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Groundwaters in wet, temperate, mountainous, sulphide-mining districts : delineation of modern fluid flow and predictive modelling for mine closure (Rosebery, Tasmania).


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Evans, LR 2009 , 'Groundwaters in wet, temperate, mountainous, sulphide-mining districts : delineation of modern fluid flow and predictive modelling for mine closure (Rosebery, Tasmania).', PhD thesis, University of Tasmania.

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There are as yet few studies of the hydrogeology of sulphide-mining districts in
wet, temperate, mountainous areas of the world. This is despite the importance of
understanding the influence of hydrogeology on the evolution and management of
environmental issues such as acid mine drainage (AMD). There is a need to
determine whether the special climatic and geological features of such districts result
in distinct groundwater behaviours and compositions which need to be considered in
mining impact studies.
The present study addresses this information gap by broadly delineating the
groundwater regime within the temperate Rosebery catchment, western Tasmania,
Australia. The mountainous Rosebery catchment contains the large, active,
underground, polymetallic Rosebery mine, based on a sulphide deposit hosted in the
Cambrian Mount Read Volcanics. Rainfall in the Rosebery region far exceeds
evapotranspiration and supports a dense cover of vegetation. The Rosebery
groundwater system provides an example of an area in which groundwater
management and hydrogeological research is in its infancy, as it is in many regions
with similar settings.
Although the area has been glaciated historically, mine data, particularly rock
quality designator (RQD) values, provides clear evidence of: (i) a weathering induced
increase in permeability within 100 m of the natural ground surface; (ii) an increased
permeability associated with shear zones and faults; and (iii) beyond a depth of 100
m, a uniform decrease in permeability with depth below the ground surface. Insights
into groundwater flow have been provided by qualitative and quantitative
observations of piezometric level in 29 exploration drillholes, 8 piezometers,
discharging groundwaters, and surface water flow monitoring points. Together piezometric heads, flow rates, and calculated material properties have provided the
framework for developing a conceptual model of the groundwater regime within the
Rosebery catchment. The groundwater flow system is typified by a deep fractured
aquifer (which contain the mine voids), overlain by surficial glacial deposits and
weathered material. Significant interaction between surface waters and groundwater
was observed throughout the catchment. Geochemistry provided support for
conceptual flows and justification of the modelling approaches.
Potential acid-generating and neutralising minerals were identified by examining
whole-rock geochemistry. The net acid generation and acid consuming potential of
Rosebery materials were quantified. The results indicated that mined materials at
Rosebery have the potential to produce a significant volume of acid mine drainage.
At the Rosebery mine, metal contaminated waters originate from localised point
sources of sulphides, such as tailings dams, waste rock, and mine workings. Waters
are contaminated by AMD, resulting in elevated levels of H2SO4 as well as the
elements Pb, Zn, Cu, Fe, Mn, Mg Cd, Al, and Ca. The Mg content of AMD indicates
that neutralisation is occurring, most likely through the dissolution of the hypogene
minerals chlorite, ankerite, and dolomite. Although background surface waters and
contaminated mine waters are acidic, regionally the groundwaters sampled are nearneutral.
The quantification of the important flows in the conceptual model of the
Rosebery catchment allows the construction of a water balance, which provides a
reasonable estimation of annual flows. The average precipitation rate across the
catchment is estimated at 8.4 m3/s. The water budget for the Rosebery catchment is:
(i) 42% of precipitation runs off to become true surface water flow (including
interflow); (ii) 24% of precipitation is lost to evapotranspiration; (iii) 17% of precipitation becomes groundwater and is discharged as baseflow into creeks and
rivers within the catchment; and (iv) 18% remains as groundwater discharging into
the regional groundwater system outside of the catchment or into the Pieman River
system. The water balance highlights the importance of groundwater in the
catchment, with a groundwater to surface water flow rate ratio of 1:1.2. The water
balance was applied to three scenarios: (i) a quantification of the contribution of the
open pit to underground flows; (ii) the filling of the southern exploration decline; and
(iii) the filling of the mine after decommissioning. This work indicated that the open
cut makes only a minor contribution to underground water flow and that the mine is
expected to fill to a decant point after six years.
A steady-state, 3D numerical groundwater flow MODFLOW model with a
geometry representative of the hydrogeologic environment around the Rosebery mine
and surrounding catchment was produced. To satisfactorily represent the dual aquifer
system, deep fracture flow on a regional scale was represented by a continuum
approach to couple with true porous media flow (and shallow fracture flow) in the
near surface aquifer. The groundwater model was calibrated to the local observations
and water balance to represent the mine whilst in operation. A predictive closure
scenario was undertaken using the calibrated model in a non-operational state by
eliminating pumping to simulate a flooded mine. Particle tracking was used to plot
potential contaminant flow pathways from the mine in the Rosebery catchment.
Numerical modelling identified that the potential hydrogeological area of influence of
the mine was chiefly controlled by the topography of the host catchment. The
topographically-driven western flows off Mount Black are redirected south, primarily
by the conduits of the mine workings. Conceptualisation of the groundwater regime
improved using computer generated 3D visualisation, and through the numerical modelling exercise. The numerical modelling suggested that the potential area of
discharge for contaminated mine waters is far more limited in extent than was
previously believed by mine personnel. This area is limited to: (i) areas along the Stitt
River and Rosebery Creeks, which are already experiencing significant acid mine
drainage contamination; and (ii) a very limited area south and north of the Pieman
River’s confluence with the Stitt River. The implications of understanding the area of
potential influence are: (i) resources for future monitoring investigations can be
focussed in this discrete area; (ii) the scale of future modelling efforts can be
restricted to this area, improving detail and limiting computational requirements; and
(iii) background monitoring beyond this area can be used to further test the model and
provide data for future model calibration and validation.
The present study at Rosebery has wider implications for researching
groundwater in wet temperate mountainous sulphide mining terrains. The
characteristic feature of such terrains is the local spatial variation in precipitation,
evapotranspiration, and therefore recharge. Estimating a representative recharge
remains the pivotal quantification for undertaking groundwater investigations in such
climates. The significant interaction of surface waters and the groundwater system in
wet, temperate, mountainous environments requires modelling to be capable of
accounting for this interaction.
Surface water and groundwater interactions present an opportunity to investigate
the groundwater regime at the surface and near surface in shallow drillholes. Gauging
stream flow for baseflow contribution provides important quantification of
groundwater flow where suitable drillhole flow data is scarce, a common state in
mined settings. Mine outflows also provide an important quantification of groundwater flow on a large scale. The present study proposed a method for
amalgamating these data which is applicable in other similar environments.
The challenges encountered in the present study provide insight into planning
for similar mining projects. Prior to the present study, there was no general
knowledge of the groundwater patterns of the region, and no clear idea of whether the
Rosebery mine had an influence on the regional groundwater regime. There was no
historical information of the type that is usually associated with groundwater resource
investigations, and the nature of exploration drillholes meant that some typical
groundwater research methodologies were not practical at Rosebery. This resulted in
the investigation taking a different research approach compared to that typically used
for groundwater evaluation in, for instance, agricultural districts.
Although the present study attempted to endure without standard
hydrogeological data, there remains a significant gap in the reliabilty of the
modelling. This is a situation that faces most mines nearing closure, when cash flow
is likely to be reduced. Time-series piezometric levels, and a larger hydraulic
property dataset were the key gaps identified in the present study. Appropriate
budgeting for the acquisition of such information should be considered a priority for
groundwater investigations of wet, temperate, sulphide-mining districts elsewhere.

Item Type: Thesis - PhD
Authors/Creators:Evans, LR
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