GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North ii of Gladstone wetland water levels is also recommended, with diversion of stormwater to the wetland proposed to mitigate the loss of catchment during GOP mining. Potential effects associated with development of a TSF within the GOP relate to the discharge of contaminants to groundwater and surface water, impacting upon water quality and potentially affecting users of water. During operation of the TSF the deep groundwater is expected to remain dewatered by ongoing underground mining activities. No discharge of water from the tailings or rock backfill to the shallow groundwater system or surface water is predicted to occur. Seepage through the TSF liner is expected to percolate to the deep groundwater system, where it will migrate with groundwater towards the deeper underground mine dewatering locations. On closure of the TSF, assuming cessation of all mine dewatering, deep groundwater levels will return to those similar to pre-mining conditions. Operation of the TSF drainage system is predicted to result in capture of upwelling groundwater and any seepages from the tailings, with inward hydraulic gradients effectively creating hydraulic containment of the TSF. Effects to groundwater and surface water during closure are therefore expected to be negligible. Closure of the TSF will also include installation of a capping layer, which will provide separation of stormwater from the tailings and be contoured to allow stormwater flow to Gladstone Wetland. This will effectively reinstate, and potentially expand, the wetland catchment from that prior to mining of GOP and operation of the TSF. The long-term effects associated with the TSF, assuming drainage is no longer operated, are assessed as follows: – Shallow groundwater: 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. While only a small part of the water discharged from the TSF is expected to have contacted tailings, some minor change in shallow groundwater quality west of the TSF is conservatively predicted to occur in the long-term. – Surface water: Changes to the Ohinemuri River water quality as a function of the TSF discharge, even when excluding potential attenuation of contaminants during migration to the river, is predicted to be negligible and within the receiving water quality criteria (RWQC). No groundwater or surface water users are predicted to be impacted by discharges from the TSF. – Monitoring and management: It is expected that during and following mine closure the quality of water being discharged from the TSF and captured by the drainage network will improve. The assessment undertaken conservatively recognises these improvements in predicting long term effects to groundwater and surface water. Monitoring of water recovered from the TSF drainage system will allow validation of these assumptions and determination of when it is appropriate to cease operating the drains. 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 migrate to impact the receiving environment. Water impacted by mine rock that is captured by the drains will be conveyed to the WTP for treatment prior to discharge to the Ohinemuri River. Prior to closure, a portion of the rock will be used to backfill GOP and the underground mine. The remaining rock will then be covered with a low permeability capping layer and rehabilitated to reduce both water and oxygen infiltration, and corresponding leachate generation. The effects associated with development of the NRS are assessed as follows: – Groundwater: Increased groundwater recharge over the footprint of the NRS is predicted to occur during operation, with any changes in groundwater levels controlled by the sub-soil drains. Groundwater quality is expected to be affected by seepage through the NRS liner, however, the sub-soil drains are predicted to capture the majority of impacted water. The un-captured seepage is predicted to influence downgradient groundwater quality. Monitoring of existing storage facilities indicates that following closure, longer term impacts to water quality should reduce. – Surface water: A reduction in direct groundwater discharge to the Ohinemuri River is likely to occur, however this is expected to be offset by discharge to the river from uphill diversion drains, treated water from the perimeter drains, and treated leachate and groundwater from the WTP. Minimal change in trace element concentrations and only a small increase in sulphate concentration in the Ohinemuri River is conservatively predicted as a result of NRS discharges.
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