Planning for closure at Westmin's Myra Falls Operations (British Columbia) began indirectly in the early 1980's with hydrogeologic and water-chemistry studies downstream of the primary waste-rock dump and mine workings. The hydrogeologic investigation was expanded significantly in 1988 and again in 1990, culminating in a Decommissioning Plan submitted in 1992. The Plan and its supporting technical documentation defined strategies to minimize acidic drainage and metal leaching in waste rock, underground workings, and pit walls. A primary target for decommissioning was Waste-Rock Dump #1. Dump #1 contains approximately 10 million metric tonnes (mt) of coarse-to-fine pyritic waste rock. This dump was built against a valley wall and is approximately 800 m long parallel to the wall by approximately 300 m wide at the base. It reaches a maximum measured height of 42 meters. Sixty-five boreholes have been drilled into the dump since 1988. Over 230 rock samples have been submitted for acid-base accounting, providing stratigraphic data on sulfide and neutralization potential. Five boreholes contain multilevel gas ports for a total of 25 ports to monitor internal levels of oxygen and carbon dioxide. One year of monitoring data showed that oxygen could vary from essentially zero to full atmospheric level at any particular depth and lateral location within the dump. Overall, sulfide-mineral oxidation within the dump was not oxygen limited. Four boreholes contain thermistor strings for a total of 20 thermistors. These thermistors have transmitted temperature data to dataloggers every 12 h for the last three years. The temperature data show a maximum value of51.6°C at intermediate depths. During precipitation and subsequent infiltration events, the upper and intermediate temperatures are cooled by the infiltration, but the deeper temperatures are raised by the infiltration because it has warmed to a maximum of 52°C. A total of fifty-one 0. 75-inch-diameter and five 2-inch-diameter piezometers have been installed within and below the dump. Four of the 2-inch wells contain pressure transducers which have transmitted groundwater levels to dataloggers every 12 h for the past three years. Fluctuations of up to 4 m have been recorded during intense storm events of approximately 10 cm of rain in 12 h. Infiltration events from individual storms were tracked downward through the dump by their effects on internal temperature and basal water-table levels. This tracking shows that infiltration can pass through up to 45 m of waste rock within 12 h, based on initial temperature responses, to 36 h, based on midpoints of water-table responses.
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