GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 50 Table G.7 Gladstone Pit model calibration target groundwater levels (current conditions scenario) Piezometer Target level (mRL) Modelled level (mRL) Calibration residual (m) North-south model GLD01 1123.63 (1) 1123.68 +0.05 GLD01a 1123.38 (1) 1123.38 0.0 GLD01b Dry (1096.0) (1) 1095.63 NA GLD03 1109.15 (1) 1108.98 -0.17 GLD03a 1105.43 (1) 1105.33 -0.10 West-east model P79s 1093.1 – 1100.6 (2) 1094.42 Calibrated to lower end of range for summer level P68s 1086.55 (3) 1085.87 -0.7 (summer level likely to be lower than target level). 1. Water levels recorded 20/3/2018 2. Range of values recorded 2018 – 2020 3. Recorded level date unknown. Assumed November 2001 following monitoring well installation (URS, 2003). G.1.2 Gladstone vein dewatering assessment G.1.2.1 Groundwater modelling methodology Groundwater modelling was undertaken using GeoStudio 2021 SEEP/W finite element numerical modelling software. One two-dimensional (2D) section was developed to model a cross section perpendicular to the strike of the Gladstone vein system where it passes beneath the Ohinemuri River southwest of the proposed GOP (Figure G.5). The assessment considers a situation where the deep groundwater southwest of the proposed GOP is not currently dewatered. The model was used to undertake an assessment of groundwater drawdown of the water table, and within the andesite of the deep groundwater system, as well as the potential for loss of surface water flow from the Ohinemuri River (OH6). The model is 950 m in length, orientated northwest-southeast perpendicular to the Gladstone vein system along the stretch of the Ohinemuri River southwest of the proposed GOP The model was set up to represent current conditions and used to assess the influence on groundwater and surface water as per the following scenarios: – Current conditions: To represent upwards vertical hydraulic gradients between the deep and shallow groundwater systems at the location of Ohinemuri River monitoring location OH6 and nearby GLD04 well series. – Gladstone vein dewatering: To predict groundwater drawdown within the shallow and deep groundwater systems and potential changes in Ohinemuri River flow in response to dewatering the Gladstone vein system to a maximum depth of 1,005 mRL, should the groundwater levels assuming a strong hydraulic connection along the vein system between the GOP and the Ohinemuri River southwest of the pit (OH6). Ohinemuri River scaling For the purposes of estimating the change in Ohinemuri River flow during the Gladstone vein dewatering scenario it was assumed that the river was located at the surface across the full length of the 950 m model section. As the results are extracted from the model on a per unit metre basis, the results were multiplied by 15 m to represent the width of the Ohinemuri River. G.1.2.2 Groundwater inflow to GOP methodology Existing data recorded during mine dewatering of the Favona system was used to undertake an empirical assessment to predict groundwater inflow rates from the deep groundwater system during excavation of the GOP. This assessment considered a scenario where the deep groundwater system southwest of the proposed GOP is not currently dewatered as a result of mine dewatering. As this assessment is supported by existing data the results are presented directly within the main body of the report and not with this technical appendix.
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