GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 40 Table G.1 GOP and TSF assessment – model scenario summary Model scenario Description Current conditions – steady state (Parent model for assessment scenarios) Calibrated base model, representing the existing natural geology and hydrogeological setting which includes the shallow groundwater system being currently under-drained due to dewatering of the Favona portal infrastructure. GOP excavation – steady state The pit is excavated to the maximum depth of 1,005 mRL. Groundwater from the shallow system can flow to the pit through the pit walls. Operational TSF – steady state The pit is backfilled with NAF and PAF rock to a minimum elevation of 1,060 mRL, and on the pit walls to a minimum width of 10 m at the crest. The rock backfill is amended with limestone and compacted in situ. A liner subgrade and geosynthetic liner are placed on top of the backfill. The TSF drainage system is located beneath the geosynthetic liner at the base of the tailings. Emplacement of tailings to a maximum of 1,103 mRL. Tailings pond level set to 1,103 mRL. Shallow groundwater system remains under-drained due to Favona dewatering. TSF closure – steady state The tailings pond has been drained and the tailings are covered with a capping layer comprising a minimum 1 m thickness of NAF rockfill. The TSF drainage system remains operational. Rewatering of the deep groundwater system has occurred following the end of underground mining operations, therefore the shallow groundwater system is no longer underdrained. Groundwater pressure within the andesite set to 1,106 mRL to represent the influence of Martha pit lake (proposed level of 1,104 mRL allowing for a degree of water table increase within underground workings). Long term TSF – steady state Tailings are covered with a NAF rockfill capping layer between 1,104 – 1,107 mRL contoured towards the southern pit boundary. The TSF drainage system is no longer operational. Groundwater pressure within the andesite set to 1,106 mRL to represent influence of Martha pit lake (proposed level of 1,104 mRL allowing for a degree of water table increase within underground workings). The 2D groundwater model cross sections were used to predict groundwater levels, flow paths and fluxes: 1. Groundwater modelling a. Predicted groundwater flow across the pit boundary, broken down into: i. groundwater predicted to discharge to surface water receiving environments during current conditions and the full life cycle of the GOP and TSF. ii. groundwater flow from the shallow groundwater system into the pit during GOP excavation. iii. groundwater flow predicted to discharge to the deep groundwater system during current conditions and the full life cycle of the GOP and TSF. b. Predicted groundwater flow to surface water receiving environments, including the breakdown of flow predicted to have interacted with tailings or rock backfill of the TSF. c. Predicted changes in groundwater levels. d. Predicted tailings and groundwater flow to, and total discharge from, the TSF drainage system proposed beneath the geosynthetic liner at the base of the tailings.
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