A clay-capping trial was established in 1996 on Magazine Dump at the Mt Lyell Copper Mine. The trial was initially constructed under the Mount Lyell Remediation Research and Demonstration Program, a co-operative venture between the State and Federal governments. The detailed engineering design for the clay cover included the following features; reshaping and compaction of dump batters, 5 m vertical bench interval, overall side-slopes of 2.5:1, emplacement of clay in four 150 mm lifts, compaction by a 10 t vibrating roller or 'track rolling' on batters with density testing for quality control. A small section at the base of the dump (<10 per cent of dump surface area) was not capped. The dump was instrumented with seven 20 m deep temperature and oxygen probe holes. Six lysimeter drums were installed under the cover. Leachate from the dump passes through a single, continuously monitored gauging weir. Five years after construction, the clay cover exhibits a high degree of moisture saturation. The lysimeter data suggest that infiltration into the dump through the well-compacted dump plateaux is approximately ten per cent of precipitation. Conductivity flux emanating from the dump shows a very gradual decreasing trend and the autumn pollutant flush events are much diminished. These findings are consistent with a successful Temperate Zone capping project. The trial also generated certain unexpected findings. Field permeability tests on the cover yielded uncertain results and oxygen remains available throughout the dump. The flow data indicate that, while total outflow has decreased as a result of storm shedding, inputs from groundwater and batter infiltration are still significant. Outflow during the driest weeks of the year may have risen following capping. Detailed assessment of the data is hampered by insufficient pre-capping information; however, there is a possibility that conductivity flux emitted when flows in the receiving environment are at their annual minimum is unchanged or worse post-capping.
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