GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 145 Gladstone wetland water levels are controlled by culvert level, with stormwater and interflow dominating the water flow to the wetland. A reduction in groundwater baseflow to the Gladstone wetland of approximately <0.5 m3/day is predicted to occur during excavation of the pit, with this change in baseflow being very small in the context of the wetland water balance and expected to result in only minor change in the wetland water balance under drought conditions (GHD, 2022d). Diversion of water to the wetland to support flow is expected to mitigate meaningful reduction in levels and flow. – Operation of the Gladstone TSF: Due to the use of drainage during TSF development and into closure, predicted effects are effectively limited to those predicted as a result of dewatering during GOP excavation. No discharge of water from the tailings or placed rock to shallow groundwater system or surface water is predicted to occur. Instead seepage through the TSF is expected to percolate to the deep groundwater system where it will migrate with groundwater through the network of hydraulically connected veins, fracturing and workings towards the deeper underground mine dewatering locations, captured and directed to the WTP. Discharge via of tailings porewater via this flow path is predicted to be in the order of 187 m3/day, with effects to groundwater quality expected to be at most minor when considering the influence of underground mining and backfilling activities. – TSF after mine closure and rewatering of the deep groundwater system: Rewatering of the underground mines and formation of Martha Lake will see changes in groundwater levels and hydraulic gradients across the wider Waihi area, resulting in small increases in total groundwater discharge to the surface water receiving environment. Following closure, the TSF drainage system will control groundwater levels and create inward hydraulic gradients. This is expected to provide hydraulic containment of the TSF, effectively limiting the potential for discharges to the environment. Effects to groundwater and surface water quality during this stage are therefore expected to be negligible. In total, approximately 1,250 m3/day of water is predicted to be captured by the drains, with this comprising predominantly deep groundwater (upwelling via the vein network) with a small proportion of tailings porewater (infiltrating downwards). For mine closure the TSF cover will be contoured to allow stormwater discharge to the south, to the Gladstone Wetland. This reinstatement of the wetland catchment is expected to mitigate the temporary reductions in run-off generated flow experienced during mining of GOP and development of the TSF (GHD, 2022d). The cover layer is also predicted to provide effective separation of stormwater from tailings, ensuring that the wetland water quality can be effectively managed. – Long term effects of the TSF following cessation of drainage: Where drainage ceases control on groundwater levels within the deeper andesite, the hydraulic connection to underground workings and Martha Lake will result in deeper groundwater flowing up into the TSF. Discharge from the TSF is predicted to occur to the shallow groundwater system west of the Gladstone TSF, where it will ultimately flow to the Ohinemuri River. This discharge is predicted to comprise a mixture of upwelled deep groundwater that migrates through rock backfill, a small volume of water that infiltrates through the tailings and water that infiltrates the TSF cover and flows through the capping layer to the west. Changes to the Ohinemuri River water quality as a function of the discharge, even when excluding potential attenuation of contaminants during migration to the river, is predicted to be negligible and within the RWQC. No groundwater or surface water users are predicted to be impacted by discharges from TSF. 7.2 Northern Rock Stack Storage of rock from WNP mining activities is proposed at the Northern Rock Stack (NRS), with this to be constructed over the existing northern stockpile north of TSF2. Placement of a low permeability soil liner and installation of leachate and sub-soil collection drains is proposed to limit the potential for leachate to impact the
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