GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 68 . Approximately 65 m3/day of groundwater is predicted to discharge from the TSF and migrate to the surface water receiving environment in the long-term TSF scenario; compared to 4 m3/day that is predicted to flow to surface water currently from the proposed pit area (Table 3.16). After rewatering of the deep groundwater system an additional 100 m3/day is predicted to discharge the Ohinemuri River from the catchment area west of Gladstone (OH6 Table 3.15). The additional flow to the Ohinemuri River from the Gladstone area is not likely to be distinguishable from additional flow anticipated to result across the wider Waihi area due to rewatering of the deep system. The capping layer on the TSF is proposed to be graded for stormwater runoff, with at least some of the discharge, if not all, being directed to the south. The surface water catchment area for the Gladstone Wetland (TB4) will therefore be restored, with a potential increase by up to74% compared to current conditions (Table 3.17). This is expected to provide additional flow to the Gladstone Wetland. Further discussion of the Gladstone Wetland is provided in the GHD (2022d) Gladstone Wetland Groundwater Assessment Summary Technical Memorandum, which should be read in conjunction with this report. Water quality Groundwater discharge from the TSF to the surface water receiving environment in the long-term TSF scenario is predicted to be in the order of 65 m3/day to the west of the pit (Table 3.16). Of this discharge, approximately 5 m3/day is predicted to comprise tailings porewater, with the remainder being groundwater and infiltrated rainwater that has migrated through rock backfill to the point of discharge from the pit. Shallow groundwater quality following mixing and geochemical equilibrium (AECOM, 2021b) is predicted to result in an increase in the concentration of iron and mercury within the shallow groundwater to levels exceeding the receiving water quality criteria. The water quality assessment also considered dilution of the flux of potential contaminants within the Ohinemuri River, with the impact to the river predicted to be negligible. It is important to note that the water quality results (Table 3.19) do not allow for attenuation of key contaminants with aquifer materials during groundwater flow. It is noted that the trace metal contaminants in particular are expected to have a strong affinity for binding to clay minerals within the shallow aquifer and iron minerals within the deeper aquifer. These adsorption processes are seen in other mine areas (such as TSF1A and rock stacks) to significantly attenuate contaminant migration in groundwater. Groundwater users Consistent with the interpretation for GOP excavation scenario, development of a TSF is not expected to influence existing groundwater users owing to the lack of hydraulic connection with groundwater users. Rewatering of the deep groundwater system is expected to return groundwater levels to approximately the same level as pre-mining levels in the vicinity of the Gladstone pit. Away from the influence of the TSF drainage system (operational TSF and TSF closure scenarios), groundwater flow paths within the deep groundwater system are expected to be towards the current Favona underground mine dewatering. When the TSF drainage system ceases operation (long-term TSF) and the Favona underground mine dewatering ceases, groundwater flow paths within the deep groundwater system will either be towards the TSF where discharge to the shallow groundwater system is predicted, or towards the Martha Pit Lake. Shallow groundwater flow in the vicinity of the TSF is anticipated to be intercepted by the Ohinemuri River. Therefore, no impact to the groundwater quality at the location of the existing groundwater users south of Gladstone is expected. Surface water users The nearest surface water user on the Ohinemuri River (AUTH141637) is located approximately 10 km downstream of the WNP area. The changes to the shallow groundwater regime as a result of rewatering of the deep groundwater system may result in minimal increases in groundwater discharge to the Ohinemuri River and associated increased river flow. Discharges to the river from the TSF are, however, interpreted to have negligible influence on water quality with no predicted exceedance of the adopted receiving water criteria. As such, surface water users are not expected to be adversely impacted by groundwater discharge from the TSF in any of the TSF scenarios.
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