GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 39 This appendix presents the methodology and results for technical assessment for excavation of the Gladstone Open Pit (GOP) and development as a Tailings Storage Facility (TSF). All elevations are presented in metres reduced level (mRL) with reference to Waihi Mine datum, which is 1,000 metres below sea level. Appendix G Contents G.1 Assessment methodology G.1.1 GOP and TSF groundwater assessment methodology Groundwater modelling and simplistic groundwater catchment model for excavation of the GOP and development of a TSF, considering a situation where the shallow groundwater system is currently under-drained as a result of existing mine dewatering. G.1.2 Gladstone vein dewatering assessment methodology Groundwater modelling and empirical assessment for dewatering of the Gladstone vein system, considering a situation where the deep groundwater system southwest of the proposed GOP is not currently dewatered. G.1.3 Surface water catchment assessment methodology Simplistic surface water catchment model to predict changes in catchment area following excavation of the GOP. G.1.4 Water quality assessment methodology Simplistic contaminant mass mixing model to predict impacts to groundwater and surface water quality after development of the TSF. G.2 Assessment results G.2.1 GOP and TSF assessment results G.2.2 Gladstone vein dewatering assessment results G.2.3 Surface water catchment assessment results G.2.4 Water quality assessment results G.1 Assessment methodology G.1.1 GOP and TSF groundwater assessment Groundwater flow paths and discharges under current conditions, GOP excavation, operational TSF, TSF closure and long-term TSF were predicted utilising two methodologies: groundwater modelling and groundwater catchment assessments. This assessment considers a scenario where the shallow groundwater system is currently under-drained as a result of existing mine dewatering GOP vein dewatering assessment is discussed in Section G.1.2. G.1.1.1 Groundwater modelling methodology Groundwater modelling was undertaken using GeoStudio 2021 SEEP/W finite element numerical modelling software. Two two-dimensional (2D) sections were developed to model cross sections through a north-south orientation and east-west orientation through the proposed GOP and TSF (Figure G.1). The model orientations were selected to understand how future mining works may impact shallow groundwater and surface water. – North-south cross section: 1.1 km in length, considers shallow groundwater flow towards local tributaries (at TB4 and TB5) and the Gladstone wetland. The model was oriented through the deepest pit section, starting from Union Hill to the north, through to the watercourses and wetland south of the pit. – East-west cross section: 1.4 km in length, considers shallow groundwater flow towards the Ohinemuri River. The model was oriented through the deepest pit section, starting from the Ohinemuri River to the east (near sampling location OH3), through to the Ohinemuri River to the west (near OH6). This model alignment also incorporates an extensive unit of permeable sandy ignimbrite west of the pit. The assessment was undertaken for the scenarios presented in Table G.1.
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