The aims of this work were: to assess the long-term reactivity of sulphidic wastes disposed of in a tailings dam at Bor, Serbia; to assess the reactivity of the sludges formed in a stream receiving acidic effluents; and to estimate the risk for the wider ecosystem. The laboratory assessment was carried out according to a slightly modified AMIRA protocol, using conventional free-draining leach columns. Deionized water and simulated acid rain were used as leaching media. The experimental results over a period of c. 70 weeks show increased solubilization for a number of metals and subsequently high risk for contamination of surface- and groundwater. Finally, feasible and effective remediation measures are proposed.