GEOHYDROLOGICAL CHARACTERISATION OF THE YARRAMBA
PALAEOCHANNEL, HONEYMOON URANIUM PROJECT
Rooke, E.R. 1 , Gallagher, M.1 and Bush, P.D. 2
1.
Water Studies Pty. Ltd.
Level 2 / 70 Hindmarsh Square, Adelaide, SA 5000
Tel 08 8232 2400 Fax 08 8224 0453
Email erooke@waterstudies.com.au
2.
Southern Cross Resources Australia Pty. Ltd.
PO Box 1513 Toowong, QLD 4066
Tel 07 3371 9700 Fax 07 3371 9800
Email pbush@southerncrossres.com.au
Abstract: The Honeymoon Uranium deposit is located 75 km north-west of Broken Hill in the Yarramba Palaeochannel.
Southern Cross Resources Australia Pty Ltd (‘SXR’) has been developing the Honeymoon Project since 1997 and intends
mining uranium using the in-situ leach (ISL) process.
An Environmental Impact Statement (SXR, 2000a) and Environmental Impact Statement, Response Supplement (SXR,
2000b) were released in 2000. Subsequently, the Federal Government announced in February 2001 a requirement for
further detailed information on the hydrology of the Honeymoon aquifers prior to finalising a decision on the
Honeymoon uranium mine proposal. This paper describes the work necessary to satisfy this requirement. It includes
descriptions of selected analyses following a program of aquifer test pumping and numerical groundwater modelling
designed to investigate hydrogeological boundaries, inter-aquifer leakage, fate of solutes generated by In-Situ Leach
(ISL) mining operations and to demonstrate that a monitoring regime can detect excursions of these solutes.
Results from analyses of the test pumping provide convincing evidence of lateral hydraulic boundaries to the
Palaeochannel (Water Studies, 2001a) and are supported by geological evidence from stratigraphic drilling (SXR, 2001).
Numerical modelling (groundwater flow and solute transport) simulating operational start -up and ISL operations for 5
years, 15 years of disposal of solutions (including the operational period), followed by up to 85 years cessation of
production, was undertaken. The models indicated minimal excursions of disposal solutions and a rapid return to ambient
native groundwater potentiometric and hydrochemical conditions. It also indicated that a hypothetical network of
monitoring bores would be effective in monitoring possible changes in potentiometric heads and changes in solute
concentrations during and after disposal.
Key words: ISL mining, hydraulic testing, analytical solutions, numerical modelling, solute transport.
REGIONAL SETTING AND PROJECT HISTORY
The Honeymoon Uranium deposit is located within the
Mesozoic-aged Frome Embayment of South Australia, 75 km northwest of Broken Hill, Figure 1. It lies over 70 km south of the southern–
most extent of the Great Artesian Basin. Regionally it occupies the
south-eastern extremity of the Lake Eyre drainage basin.
The deposit precipitated along the convex margin of a major
bend within the Cainozoic-aged Yarramba Palaeochannel, principally
within the reducing zone of the Basal Sands. The Yarramba
Palaeochannel, situated within a landscape of low sand dunes and clay
pans, slopes imperceptibly northwards from the Olary Range toward the
saltpan of Lake Frome. The Palaeochannel consists of a sequence of
fluvial sand channels, rendered discontinuous by intermediary clay and
silt formations incised into Proterozoic basement rocks. In the area
occupied by the Honeymoon mineral claims, conceptually the
Palaeochannel sediments have been differentiated into five units
consisting of three aquifers each separated by an aquitard.
Hydrochemical and hydraulic differences between the aquifers
support this conceptual model. Regional groundwater flow is northwestwards towards Lake Frome.
Figure 1 Location of the Honeymoon Uranium Project
1
Southern Cross Resources Australia Pty Ltd (‘SXR’) has been developing the Honeymoon Project since mid1997. The Environmental Impact Statement (SXR, 2000a) sought approval for the establishment of a commercial in-situ
leach (ISL) uranium operation, initially located at the adjacent Honeymoon and East Kalkaroo Deposits. SXR proposes
disposal of liquid waste by re-inject ion to the mining aquifer. It is necessary to characterise any subsequent migration of
re-injected waste and demonstrate that detrimental environmental consequences will not occur. As part of project
development, a Field Leach Trial began in April 1998 producing uranium peroxide on a demonstration basis. This
operation ceased in August 2001 after sufficient data had been collected for the design of a commercial plant.
TERMS OF REFERENCE FOR FURTHER EVALUATION OF AQUIFER
An Environmental Impact Statement, Response Supplement (SXR, 2000b) was produced in answer to public
comment. Subsequently, the Federal Minister for the Environment and Heritage announced in February 2001 a
requirement for further detailed information on the hydrology of the Honeymoon aquifers prior to finalising a decision on
the Honeymoon uranium mine proposal. This paper describes the work necessary to satisfy this requirement.
Federal and State Government agencies with a jurisdictional interest determined terms of reference (TOR) for
the evaluation work. Investigations proposed in the TOR would provide necessary information for acceptable designs for
ISL mining, liquid waste injection and management, and their associated monitoring programs. The TOR follows:
• Hydraulic testing to assess the degree and nature of hydrogeological boundaries and characteristics of the aquifers
and confining beds, including any leakage between the aquifers in the Honeymoon Uranium Project area (i.e.
