GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 146 environment. Prior to closure, a portion of the rock will be used to backfill GOP and underground mining stopes. The remaining rock will then be covered with a low permeability capping layer and rehabilitated. The assessment of effects on groundwater and surface water for the operational and closure phases of NRS is provided in Section 4.6, with the below summarising the predicted effects: – During operation of the NRS, leachate generated by rainwater infiltration is predicted to seep through the Zone A soil liner, however, the majority of leachate seeping through the liner, and mixed with groundwater, will be captured by sub-soil drains and diverted for treatment at the WTP as necessary. During NRS operation a residual 70 – 100 m3/day of leachate is predicted to migrate with groundwater towards the Ohinemuri River, with this reducing to approximately 50 m3/day after closure. – Water quality within the Ohinemuri River is expected to remain within the RWQC, with minimal change in trace element concentrations predicted as a result of NRS discharges, even when excluding adsorption reactions that occur with contaminant migration. An increase in sulphate may be measurable within the river, with this conservatively estimated to be an increase of between 1 – 2 mg/L. This is considered to be minor in the context of the downstream water quality changes. – No groundwater or surface water users are expected to be influenced by operation, or after closure, of the NRS. 7.3 Tailings Storage Facility 3 Development of TSF3 is proposed to accommodate tailings generated from the WNP. TSF3 will be positioned within the Ruahorehore Stream catchment and the stream will require diversion to accommodate construction dewatering during removal of weak deposits beneath the proposed embankment. To be developed in stages, the TSF3 will include both a low permeability soil liner and use of the HDPE liner across the base and starter embankment to control discharges. Sub-surface drains will be used to control groundwater pressure on the liner, and also allow capture of seepage through the liner. The assessment of effects resulting from construction of TSF3 is provided in Section 5.6, with the below summarising the predicted effects: – Construction dewatering: During excavation of the week deposits, dewatering to approximately 20 m below ground level will be required. The upper bound of the predicted take/diversion for dewatering is estimated to be 2,500 m3/day, which is less than 1% of the available aquifer management level and temporary in nature (120 days). The dewatering is predicted to have a zone of influence of up to approximately 610 m, within which no groundwater users have been identified. To accommodate the excavation, the Ruahorehore stream will be diverted to a new channel. Potential impacts of dewatering to the Ruahorehore stream and Ohinemuri River flow will be mitigated by diversion of abstracted groundwater to the stream. As such, the construction dewatering is considered unlikely to adversely affect the stream flow. – Tailings and embankment placed rock seepage: Construction of TSF3 is predicted to locally increase groundwater levels by up to 0.2 m. Mitigation for extended periods of flooding can be provided (if needed) by installing shallow drains, as is the current practice to allow farming in the area. During operation of TSF3 drains are predicted to capture all of the discharges from the tailings and the embankment. On closure, with ongoing operation of the toe drain only, approximately 60 m3/day of tailings porewater is predicted to discharge to ground and influence the deep groundwater quality. Water infiltrating through the embankment is predicted to continue to be intercepted by the toe drain following closure. Due to separation of the Ruahorehore stream catchment from the deeper groundwater, the changes to deeper groundwater quality are not expected to measurably impact surface water quality. Major ions (such as sulphate) may increase in deep groundwater to small degree down gradient from TSF3. The nearest groundwater user is located more than 1 km from TSF3 and any minor change is not expected to cause adverse effects to groundwater users.
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