GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 88 Figure I.3 Goodness of fit – Cross-section 1 (SEEP/W) base model Scenarios Multiple scenarios were assessed from the Cross-section 1 base model. These were split into two different model types: – Steady-state (Table I.6): Full TSF3 construction from foundation works to post-closure. This model provides outputs for predicting: Tailings porewater leakage rates. Embankment leakage rates. Changes in groundwater recharge, levels and flow associated with TSF3 construction. Changes to surface water flow and levels. The recharge input provided relates to average annual recharge. The daily outputs provided for these analyses are therefore based on average annual recharge, rather than transient or seasonal variations. The scenarios assessed using this model are summarised in Table I.6. Scenarios 3-5 were developed on the basis that the tailings permeability will change over the life of the facility, and changes to drains will occur. Tailings toward the base of TSF3 are expected to have a lower permeability than the material at surface, consistent with a silty clay-type material. On disposal, the mine tailings will consolidate, releasing porewater to decant. This is expected to reduce the permeability of tailings throughout the operational period of the facility. The following drains are proposed to intercept discharges from TSF3 (Figure I.4), as specified in EGL (2021b) Appendix A (DWG nos. 0620 and 0626): Leachate drains in the embankment above the Zone A pad. These are expected to intercept embankment infiltration and tailings pore water beneath the embankment. Sub-soil and toe drains (beneath the Zone A liner) are intended to intercept leachate that infiltrates through the soil liner and embankment. Over time, drains may become crushed during consolidation, or become filled with fines and fail to adequately capture leachate seepage. If this occurs where groundwater conditions are not artesian, seepage will percolate downwards until groundwater is encountered. Tailings toward the base of TSF3 are expected to have a lower permeability than at the top, and consistent with a silty clay-type material. To some extent, this will limit the potential discharge through any liner defects. The uphill diversion drain is interpreted to not intercept groundwater (or perched water only). No analysis has been carried out for this drain. An additional model was developed to understand the influence of Zone B within the shear key, which is only present for half of the embankment width. The location of Zone B is presented in Figure I.5. – Transient (Table I.10): Construction dewatering only. The timeframes for each scenario were provided by EGL (pers.comm. EGL, email dated 10 June 2020). This transient model allows for more refined outputs for predicting groundwater inflows and ZOI. It is noted that the thickness of the sensitive tuff for removal along the cross-section alignment is only present to 15 m bgl.
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