GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North iii – Water users: No groundwater or surface water users are predicted to be influenced by the development of the NRS. – Monitoring and management: As with other rock storage facilities, monitoring of groundwater, surface water and drainage water quality is recommended to validate the assessment findings and identify potential issues associated with water quality. Tailings Storage Facility 3 Initial foundation works for the TSF3 will require removal of topsoil and excavation of compressible soils up to 20 m deep near the toe of the proposed embankment The excavation will be infilled with structural backfill prior to TSF3 construction. Dewatering is proposed to enable the excavation works. The effects of construction dewatering have been assessed as follows: – Groundwater: Dewatering will result in local decreases in groundwater levels, with the zone of influence of dewatering relatively limited. No groundwater users have been identified within this area. – Surface water: Potential impacts to the Ruahorehore stream and Ohinemuri River flow during dewatering will be mitigated by diversion of abstracted groundwater to the stream. Use of appropriately sized sediment treatment basins or devices is expected to mitigate potential impacts to surface water quality. On completion of the foundation works the starter embankment will be constructed. A network of sub-soil drains will be installed to control groundwater levels, and a low permeability soil liner and geosynthetic liner constructed over the base and side-slopes to control seepage from the tailings. A network of collector pipes and drains will be installed to collect seepage from the TSF. Following filling of the initial stage, the crest height will be raised and the low permeability soil liner extended up the side slopes to the final TSF3 height. The geosynthetic liner will only be utilised for the base and upstream face of the starter embankment. Effects associated with development of TSF3 have been assessed as follows: – Groundwater: Seepage from tailings and infiltration through the rock embankment is predicted to mix with groundwater and be captured by the TSF3 drainage network during the operational and closure periods. In the long term, after the sub-surface drains cease operating, groundwater impacted by a small amount of tailings seepage is predicted to migrate downgradient of TSF3 with deeper groundwater. Groundwater quality in the immediate vicinity of TSF3 is expected to show a minor influence of TSF3 discharges in the long-term. The spatial extent of this influence is expected to be constrained by attenuation and geochemical processes and only major ions, such as sulphate, are likely to demonstrate increased concentrations in downgradient groundwater. – Surface water: The Ruahorehore Stream is not predicted to be a receptor of discharges from tailings or water infiltrating through the embankment, but may receive run off from the nearby collection pond buttress, or very minor amounts of leakage from the collection pond. Flows in the stream are expected to be maintained and potentially increased due to a small increase in groundwater levels. The stream quality is expected to remain within the RWQC. Groundwater sourced from the TSF3 is inferred to ultimately discharge to the Ohinemuri River. As with long term groundwater quality, minor increases in sulphate are conservatively predicted in the river in the long term. No water quality parameter concentrations are expected to exceed the RWQC as a direct result of the TSF3 groundwater discharges. Where the significant delay in flow to the river and attenuation reactions with groundwater flow are considered, it is unlikely that change in Ohinemuri river water quality resulting from TSF3 discharges to ground would be measurable. – Water users: The changes to groundwater and surface water quality, where predicted to occur, are not expected to measurably impact any groundwater or surface water users. – Monitoring and management: The monitoring and management approaches applied to the existing tailings storage facility are considered appropriate for application to TSF3. Willows Rock Stack The WRS is proposed to be developed within a gully (WRS gully) through which flows an unnamed tributary of the Mataura Stream, which is a tributary of the Ohinemuri River. The WRS gully is steeply sided, with a low permeability volcanic soil cover that is considered to provide containment of the rock stack approximately equivalent to a low permeability soil liner. Clean water diversion drains will be constructed to divert run-off away
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