This report describes the results of a study to determine which types of, and to what extent, mining wastes can contaminate ground water through leaching of acid-forming or potentially toxic constituents. The primary focus of this study was on coal mining wastes, especially overburden spoils and preparation plant refuse. In addition, the potential for contamination by leachates from clean coal stockpiles was also examined. The study approach included investigation of the amounts and geographic locations of mining wastes in the United States; the chemical character of mining wastes, especially with regard to potentially acid-forming or toxic constituents; and the extent to which acid and/or toxic components are leached from mining wastes by rainwater. To evaluate the leachability of coal wastes, a laboratory waste-leaching method was developed to specifically simulate the leaching process over a long term. Waste samples were also leached by the EPA extraction procedure and ASTM waste leaching method. Results of the three leach-test methods were compared to evaluate their relative capabilities to predict the leachability of mining wastes.