Honeymoon and East Kalkaroo).
• Numerical modelling using results from the pump tests and existing data to simulate groundwater flow in the project
area, and solute transport modelling to predict the movement of the reinjected liquid waste.
•
Characterise the chemical processes, which occur in any liquid waste plume, to det ermine the chemical and physical
changes and the probable rate of return to values comparable to the natural groundwater in the aquifer.
•
Further evaluate the effectiveness of the monitoring systems in the Basal, Middle and Upper Sand Aquifers to detect
possible excursions as a result of mining and liquid waste injection in the Basal Sand Aquifer (BSA) and
groundwater extraction from the Upper Sand Aquifer (USA), and any possible leakage into the Middle Sand Aquifer
(MSA).
•
Demonstrate that possible excursions can be detected by monitoring chemical and hydraulic pressure changes under
a fully operational scenario of mining and liquid waste injection in the Basal Sand Aquifer and extraction from the
Upper Sand Aquifer.
AQUIFER TEST PUMPING PROGRAM
An aquifer-t esting program was undertaken (Water Studies 2001a) to investigate lateral hydraulic barrier
boundary conditions on the north and south sides of the Palaeochannel within the Honeymoon mineral claims area. The
program involved test pumping, injection, monitoring and acquisition of data including drawdown / drawup a and
recovery data.
Depths of bores
drilled for the program
ranged from 108 – 133 m
below ground level (bgl) for
the BSA, 94 – 103 m bgl for
the MSA, and 85 – 90 m bgl
for the USA (SXR, 2001);
(they are indicative of depths
to the respective aquifers).
Pumping and injection bores
were screened adjacent to the
BSA; monitoring bores were
completed either within the
BSA, MSA or the USA.
Figure 2 shows the sites of
all the bores in the aquifertesting program and the
injection pipelines at ‘East
Kalkaroo’
and
‘Honeymoon’.
Figure 2 Sites of Pumping, Injection & Monitoring Bores & Injection Pipelines in the Yarramba Palaeochannel
a head
rise or surcharge
2
Table 1 summarises the schedule of test pumping, and injection.
Table 1: Test Pumping / Injection Schedule
HONEYMOON
E KALKAROO
Site
Pumping
Start
Injection
Start
Pumping
Duration
Injection
Duration
Pumping
Rate
Injection
Rate
Recovery
Duration
(H, D/M/Y)
(H, D/M/Y)
(Hours)
(Hours)
(L/S)
(L/S)
(Hours)
EMONB,
EKIB_B
1400,
21/05/01
1600,
21/05/01
12
10.3
25
17
(EMONB); 8
(EKIB_B)
0
EKPT_B
EMONB;
EKIB_B
1010,
23/05/01
1320;
23/05/01
11
9
38.5
19
(EMONB); 6
(EKIB_B)
1
M3
EKIB_B
EKPT_B
1400,
30/05/01
1330,
30/05/01
19
19.75
15
15
3
M4
KPT_B
HMEIB;
HMIB_B
1330, 4/06/01
13
17.5
35.5
10
0
HMEIB,
HMIB_B
1050, 5/06/01
12
2
Test
No.
Pumping
Bore
Injection
Bore
M1
EKPT_B
M2
M5
0900,
4/06/01
1100, 5/06/01
6
42 water bores were monitored manually during the suite of tests, including 21 monitored using electronic
pressure transducers.
Field operations of the aquifer tests are described in Water Studies (2001a). The injection bores accepted lower
rates of injection than pumping abstraction. Excavated tanks acted as balancing storages and alleviated back-pressure on
the injection pipelines.
The Monitoring Bores responded quickly to pumping and injection indicating transmission of pressure and a
a
highly transmissive aquifer. Middle and Upper Sands Monitors as well as Basal Sands Monitors responded indicating
leakage.
Selected analyses from Water Studies (2001a) are outlined below. Kruseman and de Ridder (1991) and
Vandenberg (1977) provide a definition and analysis of ‘bounded leaky’ and ‘bounded confined aquifers’ also known as
‘leaky strip’ or ‘strip aquifers’, respectively.
ANALYSES
Time – Drawdown / Recovery Analysis
Log-normal graphing (Jacob -Cooper modification of the Theis well equation) is used for assessing barrier
boundary groundwater hydraulic conditions. The response to a barrier condition when plotted log-normally is a doubling
in the slope of the graph; further doubling at a later time indicates interception of another barrier-boundary.
Log time – log drawdown plots are used to indicate leakage and strip aquifer (linear groundwater flow)
conditions. Later time flattening of the Theis curve is indicative of leakage. A convergence of plots from monitors at
differing radial distances from a pumped / injection bore is indicative of linear flow conditions, and in the case of the
Palaeochannel would indicate strip aquifer hydraulic conditions.
