2.
Purpose of this report and scope
The purpose of this memo report is to present modelling results to understand a range of possible water levels in the OCP during the project construction and operations phases, with the phasing outlined below for reference: • Modelling of the water balance during construction phase (Phase 1 - Month 0 - 11), starting May 2025. • Modelling of the following three years (Phase 2 – Month 12 - 51) of the water balance during the phased approach, implementing the OCP as the TSF. • Modelling of the following two years (Phase 3 – Month 52 - 73) of the water balance during the phased approach when Upper Mundic Gully (UMG) is the TSF. • Modelling of the final two years of the water balance during the phased approach of implementing the UMG TSF (Phase 4 – Month 74 - 94). • The objective of the water balance modelling is to understand the impact of the proposed construction and mining activities on the water levels in the OCP and to confirm the level of spill risk from the OCP, if any, based on the model configuration and assumptions used in its development.
3.
Key updates to the study
Key updates since the GHD (2022a) study are described in the following sub-sections.
3.1 Operational model The key update to the water balance model is the change to the operational structure of the system. Previously, the SSG functioned as the Tailings Storage Facility (TSF) where all tailings were deposited directly, following the construction period. This has been updated into a phased approach as follows: – Phase 1 (Month 0 - 11): Modelling of the construction phase including dewatering of SSG to OCP (11 months) – Phase 2 (Month 12 - 51): Modelling of the period after the construction period where the OCP functions as the TSF dam – Phase 3 (Month 52 - 73): Modelling of the following period where the UMG functions as the TSF dam. – Phase 4 (Month 74 – 94): Modelling of the remaining five years where the UMG functions as the TSF dam. It is noted that, prior to Upper Mundic Gully becoming the TSF for the site, it is not proposed to function as a water retaining structure. It will, however, during the construction phase, naturally retain contributing catchment runoff once the tunnel has been closed off. It is not intended to receive any other form of water from the site during the construction phase (i.e. prior to it becoming the TSF). As such, the water balance modelling presented here-in does not allow for any simulated transfers into the Upper Mundic Gully site, prior to it becoming the TSF for the site. However, in spite of the intentions outlined above, the UMG provides additional capacity in a potential worst case scenario, should the OCP be in a position where it could potentially overtop. 3.2 Upper Mundic Gully as the TSF In previous assessments associated with this site, the Sandstone Gully (SSG) area was nominated as the TSF for the site, once constructed. However, in this assessment (July 2025), the SSG area is no longer proposed to be a TSF, rather just an additional area within the mine site. Rather, the Upper Mundic Gully area is now proposed to be the TSF, which is depicted in the GoldSIM Water Balance Model (WBM) layout presented in Figure 3.1, to the north of the SSG. The basin stage storage curve used in this model was
12624987 | Heritage Minerals EA Amendment Application – Updates to Water Balance Model July 2025 (Final) 3
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