Acid rock drainage (ARD)
Acid rock drainage, or ARD, occurs when sulphide bearing minerals in ores and mineral wastes oxidise in the surface environment. It is a natural phenomenon which occurs when freshly mined sulphide minerals are exposed to air and water which causes the sulphides to oxidise to produce soluble sulphates. Mining accelerates the amount of fresh rock exposed to the air, and may enhance the potential to produce ARD.
ARD contains strong mineral acids and metal ions, and these solutions can in turn leach more constituents as they migrate through mineral bearing structures such as tailings dams, stockpiles, mine workings and waste rock dumps. Even when naturally occurring neutralising minerals and rocks counteract the acid, the resulting solutions can still have elevated levels of dissolved constituents. This is termed neutral drainage.
Untreated ARD has the potential to cause the pollution of great stretches of surface waterways - lakes, streams and rivers - and groundwater. Land affected by ARD may be unsuited to any other beneficial use. Ecosystems, especially aquatic systems, have often been destroyed by ARD from mines.
We recognise that ARD is a strategic issue for Rio Tinto and we have programmes in place to prevent and to effectively manage it.
In 2003, Rio Tinto commenced work on an ARD strategy, with the aim of reducing if not eliminating the risks and liabilities arising from this significant issue. The strategy is comprehensive, dealing with methods for improving our performance in ARD management, strengthening our decision making with regard to ARD, and making clear our commitment and intentions for reducing ARD risk. The elements of the strategy were further developed in 2004 and 2005 for consultation, review and final approval in 2006.
The most advanced element of the strategy is a Group wide review of ARD risk, which is carried out by teams of internal and external experts using a detailed technical protocol developed specifically for the purpose. The ARD risk review programme commenced in 2004 and initially focused on operating sites which represented the highest potential for ARD risk. The review continued throughout 2005, covering the next tranche of high to medium hazard sites and these will be completed throughout the Group in 2006. Remaining low hazard sites will be covered by existing assurance programmes such as the four yearly HSE review.
The reviews conducted to date have identified some significant risks associated with ARD and its management, but they have also revealed many good practices. Another element of the strategy has been to evaluate the review findings and to amend the Rio Tinto Environmental Standard and Guidance for ARD prediction and control accordingly. All businesses were required to be compliant with this standard, effective July 2005. The design of the risk review programme and the summary results of the 2004 and 2005 reviews will be reported at the 7th International Conference on Acid Rock Drainage (ICARD) in St Louis, MO, U.S.A. in March 2006.
Rio Tinto was a founding member of INAP - the International Network for Acid Prevention - and continues to play a prominent role. INAP is a global network of a number of major mining companies, working collectively to combat all aspects of ARD, including prevention, prediction and treatment. INAP sponsors priority research activities, supports and promotes technology transfer, and actively shares examples of good practice across its membership and the broader industry. INAP also promotes international exchange of information through a global alliance with recognised centres of excellence in ARD in Canada, the US, Australia and Europe. As part of the Rio Tinto strategy we will review our knowledge gaps, identify our research needs and take action to meet these, both through INAP and our own programmes.
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