Acid rock drainage (DAR) (in English Acid Rock Drainage, ARD) is a natural and complex phenomenon associated with the existence of rocks that contain sulfide minerals. This process generates acidic solutions with heavy metals, as a result of weathering and biochemical reactions that occur in the presence of water and oxygen. Unlike acid mine drainage (AMD), which also produces acidic solutions due to the presence of sulfide minerals, AMD originates exclusively in contexts linked to mining activity.

Acid rock drainage generates sulfuric acid and dissolves heavy metals, reducing the pH of the water to values ​​between 1.5 and 5. The presence of ferric or ferrous ions produces reddish-brown or greenish-blue colors, respectively. When acidic and non-acidic waters mix, surface deposits of reddish iron oxides are formed. This process deteriorates the quality of water, soil and ecosystems, with impacts that can extend downstream. [1]​.

Direct factors

The formation of ARD occurs in the presence of a sulfide mineral and oxidants:[2]​.

Indirect factors

Temperature, water availability and pH are key factors in the generation of acid rock drainage (ARD). An increase of approximately 10 °C doubles the rate of chemical reactions, accelerating the formation of DAR. Rainfall and water sources influence soil saturation and the transport of oxidation products, especially in materials with high hydraulic conductivity. In acidic conditions (pH less than 5), heavy metals remain dissolved and favor the activity of acidophilic bacteria such as Acidithiobacillus ferrooxidans. However, as the pH increases, the metals precipitate as ferric hydroxides, forming orange deposits visible in the channels.