GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 34 5. Design analysis 5.1 Groundwater 5.1.1 Overview A groundwater assessment was undertaken by GHD (2021a) to assess the impact of GOP TSF on the surrounding environment. Assessment of potential effects on groundwater and surface water has focused on the influence on the following: – Shallow and deep groundwater levels and baseflow to surface water during excavation and dewatering of the GOP, including: The Gladstone wetland and the intermittent Ohinemuri tributary at TB4, south of the pit. The intermittent Ohinemuri tributary at TB5, northwest of the pit. The Ohinemuri River. – Water quality, shallow and deep groundwater levels and changes to surface water baseflow during the following phases of the TSF: Operational TSF Long-term TSF TSF closure Potential impacts on groundwater and surface water associated with the proposed GOP TSF were assessed using conceptual interpretation, supported by the use of 2D numerical groundwater models and simplistic analytical models. The results of the groundwater study are provided in GHD (2021a). The following provides a brief summary of the key outcomes. 5.1.2 Summary During development of the GOP TSF the deep groundwater system is anticipated to continue to under-drain the shallow groundwater system due to ongoing dewatering in the surrounding underground mining. Estimated groundwater drawdown and groundwater levels in this scenario are similar to during mining, as the tailings pond and recharge to the tailings is not expected to influence the adjacent shallow groundwater system. Instead, tailings pore water not recovered at surface as decant is expected to percolate downwards, ultimately discharging to the deep groundwater where it will contribute to flow being captured by underground mine dewatering. Following capping of the GOP TSF and cessation of underground mine dewatering, represented by the long-term GOP TSF scenario in GHD (2021a), rewatering of the deep groundwater system will result in elevated groundwater levels within the deep groundwater system. As a result of this, downwards vertical hydraulic gradients currently recorded between the shallow and deep system are expected to reduce or reverse. This reduces the estimated zone of influence of the groundwater drawdown, and groundwater levels are estimated in a number of locations to be greater than current conditions, including in the vicinity of the Ohinemuri tributary to the north (TB5 – refer Figure 6 for monitoring locations) and the Ohinemuri River (OH3). Groundwater levels in the vicinity of the Gladstone Wetland (TB4) are modelled to increase by approximately 3 m in comparison to current conditions. The modelling assessment indicates that the greatest influence on groundwater levels within the receiving environment is the elevation of the GOP TSF drainage system and rewatering of the deep groundwater system (long-term GOP TSF and GOP TSF closure scenario). The proposed Martha Pit Lake level (1,104 mRL) is expected to impose a water level control on the connected mine workings. Hydraulic containment of the tailings and waste rock backfill is anticipated to be achieved by the GOP TSF drainage system, with the drain modelled to capture both tailings pore water generated from infiltrating rainwater
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