Chapter 2

2. The ARD Process
Last Update: June 2009

GARD Guide Chapter 2

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Acid rock drainage is formed by the natural oxidation of sulfide minerals when exposed to air and water. The drainage produced from the oxidation process may be neutral to acidic, with or without dissolved heavy metals, but always contains sulfate. ARD results from a series of reactions and stages that typically proceed from near neutral to more acidic pH conditions. The process of sulfide oxidation and formation of ARD is very complex and involves a multitude of chemical and biological processes that can vary significantly depending on environmental, geological and climate conditions. Stopping ARD formation, once initiated, may be challenging because it is a process that, if unimpeded, will continue (and may accelerate) until one or more of the reactants (sulfide minerals, oxygen, water) is exhausted or excluded from reaction. The ARD formation process can continue to produce impacted drainage for decades or centuries after mining has ceased.

The sulphide oxidation process central to ARD generation is explained in great detail in this chapter, in addition to the biological, chemical, and physical factors that govern the rate of sulphide oxidation.

Table of Contents

  • 2.0 The Acid Rock Drainage Process
  • 2.1 Introduction
  • 2.2 Acid Rock Drainage, Neutral Mine Drainage, and Saline Drainage
    • 2.2.1 Definition of Acid Rock Drainage, Neutral Mine Drainage, and Saline Drainage
    • 2.2.2 Natural vs Anthropogenic Drainage
  • 2.3 History of Acid Rock Drainage
  • 2.4 The Acid Generation Process
    • 2.4.1 Characteristics of Acid Rock Drainage, Neutral Mine Drainage, and Saline Drainage
    • 2.4.2 The Global and Geochemical Sulphur Cycles
    • 2.4.3 Acid Rock Drainage Sources
    • 2.4.4 The Sulphide Oxidation Process
    • 2.4.5 Reaction Products from Sulphide Oxidation
    • 2.4.6 Neutralization Reactions
    • 2.4.7 The Acid Rock Drainage Process and Migration
  • 2.5 Concluding Statement
  • 2.6 References
  • List of Tables
  • List of Figures