Toms Gully Underground Project Acid and Metalliferous Drainage Management Plan
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Toms Gully Underground Project
Acid and Metalliferous Drainage
Management Plan
July 2019
Toms Gully Underground Project Acid and Metalliferous Drainage Management Plan
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Document Control Record
Prepared by: Charles Hastie Approved by: Mark Qiu
Position: Manager Approvals and
Tenure Position: Director
Signed:
Signed:
Date: 15/07/2019 Date: 15/07/2019
Revision Status
Revision No. Description of
Revision
Date Comment Approved
1.0 First Issue 18/09/15 First issue released by GHD in 2015 for the purpose of the EIS draft.
2.0 Second Issue 8/08/18 Updated to reflect Draft EIS comments. Submitted for EIS Supplement
MQ
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Contents List of Figures ................................................................................................................................................ 5
List of Tables ................................................................................................................................................. 5
1. Introduction .......................................................................................................................................... 6
1.1. Purpose and Scope ........................................................................................................................ 6
2. Legal and Other Requirements ............................................................................................................. 9
2.1. Guidelines ..................................................................................................................................... 9
2.2. Environmental Corporate Governance ......................................................................................... 9
2.3. Inputs to the AMDMP ................................................................................................................. 10
2.4. Document Revision ..................................................................................................................... 10
3. Waste Rock and Tailings Characterization and Classification ............................................................. 12
3.1. Waste Rock Characterization ...................................................................................................... 12
3.1.1. Mineralogy .......................................................................................................................... 12
3.1.2. Oxide Waste Rock Dump..................................................................................................... 13
3.1.3. Sulphide Waste Rock Dump ................................................................................................ 13
3.1.4. Boxcut Waste Rock.............................................................................................................. 13
3.2.1. Mineralogy .......................................................................................................................... 14
3.2.2. Geochemistry ...................................................................................................................... 14
3.3. Waste Rock and Tailings Classification ....................................................................................... 15
3.4. PAF/NAF Estimated Volumes ...................................................................................................... 15
4. Conceptual Site Model ........................................................................................................................ 16
4.1. Background ................................................................................................................................. 16
4.2. Source ......................................................................................................................................... 16
4.2.1. SWRD................................................................................................................................... 16
4.2.2. OWRD .................................................................................................................................. 17
4.2.3. TSF1 ..................................................................................................................................... 17
4.2.4. TSF2 ..................................................................................................................................... 17
4.3. Pathway....................................................................................................................................... 18
4.3.1. Surface Water ..................................................................................................................... 18
4.3.2. Groundwater ....................................................................................................................... 19
4.4. Receptors .................................................................................................................................... 19
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4.4.1. Mt Bundey Creek ................................................................................................................ 19
4.4.2. Lake Bazzamundi ................................................................................................................. 19
5. Acid Mine Drainage Model and Balance ............................................................................................. 22
6. AMD Management .............................................................................................................................. 24
6 . 1 . AMD Risk Assessment ............................................................................................................. 24
6.2. AMD Management Strategy ....................................................................................................... 30
6.3. Controls ....................................................................................................................................... 30
6.3.1. Ore and Waste Rock ............................................................................................................ 30
6.3.2. Tailings ................................................................................................................................ 31
6.4. Site Drainage and Controls ......................................................................................................... 32
6.4.1. Maintenance of existing structures .................................................................................... 38
7. AMD Monitoring ................................................................................................................................. 38
7.1. Geochemical Monitoring ............................................................................................................ 40
7.1.1. Visual Methods ................................................................................................................... 40
7.1.2. Laboratory Analysis ............................................................................................................. 41
7.2. Surface Water Monitoring .......................................................................................................... 42
7.3. Groundwater Monitoring ............................................................................................................ 42
8. Contingency Planning.......................................................................................................................... 43
8.1. Overview ..................................................................................................................................... 43
8.2. Specific Measures ....................................................................................................................... 43
8.2.1. Tailings Management .......................................................................................................... 43
8.2.2. Waste Rock.......................................................................................................................... 43
8.2.3. ROM Pad / Ore .................................................................................................................... 44
8.2.4. Water Management ............................................................................................................ 44
9. Roles, Responsibilities and Training .................................................................................................... 45
9.1. Awareness, Training and Competence ....................................................................................... 45
9.2. Records, Reporting and Document Control ................................................................................ 46
10. References ...................................................................................................................................... 47
Appendix A: Site Geochemical Sampling Procedure ................................................................................... 49
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List of Figures Figure 1: Project Location ............................................................................................................................. 8
Figure 2: Site Layout .................................................................................................................................... 11
Figure 3: Conceptual Site Model at Toms Gully (GHD 2019) ...................................................................... 21
Figure 4: Location of waste material to be placed in the flooded pit ......................................................... 31
Figure 5: Systematic Floating Head Traverses to Deposit Tails. ................................................................. 32
Figure 6: Site Drainage ................................................................................................................................ 35
Figure 7: Surface Water Monitoring Locations ........................................................................................... 36
Figure 8: Surface Water Monitoring Locations Continued ......................................................................... 37
Figure 9: The process of adaptive management ......................................................................................... 40
List of Tables Table 1: 2019 TGU Materials Balance ......................................................................................................... 12
Table 2: Tailings Acid Base Accounting Summary - Median Results (GHD 2018) ....................................... 14
Table 3: Net Acid Generation (GHD 2018d) ................................................................................................ 22
Table 4: Average Annual Acid Mine Drainage Balance (GHD 2018d) ......................................................... 23
Table 5: AMD Risk, potential impact and management / mitigation control ............................................. 25
Table 6: Water management infrastructure and their catchment areas (GHD 2019) ................................ 34
Table 7: Water Storages at TGU Project (2019) .......................................................................................... 34
Table 8: Sampling Frequency ...................................................................................................................... 42
Table 9: Roles and Responsibilities ............................................................................................................. 45
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1. Introduction The Toms Gully Underground Project (TGU or the Project) is located within the Old Mt Bundey Station,
approximately 90 km south-east of Darwin. The mine was operational on occasions from 1988 and has
been in Care and Maintenance since November 2010 (Figure 1).
Primary Gold Ltd (PGO) acquired the Project in 2013 and has proposed to implement a resumption of
mining at TGU (Figure 2). The Project includes the following works to recommence underground mining
and ore processing on site:
• Construction of a new 1 gigalitre (GL) water storage dam (WSD),
• Construction of a new Boxcut and Decline,
• In-situ treatment of pit water,
• Treatment of the water displaced from the pit,
• Underground mining for approximately three to four years with all waste rock stored
underground or in-pit,
• Upgrade of the existing tailings storage facility (TSF2) to ANCOLD 2012 standard for potential
use a as water storage dam,
• Placement of existing tails from TSF1 and 2 (whether processed or not) in the pit under a water
cover,
• Upgrade of the processing circuit, and
• Establishment of a water and tailings treatment plant.
Acid and metalliferous drainage (AMD) (including neutral and saline drainage) from existing mine features
(TSFs, processing area, waste rock dumps, pits and evaporation ponds) are potentially impacting water
quality and downstream aquatic ecosystems.
The Environmental Impact Statement (EIS) Terms of Reference (ToR) issued by the Northern Territory
Environmental Protection Authority (NT EPA) for the Project requires that the EIS should contain a detailed
AMD Management Plan (AMDMP) that outlines how AMD risk will be managed on site. This AMDMP has
been developed to address the EPA’s recommendations in the ToRs as well as the NT EPAs comments on
the Draft EIS.
1.1. Purpose and Scope The purpose of this AMDMP (or, the Plan) is to describe the systems, processes and procedures used at
the Project to manage the overall risk of AMD being generated on site throughout operations (and
therefore into closure). It does so by classifying waste rock and tailings based on geochemical testing,
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providing a management strategy, and articulating management and monitoring procedures for the
handling and long term storage of waste rock and tailings on site.
This document outlines the objectives and methods for PGO to follow in pursuit of leading practice AMD
management. The principle objective is to manage AMD risk resulting from the oxidation of sulfidic
mineral waste material throughout operations, such that off-site environmental values are maintained
during operations and into closure.
