What is Acid Rock Drainage (ARD)?
Iron is one of the most common elements on earth, and it is the iron sulfide that often occurs with gold and other valuable minerals that is the main source of acid. Rain, snowmelt, or water moving over iron sulfide forms sulfuric acid. The acid dissolves metals in the rock like copper, zinc, nickel, and lead. Acid and metals are washed downstream into clean watersheds where aquatic plants and animals are exposed. This process is called “acid rock drainage” or, because it occurs at many mining sites, “acid mine drainage”. It can occur in tunnels, open pits, waste rock piles, and mill waste (tailings) (Figure 1).
