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TitleRanger Pit 1 Final Deposition of Tailings Level to +7 mRL
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Total Pages113
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Page 1

Assessment Report:

Ranger Pit 1 Final

Tailings Deposition

Level to +7 mRL

K Turner, K Tayler, A Costar,

C Zimmermann, M Bouma

and P Baker

February 2017

Release status – unrestricted



Page 2

The Department acknowledges the traditional owners of country throughout Australia
and their continuing connection to land, sea and community. We pay our respects to them

and their cultures and to their elders both past and present.

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Middlemis_2016_Pit1_review.docx 8

No information is presented on the “undifferentiated bedrock” that underlies the SLB and
which could form an additional zone of connectivity between Pit 1 and the creek. In addition
to the potential for the MBL zone to exhibit potential high hydraulic conductivity (as
discussed above), this thin zone (Figure 2) could have been subject to fracture enhancement
due to pressure unloading via the pit excavation, and then subsequent re-loading effects as
the tailings were deposited and then surcharged with additional fill material.

The following explanation relies largely on a comprehensive compilation of information on
pit slope water management (Read and Beale, 2013), but before proceeding, two concepts
may need some introduction:

 effective stress is the difference between total stress and pore pressure (total stress
is the total pressure formed by the overlying rock (lithostatic) load plus the water
(hydrostatic) load).

 hydromechanical (HM) coupling refers to the interdependence of rock properties and
their fluid behaviour, via a solid-to-fluid exchange (e.g. a reduction in total stress by
pit excavation causing a reduction in fluid pressure) and/or via a fluid-to-solid
exchange (e.g. a reduction in pore pressure leading to fracture dilation and increased

If the total stress is reduced by excavation of rock from the pit for example, the lithostatic
unloading can cause a direct HM coupling response as the fractures dilate and/or pore
pressures reduce, and an indirect hydromechanical coupling response due to increased
aperture (permeability) of the fractures that can change the pressure/flow conditions. The
magnitude of deformation from lithostatic unloading depends upon a range of factors
(including the weight of material removed and the overall size/depth of the pit), and the
effect can extend laterally beyond the pit and vertically below the base of the pit. It is
possible that fractures that have dilated due to lithostatic unloading could revert to their
initial condition on subsequent re-loading. However, in this case of tailings deposition, there
is also the potential for infill of fractures with low permeability tailings material, or possibly
smearing (“blinding”) by the tailings of the fracture network exposure in the pit wall. These
factors may act in concert with changes in effective stress due to re-loading via tailings
deposition causing pore pressure changes, closure of fractures and a decrease in

These potential conceptualisation issues have not been given adequate consideration in the
technical investigations or the risk studies. Once the conceptualisation has been properly
considered, it may be deemed necessary to apply a coupled flow-stress modelling
methodology, whereby load-induced changes in pressure are accounted for (e.g. using TNO-
DIANA or SEEPW-SIGMAW modelling packages). While it is not clear from the reports
reviewed the degree to which the consolidation modelling considered flow-stress modelling,
the Modflow numerical groundwater modelling package that has been applied in this case
definitely cannot account for load-induced effects, only flow-induced changes to pressure
(Harrington and Cook, 2011).

Conceptualisation Review Summary:

The hydrogeological conceptualisation is not underpinned with adequate background

information (e.g. from dewatering operations), technical justifications for key

assumptions (e.g. hydraulic character of the SLB) or sensitivity/uncertainty assessment

(e.g. consideration of alternative conceptualisations), and it ignores a potentially

significant pit to creek connection through and MBL zone and the SLB. Depending on the

validity of the conceptualisation, it may be deemed necessary to apply a coupled flow-

stress modelling methodology, whereby load-induced changes in pressure are accounted

for directly.

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Whilst a water balance is fundamental to any hydrological investigation or modelling study
(Barnett et al, 2012), the modelling studies presented are deficient in that they lack water
balance data or even plots of elements of the water balance (other than the calculated
consolidation flows of expressed porewater), even though there are references to various
water measurement works, including:

 there are several low key references to the MBL aquifer providing inflows and
dewatering problems for the operating mine, but no indication of volumes or rates

 ATCW (2012) mentions a vertical dewatering bore that was installed to the drainage
adit at the base of Pit 1 (prior to tailings placement) and operated on an intermittent
basis (section 2.2)

 Fitton (2015) mentions a Pit 1 flow meter (section 5.3) but it is not known whether
this is on the decant system or the dewatering bore, and no data is presented

 Fitton (2015) states in section 2 and section 4.4.2 that some of the water released due
to tailings consolidation is “lost to groundwater”. Although such seepage cannot be
measured directly (a fundamental uncertainty that has not been explored adequately
by the modelling), the other part of that process is the expressed tailings porewater
collected via the prefabricated vertical drains (wicks), which reported to decant
structures, but no information is presented on the volumes.

