GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 62 . 3.6.3 Surface water catchments The reduction in the surface water catchment areas in the operational TSF scenario is predicted to reduce runoff and interflow, and hence peak flow in the tributaries. After capping of the TSF (TSF closure and long-term TSF scenarios) the proposed gently contoured surface will direct stormwater run-off. It is expected that this will restore, or increase, surface water contribution to the catchment to the south (TB4 catchment). The predicted change in surface water catchment areas in all TSF scenarios are presented in Table 3.17. Table 3.17 Predicted change in surface water catchment area after TSF development Surface water catchment Percentage change in surface water catchment area from current conditions Operational TSF TSF Closure / Long-term TSF Ohinemuri Tributary (TB5) -16% -16% Ohinemuri River west of Gladstone (OH6)1 -20% -20% Ohinemuri River east of Gladstone (OH3)1 -20% -20% Ohinemuri Tributary (TB4) -26% Up to +74% 1. Constrained to the area between the Gladstone Pit and Ohinemuri River 3.6.4 Deep groundwater drainage to WTP The predicted flow of water impacted by the backfilled rock and tailings and the flow of deep groundwater as a result of operation of the TSF drainage system in the operational and TSF closure scenarios is presented in Table 3.18. Table 3.18 Predicted flow to the deep groundwater system and TSF drainage system in the operational TSF and TSF closure scenarios Deep groundwater receptor Flow (m3/day) Operational TSF TSF Closure Assumed deep groundwater levels Below TSF drain levels Above TSF drain levels Deep groundwater system 187 (tailings porewater) 0 Gladstone TSF drainage system 0 1,252 total (58 tailings porewater) (1,194 deep groundwater) No discharge is predicted through the TSF drainage system when deep groundwater levels are at an elevation below the proposed drains, as the materials surrounding the drainage sump will remain unsaturated, preventing the migration of tailings porewater discharge towards the drains. Where groundwater levels are at an elevation above the proposed TSF drainage system the drainage sump will be saturated allowing the generation of a groundwater gradient towards the drains. These conditions are anticipated to provide hydraulic containment of the tailings porewater, with no discharges predicted to either the deep or shallow groundwater systems. Water impacted by the backfilled rock and tailings that has discharged through the TSF drainage system will be directed to the WTP for treatment. For a situation where the deep groundwater system at Gladstone is currently dewatered as a result of mine dewatering to an elevation lower than the proposed TSF drainage system, as represented by the operational TSF scenario, then the tailings porewater from the TSF is predicted to discharge into the deep groundwater system. The tailings porewater is then anticipated to migrate with groundwater towards other (deeper) underground mine dewatering systems where it will be captured and directed to the WTP. In the event that the impacted groundwater
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