GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 109 5.4 TSF3 assessment 5.4.1 Approach An assessment of impacts to groundwater and surface water associated with TSF3 was undertaken through conceptual interpretation, supported by the use of 2D numerical groundwater model with Geostudio 2019 R2 SEEP/W finite element numerical modelling software. The assessment comprised development of the following models: – Conceptual: Development of a conceptual model for the TSF3 site (Section 5.3). – Dewatering and TSF3 construction: Development of a steady-state 2D cross-sectional model using Geostudio SEEP/W (Cross-section 1, Figure 5.12), representing the facility through different stages of life (construction, filling and post closure). This steady-state model was set-up using average annual rainfall rates. The base model calibration was set to measured site conditions, focusing on: Groundwater levels (Appendix C) measured in the primary TSF3 site aquifer (the deeper groundwater system) and the horizontal gradient across the TSF3 site. Comparative vertical gradients within well pairs at TSF3. Neutral to losing conditions in the Ruahorehore Stream (Figure 5.9). These model scenarios were used to predict the following at different TSF3 construction stages: a. Tailings porewater leakage rates. b. Embankment leakage rates. c. Changes in groundwater recharge, levels and flow associated with TSF3 construction. d. Changes to surface water flow and levels. – Additional construction dewatering: An additional construction dewatering analysis using a transient 2D SEEP/W cross-sectional numerical model (Cross-section 2, Figure 5.12) and analytical (Theis, 1935) model. This was to further support estimation of groundwater inflows and drawdowns during the preparation and foundation works stages (Section 5.5.1). – Water quality: Development of a simplistic mixing model to predict impacts of TSF3 seepage to groundwater and surface water quality. This model considered seepage rates from the above models and chemistry of discharges from TSF3 (from AECOM, 2021a) for mixing with the existing water quality of receiving groundwater and surface water. Refer to Appendix I for detailed description of the assessment methodology. 5.4.2 Rationale The TSF3 site comprises a complex setting, which has been assessed using simplified methods: – Steady state 2D cross-section: The primary purpose of this 2D model (Cross-section 1) is to quantify model TSF3 seepage out of the facility. It represents the groundwater movements in the direction of groundwater flow through the paleo-gully, which (combined with drains) is inferred to effectively channel groundwater from the TSF3 footprint to the drains beneath the embankment. This analysis is used in combination with the CGM to provide assessment. – Transient 2D cross section and Theis (1935): The purpose of these models was to predict potential groundwater drawdown, resulting from construction dewatering, near TSF3 including under the TSF1a for settlement calculation purposes. Cross-section 2 was oriented approximately perpendicular to Cross-section 1 through the sensitive tuff as shown in Figure 5.12, which is perpendicular to groundwater flow direction. This set-up provides potential for inconsistencies when predicting groundwater drawdown with distance from the excavation. Cross-section 2 was therefore used primarily to assess groundwater drawdown in close proximity to the excavation, where the excavation locally controls the hydraulic gradient and groundwater flow direction. Outside of this area, the model is expected to over predict drawdown (and also inflows).
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