GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 83 I.1 Introduction This technical appendix provides the details of the TSF3 analysis presented in Section 5.5 of the Waihi North Groundwater Assessment report. The outline of this appendix is as follows: – I.2 Assessment methodology (model set up, calibration, scenarios) – I.3 Analysis results – I.4 Water quality assessment – I.5 Sensitivity testing I.2 Groundwater modelling methodology The rationale for the model set up is described in Section 5.4 of the Groundwater Assessment report. This section describes the physical model set up, how calibration was achieved, and the scenarios applied. I.2.1 Cross-section 1 (SEEP/W) Base model set up The base model comprised a steady-state 2D cross-sectional model using Geostudio 2019 R2 SEEP/W. The section length was 2.1 km, starting from the rhyolite hills north of the TSF3 site through to the Ruahorehore Stream south of the TSF3 embankment toe (See Figure I.1 and Figure I.2). The cross-section was aligned in the direction of dominant groundwater flow of the primary (deeper) aquifer and through the paleogully infilled with sensitive tuff, which will be excavated to approximately 20 m below ground level. The undercut will require dewatering to this depth and will be backfilled during the foundation works phase. For consistency with the Waihi North TSF3 geotechnical assessment, the geology represented in the hydrogeological model was provided as a direct export from the EGL Geostudio SLOPE/W ground model (as reported in the EGL 2021 TSF3 technical report). Minor edits to the geology were made for the groundwater assessment, including merging regions of the same or hydraulically similar materials; applying a single surface material to represent alluvium, ash and colluvium given the limited surficial geological information (i.e. upper 2 m); and creating a separate “hard rock” rhyolite region under the northern hills to reflect the rhyolite ridge, where sitespecific groundwater and geological information was limited. Parameters and boundary condition calibration Through an iterative process, the input values for the final base model were derived by applying values from within measured ranges of parameters such as rainfall and hydraulic conductivity for each geological unit. The input values were varied in order to achieve base model calibration, focusing on: – A horizontal hydraulic gradient in the primary (deeper) groundwater system beneath the TSF3 site consistent with that measured during site investigations (Appendix C) – Downward vertical gradients at the northern edge of the TSF3 site, and upward gradients at the embankment toe, whilst achieving minimal flow to the Ruahorehore Stream (i.e. interflow only). To achieve this, the following boundary conditions (Table I.1) were applied in conjunction with the hydraulic parameters provided in Table I.2: – 740 mm/year (35% average annual rainfall) to groundwater at the valley ridges. – 11 mm/year (0.5%) within the TSF3 valley. – No recharge in the lower valley floor, where upward vertical gradients are present and surface ponding occurs. – Constant head boundaries at either end of the model to achieve artesian conditions at depth below the toe of the TSF3 valley.
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