mcarthur river mine overburden management project · pdf filemcarthur river mine overburden...
Post on 09-Mar-2018
217 Views
Preview:
TRANSCRIPT
McArthur River Mine
Overburden Management Project
Draft Environmental Impact Statement
Appendix IMRM NOEF 7.5m
Assessment Case Report
I
7.5m Waste Deposition Assessment
in Support of the EIS Submission
20 January 2017
7.5m Waste Deposition Assessment in Support of the EIS Submission
750/42-02
January 2017
Prepared for:
McArthur River Mining Pty Ltd 34a Bishop Street
Stuart Park NT 0820
Prepared by:
O'Kane Consultants Pty Ltd
193D Given Terrace
Paddington QLD 4064
Australia
Telephone: (07) 3367 8063
Facsimile: (07) 3367 8052
Web: www.okc-sk.com
Rev. # Rev. Date Author Reviewer PM Sign-off
1 20/01/2017 SP SP PG
DISCLAIMER
This document has been provided by O'Kane Consultants Pty Ltd (OKC) subject to the following limitations: 1. This document has been prepared for the client and for the particular purpose outlined in the
OKC proposal and no responsibility is accepted for the use of this document, in whole or in part, in any other contexts or for any other purposes.
2. The scope and the period of operation of the OKC services are described in the OKC proposal and are subject to certain restrictions and limitations set out in the OKC proposal.
3. OKC did not perform a complete assessment of all possible conditions or circumstances that may exist at the site referred to in the OKC proposal. If a service is not expressly indicated, the client should not assume it has been provided. If a matter is not addressed, the client should not assume that any determination has been made by OKC in regards to that matter.
4. Variations in conditions may occur between investigatory locations, and there may be special conditions pertaining to the site which have not been revealed by the investigation, or information provided by the client or a third party and which have not therefore been taken into account in this document.
5. The passage of time will affect the information and assessment provided in this document. The opinions expressed in this document are based on information that existed at the time of the production of this document.
6. The investigations undertaken and services provided by OKC allowed OKC to form no more than an opinion of the actual conditions of the site at the time the site referred to in the OKC proposal was visited and the proposal developed and those investigations and services cannot be used to assess the effect of any subsequent changes in the conditions at the site, or its surroundings, or any subsequent changes in the relevant laws or regulations.
7. The assessments made in this document are based on the conditions indicated from published sources and the investigation and information provided. No warranty is included, either express or implied that the actual conditions will conform exactly to the assessments contained in this document.
8. Where data supplied by the client or third parties, including previous site investigation data, has been used, it has been assumed that the information is correct. No responsibility is accepted by OKC for the completeness or accuracy of the data supplied by the client or third parties.
9. This document is provided solely for use by the client and must be considered to be confidential information. The client agrees not to use, copy, disclose reproduce or make public this document, its contents, or the OKC proposal without the written consent of OKC.
10. OKC accepts no responsibility whatsoever to any party, other than the client, for the use of this document or the information or assessments contained in this document. Any use which a third party makes of this document or the information or assessments contained therein, or any reliance on or decisions made based on this document or the information or assessments contained therein, is the responsibility of that third party.
11. No section or element of this document may be removed from this document, extracted, reproduced, electronically stored or transmitted in any form without the prior written permission of OKC.
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission iv
O’Kane Consultants 20 January 2017 750/42-02
TABLE OF CONTENTS
1 INTRODUCTION .................................................................................................... 1
1.1 Project Objectives and Scope.................................................................................. 1
1.2 Report Organization ................................................................................................. 1
2 BACKGROUND ..................................................................................................... 2
2.1 7.5m Placement Scenario ........................................................................................ 2
2.2 Advective Cover Material Specifications ................................................................. 3
3 MODELLING ASSESSMENT ................................................................................ 5
3.1 Loading and diffusion risk ........................................................................................ 5
3.2 Material properties ................................................................................................... 6
3.3 Boundary conditions ................................................................................................ 7
4 DIFFUSIVE OXYGEN INGRESS ........................................................................... 8
4.1 Oxygen Diffusion Assessment Cases ................................................................... 10
5 ADVECTION MODELLING .................................................................................. 12
6 MODELLING ASSESSMENT RESULTS ............................................................. 14
6.1 Oxygen Diffusion Results through the Alluvium Layer .......................................... 14
6.2 Diffusion Based Acidity Load Assessment ............................................................ 15
6.3 Results of Advection Load Assessment ................................................................ 18
6.4 Total Loading Assessment .................................................................................... 19
7 CONCLUSIONS AND LIMITATIONS .................................................................. 21
7.1 Assessment Conclusions ....................................................................................... 21
7.2 Assessment Limitations ......................................................................................... 21
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission v
O’Kane Consultants 20 January 2017 750/42-02
LIST OF TABLES
Table 2-1: "Core" surface areas (m2)................................................................................................ 3
Table 3-1: Integrated advective and diffusive AMD loads for entire construction period based on
Scenario 4 (500m “cells” using alluvial materials as oxygen ingress barriers) ............ 6
Table 4-1: Key geometry for loading assessment ............................................................................ 9
Table 4-2: Exposure of waste to atmosphere during placement .................................................... 10
Table 5-1: Key input parameters for Stage areas .......................................................................... 12
Table 6-1: Diffusive oxygen ingress assessment results ............................................................... 15
Table 6-2: Acidity load rate during construction stages for various PAF rock oxidation rates ....... 17
Table 6-3: Acidity load produced during construction stages for various PAF rock oxidation rates
................................................................................................................................... 18
Table 6-4: Advection loading assessment ...................................................................................... 18
Table 6-5: Detailed model results for Stage S2 .............................................................................. 18
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission vi
O’Kane Consultants 20 January 2017 750/42-02
LIST OF FIGURES
Figure 2-1: Schematic of 7.5m placement scenario (as provided by MRM) .................................... 3
Figure 4-1: Schematic of diffusive oxygen transport through the alluvium layers. ........................... 8
Figure 6-1: Calculated oxygen flux rate for various alluvium saturations overlying the PAF rock
with an oxidation rate of 5.0 x 10-7 kg O2/m3/s. ......................................................... 14
Figure 6-2: Calculated acidity load rate for the PAF rock with an oxidation rate of 5.0 x 10-7
kg
O2/m3/s for various alluvium cover cases during the PAF rock construction. ............ 16
Figure 6-3: Calculated acidity load rate for the PAF rock with an oxidation rate of 1.0 x 10-6
kg
O2/m3/s for various alluvium cover cases during the PAF rock construction. ............ 16
Figure 6-4: Calculated acidity load rate for the PAF rock with an oxidation rate of 5.0 x 10-6
kg
O2/m3/s for various alluvium cover cases during the PAF rock construction. ............ 16
Figure 6-5: Temperature and gas flux for Stage 2 showing effect of degree of saturation ............ 19
Figure 6-6: Placement method and calculated loading rates ......................................................... 20
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 1
O’Kane Consultants 20 January 2017 750/42-02
1 INTRODUCTION
McArthur River Mining (MRM) have requested that O’Kane Consultants Pty Ltd. (OKC) complete
a modelling assessment of the existing North Overburden Emplacement Facility (NOEF), as well
as specific facets of the NOEF EIS expansion. The modelling assessment is being conducted
using OKC’s DumpSim proprietary model and centres around the estimation of risks related to
pyrite oxidation that include acid and metalliferous drainage and heat generation.
