GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 58 6.3.2 Quarterly Water Balance A forecast of water gains for each year of mining is summarised from the daily model data. These gains have been compared to the allowable discharge and based on this comparison the upgraded WTP has sufficient capacity to treat and discharge water gains from the Waihi North Project for all years of mining. Given that peak treatment requirements do not occur until 2027, assumptions relating to the various input sources (most notably the groundwater inflow rates from WUG) will be validated well in advance and importantly, OGNZL will have time to adapt their water management approach in advance of the critical period. The volumes of water requiring treatment will be greater than currently experienced and as predicted for Project Martha, operators will need to maximise opportunity to treat and discharge water when river flows are high. Figure 40 (High K Scenario) and Figure 41 (Low K Scenario) show the allowable discharge against water treatment requirements predicted over the life of the mine. The difference between the mean treatment requirement and the mean WTP discharge allowance indicates spare capacity and allowance for WTP operational efficiency. The key periods during the LOM shown on Figure 40 and Figure 41 include: high treatment requirements through years 2027 to 2030 as dewatering of MUG is undertaken and increasing groundwater inflows from the WUG component are realised; reduced minewater treatment requirements from 2030 as stoping at the lower levels of the MUG is completed and dewatering requirements are reduced; and increase in cyanide water treatment from 2027 as multiple TSFs are operational and receiving tailings are discharged to the operational tailings facility. For modelling purposes it has been assumed that excess Priority One flows are also directed to the operational tailings facility, however other options for this water may be considered (refer Section 6.3.3). Overall, the projected gains of water requiring treatment on site are within the assumed WTP capacity and can be managed within assumed discharge constraints. Figure 42 shows the same data over 2028 which will likely be one of the more challenging years in terms of allowable discharge. During this period the discharge volume is constrained during low flow events over the summer period. Based on the predicted cumulative tailings deposition (refer Figure 43), the use of tailings storage facilities will generally be staged to limit periods where treatment is required of decant from more than one TSF. TSF2 will be activated near the middle of 2027 after TSF1A is completed. TSF3 will come online near the end of 2028 and will operate in parallel with GOP TSF from 2031 until the start of 2036. This dual decant period occurs later in the LOM and corresponds with a reduced WTP requirement (due to the cessation of MUG dewatering). TSF3 will be utilised solely post 2036. The increasing volume of water sourced from WUG mine will result in the proportion of cyanide water compared to non-cyanide mine water remaining low and the discharge will be able to be operated at Regime E most of the time. The RO plant is available as backup mitigation to increase the quality of treated water discharge through these periods if needed, however it has not specifically been utilised in this assessment and should be regarded as contingency.
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