The plan is applicable to Mining Leases (ML) MLN 1058, ML29812 and ML29814 and has been informed
by the Mineral Waste Geochemical Assessment (GHD 2019) shown in Appendices D, J and baseline
geochemistry and conceptual site model report undertaken by GHD (2019) Appendix N. As Toms Gully is
a brownfield site, it is noted that AMD is currently being generated at the sulfide and oxide WRDs, TSF1
and 2, the pit, and evaporation ponds 1 and 2. This Plan acknowledges the legacy AMD and incorporates
its ongoing management and monitoring into the AMD strategy for the Project.
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Figure 1: Project Location
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2. Legal and Other Requirements PGO submitted a Mining Management Plan (MMP) under the NT Mining Management Act to the Northern
Territory Department of Primary Industries and Resources (DPIR) in late 2013. Subsequently, on 28
February 2014, DPIR referred the Toms Gully Underground Project MMP 2013-2014 and 18 associated
documents to the NT EPA for consideration under the Environmental Assessment Act (EA Act).
The NT EPA subsequently determined that an Environmental Impact Statement (EIS) was required under
the EA Act and the terms of reference (ToRs) for the EIS were issued in October 2014.
PGO submitted the Draft EIS to the NT EPA. The NT EPA provided comments on the Draft EIS and
subsequent EIS Supplement submitted in response. This AMDMP is a revised version of the AMDMP
submitted with the EIS Supplement and incorporates the NT EPA/stakeholders comments as well as
results of updated studies and project changes.
2.1. Guidelines Content from the following industry guidelines were considered when preparing this Plan:
• AMIRA (2002). ARD Test Handbook. Project P387A Prediction and kinetic control of acid mine
drainage. Available at:
http://www.amira.com.au/documents/downloads/P387AProtocolBooklet.pdf
• Department of Foreign Affairs and Trade (DFAT) and Department of Industry Innovation and
Science (2016). Leading Practice Sustainable Development Program for the Mining Industry:
Preventing Acid and Metalliferous Drainage. Canberra. Available at:
https://industry.gov.au/resource/Documents/LPSDP/LPSDP-AcidHandbook.pdf
• The International Network for Acid Prevention (INAP) (2009). Global Acid Rock Drainage Guide.
Available at www.gardguide.com
• NT Environment Protection Authority (2013). Environmental Assessment Guidelines: Acid and
Metalliferous Drainage. Version 2.0. Available at:
https://ntepa.nt.gov.au/__data/assets/pdf_file/0011/287426/guideline_assessment_acid_meta
lliferous_drainage.pdf
2.2. Environmental Corporate Governance It is PGO’s mission to operate in an environmentally and socially responsible way, in order to minimise
their footprint and maximise benefits for their staff, shareholders and stakeholders well beyond the life
of their mines.
PGO formally endorsed their Environmental Policy in August 2015 and updated it in April 2017. The
Environmental Policy includes:
• A strong emphasis on rehabilitation
• Engaging widely with Project stakeholders
• Continuous improvement
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• Cultural awareness
• Maximising shareholder value while balancing the quadruple bottom line of:
o Environmental sustainability
o Social equity
o Cultural Vitality
o Economic prosperity.
2.3. Inputs to the AMDMP This Plan has been informed by Appendix D, J and N (2019 Mineral Waste Assessment and 2019 Baseline
Geochemical Assessment respectively) and compiled using the following input documents and
information sources:
• A draft Toms Gully Underground Project description (Primary Gold 2015)
• The draft Terms of Reference for the Toms Gully Gold Project (NT EPA 2014)
• PGO exploration data from the 2014 drilling program
• Geotechnical results provided by PGO from the 2015 fieldwork
• Waste characterization analysis of 2018 PGO geotechnical Boxcut samples
• Geochemical data provided by PGO from the 2015 sampling and analysis program as
recommended in GHD (2015) report.
• Geochemical baseline assessment and conceptual site model (GHD 2019)
• NT EPA comments on draft EIS and EIS Supplement
• AMD Assessment: Toms Gully Boxcut Material
• Publicly available information.
2.4. Document Revision The Plan is a dynamic document and therefore should be revised annually (as required) and appended to
the updated Mining Management Plan (MMP) upon its re-submission to DPIR.
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Figure 2: Site Layout
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3. Waste Rock and Tailings Characterization and Classification Based on the current mine schedule, a summary of estimated ore and mineral waste (waste rock and
tailings) that require management at Toms Gully over the life-of-mine is shown in Table 1.
Current mine schedules shows underground development over the mine life totaling 21,769 metres,
ramping up and down; for an average of 454 metres per month.
As all waste rock from mining development will be treated as PAF managed by in pit and/or underground
placement, and all tailings will also be treated as PAF with all material placed in the pit. Tails will be placed
in the pit whether or not reprocessed to remove gold, mixed metal oxides, sulfur and silica. No
geochemical characterisation in real time is required for mineral waste handling and placement. Rather,
the geochemical sampling will provide an inventory and historic record for closure management and mine
legacy purposes.
Table 1: 2019 TGU Materials Balance
Year Ore mined Ore stockpiled Waste rock Tailings Existing Tailings Removal
1 - - 1,004,400 - -
2 385 385 156,997 - 187,500
3 220,666 27,062 162,062 193,998 187,500
4 241,116 18,183 138,418 249,996 -
5 284,357 52,544 47,317 249,996 -
6 135,579 - - 190,113 -
Total 884,103 NA 1,509,194 884,103 375,000
3.1. Waste Rock Characterization The data from both the SWRD and OWRD indicate that while there remains unoxidised sulfides and
weatherable acid forming sulfates in both dumps, the measured pyrite oxidation rates are proceeding at
a relatively benign pace that makes management of the oxidation products achievable under an
appropriate AMD strategy and management plan. Acidity loads from both structures can be effectively
managed using traditional water management solutions, including storage treatment and seasonal
dilution to reduce elements consistent with saline and metalliferous drainage to environmentally benign
concentrations for licenced offsite release (GHD 2018).
3.1.1. Mineralogy
The majority of minerals identified in the waste rock dumps were inert, being; quartz, mica and clays -
around 80% (in the RoM Pad), 75% (SWRD), and 85% (OWRD). The remaining balance of minor minerals
were classified as non-diffracting or unidentified; which are usually amorphous secondary minerals. The
presence of up to 11.1% dolomite and 1.9% calcite in the OWRD and up to 4.8% dolomite in the SWRD
indicates some neutralizing capacity is inherent in both structures (GHD 2018). See Table 5.5 of the 2018
Baseline Geochemical Report (Appendix C) for more details on Waste Rock acid base accounting results.
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3.1.2. Oxide Waste Rock Dump
The median pH1:5 value for the 114 samples analysed was 4.4 (classified as very low) which increases to
5.6 as pHOX when a rapid oxidant is introduced. A median EC1:5 of 96 μS/cm (classified as very low using)
suggests a very low potential for saline drainage, supported by a very low median chloride value of <10
mg/kg. The median total sulfur value for the 114 samples was 0.04%. The median chromium reducible
sulfur concentration was 0.01%. This would suggest the presence of a majority of acid-forming and non-
acid forming sulfate species in the total sulfur content reported from the OWRD. The NAPP value most
likely representative of the content of the OWRD is 0.4 kgH2SO4/t. when the median total sulfur NAPP
value of 0.9 kgH2SO4/t (or the jarosite adjusted value of 0.4 kgH2SO4/t) is considered along with the
median pHOX value of 5.6, waste rock in the OWRD may be seen as being potentially acid forming (low
capacity). Using the jarosite-adjusted NAPP value of 0.4 kgH2SO4/t, a density of 2.6 t/m3 (PGO 2013) and
GHD’s estimated volume of 3,967,800 m3, there remains a total potential acid load of around 3,761 tonnes
of H2SO4 in the OWRD (GHD 2018).
It is also important to note that sulfur concentrations are relatively low in the OWRD, and there are
pockets of neutralising carbonate present at up to around 11% (GHD 2018).
