GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 56 . Table 3.12 Predicted change in groundwater table compared to current conditions and drawdown ZOI after development of the TSF Orientation from pit Operational TSF TSF closure Long-term TSF Groundwater level change at edge of pit (m) Zone of influence (m)* Groundwater level change at edge of pit (m) Zone of influence (m)* Groundwater level change at edge of pit (m) Zone of influence (m)* North -3 56 -3 36 -2 28 East -6 155 -6 120 -5 99 South -8 288 -8 88 -7 69 West -6 212 -5 124 +2 - Notes: * Where drawdown > 0.5 m - Considering model uncertainty, results are rounded The following is noted regarding influence on groundwater levels during various phases of Gladstone TSF assessment: – During development of the TSF (operational TSF scenario) the groundwater drawdown adjacent to the pit, and the drawdown of the groundwater table with distance from the pit, is predicted to be very similar to that during GOP excavation (discussed in 3.5.1). This is because no additional recharge to the shallow groundwater system is predicted to occur during this early phase of the TSF development. Instead, tailings pore water not recovered at surface as decant is predicted to discharge through the base of the pit to the deep groundwater system, with no connection to the adjacent shallow groundwater system (Figure 3.16). – After rewatering of the deep groundwater system (TSF closure and long-term TSF scenarios) as a result of cessation of mine dewatering and formation of Martha Pit Lake, the greater pressure in the deep system is expected to reduce or reverse the local downward vertical hydraulic gradients between the shallow and deep groundwater systems (Figure 3.17 and Figure 3.18). This in turn reduces the predicted groundwater drawdown at the edge of the pit and the drawdown zone of influence (Table 3.12). – In the TSF closure scenario the groundwater table to the north and south (Figure 3.17) of the pit is predicted to rise in comparison to current conditions. This is again due to the changes in vertical hydraulic gradients as a result of rewatering of the deep groundwater system. In addition, the groundwater table to the west of the pit is also predicted to increase in comparison to current conditions in the long-term TSF scenario (Figure 3.18) as a result of increased recharge to this catchment in the form of groundwater discharge from the TSF (further discussed in Section 3.6.2). – Excavation of the GOP will remove the upper sections of the intermittent TB4 tributary. In the TSF closure and long-term TSF scenarios, groundwater discharge is expected to re-establish in the lower reaches approximately 115 - 125 m downstream of the headwaters, with a predicted rise in the shallow groundwater table of approximately 3 m in the vicinity of the wetland due to reduced or reversed downwards vertical hydraulic gradients. The predicted total groundwater flow to surface water bodies for each TSF scenario is presented in Table 3.13, Table 3.14 and Table 3.15. An increase in total groundwater discharge to surface water in comparison to current conditions is predicted in the TSF closure and long-term TSF scenarios as a result of increased groundwater levels and changes in vertical hydraulic gradients. With the exception of the Ohinemuri River to the east (OH3), an additional increase in discharge to surface water is predicted in the long-term TSF scenario in comparison to the TSF closure scenario, which is due to discontinued operation of the TSF drainage system allowing groundwater levels in close proximity to the pit to increase, also reflected in the further reduction of groundwater drawdown at the edge of the pit (Table 3.12). The reason why there is no change to the discharge to the Ohinemuri River to the east (OH3) is that the TSF drainage system is predicted to have little effect on this catchment due to a large thickness of low permeability breccia separating the deep and shallow groundwater systems and no discharge from the TSF predicted to the east in the long-term TSF scenario.
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