Log –Normal (Jacob-Cooper) Graphical Plots
Figure 3 (Test M2) showed accelerating drawdown indicative of a cone of depression propagating into less
permeable aquifer material than the pumped well (i.e. a ‘soft barrier’, possibly multiple) boundary condition, indicative
of a strip aquifer (Clarke, 1989).
A muted recovery began at tpb=200 minutes (mins); i.e. 10 mins after start of injection and in response to it.
Drawdown was greater in the Basal Monitors than either the Middle or Lower Monitors. Leakage was evident.
a The
b tp
abbreviation Basal Monitors, etc alludes to bore(s) screened against and thus monitoring potentiometric pressures in the BSA, and so on.
= time from start of pumping whilst ti = time from start of injection.
3
Figure 3 Acceleration of Drawdown in Monitors showing Influence of Injection, E. Kalkaroo
For Test M3 a barrier response to
pumping occurred in EKMON_3B after tp=5
mins, emphasised when compared with distant
Basal Monitors, Figure 4. Leakage prevailed in
EKMON_3B after tp =100 mins.
Figure 4 Drawdown in Basal Monitors showing a Barrier Boundary Condition, E. Kalkaroo
Figure 5 (Test M5) indicates drawup response in monitors proximal to injection bores, HMONB and HMEIB. A
barrier boundary is intercepted by HMON_1B at only ti=1.5 mins with the rate of drawdown doubling again at ti=8 mins
and 80 mins, respectively. The barrier
boundary is detected in HMON_1M and
HMON_1U at a later time (ti=80 mins).
Figure 5 Drawup in Injection Site Monitors showing a Barrier Boundary Condition, Honeymoon
Log-Log (Theis) Plots
Figure 6 shows a log-log plot for Test M4 of Basal monitors; the slopes of the graphs approach convergence
indicating strip aquifer conditions.
4
Figure 6 Drawdowns in Basal Monitors showing Convergence, Honeymoon
Distance – Drawdown Analysis
Drawdown contours (‘cones of depression’) were compiled for Tests M3 and M4. Tests M1 and M2 could not
be interpreted due to drawup from the injection site migrating towards, and interfering with, the cone of depression at
pumping bore, EKPT_B. Test M5 had insufficient drawup observations to construct contours plots.
A circular cone of depression indicates radial groundwater flow; deviation from this form indicat es anisotropic
flow, the extreme form being linear flow.
Cross-sections of time - drawdown were constructed taking north-south and east -west sections. The slope of the
cone of depression may be compared with the slope of the Theis (Jacob modification) distance – drawdown equation for
radial flow.
Figure 7 (Test M4)
presents drawdown contours at
tp=120 mins. The contours are
elliptical with long axis parallel
with the Palaeochannel; east of
the pumping centre (KPT_B)
equi potentials tend to parallel,
indicating the induction of linear
groundwater flow. The head rise
at Honeymoon did not allow
contouring to be extended
westwards.
Figure 7 Drawdown Contours for Basal Sands Aquifer, 2 hours after start of Pumping, Honeymoon
Figure 8 is a section
drawn north–south, centred on
pumped bore, KPT_B. A line of
best fit has a linear equation
indicative
of
linear
flow
conditions.
Figure 8 Distance – Drawdown Section for Basal Sands Aquifer, 13 hours after start of Pumping, Honeymoon
The radius of influence approaches 1km. Although the Palaeochannel is 2km wide at this point, the width of the
5
Basal Sands where it intersects basement is only some 1.2km. Hence the cone of depression likely intersected the lateral
a
margins of the Palaeochannel. Using the final drawdown in KPT_B (4.94m) and the Theis well equation, the theoretical
distance-drawdown has been superimposed on Figure 8. The gradient of the observed drawdown is steeper than that of
the theoretical drawdown slope. This comparison provides further evidence for a linear flow component.
Aquifer Simulation Using An Analytical Test Pumping Program
An analytical test pumping program, ‘CG’ (Clarke, 1987) was used to examine idealised distance-drawdown
relationships for a ‘strip aquifer’ from aquifer parameters determined from test pumping. The analysis herein assumes a
‘simple strip aquifer’ bounded by two impermeable, parallel boundaries with the pumped / injected bore and monitoring
bores centred on the long-axis of the channel.
When plotted on a semilogarithmic
graph,
theoretically,
the
time/drawdown data from a steady rate
discharge test in a strip aquifer shows a
continual steepening. When plotted on a
square root of time versus linear drawdown
graph, the same data should fall on a
straight line. With reference to Figure 3,
drawdown data (before influence from
injection) for nearby Monitors have been
replotted, using the square root of time on
the abscissa to observe a near straight-line
fit, presented as Figure 9.
Figure 9 Square Root of Time v Arithmetic Drawdown, E. Kalkaroo
The fit to a
leaky strip aquifer
model is convincing
for nearby monitors in
Figure 10 (Test M4).
The
leakage
coefficient
values,
between 1945 and
2660 m are high
indicating
restricted
inter-aquifer leakage
in the vicinity.
Figure 10 Match between Observed and Simulated drawdowns using a ‘Leaky Strip Aquifer’ Solution,
Honeymoon
Utilising aquifer parameters derived from a leaky strip aquifer solution for EKMON_1B and EKMON_2B in
a
Intrinsically accounting for well–loss rather than employing the Theis well function to calculate the drawdown slope.