3.1.3. Sulphide Waste Rock Dump
The median pH1:5 value for the 79 samples analysed was 3.8 (classified as very low) decreasing to 3.2 as
pHOX when a rapid oxidant is introduced. A median EC1:5 of 1,340 μS/cm (classified as high), suggests a high
potential for saline drainage, despite the very low median chloride value of <10 mg/kg. The median total
sulfur value for the 79 samples was 0.62%, with the median chromium reducible sulfur value of 0.21%. As
for the SWRD, this would suggest the presence of a majority of acid-forming and non-acid forming sulfate
species in the total sulfur content reported from the SWRD. The NAPP value most likely representative of
the content of the SWRD is 13.5 kgH2SO4/t (GHD 2018).
Using the jarosite-adjusted NAPP value of 13.5 kgH2SO4/t, a density of 2.6 t/m3 (PGO 2013) and GHD’s
estimated volume of 3,267,800 m3, there remains a total potential acid load of around 115,031 tonnes of
H2SO4 in the SWRD. In summary, the SWRD contains a large potential acidity store and must be managed
to account for this (GHD 2018).
3.1.4. Boxcut Waste Rock
The 24 samples that were tested are predominantly NAF with less than 5% of samples reporting as
uncertain and none of the samples classifying as PAF. As over 95% of the tested samples reported as non-
acid forming, the mean and median values may also be classified as NAF. The kinetic NAG testing further
confirmed this low risk with no pH results below 4.5 (i.e. classified as acidic) and increasing pH values over
time in most cases.
The samples also pose a low risk for metalliferous (neutral) and saline drainage with metals exceedances
for aluminium and zinc generally within acceptable dilution factors. As a group, the results showed
negligible risk acid drainage risk with all but one sample reporting as non-acid forming. It should be noted
that the shallow depth samples taken directly from the boxcut area at depths of 3.1 m and 5.2 m showed
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more acidic pH values (5.7 and 5.4) than the mean for the group (7.2). This may indicate some surficial
cross-contamination with residual sulfidic material.
Low levels of metal leaching concentrations were found compared to the SSTVs with the highest
exceedance being 28 times the trigger. Considering dilution and natural attenuation factors, the
aggressive nature of the ASLP test, and that the majority of the samples returned results less than 10
times the trigger values, the material is deemed a relatively low risk of generating metalliferous drainage.
There remains a small risk of aluminum and zinc leaching at low concentrations based on the sample
analysis.
3.2. Tailings Characterisation
3.2.1. Mineralogy
Key reactive minerals reported by XRD analysis from the two tailings storage facilities (TSF1 and TSF2)
shows that unreacted pyrite and arsenopyrite remain in both tailings storage facilities, particularly at
depth in the unoxidised zone, while one near surface sample in TSF1 shows the presence of over 5%
jarosite; a secondary mineral that is sparingly soluble and contains acid-forming sulfate. Minor unreacted
dolomite is present in TSF2, consistent with the ore XRD data (GHD 2018).
3.2.2. Geochemistry
Geochemical results for the 18 samples analysed from TSF1 and the 11 samples analysed from TSF2 show
the pH1:5 is very low for TSF1 (3.0) and medium for TSF2 (6.2), they both become very low once a rapid
oxidant is added as pHOX. Chloride was below laboratory detection limits in both TSF1 and 2. EC1:5 was
very high for TSF1 (2,100 μS/cm) and TSF2 (2,110 μS/cm). There are higher total sulfur and chromium
reducible sulfur concentrations in TSF1 relative to TSF2; perhaps indicating an evolving process circuit over
time and/or ore imported from another site for processing. Neither TSF1 nor 2 contain sufficient
neutralising capacity relative to their maximum potential acidity (based on total sulfur) to offset acid
generation as demonstrated by a NPR of 0.3 and below. The NAG 7.0 approximates the SCR NAPP,
indicating the likely extent of net acid producing potential in the samples from unoxidised sulfides (GHD
2018).
The volume of emplaced tailings TSF1 and TSF2 as estimated by GHD using LiDAR, 12D software and
sampling logs are around 131,000 m3 and 90,000 m3. Using a density of 1.5 t/m3 (GHD 2018 pers. comm.),
estimated emplaced tonnages for TSF1 and TSF2 are around 196,500 and 135,000 tonnes respectively.
Jarosite-adjusted median NAPP total acidity loads for TSF1 and 2 based on these tonnages are
approximately 27,320 and 9,205 tonnes of H2SO4 respectively (GHD 2018).
In summary, TSF1 and TSF2 remain significant point sources of acidity based on the data presented in
Table 2:
Table 2: Tailings Acid Base Accounting Summary - Median Results (GHD 2018)
Units TSF1 TSF2
Count 18 11
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pH1:5 pH units 3.0 6.2
EC1:5 µS/cm 2,100 2,110
Chloride mg/kg 5 5
STOT % 5.1 3.0
STOT MPA kgH2SO4/t 155 92.4
ANC kgH2SO4/t 0.3 10.8
STOT NAPP kgH2SO4/t 155 45.4
STOT NPR ratio 0 0.3
SCR % 3.2 0.9
SCR MPA kgH2SO4/t 98.1 28.7
SCR NAPP kgH2SO4/t 97.3 9.2
% S as SCR % 57.4 44.3
pHOX pH units 2.2 3.0
NAG4.5 kgH2SO4/t 109 13.4
NAG7.0 kgH2SO4/t 118.5 23.9
Fizz value 0-5 0 1
3.3. Waste Rock and Tailings Classification An updated conceptual site model of the environmental geochemistry of the Tom’s Gully site was
produced based on the 2014/2015 and 2017 mineral waste results (GHD 2018). The mineralised geology
at Tom’s Gully includes sulfides that comprise between 10 and 40 per cent of the ore at Tom’s Gully. The
key sulfides are pyrite (FeS2) and arsenopyrite (FeAsS),with minor pyrrhotite (Fe(1-x)S where x = 0 to 0.2),
chalcopyrite (CuFeS2), loellingite (FeAs2), sphalerite (ZnS) and galena (PbS) are also present (Sener 2004).
This indicates that iron, arsenic, zinc and lead, at a minimum, may pose an environmental risk given the
level of historic disturbance on site (GHD 2018).
3.4. PAF/NAF Estimated Volumes All material (waste rock, tailings, ore, and existing stockpiles) onsite is considered to be PAF material.
Using considered assumptions and site geochemical knowledge, it has been calculated that around 95
tonnes of acidity (as CaCO3 equivalents) per year has accumulated within the pit. When converted to
tonnes of H2SO4 per year, this represents around 14 % of the mineral waste mass stored within the SWRD
oxidising each year at the rate reported herein using OxCon testing. Based on this back-calculation from
the pit acidity, total acid loads being generated from the SWRD that require active management are in
the order of 485 tonnes of H2SO4 (GHD 2018). As context, this would take around 20 truck and dog loads
of an 80% calcite blend of 80% purity to treat to a pH of 7.0 for release per year. Whilst these calculations
remain order of magnitude for input into this revised AMD Management Plan, they are provided as
indicative for context to show that the acidity being generated on site from key historic mineral waste
storage structures remains manageable (GHD 2018).
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4. Conceptual Site Model The mineralised geology at Tom’s Gully includes sulfides and has been discussed in terms of waste rock
and tailings chemistry in Section 3.3.
A simplified summary of the updated conceptual site model (CSM) from a background, source, pathway,
receptor model perspective is provided below Appendix N. See Figure 3 for the conceptual site model
schematic.
Table 5-26 in the 2018 Baseline Geochemical Report (EIS Supplement Appendix C) provides a summary of
the 2014/2015 and updated 2017 data that was used to develop the CSM.
4.1. Background Background surface water quality data at sampling upstream locations in Mt Bundey and Coulter Creeks
showed median values for all analytes below SSTVs (median values for PGO data from December 2016 to
March 2018).
