This review of existing information on acid-generating waste rock was based on searches of numerous computerized databases, several visual searches of libraries, and direct contacts with researchers, technical organizations, and mining companies. Relevant documents were examined for detailed information on theory, laboratory experiments, modelling, and field studies. Documents were also critically assessed to determine their validity, applicability, and weaknesses.

In order to organize and present the findings of the critical review, waste-rock piles were conceptually viewed as natural systems with transport of water, solids, and gases through the piles as well as reactions operating within the piles. The internal reactions included acid generation, bacterial activity, acid neutralization, and metal leaching. The effects of pile construction, phase transport and acid-control technologies on the system were also addressed. Finally, recommendations were offered for further studies and for decommissioning of waste-rock piles.

This review demonstrated that groundwater movement through, and beneath, waste-rock piles represents the primary pathway for contaminant migration. Consequently, detailed hydrogeologic studies of piles and their surrounding environments are primary requirements in understanding and predicting the migration of acid drainage. Furthermore, the process of acid generation was found to be highly complex and most published assessments of acid generation failed to consider this complexity.