As part of the DumpSim modelling scope of work, OKC considered various scenarios of waste
placement that included paddock dumping, 5m end tipping, and a “cell” scenario that included end
tipping (5m high tip heads) within 500m cells and coverage of cells with alluvial advection barriers
(2m thick). Upon review of the placement specifications presented, MRM requested that an
additional scenario be assessed that is termed herein the 7.5m case.
1.1 Project Objectives and Scope
Further assessment work to be carried out as part of the 7.5m case included:
1) Define 7.5m concept and model scenarios;
2) Set up loading model scenarios to assess diffusive flux and loading rate;
3) Set up loading model scenarios to assess advective flux and loading rate; and
4) Compare results to previous modelling.
1.2 Report Organization
For convenient reference, this report has been divided in the following sections:
Section 1: Introduction
Section 2: Background
Section 3: Modelling Assessment
Section 4: Diffusive Oxygen Ingress
Section 5: Advection Modelling
Section 6: Modelling Assessment Results
Section 7: Conclusions and Limitations
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 2
O’Kane Consultants 20 January 2017 750/42-02
2 BACKGROUND
2.1 7.5m Placement Scenario
MRM are implementing a strategy to effectively place potentially acid forming (PAF) waste rock at
their mining site, so that oxidation of sulfide minerals can be managed during waste placement,
which therefore results in limiting stored oxidation products, and thus long-term reliance on a
cover system as the “sole” means of managing seepage from the waste rock dump. Managing
oxidation of sulfide minerals involves strategic placement of run-of-mine (ROM) waste such that
advective gas transport within the dump (i.e. oxygen transport) is limited because air flow capacity
(air permeability) is controlled. The primary air flow mechanism being addressed by utilising this
strategy is convection, which results from a temperature differential within, and external, to the
dump.
To assess practical options to minimise gas flux OKC have assessed the use of alluvial materials
as an “advection barrier”. The 7.5m placement scenario utilises this concept as alluvial materials
are used as advection control barriers.
The evaluated strategic waste placement in this report is:
Lift height of 7.5m
o Bottom layer (~2 m ) is paddock dumped
o ~5.5m tip head developed over the top
Alluvium sheeting is applied every 7.5m lift (approx. 100mm thick and is heavily
compacted due to trucks running over it)
1.5m thick alluvium layer on inter-stage slopes*
100mm-200mm thick alluvium layer on all inter-stage roads (heavily compacted due to
trucks running over it)
If, for any reason the external HALO/COVER development falls significantly behind, a
1.5m thick alluvium layer to be constructed on the external slope of the CORE also.
It is noted that MS-NAF may be utilised for erosion protection on the inter-stage barriers, if placed
this will form a cover over the alluvium. Erosion is a key risk factor with respect to the in service
performance of the alluvium with regard to gas flux management, therefore management of
erosion is important as part of overall risk management.
The placement scenario is shown in Figure 2-1
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 3
O’Kane Consultants 20 January 2017 750/42-02
Figure 2-1: Schematic of 7.5m placement scenario (as provided by MRM)
Table 2-1includes data provided by MRM on the average surface area of the “core” of the waste
being placed in each Stage as part of the 7.5m placement scenario. The surface area of the
waste being placed in any given lift is a critical input to the modelling and therefore the
assumptions made in the model relating to the surface areas provided in Table 1 should be
regarded as a key input parameter.
Table 2-1: "Core" surface areas (m2)
Stage 2 Stage 3 Stage 4 Stage 5 Stage 6 Stage 7
31-Oct-19 334,400 324,500 0 0 0 0
31-Oct-20 0 183,750 282,500 0 0 0
31-Oct-21 0 0 232,500 0 0 0
31-Oct-22 0 0 120,400 734,500 0 0
31-Oct-23 0 0 0 282,200 0 0
31-Oct-24 0 0 0 0 428,175 0
31-Oct-25 0 0 0 0 462,825 0
31-Oct-26 0 0 0 0 154,275 225,700
31-Oct-27 0 0 0 0 0 315,000
31-Oct-28 0 0 0 0 0 122,100
31-Oct-29 0 0 0 0 0 89,100
31-Oct-30 0 0 0 0 0 72,600
31-Oct-31 0 0 0 0 0 39,100
2.2 Advective Cover Material Specifications
In general the advective flux barrier concept is modelled to be effective at reducing gas flux rates
to a point where reduction of internal temperatures to below 60°C over time is possible if an air
permeability of 3E-11 m2 can be achieved. It should be noted that air permeability in the context
of the modelling carried out is not a function of material thickness, therefore a 1m, 2m or 5m
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 4
O’Kane Consultants 20 January 2017 750/42-02
advective blanket will provide the same air permeability provided it is constructed to
specifications. In reality there will be a minimum thickness that is required to maintain a given air
permeability which will relate to practical construction issues such as:
Maintaining a consistent layer thickness over a “rough” surface such as ROM waste using
large machinery and over large surface areas and large volumes of material will require a
minimum thickness of material to be placed due to engineering tolerances that are greater
than “civil engineering” specification.
