining, they oxidise to produce sulfuric acid. This acid mine drainage (AMD) leaches heavy metals from waste rock and tailings, creating water with pH as low as 2–3 that can kill aquatic life for tens of kilometres downstream.

The chemistry is simple but relentless:

For most mines, the ore is a small fraction of the material moved. A copper mine grading 0.5% Cu must process 200 tonnes of ore to extract 1 tonne of copper. The remaining 199 tonnes become tailings — finely ground rock left after the ore minerals are extracted. Separately, huge volumes of waste rock (barren overburden) are removed to reach the ore body in the first place.

Tailings are often stored as a slurry behind dams called tailings impoundments. When these dams fail — as at Brumadinho, Brazil (2019) — the consequences are catastrophic. Dry stacking and backfilling underground are safer alternatives but cost more.

Reclamation (or remediation) aims to return mined land to a safe, stable, and productive state. Common strategies include:

• Reshaping slopes and covering waste rock with impermeable caps to exclude water and oxygen.
• Re-vegetating with native species suited to the altered soil chemistry.
• Treating AMD with limestone to raise pH, or constructing wetlands to filter metals.

But reclamation has limits. Underground aquifers may never regain pre-mining chemistry. Subsidence from longwall mining is permanent. And ecosystems rebuilt on mine waste rarely match the biodiversity of the original landscape.