∕ Groundwater quality variability: available groundwater quality data is spatially and temporally limited, and variability in groundwater chemistry across the TSF footprint and through time has not been fully characterised, limiting confidence in defining local background conditions and assessing incremental impacts associated with TSF seepage; ∕ Groundwater elevations across the inferred groundwater divide: the location and hydraulic behaviour of the northeast-southwest trending groundwater divide are inferred primarily from topography and limited groundwater level data. Additional groundwater elevation monitoring is required on both sides of the divide to confirm the conceptualisation of eastward groundwater flow toward the pit from the TSF area, and westward flow toward Turner Creek; ∕ Razorback Beds stratigraphy and hydraulic properties: geological setting, the actual thickness, stratigraphy and strata continuity, and hydraulic properties of the Razorback Beds beneath the UMG TSF have not been confirmed by site specific investigation, resulting in uncertainty regarding recharge behaviour, potential perched groundwater development, and seepage pathways within these units; and ∕ Groundwater recharge: Rainfall infiltrating into the sandstone units of the Razorback Beds may accumulate and forms seasonal springs which provide temporary contributions to local drainage lines and ephemeral streams incising the sandstone layer downdip westwards from the TSF area (i.e. Paddy Creek and Spring Creek), primarily during and shortly after wet season rainfall events. 5.9.2.2 Seepage potential conceptualisation Based on the current hydrogeological conceptualisation for the UMG TSF area, and acknowledging the various data gaps that exist, seepage from the facility may occur via the following mechanisms: ∕ Seepage from the embankment and foundations: Seepage may occur through the TSF embankment materials or their foundation interface where hydraulic gradients are directed outward from the TSF. Potential seepage pathways include preferential flow through embankment materials, contacts between embankment fill and foundation materials, and zones of higher permeability within foundation soils or weathered bedrock. Any such seepage is expected to be controlled by the relative permeability of foundation materials and underlying bedrock, as well as the hydraulic head within the TSF; ∕ Seepage from the base of the TSF to the groundwater system: Vertical seepage from the base of the TSF may occur where tailings water infiltrates downward into the underlying geological units and intersects the groundwater table. Given that groundwater beneath the UMG TSF area occurs predominantly within competent Paleozoic bedrock with limited primary porosity, downward seepage is expected to be governed by secondary permeability within fractures. Where fractured bedrock or saprolite is thin or absent, vertical seepage may be locally constrained and preferentially redirected laterally along the bedrock surface or within discrete fracture zones; ∕ Lateral seepage from the sides of the TSF into more permeable units: Seepage may occur laterally from the sides of the TSF where tailings or foundation materials are hydraulically connected to more permeable units, such as sandstone intervals, top and basal units within the Razorback Beds or fractured zones within Paleozoic bedrock. Under these conditions, seepage could migrate laterally away from the TSF footprint following local hydraulic gradients or layer of low permeability, potentially discharging to downslope drainage features or contributing to groundwater flow toward downgradient receptors; ∕ Influence of stratigraphy and hydraulic contrasts: Within the Razorback Beds, contrasts between permeable sandstone units and low permeability claystone or siltstone layers may locally influence seepage behaviour, including the potential for lateral diversion of seepage along stratigraphic boundaries. Such contrasts may also promote delayed or episodic seepage responses following rainfall or TSF operational stages; and
Project number: 25B061
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