Puhalovich, Alan - Groundwater 2010

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.
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________