The distal hanging wall (DHW) unit, along with the rock and soil samples collected from where the fresh
water dam is to be located to the west of SWRD returned analytical results that showed inert background
or baseline, non-mineralised conditions. This includes circumneutral median NAPP and NAG values, and
no element with a GAI exceeding a value of 3.
Background groundwater values (the Ridge Bore and Bore 11) include circumneutral to slightly alkaline
pH values, fresh, non-saline conditions with no individual element exceeding the ANZECC/ARMCANZ
(2000) stock value (GHD 2018b).
4.2. Source
4.2.1. SWRD
The SWRD contains an estimated 3.27 million m3 of mineral waste material, of which the bulk is
potentially acid forming. Mineralogical analysis shows that the SWRD contains up to 1.4% pyrite, 0.4%
arsenopyrite and up to 8.9% jarosite. Acid base accounting showed that the SWRD had a median NAPP
value of 16.2 kgH2SO4/tonne (13.5 kgH2SO4/tonne when adjusted for jarosite – assuming that all
nonsulfidic sulfur is present in that form). The median NAG value was 8.62 kgH2SO4/t. The sulfide oxidation
rate was shown to be relatively slow, with a median intrinsic acidity generation rate of 0.4
kgH2SO4/tonne/year (values were 0.9, 0.62, 0.4, 0.028 and 0.1 for a median of 0.4) (GHD 2018).
Therefore, whilst the SWRD is generating acidity, it would appear that the annual load is manageable. The
SWRD is also a saline and metalliferous drainage risk. Median historic surface water data from location
SWTG13, being surface water runoff from the SWRD prior to it entering the evaporation ponds showed
water quality consistent with sulfidic waste rock contact. However, median surface water quality collected
at SWTG9, located in a drainage line on the western side of SWRD prior to it entering Mt Bundey Creek
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downstream of SWTG1A was entirely compliant with respective SSTVs for all analytes. This suggests that
rainfall interacting with the rehabilitated western slopes of the SWRD was not entraining environmentally
deleterious elements (GHD 2018).
4.2.2. OWRD
The OWRD contains an estimated 3.97 million m3 of mineral waste material. Mineralogical analysis shows
that the OWRD contains up to 1.7% pyrite and up to 0.6% jarosite. It also contains carbonate as dolomite
(up to 11.1%) and calcite (up to 1.9%) (GHD 2018).
Acid base accounting showed that the OWRD had a median NAPP value of 0.9 kgH2SO4/tonne (0.4
kgH2SO4/tonne when adjusted for jarosite – assuming that all non-sulfidic sulfur is present in that form)
with a median NAG value of 1.1 kgH2SO4/t. The sulfide oxidation rate was shown to be very slow, with an
intrinsic acidity generation rate of <0.1 kgH2SO4/tonne/year. Therefore, whilst the OWRD is generating
acidity, it would appear that the annual load is small and manageable. The OWRD is also a saline and
metalliferous drainage risk (GHD 2018).
4.2.3. TSF1
TSF1 contains an estimated 131,000 m3 of tailings; or approximately 196,500 tonnes. Mineralogical
analysis shows that TSF1 contains up to 6.2% pyrite, up to 5.0% arsenopyrite, up to 0.4% marcasite (a
pyrite allotrope) and up to 5.5% jarosite. It also contains minor gypsum (up to 0.6%).
Acid base accounting showed that TSF1 had a median NAPP value of 155 kgH2SO4/tonne (139
kgH2SO4/tonne when adjusted for jarosite – assuming that all non-sulfidic sulfur is present in that form)
with a median NAG value of 119 kgH2SO4/t. Being finer grained material than that stored in the OWRD
and SWRD, the sulfide oxidation rate is somewhat faster, with an intrinsic oxidising zone, around the top
metre or so given the lack of oxygen replenishment in the lower, anoxic zone in the TSF; which comprises
around two-thirds of the facility. In addition, the upper horizons of the oxide zone would contain relatively
less sulfidic sulfur than the anaerobic zone given historic oxidation, though would contain jarosite as a
result. Therefore, whilst TSF1 is generating acidity, it would appear that the annual load is manageable.
TSF1 is also a saline and metalliferous drainage risk (GHD 2018).
4.2.4. TSF2
TSF2 contains an estimated 90,000 m3 of tailings; or approximately 135,000 tonnes. Mineralogical analysis
shows that TSF2 contains up to 3.9% pyrite, up to 3.9% arsenopyrite, and up to 0.1% jarosite. It also
contains minor gypsum (up to 0.8%) and up to 2.0% dolomite. Acid base accounting showed that TSF2 had
a median NAPP value of 45.4 kgH2SO4/tonne, with a median NAG value of 23.9 kgH2SO4/t. TSF2 is
generating acidity; however, it would appear that the annual load is relatively small and manageable. TSF2
is also a saline and metalliferous drainage risk (GHD 2018).
This risk was further shown for TSF1 and TSF2 via historic surface water quality data that significantly
exceeded the SSTVs for all analysed water quality parameters with the exception of lead (TSF1 and TSF2)
and manganese (TSF1). Median acidity levels of 1,100 and 1,600 mg/L CaCO3 equivalents respectively for
TSF1 and TSF2 would indicate significant sulfide and metal acidity in both structures (GHD 2018).
Toms Gully Underground Project Acid and Metalliferous Drainage Management Plan
18
In summary, the key historic structures have a significant acid generation potential based on retained
(acid-forming secondary sulfo-salts) and potential acidity (sulfidic sulfur). However, the relatively slow
oxidation rates across the four key historic mineral waste storages suggest that appropriate management
and mitigation strategies are achievable to lower AMD risk to as low as reasonably practicable GHD 2018).
4.3. Pathway The three key pathways for potential transfer of contaminants from source to receptors are:
a) Sediment
b) Surface water (Old Decant Pond, Evaporation Ponds 1 and 2, Stormwater Pond, Pit Lake and drainage
lines) and
c) Groundwater
See the Updated CSM (pp. 117 – 128) of Appendix C for full details of contaminants in the AMD pathways.
4.3.1. Surface Water
The geochemical data suggests that contaminated runoff from the SWRD in Evaporation Ponds 1 and 2
has historically been discharged (via a discharge licence) into Mt Bundey Creek along the drainage line to
the north of Evaporation Pond 2. Visual evidence of perished discharge piping over the dam wall of
Evaporation Pond 2 would support this theory. Sufficient freeboard should therefore be retained in
Evaporation Ponds 1 and 2 to actively manage the risk of overtopping such that managed and licensed
release only may be undertaken. The flooding risk reported by GHD (2018a) should be considered when
determining appropriate sizing for the OWRD bund and any other structures that require re-sizing. In
particular, modelling by GHD (2018a) indicates that the Oxbow Wetland is expected to be inundated
during the 10-year average recurrence interval (ARI) flood event, with depths during the 100- year ARI
flood event exceeding three metres at some locations, albeit with relatively low velocities.
It is likely that saline and metalliferous acidic through flow and leachate from the OWRD is overtopping
the OWRD bund and discharging into Lake Bazzamundi during periods of extended rainfall and/or wet
season storm events. This is evidenced by acidity, sulfo-salts and environmentally deleterious elements
migrating down the drainage line between the OWRD bund, and into, Lake Bazzamundi as identified by
sediment and surface water data. The OWRD bund will therefore be increased in capacity to allow storage
volume for an appropriately risk-managed recurring storm event, consistent with appropriate water
management guidelines such as:
• Manual for assessing consequence categories and hydraulic performance of structures
(Queensland Department of Environment and Heritage 2016);and/or
• Structures which are dams or levees constructed as part of environmentally relevant activities
(Queensland Department of Environment and Heritage 2017).
Surface water and sediment shed from OWRD that is not overtopping the OWRD bund appears to be
managed reasonably effectively via the existing drainage line past TSF2 and into the Oxbow Wetland,
which is effectively acting as a retention basin. Sediment and surface water geochemical data has tracked
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the reducing contaminant concentrations down the engineered drainage line until dilution effects within
the Oxbow Wetland further reduce contaminant concentrations (GHD 2018).
