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Cover System Design:

The application of a dry cover system over reactive waste rock or tailings is becoming a common technique for preventing and controlling acid mine drainage following closure of a mine site or waste storage facility. Typically, the objectives of dry cover systems are to minimize the influx of meteoric water and provide an oxygen diffusion barrier to minimize the ingress of atmospheric oxygen.

However, it is fundamental to ensuring long-term performance that cover systems are designed as unsaturated systems whose performance is a direct function of site-specific climate conditions. Therefore, achieving continuous control of oxygen ingress may not be possible due to sustained arid and semi-arid conditions. In these cases, acid mine drainage would be controlled by a moisture "store and release" cover system to limit percolation to the underlying waste. Apart from limiting water and oxygen, dry covers are expected to be resistant to erosion (i.e. maintain physical stability), and provide a medium for supporting vegetation that is consistent with the end land use.

Dry covers can be simple or complex, ranging from a single layer of earthen material to several layers of different material types, including native soils, non-reactive tailings and/or waste rock, geosynthetic materials, and oxygen consuming organic materials.

Multi-layer cover systems typically utilize the capillary barrier concept to keep one (or more) of its layers near saturation under all climatic conditions. This creates a "blanket" of water over the reactive waste material, which reduces the ingress of atmospheric oxygen and subsequent production of acidic drainage.

At many mine sites, the design and construction of a cover system for closure of a waste storage facility represents the single biggest issue, not only with respect to environmental impact and cost, but also public and regulatory scrutiny. In our experience, there has been a general tendency by regulators to develop performance criteria for cover systems that are tied directly to various cover design objectives. In many cases, this practice has led to the development of single, often very conservative, numerical values of cover performance criteria such as “net percolation”, “rate of oxygen ingress” and/or “plant density/mixture”. In our opinion, there is a need to develop cover performance criteria on a case-by-case basis and with due consideration of the short-term and long-term impacts on the receiving environment at a particular site. Further details on this approach can be found in the following paper.

O'Kane, M. and Wels, C., 2003. Mine Waste Cover System Design – Linking Predicted Performance to Groundwater and Surface Water Impacts. Proceedings of 6 th International Conference for Acid Rock Drainage, Cairns, Qld., Australia, July 2003.

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