6
Tests M1 and M2, the theoretical strip width averages 1 km, i.e. slightly less than the width of the Palaeochannel (extent
of BSA) taken normal to pumped bore, EKPT_B. A theoretical distance-drawdown plot for this strip aquifer was made at
tp=144 mins. In the vicinity of EKPT_B, transmissivity (T) values ranged between 2500 and 2800m2/d. With such large T
values of the BSA, the theoretical radius of influence extended 3km from EKPT_B. Likewise, for Test M4, using aquifer
parameters derived from KMON_1B and KMON_2B, a distance-drawdown plot at t p=144 mins, indicates that the
theoretical radius of influence extends to 2.5 km from pumped bore, KPT_B. Here T values ranged from 1600 to 1800
2
m /d.
From Test M3, transmissivity (T = 165 m2/d) in the vicinity of bore, EKIB_B, supported by the geological logs
describing mixed and intercalated sands and clays (SXR, 2001), was an order of magnitude lower than at EKPT_B and
KPT_B where clean sands were encountered. Also the leakage coefficient value of 53 m was relatively low, possibly
indicating a thin, or graded confining layer(s). The modelled aquifer width of 1.1 km agrees with the width of the
Palaeochannel taken normal to pumped bore, EKIB_B.
NUMERICAL GROUNDWATER SIMULATION
An assessment was made of the likely hydrological and water quality effects on the groundwater system within
the Honeymoon mineral claims from the proposed operations outlined in the EIS (SXR, 2000a). The work enhanced
previous modelling to include five hydrogeostratigraphic units with updated geological and hydraulic datasets from
supplementary drilling and test pumping (SXR, 2001 and Water Studies, 2001a). Solute transport modelling has
enhanced the state of knowledge concerning solute migration from proposed Honeymoon ISL operations, which had
been based previously on advection and dispersion only, using conservative chemical species (SXR, 2000a).
Methodology and Data Sources
A multi-layered numerical groundwater flow model (MODFLOW) was constructed and calibrated using
geological and monitoring data to represent the conceptual hydrogeological system. The model was used to simulate the
impact on the groundwater regime from start-up operations for the proposed Project. It incorporated an ISL wellfield
array and disposal of reverse osmosis plant (RO) reject brine and ISL barren solution bleed to the BSA at various
locations, and the abstraction of raw water from the USA for the RO plant. Inter-aquifer leakage induced by
pressurisation and dewatering effects during injection and abstraction was assessed, as was solute migration arising from
ISL injection and abstraction from the BSA.
A numerical solute transport model (MT3D99) a was constructed from the MODFLOW model, to incorporate
advection, dispersion and linear sorption processes. MT3D99 was used to simulate transient changes in the distribution of
contaminant concentration. It included a sensitivity analysis on the assumed solute adsorption coefficients to ‘bracket’
expected solute migration. The analysis was based on producing breakthrough curves showing time-variant water quality
concentrations at particular locations for chemical species of interest.
The groundwater flow and solute transport model was developed using data listed below:
• reports on hydrogeology including stratigraphic drilling and bore construction details;
•
spatial data and isopach maps for the five hydrogeostratigraphic units modelled (O.R.E.S. and SXR);
•
digital mapping of uranium mineralisation deposits within the mining lease (SXR);
•
daily rainfalls; two years’ record (May 1998 to April 2000) for the Honeymoon site (SXR);
•
groundwater levels; two years’ record (March 1999 to March 2001) (SXR);
•
daily ISL trial injection and recovery flow rates; 18 months’ record (March 1999 to September 2000) (SXR);
•
coordinates and configurations of the trial ISL well field, suggested disposal sites for process liquid and RO
reject bleed, and project water balance data (SXR, 2000a); and,
•
baseline and ISL trial groundwater chemistry datasets, which included cadmium, chromium, copper, iron,
radium, selenium, sulphate, uranium, vanadium, and zinc.
Flow Model Development, Calibration and Sensitivity Analysis
Five hydrogeostratigraphic units were modelled; three aquifers (BSA, MSA and USA) and two intervening
aquitards, the Basal Clay, and Middle Clay (MC). Fine model grids were required in the vicinity of the ISL trial locations
and nearby monitoring bores to accurately represent historical head variations for improved calibration performance, and
in the vicinity of the proposed ISL well field for operational scenario modelling.
An 18-month period was selected for transient calibration using available piezometric head data from 13 bores
for the BSA, MSA and USA. The horizontal and vertical hydraulic conductivity and storage coefficient were varied
individually in all model layers. Model outputs were most sensitive to variations in horizontal hydraulic conductivity for
the BSA.
a MT3D99
uses numerical techniques to solve the transport equation on the basis of hydraulic gradients computed by MODFLOW for each given time
step.
