GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 144 7. Conclusions and recommendations This report provides assessment of four of the components of WNP: The Gladstone Open Pit (GOP) and tailings storage facility (TSF), Northern Rock Stack (NRS), Tailings Storage Facility 3 (TSF3) and the Willows Road rock stacks (WRS) associated with Wharekirauponga underground mine (WUG). It is concluded that the potential effects to groundwater and surface water quality associated with development, operation and closure of these mine components may be measurable in localised areas, but are no more than minor. Each mine component is recommended to have a monitoring plan, outlining requirements for monitoring (for effects to be assessed against), site-specific trigger levels, and mitigation / contingency measures to manage unexpected adverse outcomes. The assessment conclusions and recommendations for each mine component are summarised below. The cumulative assessment of discharges to the Ohinemuri River catchment is provided in the GHD (2022a) Water Management Report. 7.1 Gladstone Open Pit and Tailings Storage Facility Excavation of the Gladstone Open Pit (GOP) is proposed to comprise two conjoined pits over Gladstone Hill and Winner Hill to a maximum depth of 1,005 mRL. While existing mine dewatering activities are considered to have largely created a situation where the shallow groundwater system is under-drained in close vicinity of the proposed GOP, the Gladstone vein system southwest of the pit is not expected to be currently dewatered. Following excavation to maximum pit depth, the pit will be developed as a TSF, including a drainage system to contain tailings seepage until a significant improvement in seepage water quality is apparent. Rewatering of the deep groundwater system will occur after all mine dewatering activities at Waihi cease, restoring deep groundwater levels to those similar to pre-mining conditions. Groundwater levels and flow direction will be influenced by the proposed elevation of the Martha Pit Lake (1,104 mRL) and the hydraulic connections created by underground mining. This is expected to have a controlling influence on groundwater pressure beneath the Gladstone TSF in the long-term. The assessment of effects on groundwater and surface water for Gladstone Open Pit and the Tailings Storage Facility is provided in Section 3.7, with the below summarising the predicted effects. – Excavation of the GOP and dewatering of the Gladstone vein system: Assuming the veins are currently saturated, groundwater inflow to the excavated pit from the vein is estimated to be up to 1,100 m3/day during initial inflow as groundwater is drained from storage, reducing to up to 325 m3/day as equilibrium conditions are achieved. Given the limited extent of ore body southwest of the proposed GOP and generally limited hydraulic connection outside of the immediate vein system, these estimates are expected to represent an upper bound. Inflow from the shallow groundwater system is expected to be small, and in the order of 9 m3/day. Is predicted to reduce groundwater discharge to the Ohinemuri River by up to 55 m3/day to the west (towards monitoring location OH6) and <1 m3/day to the east (towards monitoring location OH3) and 20% reduction in stormwater runoff and interflow. Such reductions in river flow are expected to be unmeasurable as the area of the OH3 and OH6 sub-catchments that interact with Gladstone Hill total approximately 30 ha, which is very small in the context of the total Ohinemuri River catchment of approximately 29,000 ha. Dewatering of the vein system is expected to result in limited impact on the deep groundwater system. Assuming the vein is currently saturated to the southwest of the proposed GOP, mining is predicted to see the desaturated zone extend to the southwest just beyond the Ohinemuri River. Loss of water from the Ohinemuri River due to dewatering in the andesite is predicted to be minimal, owing to relative separation of the shallow and deep groundwater systems. No groundwater or surface water users are predicted to be impacted by dewatering within the vein system.
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