One of the most significant environmental issues that can be associated with mining operations arises from the presence of sulphide minerals. Sometimes these are the ore minerals themselves, such as those constituting the common ores of copper and zinc. More commonly, sulphide minerals are widespread in the rocks surrounding orebodies. The minerals can be exposed to air and water in the walls of the excavations themselves - open pits and underground voids, and in the mineral wastes - tailings and waste rock, stored at or near the mine site.

Air and water can lead to accelerated weathering of sulphide minerals, producing acid solutions - acid rock drainage (ARD). ARD can contain mobile constituents of weathered minerals, including heavy metals. These solutions can enter surface and groundwater causing negative impacts to soils, water and ecosystems. ARD continues to emanate from many historical mine sites around the world and is a negative aspect of the mining industry's legacy.

ARD formation is not inevitable, and techniques have been developed to provide reliable evaluation of the risks and consequences. Many prevention and control techniques exist to reduce these risks to acceptably low levels.

Rio Tinto employs a range of best practice techniques at operations where ARD risk has been identified. These techniques include storage of wastes under water to prevent exposure to air, and the use of compaction and cover layers to control air ingress and water infiltration. Rio Tinto was a founder member of INAP - the International Network for Acid Prevention - and continues to contribute to cutting edge research into ARD through this organization.