7
Operational Flow Modelling
Dewatering and surcharge effects were simulated for a sequence of operations to demonstrate the impact of
possible operational scenarios on the groundwater (Water Studies, 2001b) as follows:
• a 5 year period of ISL in the BSA, disposal of RO brine and ISL bleed at two sites in the BSA, and pumping
water from the USA for potable water supplies and drilling;
•
cessation of ISL operations and raw water pumping, with continued disposal of solutions for a further 10 years;
•
complete cessation of disposal for 35 years resulting in a 50-year total period of simulation; and,
•
complete cessation of disposal for 85 years resulting in a 100-year total period of simulation.
Operational Scenarios
Three sites were selected for
modelling of the continuous disposal
of RO brine and barren solution
bleed over a 15-year period (Water
Studies, 2001b). Figure 11 shows
these disposal sites and indicates the
zones
of
mineralisation,
the
numerical model’s grid and boundary
conditions. It also shows an exploded
view of the FLT ISL wellfield array.
Figure 11 Numerical Model, MODFLOW, Operational Grid and Bores for the Basal Sands Aquifer
These disposal sites were identified bas ed on a substantial thickness of Middle Clay, high transmissivity in the BSA
and site logistics. Two disposal options were analysed, each utilising two of the sites. The eastern most sites were found
to be the most suitable under worst-case conditions of maximum dispersion and no adsorption. Site 3, west of the other
two sites, placed disposal solutions closer to the western boundary of the lease.
Water Balance
The flow balance results presented in Figure 12 indicated an upward leakage from the MSA to USA of 37 kL/d,
constituting 12% of the raw water outflow from the USA for the simulated initial 5-year operational phase. Dewatering
of the USA drove this upward leakage by raw water abstractions. At the same time, pressurization of the BSA by
disposal was rapidly equilibrated due to the highly transmissive BSA, and was not considered to significantly contribute
to this upward leakage.
The impact of the disposal was only about 3 kL/d of upward leakage from the BSA to the MSA during the initial
5-years of operations. Excursions from the MSA to USA rapidly returned to zero at the cessation of ISL operations and
raw water abstraction, even while disposal to the BSA continued. Approximately 10 kL/d of leakage from the MSA to
BSA was calculated during the initial 5-year period,
which most likely occurred in the vicinity of the ISL
wellfield where a net 1% of the production rate (4.4 L/s)
was maintained causing localised dewatering and
associated downward leakage due to the resultant head
differential. From the end of raw water extractions and
ISL operations until disposal was discontinued at 15years, a steady rate of 23 kL/d upward leakage from BSA
to MSA was demonstrated (3% of the total 680 kL/d
injected), which was previously damped by the net
leakage from the MSA to BSA as a result of ISL wellfield
activities.
Figure 12 Inter-Aquifer Leakage for 0 – 5 Years Operating Period
8
Following the cessation of disposal after 15 years of continuous operation, there was a rapid return to net
downward leakage caused by the head gradient prescribed by the head boundary conditions.
Predicted Drawdown
Predicted piezometric head changes in each aquifer at the end of the first two operational phases was calculated
from the difference between piezometric heads at each time and the initial steady -state heads, and was undertaken for
each disposal site combinations. For the USA, up to 3.5m drawdown was found after 5 years of operations, decreasing to
0.04m at the cessation of disposal. This demonstrates that the USA was not significantly influenced by disposal activities,
after raw water pumping ceased.
Drawdown in the MSA was of the order of 0.03m after 5 years of operations, due to upward leakage during raw
water abstractions from the USA and some downward leakage to the BSA from the net ‘production’ occurring there. This
equilibrated by the cessation of
disposal.
The predicted change in the
piezometric heads for the BSA for the
first disposal option after 5 years of
operations was 0.25 m within the ISL
wellfield, which is consistent with the
small net production maintained there.
Disposal activities lead to a maximum
increase of 0.45 m in piezometric head
centred at the areas of disposal, Figure
13.
After 15 years of disposal this rose to a
maximum of 0.85 m, gradually
decreasing in magnitude upstream and
downstream. The increase in heads at
the upstream boundary was 0.4 m,
while the increase downstream was
approximately 0.05 m.
Figure 13 Head Drawdown in the Basal Sands Aquifer after 5 Years of Operation
Operational Monitoring Bore Placements
The
proposed
placement
of
monitoring bores at 125m intervals from the
ISL wellfield (SXR, 2000a) was assessed for
its effectiveness in detecting pressure changes
within the BSA. The drawdown at a ring of
seven monitoring bores placed around the
ISL wellfield within the BSA was predicted
over a fifty-year period. No appreciable
difference was shown between the two
disposal site options.
An average drawdown between 0.1
and 0.15m was simulated during a 5-year
period of operation at the bore locations,
Figure 14.
Figure 14 Drawdown from Start of Operations at Hypothetical Monitor Locations
A surcharge mound of less than 0.65m was predicted post- ISL operations, which returned to steady state over
several weeks after cessation of disposal operat ions. It was concluded that such a placement of monitoring bores would
allow effective monitoring of water balance in the ISL wellfields.
SOLUTE TRANSPORT MODELLING
Methodology and Initial Conditions
In addition to simulating the advection and dispersion of the contaminants, solute retardation was examined.
