5.9 Impacts of Mining

An ore is rock that contains enough of a valuable mineral, like copper, iron or gold, to be worth mining. Ore grade is the concentration of the mineral in the rock. A high-grade ore has a lot; a low-grade ore has little.

Companies mine the richest, easiest-to-reach ores first. As those run out, they turn to lower-grade ores and deeper deposits. Lower-grade ore means more rock must be dug up, crushed and processed for each ton of metal, which takes more energy and water and creates more waste. The world isn't running out of copper atoms, but the cost, energy and environmental damage per ton keep rising. Mining still matters: it supplies the low-cost energy and raw materials that almost every product depends on.

Surface mining removes the overburden, which is the soil, rock and vegetation above a deposit, to reach minerals near the surface.

• Strip mining removes overburden in long strips to reach shallow coal seams.
• Open-pit mining digs a huge pit in steps to reach ores like copper and iron.
• Mountaintop removal mining blasts off the tops of mountains to reach coal seams, as in the Appalachian region of the U.S. The removed rock is often dumped into nearby valleys, burying streams.
• Placer mining sifts gold or other minerals from river sediments.

Subsurface mining uses shafts and tunnels to reach deep deposits. It disturbs less land at the surface, but it's more expensive and more dangerous for miners: tunnels can collapse, methane gas can explode, and coal miners can develop black lung disease from breathing coal dust. As easy-to-reach coal near the surface runs out, more coal has to come from deep subsurface mines, which are very expensive.

Mining produces large amounts of waste. Tailings are the finely ground rock left after the valuable mineral is separated from ore; they often contain toxic metals and processing chemicals and are stored in ponds behind dams. If a tailings dam fails, it can bury communities and poison rivers, as happened in Brazil in 2019, when a collapse killed more than 250 people. Slag is the glassy waste left after smelting (heating ore to extract metal).

When rock containing sulfide minerals, like pyrite, is exposed to air and water, it reacts to form sulfuric acid. This acid mine drainage runs into streams, lowering the pH and dissolving toxic metals like lead, arsenic and cadmium from the rock. Acidic, metal-laden water can kill fish and other aquatic life for miles downstream and can continue for decades after a mine closes.

Mining also clears habitat, adds sediment to streams from exposed soil, releases dust and particulate matter, and burns fossil fuels to run machinery. Coal mining can also contaminate groundwater and release methane, a greenhouse gas, trapped in coal seams. In the U.S., the Surface Mining Control and Reclamation Act of 1977 requires coal companies to reclaim surface-mined land, by replacing overburden and topsoil and replanting vegetation. Reclamation helps, but restored land rarely matches the original ecosystem.