Processes such as erosion, weathering, and heating (from the waste itself) will act to
deteriorate the material properties and thickness as placed. These factors cannot be
modeled to any degree of certainty and therefore there is a wide range of tolerance that must
be accepted when determining how thin a layer can become before it can be expected to
maintain performance as considered in modeling.
Based on material availability and geotechnical properties, materials referred to as “J alluvials”,
which come from the Stage J open cut area, have been selected as a potential source of
construction material for the advective flux barrier. These materials are understood to comprise
three main material classes; clays, sand, and gravel/ cobbles.
OKC have recently completed a modelling assessment to determine the optimum specification for
the alluvial materials as placed as advection barriers (McArthur River Mine NOEF Advective Gas
Flux Barrier (Thermal blanket) Material Placement Specifications – REVISED, 12 October 2016).
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 5
O’Kane Consultants 20 January 2017 750/42-02
3 MODELLING ASSESSMENT
3.1 Loading and diffusion risk
Advective and diffusive flux of oxygen is expected to occur as a continuous process along all
exposed surfaces of a waste rock facility. In general, advection is directly related to material
properties such as texture, saturation state and degree of compaction. In general, higher
saturation levels reduce oxidation risks; however, given the climate present it may not be possible
to maintain a high degree of saturation in the NOEF as a whole; particularly when pyrite and
carbon oxidation consume pore water/vapour and elevated temperatures can result in advective
drying processes. Previous drilling investigations identified waste saturation in the NOEF was
typically in the 1 to 20% range, with most results around 5%. This range was used to focus
OKC’s DumpSim assessment tool on the saturation range that under conservative assumptions
may most likely be encountered in the field, though an upper 70% saturation scenario was also
modelled to reflect the likelihood that the new waste construction methods will reduce both
advective gas flux and internal temperatures relative to the existing NOEF. Results from the
previous assessment of using alluvial barriers as part of construction are reproduced in Table 3-1
below, key points from this assessment with respect to loading risks are:
Loading rates from diffusion and advection are broadly similar for Future Stages. It should be
noted that as indicated in Table 4-1 and Table 4-2, air permeability is not a function of
material thickness, therefore a 1m, 2m or 5m advective blanket is considered by the model to
have the same air permeability provided it is constructed to specifications. In the model
scenario then advective air flux is driven by air permeability and therefore loading rates with
respect to advection are not sensitive to material thickness, diffusive risks however are
directly linked to material thickness.
The existing NOEF has a very high existing load and additional diffusive loads occurring
during the construction period are not likely to materially increase the loading (assuming that
side slopes have advection barriers constructed on them along with final CCL cover where
slopes are not planned to be built against as part of EIS expansion).
Future planned Stages of construction have loads where diffusion on average contributes
approximately 50% of the total load.
On this basis the sensitivity of the overall loading profile to diffusion can be considered to be
high for future Stages of construction but low for the existing NOEF.
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 6
O’Kane Consultants 20 January 2017 750/42-02
Table 3-1: Integrated advective and diffusive AMD loads for entire construction period based on Scenario 4 (500m “cells” using alluvial materials as oxygen ingress barriers)
Advection Diffusion Total
load kg/t load kg/t load/kg/t/H2SO4
Existing NOEF 16.5059 0.00 16.5
CW 0.3 0.4 0.7
STAGE2 0.8 0.7 1.5
STAGE3 0.6 0.4 1.
STAGE4 0.5 0.4 0.9
STAGE5 0.6 0.3 0.9
STAGE6 0.5 0.4 1.0
STAGE7 0.7 0.3 1.
RPAF_1 6.5 1.1 7.6
RPAF_2 2.7 0.6 3.3
RPAF_3 1.7 0.4 2.1
The previous assessment of the use of alluvial advective barriers assumed the following:
The thickness of these barriers would be 2m minimum
The degree of saturation would be 25%
Compaction would produce a dry density of 1.85 g/cm3.
3.2 Material properties
The material properties of the alluvium material and PAF rock used in the oxygen diffusion
assessment are as follows:
1) PAF rock
o Specific gravity (Gs): 2.75;
o Dry density: 1,900 kg/m3;
o Void ratio: 0.45
o Porosity: 0.31
2) Compacted alluvium (100 mm thick)
o Specific gravity (Gs): 2.65;
o Dry density: 2,130 kg/m3;
o Void ratio: 0.25
o Porosity: 0.20
3) 1.5m alluvium cover (1.5 m thick)
o Specific gravity (Gs): 2.65;
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 7
O’Kane Consultants 20 January 2017 750/42-02
o Dry density: 1,850 kg/m3;
o Void ratio: 0.43
o Porosity: 0.30
3.3 Boundary conditions
The upper boundary condition is the oxygen concentration in the atmosphere, i.e. 20.9% O2 by
volume. The annual average air temperature at the site is 27.8 ˚C and the annual average air
pressure is 101,330 Pa. So the oxygen concentration in the atmosphere is approximately 270
g/m3. The lower boundary condition is the oxygen concentration below the alluvium layer, which
varies and is determined on the base of oxygen diffusion within the PAF rock being equal to
oxygen consumed by the PAF rock.
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 8
O’Kane Consultants 20 January 2017 750/42-02
4 DIFFUSIVE OXYGEN INGRESS
The objectives of oxygen diffusion assessment is to evaluate diffusive oxygen ingress through
different alluvium thicknesses that is overlying PAF rock. The alluvium layer has various
construction methods and saturation conditions.