9
Linear sorption theory was used to account for the likely adsorption of dissolved solute to the mineral surfaces of the
receiving aquifer. A literature review was conducted to obtain linear sorption values representative of the Palaeochannel
(Water Studies, 2001b). A concentration contour of 1% variation was adopted to assess the migration of the solute plume
resulting from the disposal of waste solutions. This contour represents 1% of the difference between the background
concentration of a source element in the natural groundwater and the concentration of the same element in the disposal
fluid (SXR, 2000a).
SXR undertook an analysis of water quality data from leach trials by comparing with background el vel
monitoring. Water quality concentrations at increasing distances from the point of injection were monitored and then
compared with the minimum and maximum measured background values. Concentration profiles for chromium, iron,
sulphate, vanadium, and zinc were shown to exceed the maximum background values in a number of instances. Of these
only chromium, vanadium, and zinc were considered important due to the intrinsic relationship of iron and sulphate with
the ISL process.
A robust sensitivity analysis of the assumed solute adsorption coefficients was conducted to ‘bracket’ the
expected range of down-gradient migration of solute as well as the concentrations at hypothetical monitoring bores
located at 125 m from the disposal sites.
The background solute concentration was set to zero to assess the percentage reduction in concentration of
chemical species due to the solute migration processes. The concentration of the bleed stream and RO reject brine was
set to 100 units and input to the model as a point source at the disposal wells of interest for the period of operational
disposal.
Results
Solute Concentration contours
Sensitivity analyses were undertaken using a range of distribution coefficients, Kd (0 a , 1and 10mL/g) and
varying dispersivity (1 and 10m). Plots of the spatial migration of solute for each of the sensitivity runs were undertaken.
Figure 15 and Figure 16, respectively, show the results at the end of 15 continuous years of disposal, and at 35 years
following cessation of disposal with a dispersivity of 10 m.
Figure 15 1% Concentration Contours after 15 Years of Injection with 10 m Dispersivity
a
It is unlikely that the most conservative solute case, Kd = 0 (i.e. no adsorption of chemical species) is representative of actual
conditions in the Basal Sands Aquifer (refer to the discussion on Adsorption below).
10
Figure 16 1% Concentration Contours 35 Years after Cessation of Injection with 10 m Dispersivity
The distance down gradient of the most westerly disposal point was also determined to ascertain possible
excursions from the mineral claim areas.
Figure 17 indicates the 1%
concentration distribution contour in
the BSA at 85 years after cessation of
disposal.
Figure 17 1% Concentration Contours 85 Years after Cessation of Injection with 10 m Dispersivity
This demonstrated that the migration of solute was still effectively retained by the aquifer in the vicinity of
disposal areas for values of Kd between 1 and 10, whilst the mineral claim areas were only marginally transgressed down
gradient for a Kd value of 0. Generally it could be expected that after 100 years, concentration differences of more than
1% would have migrated no further than about 120 m from the point of injection.
Similar analysis was undertaken for the MSA, however no detectible concentrations were predicted at any time
period, i.e. no values within the MSA domain reached the 1% threshold.
Monitoring
In order to examine the timely
detection of solute concentration changes
during and after disposal, solute
concentration breakthrough curves were
generated for an array of hypothetical
Basal Monitoring bores situated 125 m
from each of the disposal sites
considered.
Figures 18 shows an example of a typical
breakthrough curve output from the
modelling.
Figure 18 Breakthrough Curves in a Hypothetical Monitor with 10 m Dispersivity, ‘Disposal Site 1’, E. Kalkaroo
11
•
The major outcomes of this analysis were as follows:
All monitoring points would detect appreciable concentration changes with time, which would enable an
appreciation of the relative progress of contaminant migration from the disposal sites;
•
Concentrations peaked at the time of the cessation of disposal for the lower Kd values of 0 and 1,
•
Predicted concentrations at 35 years post-disposal showed between 0 and 75% of source concentration
remaining in these areas for all sorption and dispersivity values tested - a result of the effectiveness of the
aquifer to contain the solute locally as well as the feedback of solute that had migrated up-gradient during
aquifer pressurization;
•
For a Kd of 10, which is a plausible figure for chromium, vanadium and zinc, a slowly increasing concentration
was predicted over the total 50 year period simulated, reaching no greater than 2% as a result of the highly
localised retardation of solute around the disposal locations.
•
A feature of the conservative solute cases was the cyclical (but diminishing) nature of the concentrations
reporting to the monitoring locations over time. This was due the more widely dispersed solute up -gradient
reversing once the influence of the natural gradient was re-established after cessation of disposal;
These breakthrough curves demonstrated the effectiveness of the aquifer to retard the migration of chemical
compounds over extended periods of time following the cessation of disposal activities. In cases where higher sorption
characteristics were assumed, only minor differences between the background concentration in the native groundwater
and the concentration of the same element in the disposal solution were reported at the monitoring locations over the time
periods considered. The solute plume was localised within a 125 m radius of the disposal points. The removal of
contaminants was determined to be rapid and residual concentrations were confined to the immediate vicinity of the
injection bores.
