Evolution of conceptual hydrogeological models, Ranger Uranium Mine, Northern Territory Alan A Puhalovich Energy Resources of Australia Ltd, Level 3 18-20 Cavenagh Street Darwin NT 0800 Hydrogeological investigations at the Ranger Uranium Mine, situated 250 km east of Darwin (Northern Territory), were initially undertaken in the 1970s. Since this time, technological advances in hydrogeological characterisation methods and data collected from the measurement and observations of groundwater behaviour and calibration of flow and solute transport models, have been used to develop the current conceptual hydrogeological model for the site. Initial investigations included open-hole drilling, water quality analyses and groundwater level mapping. A ‗two aquifer‘ concept for hydrogeological units (HUs) present was initially developed in 1980. It assumed the existence of soils HUs overlying deeper weathered and fresh rock HUs. Lithological characteristics were considered an important hydrogeological control. Groundwater flow patterns and catchments were assumed to be aligned with surface water characteristics, with relatively high rates of rainfall-recharge to groundwater systems in upper catchment areas and groundwater discharge to local creek and billabong systems. In the early-mid 1990s, hydrogeological investigations began to use emerging groundwater monitoring datasets and field observations of groundwater behaviour (e.g. inflows to mined pits). In addition, investigations began to include airborne geophysics methods (e.g. electromagnetics), ground geophysics (e.g. electromagnetic depth soundings, direct current and induced polarisation, self potential and transient electromagnetic surveys), coring/packer testing, creek baseflow analyses and groundwater quality ‗fingerprinting‘ of specific HUs. A ‗three aquifer‘ concept emerged, assuming alluvial soils HUs (where present), overlying residual soils/weathered rock HUs, which, in turn, overly fresh rock HUs. Groundwater level and water quality data distinguished the three ‗aquifers‘, although the similarities of these data for various lithological units lead to the conclusion that geological controls are not as important as structural and weathering controls. The relatively low permeability and storativity within site HUs suggested that groundwater systems are highly compartmentalised and not connected. In particular, deeper HUs (beneath the alluvial and residual soils units) were considered to be largely disconnected to surface water systems. In the 2000s, investigations focussed on measurement of vertical (hydraulic) gradients, groundwater ‗dating‘, modelling of surface water balances within local billabong systems, soil moisture monitoring and modelling, calibration of geophysics data with groundwater level/ permeability data and calibration of groundwater flow and solute transport models. The current conceptual hydrogeological model identifies the most active (‗Primary HUs‘) to least active HUs (‗Secondary HUs‘ and ‗Tertiary HUS‘). Primary HUs comprise relatively permeable, alluvial soils and fractured rocks (both) beneath creek lines, with complex rainfall recharge–discharge interactions present in these units. Surface waters are generally coupled to alluvial soils within this unit. Secondary HUs, within fractured and weathered metamorphic rocks, are relatively continuous but less permeable than soils/rocks in the Primary HUs. These HUs have little connection with the near-surface hydrological regimes (e.g. rainfall recharge). Tertiary HUs are the least ‗active‘ groundwater systems and comprise disconnected, ephemeral aquifers within residual soils and unweathered metamorphic rocks. In summary, a significant number of investigation and monitoring approaches have been used at Ranger Uranium Mine since the 1970s. Conceptual hydrogeological models have evolved over time, reflecting technological advances, the varied nature of investigation approaches used and monitoring data and observations. The current conceptual hydrogeological model defines groundwater behaviours for three key hydrogeological units and the roles that these play in the hydrological cycle. __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________
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