GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 63 . is not captured prior to cessation of mine dewatering, new groundwater gradients would either direct the groundwater back towards the TSF drainage system, or towards the Martha Pit Lake (1,104 mRL proposed level). For a situation where the deep groundwater system at Gladstone is not currently fully dewatered, and groundwater levels are at a higher elevation than the proposed TSF drainage system, the tailings porewater is anticipated to be hydraulically contained. This alternative operational TSF scenario would have similar results to the TSF closure scenario, which considers a situation where full rewatering of the deep groundwater system has occurred after mine dewatering has ceased. Operation of the TSF drainage system will be discontinued after a sufficient improvement in water quality of the discharge has been recorded (long-term TSF scenario). Discharges of tailings porewater to the receiving environment in the long-term TSF scenario are discussed in Section 3.6.2. 3.6.5 Water quality Following development of the TSF interaction of groundwater with rock backfill/tailings will influence groundwater quality. In the long-term TSF scenario the drainage system will no longer be operational and groundwater discharge is expected to the west of the pit (Section 3.6.2). Groundwater flowing through the TSF will interact with the NAF weathered rock capping layer, the backfilled PAF rock and tailings pore water, however the concentration of trace elements will be influenced by the rate of leaching, which will be many orders of magnitude lower through the consolidated tailings than through the rock backfill. The water quality assessment has therefore considered the following groundwater flow pathways when assessing TSF discharge: – Groundwater interaction with tailings impacted porewater. – Groundwater recharge through the NAF weathered rock capping layer. – Deep groundwater flow through leached PAF and NAF rock backfill. The results of equilibrated TSF discharge, which considered the above flow pathways, with shallow groundwater (AECOM, 2021b) have then been used to undertake a mixing model with the Ohinemuri River. The predicted effects to the receiving water quality for the long-term TSF scenario are provided in Table 3.19. The results indicate that elevated concentrations of number of parameters associated with tailings discharge are likely within the shallow groundwater west of the Gladstone pit during the long-term TSF scenario. This includes iron, mercury and zinc which are predicted to exceed the receiving water quality criteria. Mercury is typically immobile in groundwater due to volatilization and/or precipitation processes and has not been reported in significant persistent concentrations by OGNZL (2020). It is not a parameter of concern for these reasons. Although zinc is also predicted to exceed the receiving water quality criteria in the shallow groundwater in the longterm TSF scenario, the concentration represents a reduction from existing conditions recorded in GLD04s (Table 3.19). Impacts to the Ohinemuri River water quality following mixing with the discharged groundwater are predicted to be negligible and within the RWQC. Table 3.19 Predicted receiving water quality in the Long-term TSF scenario (values in mg/L) Parameter Existing shallow groundwater quality (GLD04s) Predicted values after equilibration of TSF discharge with shallow groundwater (AECOM, 2021b) Existing Ohinemuri River quality (OH6 median) Predicted values after mixing with Ohinemuri River Receiving water quality criteria pH 6.8 7.7 7.1 - 6.5 – 9.0 Al 0.088 0.07 0.016 0.016 As 0.001 0.0012 0.0010 0.0010 0.19 Ba 0.14 0.109 0.038 0.038 Total Alkalinity 65 42 15.4 15.6
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