 Section 5.3 states that the “measured” consolidation flow is 198 m3/d, but it does not
indicate how or where that measure is obtained. Adopting the “measured” settlement
volume of 227,500 m3 (better described as calculated volume) and an assumed 60% of
that comprising expressed (upwards) tailings porewater fluid (over a period of about
700 days from 16 June 2013 to 19 May 2015) generates 194 m3/d. This suggests that
the reported 198 m3/d value is indeed a model result, and the 60% factor is simply a
rule of thumb derived from model results. It is recommended that the “measurement”
term should be replaced with “estimated” or “calculated”, and that improved
documentation of the assumptions and the calculations involved is warranted in order
to meet best practice guidance.

 Despite these problems, the correct conclusions are made in Fitton sections 5 and 6
that the inflow from the “spring” at 8-12 L/s (690-1040 m3/day) dominates the Pit 1
water balance, although the spring flow estimate is very uncertain (it is not measured).

 The short term rainfall inputs (Fitton, section 6.1) are under-estimated, but this is
arguably not critical (given that the spring flow dominates). The 72-hour rainfall event
is appropriate (as this maximises the volume of runoff), and while the adoption of a
10-year ARI involves a relatively high probability of occurrence by 2027 (~63%), it also
involves a relatively low rainfall intensity (3.8 mm/hr). Rainfall-runoff estimates
usually adopt the maximum duration and maximum ARI in order to calculate the
maximum volume. In this case, an ARI 100-year event over 72 hours involves
6.3 mm/hour, generating almost twice the volume estimated. In practical terms, the
under-estimate of short-term rainfall inputs in Fitton (section 6.1) is arguably not
critical, because there appears to be capacity to store the related volumes in the pit.
Dealing with the volumes would mean that either it would take up to twice as long
(i.e. 20 days) to discharge at the reported rate of 118 L/s, or that higher capacity
pumps may need to be applied to the task to achieve the 10-day target. The report
does not justify the selection of the ARI 10-year design event, but it does seem a little
under-designed. Table 11-11 of ARR (1987) indicates that an ARI 10-year event has
~63% chance of occurrence within a 10-year period, while an ARI 100-year event has
~10% chance of occurrence in 10 years and ~18% chance of occurrence in 20 years.

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Ranger Groundwater Workshop Agenda

Monday 5 September

1000 Site Tour Participants to meet at SSB Jabiru Field Station (JFS) for a 10am departure.

1200 Lunch Travel back to JFS for lunch

1300 Opening comments Keith Tayler (SSB)

1330 Overview of closure strategy ERA

1400 Overview of Solute Transport Model(s)

• Pit 1
• Pit 3

INTERA to present

1430 Solute Transport Model reviews (30-45 mins):

• Pit 1
• Pit 3

SSB consultant to present followed by general discussion on topic

1600 Break

1630 Geochemistry review (30-45 mins):

• Site-wide

SSB consultant to present followed by general discussion on topic

1800 Conceptual Model Part 1: Chapters 1 & 2:

• Background; objectives; development of CM

INTERA to present followed by general discussion on topic

1830 Close for day

Tuesday 6 September

0800 Conceptual Model Part 2: Chapters 3 & 4:

• Regional CM
• Site-wide Scale CM

INTERA to present followed by general discussion on topic

This includes geology along with features and processes on a regional and site-wide

1030 Break

1100 Conceptual Model Part 3: Sections 5.2 & 5.3:

• Pit 3 CM
• Pit 1 CM

INTERA to present followed by general discussion on topic

1300 Lunch

1330 Conceptual Model Part 4: Sections 5.4, 5.5, 5.6 &

• Processing Plant Area CM
• R3 Deeps CM

INTERA to present followed by general discussion on topic

1600 Break

1630 Conceptual Model Part 5: Section 5.8 (plus low-K
cap assessment):

• Landform Waste Rock CM

INTERA to present followed by general discussion on topic

1730 Close for day

Wednesday 7 September

0800 Conceptual Model Part 6: Section 5.9:

• Screening evaluation of areas of

INTERA to present followed by general discussion on topic

0900 Conceptual Model – ERA's Perspective ERA

0930 Closing comments Keith Tayler (SSB) & general discussion

1100 Closure (morning tea)

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Acronym Definition
BPT Best practicable technology
Ca Calcium
COPC Contaminants/constituents of potential concern
DEWNR Department of Environment, Water and Natural Resources, South Australia
DJ David Jones, D R Jones Environmental Excellence
DME Department of Mines and Energy
ERA Energy Resources of Australia Limited
ET Evapotranspiration
GAC Gundjeihmi Aboriginal Corporation
GA Geoscience Australia
INTERA INTERA Geoscience and Engineering Solutions
ITWC PFS Integrated Tailings, Water and Closure Pre-feasibility Study
LAA Land Application Area
MBL Mine Bore L - MBL is a zone (based on the MB-L bore) of relatively higher

permeability in the south-east part of Pit 1
Mg Magnesium
Mn Manganese
NLC Northern Land Council
OWS Office of Water Science Branch, Department of the Environment and Energy
PPA Processing Plant Area
PEST Model-Independent Parameter Estimation and Uncertainty Analysis
PTF Pit tailings flux (expressed tailings water)
R3D Ranger 3 Deeps
RCM Ranger Conceptual model
RL Reduced level
SSB Supervising Scientist Branch, Department of the Environment and Energy
TAN Total ammonia N
TSF Tailings Storage Facility
U Uranium

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