Discussion on Adsorption
The partition coefficient (K d) is the ratio of mass of solute (g) adsorbed per mass of sorbent (g) divided by the
mass of solute (g) per volume of solution (H2O) (mL). The partition coefficient is a function of the electrochemical
properties of the water and dissolved material. The Kd provides a quantitative estimate of the sorption process and
retardation of the movement of chemical species through the aquifer. Knowledge of the Kd and of the bulk density of
porous media and porosity for porous flow, allows calculation of a retardation factor.
Numerous studies have been undertaken to ascertain the variability of Kd in different hydrogeological
environments and variable aqueous phase chemistry. Numerical values for Kd obtained from the literature show a wide
range (Water Studies, 2001b). Table 2 lists values (Baes et. al, 1984) pertinent to the groundwater and disposal solution
analyses obtained during the Honeymoon Field Leach Trail (SXR, 2000a, 2000b). Chromium, iron, vanadium, zinc and
pH were outside the natural background levels in the area of the disposal bore during the Field Leach Trial (SXR, 2000b).
Table 2 Reported Kd Values
Species
Average Value
Range of Values
Disposal Solution within
(g/mL)
(g/mL)
Baseline
yes
U3 O8
450
10.5 – 4,400
Fe
25
1.4 – 1,000
no
Mo
18
0.37 - 400
yes
V
1,000
no
Cl
0.25
yes
Si
30
Ca
4.1
1.2 – 9.8
yes
no
Mg
4.6
1.6 – 13.5
yes
Al
1,500
Cu
35
1.4 – 333
Zn
38
0.1 – 8,000
Se
300
no
no
no
yes
Cr
850
no
Na
100 mg/L
yes
F
150
Ra-226
450
10.5 – 4,400
no
yes
Source: Baes et al 1984
12
The data in Table 2 show that the elements having elevated levels all exhibit average Kd values significantly
higher than the upper limit of 10g/mL used in the sensitivity analyses (Water Studies, 2001b). This value produced a
residual elevation of chemical species concentrations of less than 2% over a period of 50 years.
In terms of the robustness of the modelling methodology, Pirlo (Per. Comm., 2001 & 2001) stated, “the sorption
of ions onto mineral surfaces in the aquifer would be an important sink for dissolved elements ands species in solutions
associated with the Honeymoon Uranium Project” and, “metal oxides (especially those of iron and aluminium) and
aluminosilicate minerals (clays) could adsorb electrolytes strongly due to their high reactivity’s and large surface areas.”
Hence, he concluded that sorption could significantly diminish the mobility of dissolved components in solution.
The process of contaminant removal will be enhanced by the presence of reductants such as pyrite and organic
matter, which are abundant in the aquifers of the Palaeochannel.
Conclusions
Aquifer Test Pumping
The suite of aquifer test pumping analyses provided convincing hydraulic evidence of a laterally, bounded
channel (restricted to the north and south by low permeability boundaries). This conclusion is in agreement with the
recent program of stratigraphic drilling (SXR, 2001), which verified the Palaeochannel’s morphology. Important
conclusions from testing follow:
• Cones of depression were quickly established and migrated only marginally thereafter indicating a highly
transmissive, leaky aquifer. T values for the BSA ranged from 165 m2 /day (at EKIB_B) to 2800m2 /day at other
sites; strongly skewed towards larger values;
•
Inter-aquifer leakage was more dominant from the MSA to the BSA than from the USA to the BSA;
•
Barrier boundary conditions were encountered in the vicinity of monitoring bores at East Kalkaroo (Test M1;
injection site, Test M2; injection, Test M3; pumping) and Honeymoon (Test M4; injection and Test M5;
injection). Soft (possibly multiple) barrier boundaries were observed in Tests M1, M2 and M4.
•
Convergences of Basal Monitors log time – log drawdowns’ were observed for Test M2 (pumping site), Test M3
(injection site) and Test M4 (pumping and injection sites) indicating strip aquifer conditions.
•
Distance – drawdown analysis of later time data was used to construct cones of depression. The slope of the
drawdown on the sections was compared to that given by Theis for radial flow. Tests M3 and M4 exhibited
elliptical contours, the long-axis of which paralleled the Palaeochannel. In Test M3 radial flow remained
dominant, whilst Test M4’s drawdown indicated linear flow. The north-south sections had a steeper gradient
than the east-west ones indicating ‘restricted’ flow; and,
•
An analytical program (Clarke, 1987) was used to achieve best fits of field data from the Basal Monitors with
simulated aquifer conditions constructed using mathematical well equations. The best fits were obtained using a
‘leaky strip aquifer’ solution for Basal Monitors in Test M1, Test M3, and Test M4. Modelled widths averaging
1 to 1.1km are in good agreement with the width of the Palaeochannel (BSA junction with basement).
Numerical Simulation
A numerical groundwater model (MODFLOW), representing the hydrogeological conceptual model of the Honeymoon
mineral claims area of the Yarramba Palaeochannel, was constructed, calibrated and run to simulate the effects on the
groundwater operations from start-up operations for the proposed Honeymoon Uranium Project. It incorporated an ISL
wellfield array and disposal of RO reject brine and ISL barren solution bleed at various alternative locations. The
abstraction of raw water from the USA for RO plant feed was also included.