Following previously licenced wet season discharge at LDP1 (SWTG12), surface water monitoring (PGO
2018) suggests that the mixing zone within Mt Bundey Creek immediately below LDP1 (at SWTG 12) is all
but complete by the time the water reaches the Arnhem Highway Bridge (SWTG2) – where all key analytes
are below SSTVs except copper (GHD 2018).
Aquatic ecology monitoring (GHD 2018c) indicates that the aquatic fauna take slightly further downstream
to recover, with species impacted beyond SWTG2, though fully recovered to, or better than, background
levels by monitoring point SWTG3 – located around 300m upstream of the confluence with Coulter Creek.
4.3.2. Groundwater
GHD (2018b) reported that contamination, of at least shallow groundwater, has occurred around the
SWRD and OWRD, Evaporation Ponds 1 and 2 and TSF1 and TSF2, based on monitoring exhibiting localised
areas of elevated sulfate or low pH and elevated metals. It is possible that contamination to the west (well
G8 that showed pH, aluminium, cadmium, copper and nickel outside ANZECC/ARMCANZ (2000) stock
trigger values) and north (wells OB10 and 11 that showed EC and sulfate outside ANZECC/ARMCANZ
(2000) stock trigger values) of the SWRD and Evaporation Pond 2 extends through shallow aquifers to Mt
Bundy Creek, approximately 130 to 300 m to the northwest. Based on current relative groundwater
elevations, the hydraulic gradient is inwards to the pit, which is likely to capture groundwater flow from
beneath the various mine–related contamination sources noted above.
4.4. Receptors
4.4.1. Mt Bundey Creek
Once surface water leaves site, it passes via SWTG2 on Mt Bundey Creek at the Arnhem Highway Bridge.
The median water quality data for this sample location indicates that it is entirely compliant with all SSTVs,
except copper. Further downstream of SWTG2 on Mt Bundey Creek is SWTG3. The median water quality
data for this sample location indicates that it is entirely compliant with all SSTVs, with the exception of
zinc (0.0039 mg/L against the SSTV of 0.0031 mg/L) and copper (0.002 as against the SSTV of 0.0018 mg/L).
The furthest surface water sampling location is SWTG16, located some 15 kms downstream in Hardy’s
Lagoon. Interestingly, water quality in Hardy’s Lagoon shows that copper (0.003 mg/L) and EC (42 μS/cm)
exceed SSTVs. This may indicate influences other than Tom’s Gully, given the lack of sulfate in the water
(1 mg/L), a value that is below the upstream baseline of 2 mg/L.
4.4.2. Lake Bazzamundi
The five sediment samples collected within Lake Bazzamundi (TGSED18 to 22 inclusive) returned pH1:5
values that ranged between 3.7 and 5.1, EC values between 142 and 2,130 μS/cm, and STOT NAPP values
between 1.5 and 14.1 kgH2SO4/t. These data would infer overtopping of the bund during wet season with
AMD water containing sulfo-salts finding its way down the drainage path into Lake Bazzamundi. Water
quality in Lake Bazzamundi as per median historic data at SWTG5 shows a median pH value of 5.7, with
aluminium, cadmium, cobalt, copper, nickel, uranium and zinc exceeding their respective SSTVs. Median
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sulfate (64 mg/L, acidity (7 mg/L CaCO3 equivalents) and EC (170 μS/cm) are approaching background
concentrations, likely as a function of dilution in the Lake Bazzamundi wetland.
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Figure 3: Conceptual Site Model at Toms Gully (GHD 2019)
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5. Acid Mine Drainage Model and Balance Based on the TGU Projects conceptual site model developed by GHD (2018), a mass balance of acid,
expressed in mass of CaCO3 that may be used to treat the water to neutralise the acidity was developed.
GHD (2018) identified the significant sources of potential acid generation as the sulfide waste rock dump,
oxide waste rock dump, TSF1 and TSF2 and estimated net acid generation. The ROM stockpile,
metallurgical tailings and waste rock were also identified as an acid and metalliferous drainage risk, but
as the risk was deemed lower it was not quantified in terms of net acid generation. The undisturbed
catchments and in situ rock were deemed as a neutral and not a significant source of acid generation. For
the purpose of the acid mine drainage model, the neutralising potential of sediment in surface water
storage was ignored.
The net acid generation of the different site land uses are summarised in Table 3. A nominal net acid
generation rate of 0.1 kg CaCO3/ tonne/year was adopted for the disturbed hardstand (predominately
the ROM stockpile) and pit areas. It was assumed that any new tailings produced would be lower risk than
the existing tailings.
The acid mine drainage model does not consider the any potential of seepage of water from WRDs and
surface water storages that may report offsite. The actual flow rates of seepage are difficult to quantify
at this stage, but are considered minor and of lower risk than any potential discharge of water directly
from surface water storages
Table 3: Net Acid Generation (GHD 2018d)
Land Use Net acid generation (kg CaCO3/tonne/year)
TSF1 34
TSF2 3.0
Sulfide WRD 0.4
Oxide WRD 0.1
Hardstand 0.1
Pit 0.1
Undisturbed 0.0
The forecast average annual acid balance for Toms Gully mine is summarised in Table 4 The potential acid
mine drainage is expressed in terms of mass of CaCO3 that may be used to treat the water to neutralise
the acidity.
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Table 4: Average Annual Acid Mine Drainage Balance (GHD 2018d)
Acid flux Year ending June 2019
(tonne CaCO3)
Year ending June 2020
(tonne CaCO3)
Year ending June 2021
(tonne CaCO3)
Year ending June 2022
(tonne CaCO3)
Year ending June 2023
(tonne CaCO3)
Inputs
Direct rainfall onto storages
0 0 0 0 0
Catchment runoff
419 499 507 512 487
Groundwater inflows
0 0 0 0 0
ROM ore moisture
0 0 0 0 0
Total inputs 419 499 507 512 487
Outputs
Evaporation 0 0 0 0 0
Uncontrolled off site discharge
1 1 1 1 2
Discharge from New WSD (or supply to third party)
5 9 11 12 13
Seepage losses 22 19 20 20 23
Dust suppression losses
0 0 0 0 0
Tailings moisture losses
152 297 291 277 61
Treated by WTP 693 208 200 198 218
Total outputs 873 534 524 509 317
Change in Storage
Surface water storages
-454 -36 -17 3 169
Total change in storage
-454 -36 -17 3 169
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6. AMD Management PGO recognises that planning for closure is a fundamental component of mine planning (INAP2009, DTIR
2007). Therefore, identifying any PAF material either historical or within the context of future mine plans
and schedules is essential for effective management. Therefore PGO has developed AMD design and
operational controls to minimise the potential risks.
PGOs overall AMD strategy has been aligned to leading standard practice as per the following:
• subaqueous tailings / waste rock deposition – INAP 2009 Section 6.6.7; DITR 2007 Section 7.1.6;
DERM 1995 Section 7; DITR 2006, Appendix A; various MEND reports at: http://mend-
nedem.org/category/prevention-and-control/water-covers/
• Store-release TSF cover design - INAP 2009 Section 6.6.6, DITR 2007 Section 7.1.4, NT EPA 2013
Section 8; DERM 1995 Section 7; DITR 2006, Appendix A; various MEND reports at: http://mend-
nedem.org/category/prevention-and-control/dry-covers/
• Subterranean PAF waste rock storage – DITR 2006, Appendix A
• Drainage controls – DITR 2007 Section 7.1.1, DRET 2008
• Ongoing monitoring – INAP 2009, Chapter 8; DITR 2007, Chapter 8; NT EPA 2013 Section 9; DERM
1995 Section 7.2; various MEND reports at: http://mend-nedem.org/category/monitoring-
category
• Acid Mine Drainage – Environmental Notes on Mining, updated September 2009 (DMP 2009)
AMD Management for the TGU Project can be divided into two key strategies:
1. Managing existing AMD sources (WRDs, TSF, RoM Pad and Stockpiles, Evaporation Ponds and Pit)
2. Removal of existing AMD sources (TSF1 and 2)
3. Managing Proposed AMD sources (waste from underground mining, tailings and ore stockpiles)
6 . 1 . AMD Risk Assessment As part of the TGU Project’s NT EPA EIS Supplement, a risk assessment framework was developed for the
entire project. This included undertaking a risk matrix for potential AMD risks on site. See the EIS
Supplement for the comprehensive risk framework document and assessment. Risks associated with AMD
were taken from the Project Risk Framework and are summarized and addressed in Table 5
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Table 5: AMD Risk, potential impact and management / mitigation control
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Risk Potential Impact Mitigation / Management Control Timing Performance Indicator
Failure / overtopping
of TSF1 and/or TSF2
leading to
uncontrolled release
of tailings material
(Prior to tailings
removal)
Contamination of Mt
Bundey Creek and
downstream ecosystems
• Ensure appropriate freeboard in TSFs
at all times (weekly inspections during
operations)
• Tailings placement in the pit under a
water layer.
