6.6.9 Proposed updates to groundwater model It is proposed that a phased approach to updating the groundwater model to support the EA amendment to include the Upper Mundic TSF will occur. The first phase involves updating the site- wide groundwater flow model developed by RGC (2011) and supporting conceptual model to better understand local groundwater conditions in the Upper Mundic valley and the adjacent catchments that could be affected by the construction of the Upper Mundic TSF (e.g. Spring Creek and Patty Creek). The second phase of modelling would involve the development of a solute transport model to simulate the transport of selected constituents in groundwater within the Mount Morgan Mine Site and provide simulated contaminant loads to the Dee River during the process of reprocessing tailings and post-rehabilitation. Further details are provided in the sub-sections below. Phase I Model Update The Phase I model update involves updating the groundwater flow model to include the latest available data for the new (and existing) monitoring bores within the Upper Mundic valley since the earlier site-wide groundwater model (RGC, 2011) was completed. The primary objective of the Phase I model update is to develop a calibrated numerical groundwater flow model that addresses the groundwater modelling requirements specified by DETSI and provides a robust basis for an assessment of potential contaminant migration pathways (via particle tracking) and potential impacts to receptors due to proposed operation of the Upper Mundic Gully TSF. Particle tracking, for instance, is needed to illustrate flowpaths within the Upper Mundic Valley to demonstrate whether hydraulic containment can be achieved within the valley once the Upper Mundic TSF has been constructed. 6.6.9.1 The Phase I model update will commence once the additional (proposed) bores described in Section 4.1 have been installed and a factual drilling report is available. The first step of the Phase I modelling will be to extend the boundaries of the previous model (see RGC, 2011) to include the nearest drainage to the west of the Upper Mundic Gully (i.e. Spring Creek and Paddy Creek) so the potential for seepage and transport across the topographic divide into those unimpacted neighbouring drainages can be assessed. The updated model domain will be the same as in RGC (2011) for other areas and will include the entire footprint of the Upper Mundic Gully TSF, the OCP and Sandstone Gully. A review will occur of the water level and water quality monitoring data for existing monitoring bore MB1 (downstream toe of Western Dump near the OCP) and bores WGMB01, WGMB02, and WGMB03, which were installed in December 2025, and the data for the new bores to be installed in 2026. All of these data will be compiled and reviewed as part of updating the Conceptual Hydrogeological Model (CHM) for the Upper Mundic area, including the adjacent catchments and water level data for the OCP. The site-wide groundwater model will not be formally updated as part of Phase I, as the focus od on the areas of the existing groundwater model that are relevant to the Upper Mundic TSF. Other available data will be compiled and organized to support an update of the site-wide groundwater model (see Section 4.2.2). The CHM will be updated in accordance with the Australian Groundwater Modelling Guidelines and other relevant groundwater modelling guidelines for the mining industry, e.g., ENV (2012), and will therefore include an updated hydrostratigraphic model, a review of groundwater level data and water level data for the OCP, inferred groundwater flow fields, a review of relevant water quality results, and an updated groundwater budget, amongst other accepted (standardized) elements of a CHM. The numerical groundwater flow model will be updated to provide a numerical representation of the revised and updated CHM. The numerical model update will likely involve converting the existing groundwater model from FEFLOW to MODFLOW USG. The numerical model will be developed using model parameters, assumptions, boundary conditions, and model design consistent with the CHM. The model calibration period will be decided as part of updating the CHM
Project number: 25B061
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