GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 137 6.3 WRS assessment 6.3.1 Approach An assessment of impacts to groundwater and surface water associated with WRS was undertaken through conceptual interpretation, supported by the use of 2D numerical groundwater model with Geostudio 2019 R2 SEEP/W finite element numerical modelling software. The assessment comprised development of the following models: – Conceptual: Review of the conceptual model for the WRS site (Section 6.2). – Operational: Development of a steady-state 2D cross-sectional model using Geostudio SEEP/W (Figure 6.9), representing the WRS at the fully constructed height. The cross-section is perpendicular to the WRS gully. This steady-state model was set-up using a constant head bottom boundary to provide a consistent downward gradient to the deeper aquifer level, which is separated from the base of the WRS gully flows (Tributary 2). Recharge was applied to the emplaced rock, with a drain placed in the WRS gully beneath the rock stack. The model was used to predict WRS leachate generation rates, drain capture rates and leachate loss to ground. In addition, the potential for changes to groundwater levels was also considered. – Water quality: Due to the low volume of predicted seepage (in relation to the flow in the receiving environment) equilibrium modelling was not undertaken and a simple mass balance approach was utilised. This model considered seepage rates from the above models and chemistry of discharges provided in the Water Management Report (GHD, 2022a) for mixing with the existing water quality of receiving groundwater and surface water. The 95th percentile values for TSF1A have been conservatively applied, due to limited data available on the leachability of the WKP rock stack materials. – The contaminants of concern listed for the WRS are limited compared with the other project areas (e.g. barium and chromium are not included). The parameters presented focus on the likely consented elements in the receiving environment and are adopted here as presented in the GHD (2022a) Water Management Report. Refer to Appendix J for detailed description of the assessment methodology.
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