GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 54 Mataura (Existing) Mataura (Modelled Data) Consented Receiving Water Quality# Nickel (g/m3) < 0.0005 < 0.0005 0.003 0.04 0.160 Zinc (g/m3) 0.002 0.002 0.003 0.027 0.100 Antimony (g/m3) < 0.0002 < 0.0002 < 0.0002 0.03 0.030 Arsenic (g/m3) < 0.001 < 0.001 < 0.001 0.19 0.190 Mercury (g/m3) < 0.00008 < 0.00008 < 0.00008 0.000012 0.000012 Cadmium (g/m3) < 0.0001 < 0.0001 < 0.0001 0.0003 0.001 Chromium VI (g/m3) < 0.001 < 0.001 < 0.001 0.01 0.01 Lead (g/m3) < 0.0001 < 0.0001 < 0.0001 0.0004 0.0025 Major Cations / Anions Ca2+ 2.4 2.4 6.5 N/A Mg2+ 1.4 1.4 3.3 N/A Na+ 7.0 7.0 7.9 N/A K+ 1.2 1.2 1.3 N/A SO4 2+ 9.0 9.1 27 N/A #Consented values are based on the current OGNZL RC 971323 (and RC 971285, 971311, 971312, 971303-971306) for OH5 / OH6. They have been presented here for comparison as a basis for assessing the impact within the Mataura Stream *Trace element concentrations are dissolved 6.3 Waihi WBM Outputs 6.3.1 MUG Mine Dewatering The WBM was used to predict the dewatering rate that can be achieved under the current consented WTP discharge conditions and capabilities. The available dewatering rate is limited by excess treatment plant capacity after Priority One (comprising process area runoff, seepage, WUG minewater and collection pond water) waters have been treated and any restrictions of discharge volumes during low flow periods. The WBM was used to assess the MUG minewater dewatering rate requirements to meet dewatering targets over the LOM; which accounts for periods when minewater treatment may be constrained by the need to prioritise treatment of water from other sources. Figure 36 shows the predicted pump rates over time required to meet dewatering targets. The 50th percentile results typically operate at the nominated peak capacity of 15,500m3/d through to 2024 to achieve the target drawdown. Figure 37 demonstrates the total volume of the deficit with results showing that in general the deficit is small up to 2027 after which the WBM predicts a gradual increase. This deficit is tied to the increasing volume of water from WUG which the model prioritises for treatment over MUG water. The predicted underground water elevation in relation to the MUG underground stopes is provided in Figure 38 (high K scenario) and Figure 39 (low K scenario). The majority of the stopes throughout the LOM show a low risk of inundation. However, during the period 2027 through to 2028 the model shows a modelled risk of inundation within the lower stopes. This risk is greater within the high K scenario. Operationally, this risk could be reduced by additional advanced dewatering efforts prior to 2027 (currently the model limits advanced dewatering to 20 m below the target level). By utilising capacity within the WTP and discharge
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