A numerical solute transport model (MT3D99) was constructed, based on the calibrated flow model, to incorporate
advection – dispersion and simplified adsorption processes. This model was used to carry out a robust sensitivity analysis
on the assumed solute adsorption coefficients to ‘bracket’ expected solute migration. The analysis was based on the
production of breakthrough curves showing time – variant water quality concentrations at particular locations for
chemical species of interest.
Simulation of Operations
Dewatering and surcharge effects were simulated for a sequence of operations to demonstrate the effects of each
operation on the groundwater (Water studies, 2001b). The flow balance results indicated:
• an upward leakage from the MSA to the USA of approximately 37 kL/d. Dewatering of the USA drove this
upward leakage by raw water abstractions. Pressurisation of the BSA at the same time by disposal was rapidly
equilibrated due to the highly transmissive nature of the BSA and was not considered to significantly contribute
to this upward leakage;
•
the impact of the disposal was restricted to about 3 kL/d of upward leakage from the BSA to the MSA during
the simulated initial five-years of operations;
•
excursions from the MSA to the USA, at all the disposal sites, rapidly returned to zero at the cessation of ISL
operations and raw water abstraction, even while disposal to the BSA continued;
13
•
leakage from the MSA to the BSA was estimated to be approximately 10 kL/d. This most likely occurred in the
vicinity of the ISL wellfield where a net 4.4 L/s production rate was maintained causing minor localised
dewatering and associated downward leakage due to the resultant head differential;
•
from the end of raw water abstractions and ISL operations until disposal was discontinued, a steady rate of 23
kL/d upward leakage from the BSA to the MSA was demonstrated;
•
following the cessation of disposal, there was a rapid return to downward leakage resulting from gradient
prescribed by the head boundary conditions.
The proposed placement of monitoring bores at 125 m intervals from the ISL wellfield (SXR, 2000a) was assessed for its
effectiveness in detecting pressure changes within the BSA for operational monitoring purposes. The drawdown at a ring
of seven monitoring bores placed around the ISL wellfield within the BSA, was predicted over a fifty-year period.
Further details of the proposed monitoring system will be developed as part of the Environmental Monitoring and
Management Plan for the project (SXR, 2000a).
Adsorption
It is unlikely that the conservative solute case with an adsorption value of zero is at all representative of actual conditions
in the BSA (SXR, 2000a). It is far more likely that partial solution would occur between the source solute ions and
mineral in the aquifer matrix (Water Studies, 2001b).
In cases where higher sorption characteristics were assumed, only minor differences between the background
concentration in the native groundwater and the concentration of the same element in the disposal solution were reported
at the monitoring locations over the time periods considered. The results show pronounced localisation of the solute
plume within a 125 m radius of the disposal points.
The published data show that the elements having elevated levels in the disposal solution all exhibit average partition
coefficient values significantly higher than the upper limit of 10 g/ml used in the sensitivity analysis (Water Studies,
2001b). This value produced residual elevation of chemical species concentrations of less than 2% over a period of 50
years. The removal of contaminants was determined to be rapid and residual concentrations confined to the immediate
vicinity of the injection bores. It is reasonable to anticipate; therefore, that disposal solutions will equilibrate to natural
background levels well within the proposed seven-year monitoring period following cessation of operations (SXR,
2000a).
REFERENCES
Baes CF, Sharp RD, Sjoreen AL, and Shore RW (1984) A Review and Analysis of Parameters for Assessing Transport of
Environmentally Released Radionuclides through Agriculture, ORNL-5786, US Department of Energy, 1984.
Clarke D (1987) CG Test Pumping Analytical Software Program.
Clarke D (1989) Groundwater Discharge Tests: Simulation and Analysis (Developments In Water Science, 37). Elsevier
Science Ltd; ISBN: 0444430377.
Kruseman GP and De Ridder, NA (1991), Analysis and Evaluation of Pumping Test Data. Second Edition. International
Institute Of Land Reclamation and Improvement, Publication 47.
Pirlo MC (2001) Terms of Reference for Additional Evaluation of Aquifer, draft report prepared for Environment
Australia, May 2001, unpublished.
Southern Cross Resources Australia Pty Ltd (2000a) Honeymoon Uranium Project – Environmental Impact Statement,
May 2000, ISBN 0646394754.
Southern Cross Resources Australia Pty Ltd (2000b) Honeymoon Uranium Project, Environmental Impact Statement,
Response Supplement, November 2000, ISBN 095781951X.
Southern Cross Resources Australia Pty Ltd (2001) Honeymoon Uranium Project – Report on Channel Definition
Drilling April-May 2001, July 2001, unpublished.
Water Studies (2001a) Honeymoon Uranium Project – Aquifer Test Pumping Programme – Report WSDJ00161, report
prepared for Southern Cross Resources Australia Pty Ltd, July 2001, unpublished.
Water Studies (2001b) Honeymoon Uranium Project – Groundwater Flow and Quality Modelling – Report
WSDJ00161/1, report prepared for Southern Cross Resources Australia Pty Ltd, July 2001, unpublished.
Vandenberg A (1977) Type Curves for Analysis of Pump Tests in Leaky Strip Aquifers. J. Hydrol., Vol.33, Pp. 15-26
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