GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 20 Figure 9 Existing Collection and Contingency Pond Locations 2.6 Mine Dewatering Minewater from the Martha Pit and Underground is made up of surface runoff and groundwater which is removed continuously to keep the mines dewatered. Experience has shown that once groundwater levels have been initially reduced, pump rates tend to stabilise until further drawdown is required. Mine dewatering is currently undertaken with pumps located in the underground Correnso mine. The Martha Pit is hydraulically linked to the underground workings. Although mining in the pit has been on hold, the pit has not filled because it is connected to the underground workings, and the pumping of water to keep the water level in the Correnso and MUG workings down has the effect of preventing the accumulation of water in and below the base of the pit. The Project Martha consents allow for dewatering to 500 mRL. GWS Ltd completed a study on Project Martha (GWS 2018) that showed that an average dewatering rate of 15,500 m3/d needs to be achieved from January 1 2020 through to early 2026 to dewater from an assumed starting level of 700 mRL to the final target level of 500 mRL. To date, the dewatering mRL reduction has been quicker than anticipated. There are constraints on the rate treated water can be discharged when flows in the Ohinemuri River are low under conditions in the current resource consent (971318). To compensate for this, it is proposed that dewatering will be undertaken at higher than the average rates where the operating regimes defined in Table 1 allow. This will allow MUG dewatering to be ahead of schedule and provide contingency during constrained periods.
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