Diffusive oxygen ingress to the underlying PAF rock considered within this report occurs through
diffusion transport in the gas phase and as dissolved oxygen in the water phase (i.e. water
infiltrating through the alluvium material layer and entering into PAF rock). The difference in
oxygen concentration between the atmosphere (20.9% O2 by volume) and the upper PAF rock
profile produces a diffusion gradient driving the movement of oxygen. Fick’s first law is applicable
to calculate oxygen diffusion. The effective diffusion coefficient is influenced by the degree of
saturation within the alluvium layer; a high degree of saturation results in a low effective diffusion
coefficient and low oxygen transport because the oxygen diffusion coefficient in water is smaller
than the oxygen diffusion coefficient in air by four orders of magnitude (Aachib et al., 2004). The
oxygen concentration below the alluvium layer is affected by oxidation rate of the underlying PAF
rock and the alluvium layer conditions, such as thickness and degree of saturation. Diffusive
oxygen flux decreases with:
Increasing the alluvium layer thickness because of increasing oxygen transport pathway
Increasing density because of reduced gas permeability
Increasing saturation because of reduced gas permeability
The simplified oxygen diffusion model is presented in Figure 4-1 which includes 100 mm thick
alluvium layer overlying the 7.5 m thick PAF rock lift during the PAF rock lift placement, and 1.5 m
thick alluvium layer overlying outside of the PAF rock slope constructed during the build of the
Stage.
Figure 4-1: Schematic of diffusive oxygen transport through the alluvium layers.
The major assumptions employed in the oxygen diffusion assessment model include:
The core area for each stage is the “cell” of waste being placed.
PAF
1.5 m thick
alluvium
Diffusive
Oxygen Flux
100 mm thick
alluvium
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 9
O’Kane Consultants 20 January 2017 750/42-02
When a 7.5 m thick PAF lift is placing, the 100 mm thick alluvium is followed over the top
of the lift progressively such that little or no bare waste rock is exposed.
The tipping face is ignored as the area is small relative to the plateaux during the lift
placement.
The 1.5m alluvium is placed on the outside of the slope during lift construction.
The PAF rock has a 20% degree of saturation and oxygen consumption in the PAF rock
is not restrained by the PAF rock.
The alluvium may have a variable degree of saturation, it has better water holding
capacity than the PAF and therefore the “base case” is that degree of saturation is 30%.
Acidity load is calculated on the base of one kilogram oxygen consumption being converted into
1.7 kilogram sulfuric acid (H2SO4). Stage areas and total mass of PAF rock in each stage are
used in acidity load evaluation. The covered areas by 100 mm thick alluvium and 1.5 m thick
alluvium in each stage and waste rock mass are listed in Table 4-1.
Table 4-1: Key geometry for loading assessment
Stage Average area covered by 100 mm
alluvium (m
2)
Average area covered by 1.5 m alluvium
(m2)
Total Mass of PAF
(tonnes)
2 334,400.00 1,177,502.39 63,587,430
3 254,125.00 263,560.66 52,279,034
4 211,800.00 769,694.01 73,456,650
5 508,350.00 548,997.24 78,819,589
6 348,425.00 917,541.88 113,388,011
7 187,975.00 855,995.30 94,378,825
Diffusion is considered in modelling as both a temporary and more permanent phenomena, as
follows.
During construction of the NOEF and placement of waste individual “blocks” of waste are
exposed to the atmosphere for only a finite time as further lifts are placed on top. Diffusion
can be thought of as a surface effect that will be temporary for any given block of waste
material as waste is progressively buried as waste placement progresses as a series of lifts
that “over dump” each other.
When considering the outer layers of the NOEF, the effect of diffusion will not be a temporary
phenomenon and so this long term form of oxygen ingress requires consideration. For the
purposes of modelling, all outer surface of the NOEF in their final position are considered to
be permanent.
Table 4-2 describes the sequence of waste placement in the NOEF as considered by modelling;
each Stage of the NOEF construction will have waste placed that is initially directly exposed to the
atmosphere but is then covered up as a result of the progressive placement of the cover system
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 10
O’Kane Consultants 20 January 2017 750/42-02
on the batters and plateaux areas. Data for the surface areas and construction periods has been
taken from data supplied by MRM and is shown in Table 4-2.
The years that the waste surface is directly exposed corresponds to the construction period
for the given Stage, and the surface area of waste that is covered with the 100mm of alluvium
Years surfaces covered indicates the period during construction where the majority of the
waste surface area has been covered by batter and plateau 1.5m alluvials
Table 4-2: Exposure of waste to atmosphere during placement
Stage Years waste exposed with 100mm alluvium
Years surfaces covered (batters and plateaux covered with 1.5m
alluvium)
STAGE2 1 13
STAGE3 2 13
STAGE4 3 12
STAGE5 2 10
STAGE6 3 8
STAGE7 4 6
4.1 Oxygen Diffusion Assessment Cases
The oxygen diffusion assessment includes the following scenarios:
1) The PAF rock is assumed to have a pyrite oxidation rate within the range established as part
of kinetic laboratory testing (noting these are bookends and represent a maximum and
minimum value).
5.0x10-7
kg O2/m3/s (Base Case)
1.0x10-6
kg O2/m3/s
5.0x10-6
kg O2/m3/s
2) The degree of saturation for alluvium (compacted and non-compacted) is:
30%
50%
70%
Because the compacted alluvium (100 mm thick) saturation is not necessarily the same as the
non-compacted alluvium (1.5 m thick) saturation, the combination of above three alluvium
saturation scenarios can form the following nine cover cases:
1) 100 mm alluvium with 30% saturation + 1.5 m alluvium with 30% saturation (i.e. 100 mm
alluvium 30%S + 1.5 m alluvium 30%S),
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 11
O’Kane Consultants 20 January 2017 750/42-02
2) 100 mm alluvium with 30% saturation + 1.5 m alluvium with 50% saturation (i.e. 100 mm
alluvium 30%S + 1.5 m alluvium 50%S),
3) 100 mm alluvium with 30% saturation + 1.5 m alluvium with 70% saturation (i.e. 100 mm
alluvium 30%S + 1.5 m alluvium 70%S),
4) 100 mm alluvium with 50% saturation + 1.5 m alluvium with 30% saturation (i.e. 100 mm
alluvium 50%S + 1.5 m alluvium 30%S),
5) 100 mm alluvium with 50% saturation + 1.5 m alluvium with 50% saturation (i.e. 100 mm
alluvium 50%S + 1.5 m alluvium 50%S),
6) 100 mm alluvium with 50% saturation + 1.5 m alluvium with 70% saturation (i.e. 100 mm
alluvium 50%S + 1.5 m alluvium 70%S),
7) 100 mm alluvium with 70% saturation + 1.5 m alluvium with 30% saturation (i.e. 100 mm
alluvium 70%S + 1.5 m alluvium 30%S),
8) 100 mm alluvium with 70% saturation + 1.5 m alluvium with 50% saturation (i.e. 100 mm
alluvium 70%S + 1.5 m alluvium 50%S), and
9) 100 mm alluvium with 70% saturation + 1.5 m alluvium with 70% saturation (i.e. 100 mm
alluvium 70%S + 1.5 m alluvium 70%S).
