provides a framework to guide targeted investigations, monitoring, and refinement of modelling, with the following subsections focusing on the field investigations required to improve the hydrogeological conceptualisation and modelling definition. Hydrogeological conceptualisation and knowledge gaps Based on the information provided in the preceding sections, the key aspects relevant to the hydrogeological conceptualisation of the UMG TSF are as follows: ∕ 5.9.2.1 Groundwater within the UMG TSF area occurs predominantly within Paleozoic bedrock, which consists mainly of competent and fresh volcanic rocks. Groundwater flows through secondary porosity, along fractures and joints. In contrast to areas east of the OCP, the fractured bedrock and saprolite zones are thinner or locally absent beneath the UMG TSF area, whereas previous conceptualisations to the east have identified saprolite and fractured zones with thicknesses of up to approximately 10 m (combined thickness of 20 m); ∕ The Razorback Beds comprise a stratified sedimentary sequence that can be summarised as: – A basal coarse unit consisting of coarse sandstone interbedded with siltstone and local conglomeratic horizons; – An overlying thick claystone layer; and – An upper sandstone unit, thick bedded and composed of fine to medium ‑ grained sandstone. ∕ The Razorback Beds unconformably overlie the Paleozoic bedrock, lying on an irregular erosional surface gently dipping to the west. Within the proposed TSF area this contact varies between approximately 335 m AHD and 350 m AHD. West of the proposed TSF, on the opposite side of the northeast-southwest trending topographic relief, this contact occurs at lower elevations of approximately 320 m AHD; ∕ Groundwater elevations beneath the UMG TSF area are higher (> 300 m AHD) than water levels in the OCP (sitting at 271 m), which acts as a localised groundwater sink. Groundwater flows eastward from the TSF area toward the pit on the eastward side of the northeast- southwest trending topographic relief that under current conditions is interpreted to as also reflect a groundwater divide. To the west of the TSF area and to the west of the topographic divide, groundwater is interpreted to flow westward toward Turner Creek, which is an ephemeral drainage feature with no permanent flow; ∕ Recharge to the Paleozoic bedrock is from direct rainfall, occurring predominantly during the wet season. Recharge is expected to be enhanced within the Western spoil dump, where coarse, loosely placed materials allow increased infiltration and percolation below the surface. Recharge to the Razorback Beds is likely higher than to the Paleozoic bedrock due to their sandstone composition. This may result in the development of temporary perched water lenses above low ‑ permeability claystone layers (middle unit within the Razorback Beds), with potential for localised and ephemeral discharge to nearby drainage lines following significant rainfall events; and ∕ Groundwater quality is impacted by historical mining activities, particularly the presence of the Western spoil dump. Ongoing weathering and oxidation of mine materials promote the dissolution and mobilisation of metals and ions, which migrate vertically and laterally into the locally groundwater. At the time of this assessment the following knowledge gaps have been identified: ∕ Seasonal groundwater level variability: groundwater level data beneath and surrounding the UMG TSF area are limited, and the magnitude of seasonal fluctuations between wet and dry periods remains poorly defined, introducing uncertainty into the conceptualisation of groundwater gradients and flow directions under transient (wet season) conditions;
Project number: 25B061
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