GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 108 J.1 Introduction This technical appendix provides the details of the WRS analysis presented in Section 6.4 of the Waihi North Groundwater Assessment report. The outline of this appendix is as follows: – J.2 Numerical analysis (methodology and results) – J.3 Water quality assessment – J.4 Sensitivity analysis J.2 Numerical analysis J.2.1 Methodology The rationale for the model set up is described in Section 6.3 of the Groundwater Assessment report. This section describes the physical model set up, how calibration was achieved, and the scenarios applied. The base model comprised a steady-state 2D cross-sectional model using Geostudio 2019 R2 SEEP/W. The section length was 225 m, with the gully centred at 0 m along the X axis (refer to Figure 6.11 of the main report). The model boundaries represent the ridges and were approximated from topography. A constant head boundary was applied to the base of the model to represent deeper groundwater and rainfall was applied to the entire model to achieve a consistent downward gradient across the model, as well as sufficient separation distance from the gully to the deeper aquifer water level. Emplaced rock for the operational scenarios was conservatively applied 40 m height above the gully (compared with the proposed 20 m maximum height above natural ground). The model scenarios are summarised in Table J.1. Model set up details are provided in Table J.2. Hydraulic properties of the geology and WRS material are provided in Table J.3. Table J.1 WRS base model calibration and scenario summary (steady state) Scenario Description Base model – Base of gully set to 1,180 m RL. – Whiritoa Andesite rock as primary geology, with overlying weathered tuff and residual soils. – Rainfall recharge applied to full model and constant head groundwater level applied. This achieved a conservative deeper aquifer groundwater level of 1,172 m RL (12 m below the base of the gully). – Groundwater flow downwards, approximating existing conditions. WRS operation (40 m) Scenario 1: 30% rainfall recharge – Rock placed to maximum height of 40 m (to 1,220 m RL), directly on lower permeability residual soil. – Rainfall recharge rate applied directly to surface of the rock stack. – Rock drain at base of gully (assumed 0.5 m thick). Scenario 2: 50% rainfall recharge
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