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 12
O’Kane Consultants 20 January 2017 750/42-02
5 ADVECTION MODELLING
Advective flux is considered in modelling as both a temporary and more permanent phenomena.
During the construction of the NOEF waste is progressively buried as waste placement
progresses as a series of lifts that “over dump” each other. Advective fluxes unlike diffusive
fluxes are not as sensitive to this form of construction as air flow can extend tens or hundreds of
meters into a waste dump depending on in situ air permeability. However consumption of oxygen
in air flow that is entering a dump by advection may be more rapid than the supply of oxygen by
the advective mechanism. In this way oxygen limited systems may be created even where active
advection is occurring. From this perspective, advection may be considered to be permanent in
that the process is continuous, but at the same time temporary as over dumping of material may
result in cutting off of advective oxygen fluxes if consumption rates in the overlying material are
higher than supply rates.
Advective flux is an important risk driver as oxygen ingress rates are typically higher than for
diffusive fluxes; the two key risk parameters assessed as part of this study are temperature and
AMD loading rate. Generally, temperatures close to or over 100oC indicate risks of spontaneous
combustion.
Advection was considered to be a significant process during the construction period, the duration
over which waste is exposed and the surface area of exposed waste are both important
parameters to establish to determine advective fluxes. Information on the construction
sequencing was used to determine the main construction period for each Stage and the average
annual exposed waste surface area, Table 5-1 shows the assumptions made.
Table 5-1: Key input parameters for Stage areas
Stage Mass of material Exposure period years
tonnes
STAGE2 51066468.62 1
STAGE3 56713038.62 2
STAGE4 70143383.95 3
STAGE5 80509833.66 2
STAGE6 115383932.4 3
STAGE7 92041440.61 4
Table 5-1 shows the key input values used for assessing Stage areas as part of gas flux
assessment:
Mass of material: This is the total mass of material in the Stage at completion. Based on the
model geometry and construction sequence presented herein it is apparent that as Stages
are constructed over time the mass of material would be a transient value with respect to
time; however, to simplify modelling a fixed value was utilised.
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 13
O’Kane Consultants 20 January 2017 750/42-02
Exposure duration: This is the length of time that the material in the Stage is exposed directly
to the atmosphere. Based on the model geometry and construction sequence presented
herein it is apparent that as Stages are constructed over time the surface area would be a
transient value with respect to time; however, to simplify modelling a fixed value was used.
At the time of model construction, Stage surface areas and material mass values were not varied
on temporal basis as would be the case in reality (i.e. the surface area would in reality increase as
the Stage is being constructed and then decrease as the next Stage is constructed over it).
Detailed analysis of surface area variations over time would provide additional accuracy to the
model; however, for the purposes of this assessment the use of an averaging approach was
considered valid. This is consistent with the methodology used in previous modelling in order to
obtain relative values for comparison.
Advective flux and diffusive flux rates calculated by the model are directly tied to exposed surface
areas; therefore, this assumption is a key value.
Diffusive flux is assumed to occur only directly onto exposed surfaces. As soon as a surface
is constructed over diffusion into this mass of waste is assumed to reduce to effectively zero.
This assumption is validated based on diffusion model results, which show diffusion is more
significant in the top 2 m of the profile, and given that the waste is being placed in either 2 m
or 5 m lifts.
Advective flux, like diffusive flux, is tied to the total surface area exposed, as gas flux into and
out of waste requires exposed waste surfaces to be connected to the atmosphere. The flux of
gas is not assumed to be limited to a specific depth, rather the model allows gas flux through
the entire waste profile. However because of the low flux rates calculated as a result of the
reduced air permeability related to the waste placement method (and by alluvial cover
material when modelled), the flux of oxygen is effectively limited to the upper 5 m to10 m of
the waste profile. As a result, as a new lift is constructed advective flux of oxygen is
effectively cut off to the underlying lift. Because it is also assumed that areas of the NOEF
that have reached their final extent will have the final cover system progressively placed over
it, then this also acts to limit to advective flux in the model. For example, if a lift has reached
the final construction level in the outer embankment, this area is assumed to be covered with
the final cover system.
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 14
O’Kane Consultants 20 January 2017 750/42-02
6 MODELLING ASSESSMENT RESULTS
6.1 Oxygen Diffusion Results through the Alluvium Layer
The calculated oxygen flux rate into the PAF waste rock through 100 mm and 1.5 m alluvium for
the base case (i.e. POR of 5.0 x 10-7
kg O2/m3/s) is presented in Figure 6-1 and Table 6-1. The
calculated oxygen flux rate through the alluvium layer for other POR values (i.e. sensitivity
analysis of POR) is also listed in Table 6 for comparison.
Figure 6-1: Calculated oxygen flux rate for various alluvium saturations overlying the PAF rock with an oxidation rate of 5.0 x 10-7 kg O2/m
3/s.
The oxygen flux rate decreases with increasing saturation in the alluvium layer and the alluvium
layer’s thickness. The oxygen flux rate decreases from ~13,000 g O2/m2/yr at 30% saturation to
~4,000 g O2/m2/yr at 70% saturation when having the 100 mm alluvium layer overlying the PAF
rock, and the oxygen flux rate decreases from ~5,800 g O2/m2/yr at 30% saturation to ~400 g
O2/m2/yr at 70% saturation when having the 1.5 m alluvium layer overlying the PAF rock. The
difference of oxygen flux rate between through 100 mm alluvium and 1.5 m alluvium increases
with increasing saturation in the alluvium layer. For example, the oxygen flux rate through the
100 mm alluvium is approximately 2.5 times greater than the oxygen flux rate through the 1.5 m
alluvium when saturation is 30%, while the oxygen flux rate through the 100 mm alluvium is
approximately 10 times larger than the oxygen flux rate through the 1.5 m alluvium when
saturation is 70%. As a result, it is important to choose a proper cover layer thickness and
maintain large saturation in the layer to minimise oxygen ingress into the PAF rock.
