GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 77 H.3 NRS assessment sensitivity analysis Sensitivity analysis has been undertaken for a number of parameters to inform an uncertainty assessment of the groundwater modelling used for the NRS assessment. This provides a range of results which may be realised for the NRS, and the sensitivity of the assessment results to variations in parameter values. The sensitivity analysis has been undertaken using the Geostudio 2021 SEEP/W cross section model for the NRS Closure scenario as a base model. The parameters assessed are presented in Table H.9. Table H.9 Sensitivity analysis for NRS Closure scenario Sensitivity parameter Initial base model parameter value Modified parameter value Rock stack hydraulic conductivity function (AEV) Silty sand1 Silty sand with AEV2 reduced (see Figure H.4) Silty sand with AEV2 increased (see Figure H.4) Aquifer hydraulic conductivity function Silty sand1 Sand1 (see Figure H.5) Clayey Silt1 (see Figure H.5) Zone A liner hydraulic conductivity function Clayey silt Sandy clayey silt1 (see Figure H.6) Clayey silt (fine tailings)1 (see Figure H.6) Rock stack hydraulic conductivity 1 x 10-6 m/s 1 x 10-5 m/s 1 x 10-7 m/s Aquifer hydraulic conductivity 6 x 10-6 m/s 6 x 10-5 m/s 6 x 10-7 m/s Zone A liner hydraulic conductivity 1 x 10-8 m/s 1 x 10-7 m/s 1 x 10-9 m/s Groundwater table elevation Upgradient groundwater boundary – 1,115 mRL Downgradient groundwater boundary – 1,100 mRL Upgradient groundwater boundary – 1,114 mRL Downgradient groundwater boundary – 1,099 mRL Upgradient groundwater boundary – 1,116 mRL Downgradient groundwater boundary – 1,101 mRL 1. SEEP/W 2012 Database 2. AEV = Air entry value (minimum matrix suction at which point air enters the pores and saturation reduces) 2. AECOM 2020. Infiltration rates provided for TSF1A embankment slope. Email communication provided by AECOM, 27 July 2020. The groundwater model sensitivity analysis results have been assessed by comparing the predicted rock stack leachate seepage rates through the Zone A liner and the predicted leachate capture at the leachate collection drains. The results are presented in Figure H.7. Increased leachate capture at the leachate collection drains is largely controlled by the potential for perching on the Zone A liner, which promotes greater saturation and horizontal flow towards the leachate drains. The modelled results have greatest sensitivity to hydraulic conductivity of the Zone A liner, with leachate capture increasing from 2 m3/day to 177 m3/day when the hydraulic conductivity is reduced by one order of magnitude, causing perching of leachate on the liner. To a lesser degree, reducing the hydraulic conductivity of the shallow aquifer also results in an increase in the predicted leachate captured at the leachate drains, as a result of reducing the vertical hydraulic gradient across the liner. The results demonstrate moderate sensitivity to altering the hydraulic conductivity function of the shallow aquifer and the Zone A liner. By reducing the AEV, this increases the pore sizes and reduces the hydraulic conductivity of these materials. Although the sensitivity analysis results indicate relatively low sensitivity to changes in groundwater level beneath the rock stack, the results indicate that the closer the groundwater table is to the Zone A liner, the more leachate is predicted to be captured. This is because higher groundwater levels reduce the vertical hydraulic gradient across the Zone A liner. The sensitivity analysis results indicate low sensitivity to the rock stack parameters assessed.
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