GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 47 . passes beneath the Ohinemuri River southwest of the proposed GOP. The model was used to predict groundwater drawdown within the shallow and deep groundwater systems, and potential changes in Ohinemuri River flow. The model set-up considers a situation where the deep groundwater system southwest of the proposed GOP is not currently dewatered as a result of current mine dewatering (Section 3.3.3). The assessment modelled the following scenarios: i. Current conditions: Base model representing the existing environment. ii. Gladstone vein dewatering: Dewatering of the Gladstone vein system to maximum proposed GOP depth (1,005 mRL). b. Existing data recorded during mine dewatering of the Favona system was used qualitatively to predict groundwater inflow rates from the deep groundwater system during excavation of the GOP. This assessment considered a scenario where the deep groundwater system southwest of the proposed GOP is not currently dewatered as a result of mine dewatering. 4. Surface water catchment assessment: Development of a surface water catchment model to predict changes in catchment area following excavation of the proposed GOP and capping of the TSF. 5. Water quality assessment: Development of a contaminant mass mixing model to predict impacts to groundwater and surface water quality after operation of the TSF drainage system is discontinued (long-term TSF scenario). Results from AECOM (2021b) geochemical equilibrium modelling to generate a groundwater quality equilibrated for mineral saturation for the TSF discharge were used alongside the predicted groundwater flows from the long-term TSF assessment and existing water quality data in a mixing model to provide fully mixed water quality predictions for the Ohinemuri River. No water quality assessment was required for the GOP excavation and operational and TSF closure scenarios, as no discharges to the environment are predicted to occur. Discharge of dewatered groundwater and stormwater from the GOP during excavation, and groundwater discharged from the TSF drainage system during operation and TSF closure scenarios, will be treated at the WTP prior to discharge to the Ohinemuri River (further discussed in the Water Management Studies Report (GHD, 2022a)). Refer to Appendix G for detailed descriptions of the assessment methodologies. 3.5 GOP excavation analysis results The below sections provide summarised assessment results, with additional detail provided in Appendix G. 3.5.1 Groundwater levels and flow Shallow groundwater levels The predicted groundwater drawdown of the shallow water table from current conditions at the edge of the pit in the GOP excavation assessment scenario is presented in Table 3.5. Groundwater drawdown at the edge of the pit is expected to range between 3-8 m, with a drawdown zone of influence (ZOI) extending up to approximately 290 m. The magnitude of drawdown corresponds with the depth of the saturated material of the shallow groundwater system exposed at the pit rim. Distance drawdown graphs and the predicted groundwater water table with distance from the pit are presented in Appendix G. Predicted shallow groundwater levels and groundwater flow paths for the GOP excavation scenario are also presented in the cross sections in Figure 3.12. Groundwater levels at the Gladstone Wetland are predicted to reduce by approximately 0.5 m.
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