0
2000
4000
6000
8000
10000
12000
14000
30% 50% 70%
Oxyg
en
Flu
x R
ate
(g
O2/m
2/y
r)
Saturation
100 mm Alluvium 1.5 m Alluvium
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 15
O’Kane Consultants 20 January 2017 750/42-02
Table 6-1: Diffusive oxygen ingress assessment results
30% Saturation 50% Saturation 70% Saturation
100 mm Alluvium
1.5 m Alluvium
100 mm Alluvium
1.5 m Alluvium
100 mm Alluvium
1.5 m Alluvium
POR of 5.0 x 10-7
kg O2/m3/s
O2 Penetration Depth into PAF (m)
0.83 0.37 0.62 0.15 0.28 0.03
O2 Concentration Below Alluvium (%)
16.8 3.7 12.3 0.7 2.6 0.2
Diffusive Oxygen Ingress (g/m
2/yr)
13,098 5,785 9,605 2,401 4,366 437
POR of 1.0 x 10-6
kg O2/m3/s
O2 Penetration Depth into PAF (m)
0.50 0.20 0.44 0.07 0.14 0.01
O2 Concentration Below Alluvium (%)
15.9 2.0 8.7 0.8 1.4 0.2
Diffusive Oxygen Ingress (g/m
2/yr)
15,718 6,331 13,753 2,292 4,475 437
POR of 5.0 x 10-6
kg O2/m3/s
O2 Penetration Depth into PAF (m)
0.21 0.04 0.11 0.01 0.03 0.003
O2 Concentration Below Alluvium (%)
10.5 0.9 5.5 0.3 0.6 0.06
Diffusive Oxygen Ingress (g/m
2/yr)
32,854 6,658 17,137 2,292 4,693 437
Under the same alluvium thickness and saturation, the oxygen flux rate increases with the
increasing oxidation rate of the PAF rock, due to faster oxygen consumption resulting in lower
oxygen concentration just below the alluvium layer. Having said this, the oxygen flux rate would
not increase with increasing oxidation rate of the PAF rock when the alluvium layer is relatively
thick and has large saturation. In this situation, the oxygen flux rate is totally controlled by the
alluvium layer. The oxygen flux rate through the 1.5 m alluvium with 70% saturation illustrates
this case as presented in Table 6-1.
6.2 Diffusion Based Acidity Load Assessment
The calculated acidity load rates for the various alluvium cover cases are presented in Figure 6-2
to Figure 6-4 and Table 6-2 for different oxidation rates of the PAF rock. In general, Stage 2 has
the largest acidity load rate followed by Stage 4, while Stage 7 has the least acidity load rate.
This is contributed to the larger area of the PAF rock is covered by the 1.5 m alluvium compared
to the area of the PAF is covered by the 100 mm alluvium. Being consistent with oxygen diffusion
rate, the calculated acidity load rate decreases with increasing saturation in the alluvium layer.
The acidity load rate may be lower than ~0.2 kg H2SO4 per tonne of PAF when the alluvium layer
achieves 70% saturation. Again, the acidity load rate increases with the increasing oxidation rate
of the PAF rock. However, when the saturation in the alluvium cover layer is large enough, the
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 16
O’Kane Consultants 20 January 2017 750/42-02
acidity load rate does not show significant increase with the increasing oxidation rate of the PAF
rock.
Figure 6-2: Calculated acidity load rate for the PAF rock with an oxidation rate of 5.0 x 10-7
kg O2/m
3/s for various alluvium cover cases during the PAF rock construction.
Figure 6-3: Calculated acidity load rate for the PAF rock with an oxidation rate of 1.0 x 10-6
kg O2/m
3/s for various alluvium cover cases during the PAF rock construction.
Figure 6-4: Calculated acidity load rate for the PAF rock with an oxidation rate of 5.0 x 10-6
kg O2/m
3/s for various alluvium cover cases during the PAF rock construction.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
STAGE2 STAGE3 STAGE4 STAGE5 STAGE6 STAGE7
Lo
ad
(k
g H
2S
O4
/ to
n P
AF
)
100 mm alluvium 30%S + 1.5 m alluvium 30%S 100 mm alluvium 30%S + 1.5 m alluvium 50%S100 mm alluvium 30%S + 1.5 m alluvium 70%S 100 mm alluvium 50%S + 1.5 m alluvium 30%S100 mm alluvium 50%S + 1.5 m alluvium 50%S 100 mm alluvium 50%S + 1.5 m alluvium 70%S100 mm alluvium 70%S + 1.5 m alluvium 30%S 100 mm alluvium 70%S + 1.5 m alluvium 50%S100 mm alluvium 70%S + 1.5 m alluvium 70%S
0.0
0.5
1.0
1.5
2.0
2.5
3.0
STAGE2 STAGE3 STAGE4 STAGE5 STAGE6 STAGE7
Lo
ad
(k
g H
2S
O4
/ to
n P
AF
)
100 mm alluvium 30%S + 1.5 m alluvium 30%S 100 mm alluvium 30%S + 1.5 m alluvium 50%S
100 mm alluvium 30%S + 1.5 m alluvium 70%S 100 mm alluvium 50%S + 1.5 m alluvium 30%S
100 mm alluvium 50%S + 1.5 m alluvium 50%S 100 mm alluvium 50%S + 1.5 m alluvium 70%S
100 mm alluvium 70%S + 1.5 m alluvium 30%S 100 mm alluvium 70%S + 1.5 m alluvium 50%S
100 mm alluvium 70%S + 1.5 m alluvium 70%S
0.0
0.5
1.0
1.5
2.0
2.5
3.0
STAGE2 STAGE3 STAGE4 STAGE5 STAGE6 STAGE7
Lo
ad
(kg
H2S
O4 /
to
n P
AF
)
100 mm alluvium 30%S + 1.5 m alluvium 30%S 100 mm alluvium 30%S + 1.5 m alluvium 50%S
100 mm alluvium 30%S + 1.5 m alluvium 70%S 100 mm alluvium 50%S + 1.5 m alluvium 30%S
