GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 48 . Table 3.5 Predicted change in groundwater table compared to current conditions and ZOI for GOP excavation Direction from pit Groundwater level change at edge of pit (m) Drawdown zone of influence (m)* North -3 60 East -6 155 South -8 290 West -6 210 Notes: * Where drawdown > 0.5 m - Considering model uncertainty, results are rounded. Shallow groundwater inflow and stormwater runoff is likely to be captured and discharged to the WTP during excavation of the GOP, however if the deep groundwater system is under-drained, seepage to the andesite may occur. After reaching saturated conditions, the seepage will migrate with deep groundwater flow to underground mine dewatering systems. The Gladstone vein dewatering assessment modelled a maximum of 0.04 m of groundwater drawdown of the shallow water table from current conditions during pit excavation and dewatering at the location of the Ohinemuri River southwest of the pit (Figure 3.13). The model results also reported a reduction in Ohinemuri River baseflow of 1.5 m3/day southwest of the proposed GOP in response to vein dewatering to 1,005 mRL (Table 3.6). The low magnitude of expected groundwater drawdown and change in river flow is an expression of the separation of the shallow and deep groundwater systems. Potential impacts to the Ohinemuri River as a result of excavation of the pit and resulting changes in the shallow groundwater system are discussed in Section 3.5.2. Table 3.6 Predicted change in Ohinemuri River flow compared to current conditions in Gladstone vein dewatering scenario Surface water body Predicted change in Ohinemuri River flow (m3/day) Ohinemuri River -1.5 Deep groundwater levels As presented in Table 3.5, the influence on shallow groundwater is expected to be constrained to within 290 m of the Gladstone Pit, and within this area a slight loss of recharge to the deep groundwater system. This reduction is likely small in magnitude and expected to have an insignificant influence on deep groundwater levels. The Gladstone vein dewatering assessment assumes that a strong hydraulic connection is present along the vein system between the GOP and the Ohinemuri River, which will allow the vein system to dewater to an elevation of 1,005 mRL to model the maximum potential groundwater drawdown and resulting influence on the river. The predicted extent of unsaturated andesite perpendicular to the Gladstone vein where it is dewatered to the maximum depth of 1,005 mRL is presented in Table 3.7. Table 3.7 Deep groundwater drawdown from current conditions and ZOI for Gladstone vein dewatering Direction from vein system Predicted extent of unsaturated andesite within deep groundwater system (m from Gladstone vein)* Northwest 80 (with additional 20 m extent of unsaturated conditions approximately 180 – 200 m from the Gladstone vein) Southeast 230 Notes: * Where pore water pressure is less than zero - Considering model uncertainty, results are rounded.
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