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Transient flow and transport in unsaturated heterogeneous media : field experiments in mine waste rock Nichol, Craig Ferguson

Abstract

It is essential to understand how water is transported and stored in mine waste rock to predict the rates and quantities of mineral weathering products released to the environment. The lack of understanding of flow mechanisms is the largest of the difficulties facing efforts to predict waste rock leaching. A large scale (8m x 8m x 5m high) constructed waste rock pile experiment (CPE) has been built upon a contiguous grid of 16 drainage lysimeters to better characterize the flow of water in unsaturated waste rock. The measurement of water flow parameters in a waste rock experiment required the development and testing of new instrument methods to determine water content, using time domain reflectrometry (TDR), and matric suction, using thermal conductivity sensors. The use of a resistive coating on the TDR probe conductors is found to be a successful strategy for obtaining measurable TDR signals and hence water contents. The thermal conductivity sensors are successful in certain soil conditions, but not those present in the CPE. Water flow data from the first two and a half years of the experiment are examined. Average net infiltration to an uncovered waste rock pile is 55% of precipitation. Large rainfall events have net infiltration of 55% to 85%. Water flow is spatially heterogeneous at scales less than 2m. In-situ instrumentation is found to be a poor (non-conservative) predictor of water flow. Data are analysed from the first year of a tracer test carried out under transient infiltration. Physical mechanisms of water flow are identified including noncapillary flow, flow in macropores, and water flow in the granular matrix. Water chemistry is different between different pore sizes, and between spatially distinct areas of the pile. The residence time distribution has an estimated mean of 3.0 to 3.9 years under average infiltration conditions.

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