100 mm alluvium 50%S + 1.5 m alluvium 50%S 100 mm alluvium 50%S + 1.5 m alluvium 70%S
100 mm alluvium 70%S + 1.5 m alluvium 30%S 100 mm alluvium 70%S + 1.5 m alluvium 50%S
100 mm alluvium 70%S + 1.5 m alluvium 70%S
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 17
O’Kane Consultants 20 January 2017 750/42-02
Table 6-2: Acidity load rate during construction stages for various PAF rock oxidation rates
Alluvium Cover Case
Total Acidity Load Rate during Construction Stages (kg H2SO4 / tonne PAF)
Stage 2 Stage 3 Stage 4 Stage 5 Stage 6 Stage 7
POR of 5.0 x 10-7
kg O2/m3/s
100 mm alluvium 30%S + 1.5 m alluvium 30%S 2.44 0.84 1.40 0.95 0.83 0.70
100 mm alluvium 30%S + 1.5 m alluvium 50%S 1.08 0.47 0.69 0.56 0.46 0.39
100 mm alluvium 30%S + 1.5 m alluvium 70%S 0.29 0.26 0.28 0.33 0.25 0.21
100 mm alluvium 50%S + 1.5 m alluvium 30%S 2.41 0.79 1.35 0.88 0.77 0.65
100 mm alluvium 50%S + 1.5 m alluvium 50%S 1.05 0.42 0.64 0.49 0.41 0.35
100 mm alluvium 50%S + 1.5 m alluvium 70%S 0.26 0.20 0.23 0.26 0.19 0.17
100 mm alluvium 70%S + 1.5 m alluvium 30%S 2.36 0.70 1.28 0.77 0.69 0.58
100 mm alluvium 70%S + 1.5 m alluvium 50%S 1.00 0.33 0.57 0.37 0.33 0.28
100 mm alluvium 70%S + 1.5 m alluvium 70%S 0.21 0.12 0.15 0.14 0.11 0.10
POR of 1.0 x 10-6
kg O2/m3/s
100 mm alluvium 30%S + 1.5 m alluvium 30%S 2.68 0.95 1.55 1.07 0.92 0.78
100 mm alluvium 30%S + 1.5 m alluvium 50%S 1.06 0.51 0.71 0.60 0.49 0.42
100 mm alluvium 30%S + 1.5 m alluvium 70%S 0.31 0.30 0.32 0.39 0.29 0.25
100 mm alluvium 50%S + 1.5 m alluvium 30%S 2.66 0.91 1.52 1.03 0.89 0.76
100 mm alluvium 50%S + 1.5 m alluvium 50%S 1.04 0.47 0.68 0.56 0.46 0.39
100 mm alluvium 50%S + 1.5 m alluvium 70%S 0.30 0.27 0.29 0.35 0.26 0.22
100 mm alluvium 70%S + 1.5 m alluvium 30%S 2.58 0.76 1.39 0.83 0.75 0.63
100 mm alluvium 70%S + 1.5 m alluvium 50%S 0.96 0.32 0.54 0.36 0.32 0.27
100 mm alluvium 70%S + 1.5 m alluvium 70%S 0.21 0.12 0.16 0.15 0.12 0.10
POR of 5.0 x 10-6
kg O2/m3/s
100 mm alluvium 30%S + 1.5 m alluvium 30%S 2.96 1.26 1.87 1.48 1.22 1.04
100 mm alluvium 30%S + 1.5 m alluvium 50%S 1.21 0.78 0.95 0.97 0.75 0.64
100 mm alluvium 30%S + 1.5 m alluvium 70%S 0.46 0.58 0.57 0.76 0.55 0.48
100 mm alluvium 50%S + 1.5 m alluvium 30%S 2.82 1.00 1.64 1.14 0.98 0.83
100 mm alluvium 50%S + 1.5 m alluvium 50%S 1.07 0.53 0.73 0.63 0.51 0.44
100 mm alluvium 50%S + 1.5 m alluvium 70%S 0.33 0.33 0.34 0.42 0.31 0.27
100 mm alluvium 70%S + 1.5 m alluvium 30%S 2.71 0.80 1.46 0.87 0.79 0.67
100 mm alluvium 70%S + 1.5 m alluvium 50%S 0.96 0.33 0.55 0.37 0.32 0.27
100 mm alluvium 70%S + 1.5 m alluvium 70%S 0.22 0.12 0.16 0.15 0.12 0.10
The total acidity load for the various oxidation rates of the PAF during the PAF placement is listed
in Table 6-3. The total acidity load can change from ~600,000 tonnes H2SO4 for the case having
the alluvium layer with 30% saturation to ~65,000 tonnes H2SO4 for the case having the alluvium
layer with 70% saturation. Again, it shows the importance to maintain a high saturation in the
cover layer for minimising oxygen flux into the underlying waste rock dump and hence the
resultant acidity load within the dump. It should be noted that the acidity load produced during the
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 18
O’Kane Consultants 20 January 2017 750/42-02
waste rock placement may not be equal to the acidity load discharging to the surrounding
environment that is affected substantially by seepage from the dump.
Table 6-3: Acidity load produced during construction stages for various PAF rock oxidation rates
Alluvium Cover Case
Total Acidity Load Produced during Construction Stages (tonne H2SO4)
POR of 5.0 x 10-
7 kg O2/m
3/s
POR of 1.0 x 10-
6 kg O2/m
3/s
POR of 5.0 x 10-
6 kg O2/m
3/s
100 mm alluvium 30%S + 1.5 m alluvium 30%S 536,596 597,118 744,770
100 mm alluvium 30%S + 1.5 m alluvium 50%S 277,540 287,922 410,504
100 mm alluvium 30%S + 1.5 m alluvium 70%S 127,120 145,859 268,441
100 mm alluvium 50%S + 1.5 m alluvium 30%S 511,611 583,064 632,338
100 mm alluvium 50%S + 1.5 m alluvium 50%S 252,555 273,868 298,072
100 mm alluvium 50%S + 1.5 m alluvium 70%S 102,135 131,805 156,009
100 mm alluvium 70%S + 1.5 m alluvium 30%S 474,134 516,698 543,330
100 mm alluvium 70%S + 1.5 m alluvium 50%S 215,078 207,502 209,063
100 mm alluvium 70%S + 1.5 m alluvium 70%S 64,658 65,439 67,000
6.3 Results of Advection Load Assessment
Table 6-4 shows the results of advection loading assessment. Loads vary between 1.5-3.9 kg/t
and in general are controlled by the surface area of the core area of the Stage and the
construction period.
