| Project Description | WNP Wharekirauponga Underground Mine - Assessment of Effects of Discharges to Air | 4397169-66885702-23 | | 14 Figure 2-2 Example tunnel portal (Source: OGNZL) 2.2.4 Rock stack /clean and dirty water collection ponds The Willows Rock Stack (WRS) of approximately 5.5ha in area will be developed as shown in Figure 2-3. The WRS will have a total capacity to store up to 900,000m3 of rock. The stack will be constructed in 10m lifts up to a height of 20m above natural ground level. Rock will be progressively returned underground to backfill the mine during the stoping operation. This will be undertaken with a loader and truck via the Willows Road Portal. Once the WRS is exhausted, rock for backfilling stopes will be obtained from other Waihi mining operations. Rock stored in the NRS will be transported via reversal of the existing loadout conveyor from the NRS to an interim stockpile adjacent to the WUG portal. Ore trucks will be backloaded with this rock for returning to the mine. Runoff water and drainage water from the rock stack will be collected in a collection pond located to the east of the stack. This pond, which will have an approximate capacity of 19,000m3, is designed to contain a 1 in 10-year storm event over a 24- hour period. Collected water will be pumped to the existing Waihi WTP for treatment. A separate runoff retention pond will also be located to the east of the Surface Facilities Area. This pond will collect and treat stormwater from the facilities area before discharge to the nearby Mataura Stream.
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