GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 65 . from the pit rim (Table 3.5). Groundwater drawdown at the Gladstone Wetland is predicted to be approximately 0.5 m. A negligible reduction in shallow groundwater levels adjacent to the Ohinemuri River due to dewatering of the Gladstone vein system southwest of the GOP is anticipated, with a reduction in Ohinemuri River baseflow of approximately 1.5 m3/day (Table 3.6). Changes in shallow groundwater discharge to the Ohinemuri River as a result of pit excavation are further discussed in the groundwater discharge to receiving environment section below. The conceptual groundwater model (Section 3.3) presents evidence that the shallow groundwater system is already under-drained as a result of dewatering of the deep groundwater system, with no adverse effects recorded to date. Abstraction of groundwater during excavation of the GOP is therefore not expected to adversely impact groundwater quality. While there may be some localised changes in soil moisture in close proximity to areas of earthworks and dewatering, no impacts are expected beyond the immediate vicinity of the GOP. Intermittent wetting by rainfall is considered to provide the greatest influence on soil moisture levels across the Waihi North Project area. Deep groundwater The Gladstone vein system and mineralised fault zone extends in a northeast – southwest direction through Gladstone Hill and has been interpreted by OGNZL geological model (2021) to extend beneath the Ohinemuri River at the approximate location of GLD04 (Figure 3.8). Groundwater levels in GLD04d likely represent water pressures within the south-western extent of the Gladstone vein system and indicate that the full extent of the mineralised fault does not have a strong hydraulic connection to existing dewatered areas. This suggests that whilst permeable, there are hydraulic disconnects within the veins, and this situation can be expected further along the southwest alignment. In addition, continued saturated conditions in monitoring locations P79d and P61, which are installed within andesite country rock in close proximity to the mineralised vein system, confirm the limited drawdown influence laterally from the vein as a result of Favona dewatering (Figure 3.8). Historical dewatering records for Favona are considered to provide reasonable approximations for inflow rates to the GOP during excavation in the event that the vein southwest of the pit is not dewatered (Table 3.9), and could be in the order of: – 1,100 m3/day of deep groundwater discharge to the GOP during initial excavation until after the maximum depth of 1,005 mRL is reached. – 325 m3/day long-term groundwater inflow after dewatering has reached steady state conditions. The groundwater inflow rates are considered to represent an upper bound given the limited extent of the ore body southwest of the GOP and the generally limited hydraulic connection outside of the immediate vein system. Groundwater drawdown within the deep groundwater system has been modelled to result in unsaturated andesite extending up to 230 m perpendicular to the vein strike (Table 3.7), however the very low permeability of the surrounding country rock is expected to limit the extent of the influence on the deep groundwater system. The expression of dewatering along strike of the veins (Figure 3.14) is anticipated to be limited by hydraulic connectivity in the vein network similar to the contrasting deep groundwater levels currently observed northeast and southwest of the proposed GOP. Groundwater discharge to the receiving environment and changes to surface water catchments The small reduction in shallow groundwater levels as a result of excavation of the proposed GOP is predicted to result in locally reduced groundwater flow to nearby surface water receiving environments. The ZOI is expected to be constrained to within 290 m of the Gladstone Pit (Table 3.5), with no measurable effects to shallow groundwater levels predicted to extend beyond the Ohinemuri River. The surface area of the proposed pit (18.7 ha) currently contributes runoff and interflow to the surface water catchments of the Ohinemuri River and its tributaries. Excavation of the pit is therefore predicted to also reduce surface water runoff and interflow, with this including the removal of the upper sections of the intermittent watercourses above the Gladstone Wetland. The following changes in discharges to surface water bodies are predicted (Table 3.10 and Table 3.11): – Groundwater discharge to the Ohinemuri River west of the proposed Gladstone Pit (OH6) is predicted to reduce by approximately 55 m3/day, however groundwater discharge to the Ohinemuri River to the east
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