GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 51 G.1.2.3 Groundwater model set up Development of the steady-state cross-sectional numerical model (SEEP/W) representing existing shallow and deep groundwater systems and the Gladstone vein system. The model is orientated perpendicular to the strike of the Gladstone vein system where it passes beneath the Ohinemuri River southwest of the proposed GOP. The model was used to predict changes in groundwater levels and surface water depletion of the Ohinemuri River, considering a scenario where the deep groundwater system is not currently dewatered as a result of current mine dewatering. The geology of the SEEP/W model was sourced from the OGNZL leapfrog model (2021), with groundwater conditions reflective of measured within the GLD04 well series, and a surveyed level of the Ohinemuri River. The models were then modified to represent dewatering of the vein system to an elevation of 1,005 mRL during the GOP excavation. An annotated SEEP/W section is presented in Figure G.5. Model set up information is provided in Table G.8 and Table G.9. Table G.8 Gladstone vein dewatering – model section perpendicular to vein boundary conditions Boundary condition Adopted condition Justification Model scenario Ohinemuri River Water total head – 1,084.65 mRL Surveyed elevation close to OH6 sample point (21/02/2019) Current conditions and vein dewatering scenarios Andesite groundwater level Water total head – 1085.5 mRL Groundwater level recorded in GLD04d (October 2020) Gladstone dewatering Water pressure head – 0 m Set at 1,005 mRL within vein - assumes strong hydraulic connection along the vein system between GOP and Ohinemuri River to the southwest. Vein dewatering scenario Table G.9 Gladstone vein dewatering – adopted hydrogeological parameters Unit Hydraulic conductivity (m/s) Anisotropy Justification Ash/Regolith 6.0 x 10-6 0.5 Calibrated in GOP and TSF model assessment (G.1.1.4) Rhyolitic Tuff (RT) 2.7 x 10-7 0.5 Sandy ignimbrite 1.0 x 10-5 0.5 Weathered andesite 5 x 10-9 0.1 Andesite 4.2 x 10-7 1.0 Recorded range away from faults/crush zones 1 x 10-8 – 1 x 10-5 m/s(1) Fractured Andesite 4.2 x 10-7 2.0 Anisotropy reflects expected fracture alignment associated with Gladstone vein. Gladstone Vein 1 x 10-5 1.0 Fault / crush zone(1) 1. URS, 2003. Favona Underground Mine. Assessment of Groundwater Issues.
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