GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 70 H.1.3 NRS assessment Flow paths of infiltrating rock stack recharge, rates of leachate capture and seepage through the Zone A liner, rates of sub-soil drain capture and migration of groundwater and leachate seepage beneath the NRS were predicted using groundwater modelling for NRS operation and closure. Groundwater modelling was undertaken using GeoStudio 2021 SEEP/W finite element numerical modelling software. A two-dimensional (2D) section was developed to model a cross section through an approximately eastwest orientation through the proposed NRS. The model orientation was selected to represent the average distance between leachate collection drains and sub-soil drains allowing estimation of the average pressure head generated by a build-up of leachate on the liner and resulting leachate seepage and capture. The model considered a 467 m cross section of the proposed rock stack across an area where existing ground level slopes from the base of the elevated rhyolite dome in the east, with topography reducing to the west. An annotated plan view diagram of the location and orientation of the cross section is presented in Figure H.2. The NRS assessment was undertaken for the scenarios presented in Table H.1. Table H.1 NRS assessment – model scenario summary The 2D groundwater model was used to predict: – Leachate flux to leachate collection drains. – Seepage rate of leachate through the Zone A liner. – Leachate and groundwater flow to sub-soil drains. – Leachate and groundwater migration within the shallow aquifer west of the NRS. As the results are extracted from the groundwater model on a per unit metre basis, they were scaled up to represent the whole NRS. Results from the model section were therefore multiplied by 557 m (approximate width of NRS perpendicular to groundwater flow). The SEEP/W model was structured using EGL3 design drawings to represent conditions after construction of the NRS. Site data, including geology, permeability and water levels from monitoring wells was utilised to model the shallow groundwater system. Bore logs, groundwater levels and permeability test results from monitoring bores are presented in Appendix B, Appendix C and Appendix D, respectively. Annotated SEEP/W sections are presented in Figure H.3. Model set up information is provided in Table H.2. The adopted hydrogeological properties are presented in Table H.3. 3 EGL, 2021a. Waihi North Project. Tailings storage and rock disposal. Volume 4. Northern Rock Stack RL173 Proposed Rock Disposal Facility Technical Report. EGL Ref 9018. Report prepared for Oceana Gold (New Zealand) Limited. OGNZL Document reference: WAI-983-080-REPLC-0006. 22 October 2021. Scenario Description NRS Operation (steady state) Rock stack to maximum elevation 1,173 mRL placed on Zone A liner. Rainfall recharge applied to exposed rock stack. Rainfall recharge rate for Zone F representative of overlying Zone G & H capping layer. Leachate collection drains located on top of Zone A liner surrounded by Zone F material, and sub-soil drains located beneath the liner. Groundwater flow east to west, with groundwater levels approximating existing conditions. NRS Closure (steady state) Rock stack and Zone F to maximum elevation of 1,146 mRL. Rainfall recharge rate for Zone F representative of overlying Zone G & H capping layer. Leachate collection drains located on top of Zone A liner surrounded by Zone F material, and sub-soil drains located beneath the liner. Groundwater flow east to west, with groundwater levels approximating existing conditions. Zone A: A 0.75 m thick low permeability soil liner at the base of the rock stack. Zone F: A 1.5 m minimum thickness layer separating the rock from the capping layer. This material forms a drainage layer and is a permeable slightly silty, sandy gravel PAF material.
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