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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


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


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


kg



kg


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


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



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



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



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).



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.



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 Void ratio: 0.25

o Porosity: 0.20

3) 1.5m alluvium cover (1.5 m thick)





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.



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



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



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),
















alluvium 70%S + 1.5 m alluvium 50%S), and


alluvium 70%S + 1.5 m alluvium 70%S).



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.



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.



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



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


0.50 0.20 0.44 0.07 0.14 0.01


15.9 2.0 8.7 0.8 1.4 0.2


2/yr)

15,718 6,331 13,753 2,292 4,475 437

POR of 5.0 x 10-6

kg O2/m3/s


0.21 0.04 0.11 0.01 0.03 0.003


10.5 0.9 5.5 0.3 0.6 0.06


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



acidity load rate does not show significant increase with the increasing oxidation rate of the PAF

rock.


kg O2/m

3/s for various alluvium cover cases during the PAF rock construction.


kg O2/m



kg O2/m


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


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 70%S + 1.5 m alluvium 70%S

0.0

0.5

1.0

1.5

2.0

2.5

3.0


Lo

ad

(kg

H2S

O4 /

to

n P

AF

)





100 mm alluvium 70%S + 1.5 m alluvium 70%S



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









POR of 1.0 x 10-6

kg O2/m3/s










POR of 5.0 x 10-6

kg O2/m3/s










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



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









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



(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



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.



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.



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

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