GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 62 Table G.19 Predicted flux to the deep groundwater system and TSF drainage system during operational TSF and TSF closure scenarios (SEEP/W) Receiving environment Flux (m3/day) Operational TSF TSF Closure Deep groundwater system 187 (tailings porewater) 0 TSF drainage system 0 58 (tailings porewater) 1,194 (deep groundwater) Total 187 1,252 Sensitivity Analysis Sensitivity analysis has been undertaken to inform an uncertainty assessment. This provides a range of results which may be realised following development of the GOP as a TSF, and the sensitivity of the assessment results to variations in parameter values. The sensitivity analysis has been undertaken for the two Geostudio 2021 SEEP/W cross section models radial from Gladstone Hill for the TSF closure scenario. The following parameters were assessed: – Hydraulic conductivity of the geosynthetic liner (Table G.20) – Tailings cap infiltration rate (Table G.21) Table G.20 Sensitivity analysis of TSF Closure model scenario – hydraulic conductivity of the geosynthetic liner (SEEP/W) Flux to TSF drainage Hydraulic conductivity reduced – 5x10-11 m/s Base model – 1x10-10 m/s Hydraulic conductivity increased – 5x10-10 m/s Tailings porewater 54 58 59 Groundwater 1193 1194 1192 Total drain discharge 1247 1252 1250 Table G.21 Sensitivity analysis of TSF Closure model scenario – TSF capping layer infiltration rate (SEEP/W) Flux to TSF drainage Infiltration rate reduced – 9x10-9 m/s Base model – 1x10-8 m/s Infiltration rate increased – 2x10-8 m/s Tailings porewater 56 58 60 Groundwater 1183 1194 1220 Total drain discharge 1239 1252 1280 The results of the sensitivity analysis indicate that the flux of tailings porewater and groundwater to the TSF drainage system beneath the geosynthetic liner has low sensitivity to the hydraulic conductivity of the liner, and to the infiltration rate of the TSF capping layer. It is also noted that altering these parameters did not impact the groundwater flow regime outside of the TSF, with no tailings or rock backfill impacted porewater predicted to discharge to the shallow groundwater system or the surface water receiving environment.
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