The Ernest Henry Mining (EHM) sublevel cave provides a hydraulic connection between the pit and the underground (UG) mine. This connection results in the potential of water inflows into the UG workings during major rain events.
An UG reservoir and pumping system has been installed to mitigate the risks from storm water inflows. Half of the UG reservoir volume is located in hydraulically conductive and fractured ground, which results in recharge to the mine aquifer during periods of water storage. To manage the additional strain that this recharge puts on the UG pumping system an optimization of the reservoir fill strategy and pumping sequence was carried out.
Prior to the 2015/16 wet season the pumping and reservoir system was tested by filling the leaking section of the reservoir with water from the active dewatering system. The reservoir was filled at a rate of 150 to 230 l/s for seven days and water levels in the reservoir as well as in the adjacent fractured aquifer were monitored. Detailed monitoring of water levels and pumping volumes allowed to quantify water loss from the reservoir and to determine recharge times and storage volumes in the fractured aquifer.
Water loss from the reservoir increased linearly with water level in the reservoir and reached a total loss of 180 l/s at a water level of 35 m. The first water level increase in the fractured aquifer was observed 41 hours after water was filled into the reservoir. The water loss into the fractured aquifer allows to utilize the fracture volume as additional storm water storage during rain events, which provides an estimated 50 ML of additional water storage. The test results allowed to develop optimized reservoir fill strategies and pumping schedules for major rain events.