• Groundwater monitoring (to monitor
seepage)
• Implementation of AMDMP
• Implementation of Water
Management Plan (WMP)
• If re-processing of tailings material
occurs this will reduce the acid
producing profile of the tailings and
volumes of material
• Water treatment plant on site to treat
water for AMD
Design
Operations
Closure
• Removal of tailings
• Surface water
monitoring data
within Site Specific
Trigger Values (SSTVs)
• No incidents of
overtopping
Seepage from TSF1
or TSF2
Contamination of
surface waters and
groundwater quality and
downstream ecosystems
• Repurposing TSF1 (sediment basin)
and TSF2 (water dam) will address
existing seepage
• Engineered design
• Weekly Inspections
• Tailings removal from TSFs
• Treatment of water for AMD
• Groundwater monitoring bores
Design
Operations
Closure
Groundwater monitoring
data within SSTVs or
water quality
requirements
Runoff or seepage
from existing WRDs
Contamination of
surface water and
• Geotechnical inspection
• Continued use and management of
evaporation ponds
Design
Operations
Closure
Water monitoring data
within SSTVs
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groundwater quality and
ecosystems
• Improvement of site drainage
(increase capacity of bunds and
ensure integrity of all drains leading to
evaporation ponds from WRDs)
• Use of suitable boxcut waste rock for
capping.
• Investigation and consideration of
long term treatment and associated
closure options for WRDs (e.g. capping
of WRDs)
• Water treatment plant onsite
• Groundwater monitoring bores
WRD investigation
program
Seepage from
evaporation ponds
Contamination of
groundwater
• Treat water ex-pit and use ponds for
short term storage
• Manage water inventory (water
balance across site)
• Containment and capture of
contaminated water and treatment
(via proposed water treatment plant)
• Ongoing identification of all sources of
contaminated water
• Surface and groundwater monitoring
with associated bores
Design
Operations
Closure
Water monitoring data
within SSTVs
Inappropriate
storage and disposal
of proposed waste
rock
• Contamination of
surface water and
groundwater
systems
• Baseline waste characterisation work
completed including the Boxcut
material.
Design
Operations
Monitoring of waste rock
movement and
positioning in pit.
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• Storage outside of
footprint
• Waste rock deposited in base of pit
under a water covered during
operation and at closure
• Boxcut material suitable for capping
waste rock dumps
• Implementation of AMDMP
• Implementation of WMP
• On-going and weekly inspections of
project areas
• Assume all underground waste rock is
PAF
Geochemical sampling of
waste rock to identify
changes in chemistry.
Waste rock positioned
below water level.
Water monitoring data
within SSTVs
Indiscriminate use of
existing waste rock
for construction
• AMD leading to
contamination of
surface water and
groundwater
systems
• Storage outside of
footprint or
structure failure
• No disturbance to WRDs
• Implementation of AMDMP
• Implementation of WMP
• On-going and regular inspections of
project areas
• Assume all waste rock is PAF
Design
Operations
Monitoring of waste rock
movement and
positioning
Water monitoring data
within SSTVs
No disturbance to WRDs
Inappropriate
storage of ore on
ROM Pad or
elsewhere
AMD leading to
contamination of
surface water and
groundwater systems
• Implementation of AMD Management
Plan
• Maintenance and upgrade (if
necessary) of drainage controls and
surface drainage contours
• Operating procedures and mine
schedules
• Reprocessing of existing ore stockpiles
Operations Water monitoring data
within SSTVs
All ore stockpiles re-
processed
Monitoring of surface
water flow during rain
events.
Monitoring of ore pile
positioning on RoM pad
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Delay in attaining full
submergence of PAF
waste rock in pit
AMD leading to
contamination of
groundwater systems
• Where practical waste rock positioned
in the lowest parts of the pit with the
stacker.
Closure Submergence of PAF
underground waste
material within 48 hours
Overtopping of pit
containing
submerged PAF
material and water
level fluctuations
Mixing of pit water with
surface water leading to
potential AMD products
released
• Investigate the establishment of an
insitu sulfate reducing bacteria system
thus reducing potential AMD
formation.
• Complete modelling to understand
the final water level height of the pit.
Closure Ensure 10 m of permanent
water cover
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6.2. AMD Management Strategy As noted in the sections above, all waste rock and tailings generated on site will be treated as PAF and the
management is twofold; managing the existing AMD sources and minimizing the risk of proposed AMD
sources (waste rock, tailings and ore).
6.3. Controls As part of the AMD management strategy and minimizing the risk and volumes of AMD on site a number
of operation controls have been designed.
6.3.1. Ore and Waste Rock
• All waste rock is to be disposed in the pit.
• No future waste rock is to be deposited beyond the pit perimeter (Figure 4).
• No future waste rock is to be used for construction purposes, other than the DHW unit.
• No existing waste rock within the WRD will be used for construction purposes.
• Boxcut material will be visually monitored during excavation for the presence of sulfides. If
sulfides are identified material to be placed in the pit (under a water cover) to prevent acid
forming material being positioned on the waste rock dumps.
• The existing sulfide and oxide waste rock dumps are to be maintained to ensure their integrity.
• Waste rock will only be stored underground and/or on the ROM Pad prior to placement in the pit.
• Ore will only be stored underground and/or on the ROM Pad.
• During operations, waste rock will be placed flooded in the pit to minimize oxygen availability to
PAF waste rock. The water will work as an oxygen barrier. The dissolved oxygen concentration in
water is 8.6mg/L at 25ºC, which is approximately 25000 times lower than in the air. Organic
matter and other reduced compounds can rapidly consume the dissolved oxygen in the water,
which is then not available for sulfide oxidation (DMP 2009).
A design review is to be completed with each annual Mine Management Plan (MMP) that verifies the
AMD standards are being implemented for the MMP term.
From the baseline geochemical work it is estimated that the lag time for the breakdown of sulfides to
generate acid mine drainage is between 6 to 12 months (GHD 2018). Taking into consideration the lag
time of the PAF materials and to limit the egress of oxygen and water the following measures have been
adopted during operations:
• Placement of waste rock in the pit within 48 hours of excavating the material.
• Prior to pit emplacement position all waste rock on only the RoM or process area.
• Monitoring the pit water and if required adjust water quality by the use of lime, caustic or virtual
curtain.
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Figure 4: Location of waste material to be placed in the flooded pit
At an operation level suitable locations for the underground placement of waste rock are to be
determined by the Mining Engineer in consultation with the Mine Manager. On a daily basis the
positioning of waste rock in the pit will be communicated to the stacker operator for implementation.
During operations surveying and sampling will occur to assess the depth of the material in the pit and acid
producing potential of the material this information will be used to inform the closure strategy for the pit.
6.3.2. Tailings
Design and operational controls for the tailings include:
• It is proposed tailings from TSF1 and TSF2 will be placed in the flooded pit (whether reprocessed or
not) within 18 months from the commencement of the Boxcut.
• All future tails will be placed in the already flooded pit.
• During operations, tailings will be placed in the flooded pit to minimize oxygen availability to PAF tails.
