GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 67 . 3.7.2 TSF summary and discussion Final landform After mining of GOP ceases, the pit is proposed to be developed into a TSF. The excavated pit will be backfilled with PAF and NAF rock to a minimum elevation of 1,060 mRL to form a base for placement of a liner subgrade and geosynthetic liner. Tailings will then be placed to a maximum elevation of 1,103 mRL. After settlement of the tailings, the tailings pond will be drained and the tailings then covered with a capping layer with a minimum of 1 m low permeability NAF weathered rockfill. Limestone will be added to the backfilled rock at appropriate rates to provide mitigation of mine rock drainage, while the cover will control long term sulphide oxidation and potential acid generation. Groundwater During development of the TSF the deep groundwater system is anticipated to continue to under-drain the shallow groundwater system due to ongoing dewatering of the Favona mine. Predicted groundwater drawdown and groundwater levels in this scenario are similar to those in the GOP excavation scenario as the tailings pond and recharge to the tailings is not expected to influence the adjacent shallow groundwater system. Instead, tailings pore water not recovered at surface as decant is expected to percolate downwards, ultimately discharging to the deep groundwater where it will contribute to flow being captured by Favona mine dewatering. Following capping of the TSF and cessation of underground mine dewatering, represented by the long term TSF scenario, rewatering of the deep groundwater system will result in elevated groundwater levels within the deep groundwater system. As a result of this downwards vertical hydraulic gradients currently recorded between the shallow and deep system are predicted to reduce or reverse. This reduces the predicted zone of influence of the groundwater drawdown, and groundwater levels are predicted in a number of locations to be greater than current conditions, including in the vicinity of the Ohinemuri tributary to the north (TB5) and the Ohinemuri River (OH3). Groundwater levels in the vicinity of the Gladstone Wetland (TB4) are predicted to increase by approximately 3 m in comparison to current conditions. The modelling assessment indicates that the greatest influence on groundwater levels within the receiving environment is the elevation of the TSF drainage system and rewatering of the deep groundwater system (TSF closure and long-term TSF scenario). The proposed Martha Pit Lake level (1,104 mRL) is expected to impose a water level control on the connected mine workings. Hydraulic containment of the tailings and rock backfill is anticipated to be achieved by the TSF drainage system, with the drain predicted to capture both tailings pore water generated from infiltrating rainwater (approximately 58 m3/day) and deep groundwater (approximately 1,194 m3/day), upwelling from the deep groundwater system (Table 3.18). After the TSF drainage system is no longer operated (long-term TSF scenario), groundwater discharge from the TSF (approximately 65 m3/day (Table 3.16)) is predicted to be predominantly west towards the Ohinemuri River (OH6). This dominant flow path results from the presence of the relatively permeable young volcanics on the western pit face at an elevation that allows contact with the saturated rock backfill. Water being discharged from the TSF is predicted to be primarily groundwater upwelling through the rock backfill, and rainwater infiltrating the cover and migrating above the tailings towards the pit rim. Only a relatively small proportion of water is expected to be influenced by tailings (less than 10%), due to the low permeability of consolidated tailings relative to emplaced rock backfill. Surface water Prior to complete rewatering groundwater flow to surface water during TSF operation is expected to be consistent with that for GOP excavation. Likewise, during this time it is predicted that there would be no groundwater discharges from the TSF to surface water. However, the changes in groundwater levels and hydraulic gradients after complete mine rewatering are expected to result in the development of flow paths from the TSF to shallow groundwater to the west and the Ohinemuri River (OH6), with this also increasing net groundwater flow to the river.
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