GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 61 . 3.6.2 Gladstone TSF discharge to receiving environment Discharge of groundwater that is predicted to migrate to a surface water body from the TSF is presented in Table 3.16. Table 3.16 Estimated discharges from the pit area to surface water in the current conditions scenario, and from the TSF in the operational, TSF closure and long-term TSF scenarios Pit section Estimated groundwater discharge to surface water bodies (m3/day)* Current conditions Operational TSF TSF closure Long-term TSF North (TB5) 0 0 0 0 East (OH3) 0 0 0 0 South (TB4) 0 0 0 0 West (OH6) 4 0 0 65 (5 - tailings porewater) Total 4 0 0 65 Notes: - Considering model uncertainty, results are rounded Estimated groundwater discharge from the proposed Gladstone Pit area (current conditions) that is predicted to migrate to a surface water body is only 4 m3/day to the Ohinemuri River (OH6). This relatively low flow is primarily due to the limited thickness and saturation of the shallow groundwater system across the elevated Gladstone Hill and Winner Hill areas. Leakage to the deep groundwater system due the downwards hydraulic gradients also removes a small amount of shallow groundwater which would otherwise flow to surface water (Figure 3.5 and Figure 3.6). During development of the TSF (operational TSF scenario), there is no predicted discharge of groundwater from the TSF to the shallow groundwater system, as all recharge through the TSF is predicted to discharge to the deep groundwater system (Figure 3.16). In the TSF closure scenario, groundwater flux into the TSF is from groundwater upwelling following rewatering of the deep groundwater system, with rainwater infiltration through the capping layer also contributing water to the TSF. However, the TSF is hydraulically contained by the drainage system which generates a groundwater gradient towards the base of the tailings (Figure 3.17), again resulting in no discharges to the shallow groundwater system. In the long-term TSF scenario, the TSF drainage system is no longer operational which allows groundwater to flow into the TSF and discharge to the adjacent shallow groundwater system where it migrates to surface water bodies (Figure 3.18). The relatively permeable rock backfill which forms the TSF sub-grade provides a preferential flow path for groundwater upwelling from the deep groundwater system with limited contact with the tailings, while infiltration through the capping layer migrates towards the pit rim in rock backfill placed above the tailings. As such, only a small volume of water migrating through the TSF to the shallow groundwater system is predicted to be influenced by tailings. Discharge from the TSF in the long-term TSF scenario is predicted to occur through the western area of the pit. The maximum rock backfill and TSF capping layer elevation of 1,107 mRL means that the TSF only contacts the relatively permeable young volcanics that host the shallow groundwater system at the western edge of the pit. To the west the shallow groundwater system extends as low as approximately 1,100 mRL at the pit boundary. In the long-term TSF scenario, the groundwater level with the TSF is predicted to be higher than this, creating saturated flow outwards from the TSF towards the Ohinemuri River (OH6). To the north and south, the TSF is only in contact with lower permeability andesite and/or breccia. To the east, a hydraulic divide is predicted close to the pit boundary, creating inwards hydraulic gradients from the shallow groundwater system towards the TSF, inhibiting discharge in this direction.
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