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DETSI Comment
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Report
The WMP is proposed to be updated in May 2026 to coincide with the expected commencement of processing and discharge of tailings into OCP. The only potential mechanism where the catchment reporting into the OCP could occur is from a spill or seepage. A DBA and consequence category assessment CCA for Upper Mundic Gully TSF determined that the overtopping/ environmental spill and seepage consequence categories were considered to be “low’ for both ANCOLD and DETSI. The design criteria proposed for the Upper Mundic Gully TSF defines the water storage capacity as the greater between the Design Storage Allowance (DSA) and the 120-hour PMP volume for both stages. Considering the significant storage containment allowance, overtopping failure was not considered a credible failure scenario Geochemical characterisation of the processed tailings to be discharged has been extensively studied by Hydrobiology in the report titled “Mt Morgan Open Cut Pit Water Quality Modelling Stage 2 (March 2025)”, provided in Appendix F. This report was commissioned to model the geochemical properties of the processed tailings as the mineral processing requires use of highly degraded OCP water. The resulting reprocessed tailings will be ultimately discharged into the Sandstone Gully TSF, the OCP and the Upper Mundic Gully TSF (subject to this application being approved). The 6Mt of tailings authorised for discharge into OCP as per condition W13 has the same tailings properties for the proposed discharge into the Upper Mundic Gully TSF. Therefore, as was the case with the amendment to the EA to allow discharge into OCP, there will be an improvement in water quality that may report to the Open Cut Pit if a loss of containment event from upper Mundic Gully TSF were to migrate to the OCP.
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Current condition W10: The holder of this environmental authority must only undertake mining activities that result in no change to, or a reduction of the effective catchment reporting to the Mount Morgan Open Cut Pit.
Section 4.3.2, Table 19 and Appendix C
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Current condition W11: The holder of this environmental authority must only undertake mining activities that result in no change to or an improvement in water quality reporting to the Mount Morgan Open Cut Pit
Section 4.3.12
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