The water will work as an oxygen barrier. The dissolved oxygen concentration in water is 8.6mg/L at
25ºC, which is approximately 25000 times lower than in the air. Organic matter and other reduced
compounds can rapidly consume the dissolved oxygen in the water, which is then not available for
sulfide oxidation (DMP 2009).
• Deposition of tailings in the pit using a floating head to discharge tails 10m below the water surface.
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• Tailings systematically discharged across the pit to create an even tails surface beneath the water
Figure 5.
• Tailings to be deposited with a minimum water cover of 25m.
• All metallurgical tailings to be treated as PAF and placed in the pit.
• Implementation of this AMD Management Plan and AMD site sampling procedure (refer to Appendix
S).
• Implementation of the TGU Water Management Plan that includes the AMD water monitoring
analytes.
• Implementation of an operating manual / procedure for tailings deposition.
• Maintaining a minimum freeboard across the pit via the treating of water displaced out of the pit.
• Weekly inspections of pit deposition during operations for freeboard.
• Surveying of tailings in the pit to assess tailings levels and evenness of distribution.
Figure 5: Systematic Floating Head Traverses to Deposit Tails.
6.4. Site Drainage and Controls As the OWRD and SWRD can generate contaminated water from runoff and seepage, therefore,
appropriately designed and operated water management structures are required. As outlined in the TGU
Water Management Plan, the design and operational level controls in place to manage water on site are
as follows:
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• A series of run-on diversion bunds and run-off drains manage water on site to minimise the risk of
clean water being cross-contaminated by contaminated water (Figure 6). In addition bunds including
the OWRD will be upgraded to prevent overtopping.
• Mine affected water is diverted to stormwater ponds and non-mine affected water is transferred
through the site via the bunds and run off drains.
• Water captured within stormwater ponds is assessed through the established network of surface
water monitoring sites at TGU that would remain under the TGU Water Management Plan (refer to
Figures 7 - 8) and treated for discharge from site at locations approved under the required legislation.
The following operational controls will apply:
• The TSF water circuit is designed to be closed during operations
• Drainage from the ROM pad shall be directed to the Stormwater Sump which shall be managed to
prevent overflow
• Water from the Stormwater Sump shall not be released directly to the environment. It shall be utilised
in the process as first priority or treated.
• Details of routine operational and emergency water transfers are shown in the TGU Water
Management Plan.
• Monitor rainfall conditions and water levels across the site with the water treatment plant operated
to manage peak water periods to maintain constant water volumes across site
• Mine affected water will be treated using the Bioaqua Process (or contingency option) in the water
treatment plant and placed in the proposed water storage dam before being discharged or transferred
to a third party for agricultural use.
Controls requiring immediate action:
• The OWRD bund should therefore be increased in capacity to allow storage volume for an
appropriately risk-managed recurring storm event, consistent with appropriate water management
guidelines (GHD 2018)
• Runoff from the SWRD in Evaporation Ponds 1 and 2 has been discharged as per the discharge licence
into Mt Bundey Creek along the drainage line to the north of Evaporation Pond 2. Visual evidence of
perished discharge piping over the dam wall of Evaporation Pond 2 would support this theory.
Sufficient freeboard should therefore be retained in Evaporation Ponds 1 and 2 to actively manage
the risk of overtopping such that managed and licensed seasonal release only may be undertaken
(GHD 2018).
The catchment areas of each water management feature were delineated based on topographic
information. The land use of site, for the purpose of the site water balance, was delineated based on aerial
imagery and site observations. The catchment area and land use distributions for each water management
feature are summarised in Table 6.
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Table 6: Water management infrastructure and their catchment areas (GHD 2019)
Catchment Hardstand (ha)
Oxide WRD (ha)
Pit (ha)
Sulphide WRD (ha)
TSF (ha)
Undisturbed (ha)
EP1 - - - 15.0 - -
EP2 - - - 10.6 - -
New WSD - - - - 10.5 -
Drainage Bund - - - 2.4 - 15.7
PWP - 22.5 - - - -
Stormwater Pond 0.5 - - - - -
Toms Gully Open Pit
10.4 - - - - 6.7
TSF1 - - 32.3 1.6 - -
TSF1 decant pond - - - 1.1 6.7 1.4
TSF2 - - - 1.1 2.1 0.1
New TSF - - - - 8.7 -
The capacity of surface water storages and the maximum surface areas were provided by Primary Gold.
Compared to the previous water balance report (Coffey 2015), the design of the New WSD has been
reduced to suit the revised water management system and its offline configuration. The geometric
properties of the surface water storages are summarised in Table 7.
Table 7: Water Storages at TGU Project (2019)
Water management feature
Capacity (ML) Spill level (m RL)
Maximum inundated area (Ha)
Shape factor
EP1 346 1029.35 4.67 5
EP2 354 1025.76 4.79 5
New WSD 1000 (once constructed)
Unknown 16.0 2
Drainage bund 5.0 Unknown 7.4 2
PWP 1.4 Unknown 0.03 3
Stormwater pond 12.5 Unknown 0.6 2
Toms Gully Open Pit 4660 1019.0 9.0 3
TSF1 (including decant pond)
135.2 Unknown 7.4 5
TSF2 408.9 (once upgraded)
1026.5 8.7 (once upgraded) 5
New TSF Unknown Unknown 9.0 5
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Figure 6: Site Drainage
Toms Gully Underground Project Acid and Metalliferous Drainage Management Plan
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Figure 7: Surface Water Monitoring Locations
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37
Figure 8: Surface Water Monitoring Locations Continued
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38
6.4.1. Maintenance of existing structures
PGO recognises that the integrity of the existing structures on site needs to be maintained throughout
operations and into closure to minimise the risk of AMD. The following maintenance and monitoring is
proposed:
• Weekly visual inspections of drainage and sediment control
• Sufficient freeboard should be retained in Evaporation Ponds 1 and 2 to actively manage the risk of
overtopping such that managed and licensed seasonal
• Sufficient freeboard should be retained for TSF 1 and 2 to actively manage the risk of overtopping
while tailings are in place and if TSF 1 and 2 are repurposed.
• The OWRD bund should be increased in capacity to allow storage volume for an appropriately risk-
managed recurring storm event
• Ameliorate any eroded areas identified during visual inspection on run on and run off bunds, TSFs,
evaporation ponds and the sulfide and oxide WRDs
• Clean drainage lines surrounding key site infrastructure of observable secondary salts at the end of
dry season to minimise the risk of a contaminated ‘first flush’ event. Secondary salts would be treated
as PAF for management purposes and placed into the pit.
• Identify any areas of water ingress on the oxide and sulfide WRDs and ameliorate as necessary
• Monitor surface and groundwater as described in Section 6 and as per the TGU Water Management
Plan.
7. AMD Monitoring AMD monitoring provides feedback to confirm that the design and operational controls are effective for
their stated aim. In that regard, the following will be monitored:
• Tailings and waste rock (including ore) to validate the existing geochemical classifications and to
provide an historic inventory for site archives and legacy management
• Sources and use of construction materials
• Water (surface water and groundwater) (Figure 7 and 8)
PGO will utilise an adaptive management approach to meet the SSTVs developed for the site (refer to the
TGU Water Management Plan). The concept of adaptive management is a structured, iterative approach
to decision making with the ability to gradually reduce uncertainty over time through monitoring and
adapting to environmental, economic and social changes. In circumstances where potential impacts
cannot be entirely avoided, the adaptive management approach allows for an evaluation of the preferred
mitigation controls which can then be progressively improved and refined. This approach is particularly
relevant to the longer term management strategies for AMD sources such as the WRDs.
The process of adaptive management is shown in Figure 9. Specifically, the AMD monitoring program
would aim to:
Toms Gully Underground Project Acid and Metalliferous Drainage Management Plan
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• detect environmental change and, specifically, identify those changes resulting from the Project
• determine actual versus predicted change
• contribute to the assessment of the effectiveness of environmental management procedures
• provide data for the assessment of adherence to the environmental management plan, approval and
licence conditions
The AMD monitoring program would be reviewed annually and modified to assure continued
appropriateness. Reviews would consider the frequency and duration of monitoring and evaluate the
ongoing need for individual programs. Records of all monitoring activities would be retained to facilitate
auditing.
