Upper Mundic Gully is located north of the historic Sandstone Gully TSF and immediately west of the main Open Cut Pit (OCP) and Western spoil dump. Review of publicly available aerial imagery from the Queensland Government QImagery archive indicates that progressive emplacement of the Western spoil dump between approximately 1950 and the late 1960s partially infilled the gully entrance, resulting in stormwater ponding within Upper Mundic Gully. To manage this condition, a stormwater diversion tunnel was constructed to the northeast of Upper Mundic Gully in the early 1980s to divert ponded water to Dam 8, located east of the gully. The proposed TSF within Upper Mundic Gully is situated in a confined valley setting bounded by steep, vegetated slopes and located upstream of an existing mine spoil dump. This geomorphic setting provides a degree of natural topographic containment for tailings storage. In contrast, the central mine area comprises several historic open pits that have transitioned to permanent pit lakes following mine closure, reflecting the re‑establishment of groundwater levels over time. The UMG TSF concept design report [1] indicates the following key elements of the TSF design relevant to the hydrogeological and seepage conceptualisation: — The embankment is constructed across Upper Mundic Gully, using mine spoil materials, with staged crest elevations up to RL 390 m. This includes provision for a bituminous lining system in the embankment upstream side to limit seepage risk. — The embankment ties into natural gully abutments and adjacent landforms, including areas of the existing Western spoil dump, which forms part of the confining topography around the facility. — TSF operations include an operational decant pond within the TSF basin to manage supernatant water during tailings deposition. — Pond location and size will be controlled as part of the TSF water balance, with provision to maintain separation between the decant pond and the embankment as far as practicable under the concept design. A decant pond level is maintained 1 m above the tailings surface deposition.
Project No PS213278 Mount Morgan Mine - Upper Mundic Gully TSF Seepage Management Plan Heritage Minerals
WSP May 2026 Page 2
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