See the infographic from High Country News.
Abandoned mines — about 31,000 of them — linger like ghosts on the West’s public lands. It’s harder to find exact numbers for old mines on private land, but Colorado, for example, has about 14,000, compared to 3,299 public-land sites. In the San Juan Mountains, water from snowmelt and rainfall picks up mining remnants like arsenic, lead, cadmium and zinc from tailings piles, delivering them to the headwaters of the Animas River. This mineral runoff can harm fish, insects and other small river organisms. To help revegetate and stabilize the acidic, plant-hostile soils around mine sites, scientists are borrowing a technique from agricultural research: They apply biochar, a charcoal formed by heating plant and wood waste in the absence of oxygen, to the soil. The carbon-dense char, used to boost soil fertility on farms, helps plants get established in barren mine sites, reducing contaminated runoff.