Table 6-4: Advection loading assessment
Stage months construction Monthly load total load load kg/t
2 12 9653.0 115836.1 1.8
3 24 3293.5 79043.0 1.5
4 36 4111.4 148009.8 2.0
5 24 12848.9 308374.0 3.9
6 36 6773.4 243841.8 2.2
7 72 3637.7 261916.7 2.8
Table 6-5 shows detailed results for advection modelling for Stage 2, key points from the results
are:
Temperatures and loading rates are significantly controlled by % saturation with model
runs at lower degrees of saturation recording higher loading rates and temperatures
Temperatures greater than 100 degrees Celsius are predicted at lower saturation levels
indicating potential risks of spontaneous combustion.
Table 6-5: Detailed model results for Stage S2
% Saturation
Air Permeability
Gas flux rate
Acidity generation
Net Acidity (t
NOEF Temperature
Oxygen flux rate
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 19
O’Kane Consultants 20 January 2017 750/42-02
(m3/m2/s) total (kg H2SO4/d)
H2SO4/month) (°C) (kg O2/s)
70
3.17E-10 5.73E-06 74409.6 2232.3 66.1 0.5
50
4.42E-10 1.06E-05 136516.5 4095.5 81.1 1.0
30
7.14E-10 2.21E-05 264765.2 7943.0 100.3 2.0
20
8.88E-10 2.91E-05 321766.9 9653.0 105.3 2.7
10
9.96E-10 3.58E-05 346170.4 10385.1 115.1 3.3
Model results shown in Table 6-5 are displayed in graphical format in Figure 6-5 where the link
between saturation gas flux and temperature can be clearly seen.
Figure 6-5: Temperature and gas flux for Stage 2 showing effect of degree of saturation
6.4 Total Loading Assessment
The results of diffusive and advective flux models have been summarised in Figure 6-6 which
presents the results along with those calculated previously for other placement methods at the
site.
1.0E-12
1.0E-11
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1
10
100
1000
70
50
30
20
10
Ga
s F
lux
(m
3/m
2/s
)
Te
mp
era
ture
(ºC
)
Saturation %
Temperature
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 20
O’Kane Consultants 20 January 2017 750/42-02
Figure 6-6: Placement method and calculated loading rates
Key points from Figure 6-6 are:
The base case assumption for the 7.5m case is degree of saturation in the alluvials of 30%
and in the waste rock of 20%. If a 50% saturation case is assumed, then loading rates are
significantly lower.
Results for the 7.5m placement case are comparable with the paddock dumping scenario, but
lie above that of the 500m cell scenario.
It is noted that results from the 7.5m scenario are not directly comparable to scenarios modelled
in the NOEF DumpSim assessment as one of the assumptions from that assessment was that a
CCL cover was placed over the slope and plateaux areas once a Stage was completed. In
comparison in the 7.5m case modeled herein, 1.5m of alluvial material was assumed to comprise
the outer cover in these areas for the duration of NOEF construction.
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 21
O’Kane Consultants 20 January 2017 750/42-02
7 CONCLUSIONS AND LIMITATIONS
7.1 Assessment Conclusions
Based on the assessment completed the following conclusions are made:
The assumption regarding the degree of saturation of the alluvial materials has a key control
on calculated loading rates from diffusion. For conservatism an assumption that the waste
rock would have a degree of saturation of 20% and the alluvium of 30% was made, however
given the appreciable water holding capacity of the finer textured alluvium materials, the 1.5m
thick cover may in the medium term maintain a higher degree of saturation. Further
assessment is recommended to explore this key assumption.
The loading assessment indicates that based on loading rates, the 7.5m case is comparable
to paddock dumping, and if saturation levels of around 50% can be achieved, then the
scenario may be superior to paddock dumping.
Assessment of waste rock temperature indicates that if the most reactive material (PAF(RE))
is placed by end tipping, a potential risk of spontaneous combustion exists with the 7.5m
case. It is understood that PAF(RE) is not to be disposed of in 7.5m lifts which mitigates this
potential risk factor.
The thickness of the alluvium is shown to have a significant control on oxygen ingress rates,
the thicker the alluvium layer used, the higher the potential performance that can be
expected. The volume of alluvium available is finite and as such scheduling of the material
should be completed to plan the placement of alluvium over time to maximise the thickness
where possible.
7.2 Assessment Limitations
The oxygen diffusion assessment presented in this section was simplified into the conceptual
model and boundary conditions so that analytical calculation can be applied. The following
limitations should be noted when interpreting the results of the assessment for the PAF rock
placement below the alluvium layer.
The conceptual model for oxygen diffusion assumes a steady state oxygen transport with
oxygen concentration in the atmosphere at the environmental rock surface and below the
alluvium layer. The model does not account for any transient oxygen transport through the
alluvium layer.
The conceptual model assumes that the alluvium layer can be represented by homogeneous
material properties. The potential influence of local heterogeneity was not considered. A
larger saturation and/or lower porosity caused by local heterogeneity may become significant
for total oxygen ingress.
McArthur River Mining Pty Ltd 7.5m Waste Deposition Assessment, in Support of the, EIS Submission 22
O’Kane Consultants 20 January 2017 750/42-02
Annual average air temperature and air pressure as well as constant saturation were utilised
in the oxygen diffusion assessment. In a field condition, these parameters should change
with time and climatic conditions.
The key advantage to the oxygen diffusion assessment results summarized herein is its simplicity
and the ability to enhance judgment. Hence, rather than a focus on the absolute results
calculated, it is recommended that the assessment results be viewed as a tool to understand key
factors that will influence diffusive oxygen ingress (hence the acidity load) through the alluvium
cover layer, and develop engineering decisions based on this understanding.
For further information contact:
O'Kane Consultants Pty Ltd
193D Given Terrace
Paddington QLD 4064
Australia
Telephone: (07) 3367 8063
Facsimile: (07) 3367 8052
Web: www.okc-sk.com
top related