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Acid rock drainage (ARD) is a world-wide phenomenon that may occur naturally, as in the formation of gossans that are associated with oxidised mineral deposits, or in less complex situations in which streams traverse sulfide-bearing, typically pyritiferous, rocks whose minerals are incapable of neutralising the acidity that is generated by the oxidation of the host-rock sulfides. In most ARD situations, however, anthropogenic activity has been directly responsible for causing or exacerbating ARD situations by exposing or increasing the exposure of sulfide-bearing materials so that they are susceptible to atmospheric oxidation and the attendant acid generation.

The release of acidic effluents can be directly harmful to aquatic life, and there may also be damage to subaqueous and subaerial habitats. A second effect, which probably has a greater environmental impact than the acidity itself, is that the low-pH solutions increase the solubilisation of potentially toxic metals and semimetals such as Pb, Zn, Cu, Cd, Cr, Ni, Co, Al, Sb, and As. Treatment of surficial acidic effluents is commonly feasible, albeit expensive, through simple procedures such as the addition of lime to raise the pH of the collected waters; control of dissolved metals, however, is generally more complex and costly, as is discussed in the second part of this paper.

Figure 1 provides a perspective on ARD in relation to acid rain and some familiar acidic substances. Pure rain has a pH of 5.6, which represents equilibrium between pure water and atmospheric CO2. ARD is generally regarded to be present if effluent pH is between 5 and 5½ because in this pH range there is a negative impact on biota.

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