AMD is formed when exposure of the overburden occurs due to mining practices that place it in contact with water and oxygen. When neutralizing compounds are absent water draining from surface and deep mines can become acidic and contaminated with high concentrations of dissolved Fe, Mn, Al, and SO₄ (Edenborn et al., 1993). AMD discharges vary in their characteristics depending upon the soil and rock composition in which the drainage developed. The type of drainage is highly correlated to the amount of pyrite present and the degree of physical weathering that has occurred. Mine drainage may be either net acidic or net alkaline. Net acidic drainage lacks adequate buffering capacity and net alkaline drainage contains net neutralizing potential, generally as HCO₃^-, to accept proton acidity (Kepler and McCleary, 1994). It is the acidic mine drainage that dissolves metals such as Fe, Al, and Mn from affected overburden creating AMD.

Pyritic material is necessary for AMD formation. AMD is leached from the rocks containing pyrite by the metabolic activity of Thiobacillus ferroxidans, which promote the following reactions:

FeS₂ + 3.5 O₂ + H₂O à Fe²⁺ + 2 SO₄^2- + 2 H⁺ (1)

Fe²⁺ + 0.25 O₂ + H⁺ à Fe³⁺ + 0.5 H₂O (2)

Fe³⁺ + 3 H₂O à Fe(OH)₃ (s) + 3 H⁺ (3)

These microorganisms have been shown to catalyze the reaction and increase the rate at which AMD forms by as much as 10⁶ times. Thiobacillus spp. are acidophilic and chemoautotrophic. They thrive in an environment low in organic matter, and oxidize both Fe and S as energy sources (Robb and Robinson, 1995). Thiobacillus spp. appear to be most active between pH 2.0-5.0, which are conditions that promote AMD formation (Sobek et el., 1990). Water and oxygen are necessary components in the formation of AMD. Other important factors are the hydrologic regime, particle size of surrounding material, physical weathering of rocks, and the permeability of the soil (Robb and Robinson, 1995). AMD formation is a complicated process, but unfortunately, conditions are conducive for it on numerous mine sites.

The first noted effects of AMD are red-yellow precipitates of iron hydroxide in the discharges and the eradication of fish and other aquatic flora and fauna in the receiving streams (Paine, 1987). When the pH falls below 3.5, Fe³⁺ precipitates as Fe(OH)₃, which coat stream bottoms. The iron precipitate is called "yellow-boy". AMD has numerous effects on the stream including oxygen removal, acidification, depletion of buffering capacity, and a decrease in light penetration, which in turn affects photosynthesis and the clarity of the water. The precipitate smothers out bottom dwellers decreasing food resources (Robb and Robinson, 1995). Other metal oxides can also precipitate. Al(OH)₃ appears white, while MnO₂ appears black. Both may be directly toxic or may fill in spaces between rocks, depleting organism habitat (Robb and Robinson, 1995).

This is a discharge in the Shamokin Creek Watershed. The white precipitate is aluminum settling out when the pH increases to or above 4.5.