GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North i Executive summary Overview GHD was commissioned by OGNZL to deliver a number of studies relating to groundwater for the Waihi North Project (WNP). This report provides the assessment of effects to groundwater and surface water of the proposed Gladstone Open Pit (GOP) and the new rock and tailings storage facilities of the WNP, which are proposed to include: – GOP Tailings Storage Facility (TSF), to be emplaced within the GOP after closure. – Northern Rock Stack (NRS), to be located north of the existing Tailings Storage Facility 2 (TSF2). – Tailings Storage Facility 3 (TSF3), to be located east of Tailings Storage Facility 1A (TSF1A). – Willows Rock Stack (WRS), near the Wharekirauponga Underground Mine (WUG) access portal. Assessment considers the potential for each individual component to influence groundwater flow and levels, baseflow in surface water and water quality. 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 at most minor in nature. Each mine component is recommended to have a monitoring plan, outlining requirements for monitoring, site-specific trigger levels, and mitigation / contingency measures to manage unexpected adverse outcomes of undertaking the activities at each site. The assessment conclusions and recommendations for each mine component are summarised below. This report is subject to, and must be read in conjunction with, the limitations set out in Section 1.5 and the assumptions and qualifications contained throughout the Report. 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. Following cessation of mining at GOP, the pit will be developed as a TSF, including partial backfilling with rock, installation of a geosynthetic liner to reduce discharges and a TSF drainage system to control groundwater. When underground mining ceases in Waihi dewatering will cease and deep groundwater levels will recover. The deep groundwater flow direction and levels are expected to be controlled by the Martha Pit Lake level. This rewatering of the deep groundwater system is assumed to occur during the period of TSF closure and be the background condition for the TSF in the long term. Potential effects associated with the GOP are considered to relate to dewatering and loss of surface water catchment, which includes the lowering of groundwater levels, reduction in flow to surface water and depletion of water resources which may affect users of water. The effects associated with the GOP dewatering are assessed as follows: – Deep groundwater: Existing mine dewatering activities are considered to have largely dewatered the vein system and andesite northeast of the proposed GOP. Dewatering during mining is predicted to result in this vein desaturation extending to the southwest and potentially a small distance beyond the Ohinemuri River. Desaturation of the host andesite outside of the veins is, however, expected to be limited, as seen elsewhere in Waihi. The effects to the deeper groundwater system are therefore considered to be limited. – Shallow groundwater: The GOP excavation is predicted to result in only localised drawdown of the water table, with measurable changes only predicted immediately adjacent to the pit. – Surface water: The change in groundwater flows and run-off to the Ohinemuri River, due to loss of catchment, are considered to be negligible in the context of river flow. Changes to groundwater flow to the Gladstone wetland is expected to be small and unmeasurable given the significantly greater influence of runoff in the wetland water balance. – Water users: There are no groundwater users within the predicted zone of influence of GOP dewatering and the nearest groundwater and surface water takes are not expected to be adversely affected by dewatering. – Monitoring and management: Monitoring of inflows to the pit and groundwater levels in both shallow and deep groundwater systems around the perimeter are recommended. In particular, monitoring effects within the veins southwest of the GOP will allow validation of the predicted extent of dewatering effects. Monitoring
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