Definition

Acid mine drainage (ADM) is the outflow of acidic water from a mining site.  It is most commonly caused by hard rock mining where the precious metals are bound to sulfurous rock like iron sulfide (FeS2). Once this rock has been excavated and pulverized it reacts with the air, surrounding moisture, and microorganisms that accelerate the production of sulfuric acid (H2SO4). 

The oxidation of pyrite is a natural process. However, when it is crushed, it has 100x more surface area than the naturally occurring substance making it more readily available to react with the elements.  When this happens, the generation of H2SO4 exceeds the natural buffering capabilities found in host rock and water resources. The sulfuric acid that is produced mixes into the surrounding groundwater and leaches heavy metals like arsenic, lead, and mercury from the surrounding rocks.  The acid water ultimately contaminates waterways further downstream making it difficult for the flora and fauna to survive.

AMD is less concerning while the mine is in operation, because the water table levels are kept low due to pumping. When the mine closes, on the other hand, those pumps are turned off and the water table rebounds. Throughout history every hard-rock mine that has sulfur-bearing rock as its host media has produced ADM.  Most sites have to be perpetually treated. Mining sites in Europe, from as early as the Roman Empire (~467 AD), are still producing acid mine drainage today.  
The vast majority of sulfide mines across the country have been claimed as EPA superfund sites. The costs of such sites are not the responsibility of the companies that operated these mines; they are covered by government funds. The companies have either gone bankrupt or the funds they set aside have been insufficient to cover treatment costs.

AMD is toxic to aquatic organisms, destroys ecosystems, corrodes infrastructure, and taints water in regions where freshwater is already in short supply -  Geoffrey S. Simate

According to the U.S. Forest Service, 20,000 to 50,000 mines currently generate enough acid water to cause damage to some 8,000 to 16,000 km of streams.  

Minnesotans' Concerns

• Partridge River runs adjacent to proposed plant site and flows into the St. Louis River and then into Lake Superior

• Groundwater pollution could flow north into the Boundary Water Canoe Area Wilderness watershed via Birch Lake after the mine’s closure

• Many mining sites have to be treated perpetually once acid mine drainage becomes a problem

• Fish Populations
• Heavy Metal Poisoning

• Plant Life