Acid rock drainage (ARD) is a natural part of our global heritage. It occurs when sulphide minerals in rocks are exposed to the weathering effects of oxygen and water, and results in the chemical change of the sulphur within minerals to sulphuric acid. The acidity in ARD is first generated from the oxidation of sulphide minerals, a process often accelerated by microbiological activity. Acid drainage results when the acidity is entrained by water flowing through areas of sulphide oxidation.
While ARD is a natural process, its potential magnitude can be greatly increased by mining. In most undisturbed areas sulphide oxidation is restricted to the thin unconsolidated surface layer and a few small outcrops. In mining, the extraction, cracking and crushing of the sulphide bearing bedrock in underground workings, pits, tailings and rock dumps greatly increases the exposure of rock to oxygen and drainage. The potential for environmental impact is further accentuated at some precious and base metal mines by the abnormally high concentrations of one or more trace elements.
These guidelines are an attempt to provide formal-flexible guidance on ARD, while not limiting the options and approaches. Users are cautioned to consider the specific site and materials, when deciding which and how rules and procedures should apply. At some points, worst-case assumptions are used as the screening criteria for determining whether additional testwork is required. The final criteria used for waste handling and remediation decisions should be based on site-specific test results.