Early Hydrological Monitoring of Cadia's Instrumented Trial Waste Rock Dump

摘要

Understanding how rainfall infiltrates, is stored within and is transported through surface waste rock dumps is essential to predicting the release rates of mineral weathering products to the environment. Infiltration is commonly assumed to be 50% of annual rainfall. There are however a lack of field data to confirm this. Further, there is poor understanding of the fate of rainfall infiltration; the proportion that goes into storage within the dump and the proportion that emerges from the base of the dump as seepage. In addition, there is poor understanding of the size of rainfall event necessary to trigger base seepage and the time delay before base seepage emerges following the rainfall event, and how these parameters change as the dump wets-up. Waste rock dumps are heterogeneous in nature, with conventional end-dumping of waste rock from a tip head resulting in a base rubble zone, alternating angle of repose coarse and fine-grained layers within the dump, and a traffic-compacted top surface, creating highly non-uniform pore distributions and preferred flow paths. These preferred flow paths are likely to dominate the transport of water through the dump. A 15 m high trial waste rock dump comprising potentially acid forming material has been constructed at Cadia Hill Gold Mine in New South Wales, Australia. It has been instrumented with two surface lysimeters and 24 base lysimeters to monitor infiltration through the top of the waste rock dump and base seepage beneath the top surface and the side slopes of the dump. The paper describes the design and construction of the trial dump and its instrumentation, and the results of the first 2 years and 8 months of monitoring to 1 September 2008.