The following section provides an overview of the strategy and rationale, with Appendix A (providing the
detail for the in-situ material validation sampling and analysis. Note that the surface and groundwater
monitoring is wholly captured by the TGU Water Management Plan; therefore, it has not been reproduced
here.
Geochemical sampling and analysis on waste rock and tailings materials are to be undertaken using visual
and analytical methods. Each method is explained below.
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Figure 9: The process of adaptive management
7.1. Geochemical Monitoring As the strategy adopted by PGO assumes that all ore and waste rock is PAF, the geochemical monitoring
program is for the purpose of maintaining an inventory of waste types to allow records to be maintained
for development of the TGU components of the site. The data will help facilitate closure and legacy
management strategies, plans and monitoring.
7.1.1. Visual Methods
The Site Manager or delegate will undertake weekly inspections of PAF management, PAF deposition and
water management structures to ensure their integrity. The Site Manager or delegate will also inspect to
ensure that no PAF material has been won from the engineered landforms for use in construction.
Records will be kept and photographic evidence of any management inconsistencies and structural
integrity failing captured, with the Mine Manager notified for action. Examples include evidence of
erosion and sediment transport downslope after a storm event, poorly maintained sediment traps, or a
ruptured run on bund. This is applicable to historic and future mineral waste and water management
structures.
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On-site visual monitoring will be undertaken down-catchment of the toe of the WRDs, TSFs and ROM Pad
/ Process area, in addition to downstream of the WRDs, ROM Pad and TSF runoff dams. PGO Staff will be
alert to the formation of secondary salts on waste rock surfaces and in drainage lines, particularly in dry
season. The sources of the salts will be investigated to assist in determining future amelioration strategies
for the WRDs and site remediation for the ROM pad and TSFs.
Such salts are readily dissolved into solution during ‘first flush’ rain events and will compromise water
quality. In wet season, staff should note any discolouration of drainage water, particularly red-brown
(sometimes Fe, sometimes tannins), clear (often dissolved Al due to acidic pH conditions), or blue-green
(often dissolved Cu) rich discharge. The results of the visual inspections will be documented and
communicated to the Site Manager.
Excess sedimentation in flow channels or sumps may also be indicative of active erosion and material
transport and implies a lack of integrity of up-slope engineered management structures – which would be
investigated with remedial actions undertaken as required for stabilisation.
Material removed from drainage lines or sumps that are cleaned at the end of dry season will be managed
as PAF material and placed in the pit.
The supervising geologist will undertake visual inspection of development material (ore/waste) and
construction material collected for geochemical sampling according to Appendix A and note the presence
of any visual sulfidic material (particularly pyrite and/or arsenopyrite). This inspection will be documented
with the laboratory results cross referenced to the visual sample for data quality control and inventory,
and appropriate emplacement of the mineral waste material.
7.1.2. Laboratory Analysis
Following visual inspection, a subset of the development waste and tailings samples will be forwarded to
a NATA accredited laboratory to be analysed for:
• acid base accounting; and
• metals.
Details of this laboratory based analytical program are provided in Appendix A. The data will have two
purposes:
• supplementing the visual data generated; and
• establishing quantitative data for inventory and legacy management purposes.
The sampling and analysis frequency is provided in Table 8. The sample numbers in Table 8 are based on
an industry accepted formula provided in Equation 1 below (Price, 1997).
Equation 1: n = 25 * √x.
(Where x = Million tonnes (MT) of material per major lithological unit).
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Therefore, based on Equation 1, a rule of thumb for representative sampling is around 25 samples per
million tonnes of material - per major lithology. As all waste rock will be stored together underground or
in pit and each individual waste rock unit has been geochemically assessed, the ‘lithological’ units for
waste rock and tailings management purposes become simply ‘ waste rock’ and ‘tailings’.
Waste rock and tailings will be sampled and analysed approximately every month. The sampling frequency
will be reviewed upon the mine schedule and amended as required.
Table 8: Sampling Frequency
Mineral waste unit
Approx. life of mine tonnes
Approx. sample number required
Assumed mine life
Approximate sampling frequency
Waste Rock 1,509,194 48 48 ~1 per month
Tailings 884,103 48 48 ~1 every 40 days
7.2. Surface Water Monitoring The locations, sampling procedures, schedule and analytes for AMD surface water monitoring are entirely
consistent with the TGU Water Management Plan and are therefore not reproduced here. Analytes with
specific reference to AMD monitoring include pH, EC, acidity and alkalinity, sulfate and metals. Baseline
water quality data is also included in the TGU Water Management Plan.
Decreasing alkalinity is generally a good early indicator of deteriorating conditions in leachate from a WRD
containing PAF material, and can therefore be tracked as an ‘early warning’ mechanism.
Metals concentrations and declining pH values generally lag behind declining alkalinity; therefore,
corrective actions can be implemented early should alkalinity decline in the pit.
Other trends that highlight the onset of AMD include increasing sulfate, increasing sulfate / alkalinity ratio,
decreasing pH values and an increase in soluble metals as a result. Given that Toms Gully is a brownfields
site with known on site AMD water chemistry, the focus will be on improving trends as best as is
practicable through operations and ensuring offsite water chemistry compliance is maintained.
Given the development strategy adopted by PGO, the risks of the TGU Project creating new AMD impacts
on surface water quality are greatly reduced. Any deterioration in surface water quality will be
investigated to determine the source. Adherence to SSTVs and appropriate actions will be undertaken.
7.3. Groundwater Monitoring The locations, sampling procedures and schedule and analytes for groundwater monitoring are entirely
consistent with the TGU Water Management Plan. Analytes with specific reference to AMD monitoring
include pH, EC, acidity and alkalinity, sulfate and metals. Baseline groundwater quality data is also
included in the TGU Water Management Plan
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GHD (2018) concluded that contamination, of at least shallow groundwater, has occurred around the
SWRD and OWRD, Evaporation Ponds 1 and 2 and TSF1 and TSF2, based on monitoring exhibiting localised
areas of elevated sulfate or low pH and elevated metals. It is possible that contamination to the west and
north of the SWRD and Evaporation Pond 2 extends through shallow aquifers to Mt Bundy Creek,
approximately 130 to 300 m to the northwest. Based on current relative groundwater elevations, the
hydraulic gradient is inwards to the pit, which is likely to capture groundwater flow from beneath the
various mine–related contamination sources noted above. Therefore, the bulk of groundwater
contaminants leaching from point sources on site is reporting to the pit lake. The existing groundwater
monitoring network will be maintained with additional bores added as per recommendations from the
groundwater study (GHD 2018b).
8. Contingency Planning
8.1. Overview PGO will develop contingency plans for those failure modes where residual risk remains after the
application of AMD prevention and control approaches. A contingency plan will include targeted
monitoring, trigger levels for actions, and specific responses in case a certain event occurs. For example,
if a failure mode is the potential for increased AMD seepage from the pit, then monitoring can be
established for changes in seepage sulfate concentrations and/or acidity as an early indicator of potential
ARD formation. If significant increases in sulfate concentrations are measured, then contingency
measures such as additional drainage collection will be implemented.
PGO will develop contingency plans specific to AMD management at TGU which will include an
exceedance in the surface water monitoring results against site-specific trigger values at DP1 and DP2,
and at SWTG2. The approach will be to undertake a ‘root cause’ analysis whereby the causal link for the
water chemistry exceedance would be determined. Adaptive management would then seek to implement
an appropriate alternate management strategy to eliminate any future risk of a repeat, given the nature
of the incident.
Future revisions of this document will inform forward AMD risk management by providing feedback based
on additional water chemistry and geochemical monitoring to inform AMD risk, and therefore, any
adjusted management strategy.
8.2. Specific Measures
8.2.1. Tailings Management
Contingency measures for tailings management include the use of flocculants to enhance settlement.
8.2.2. Waste Rock
Controls for waste rock management include waste rock to be preferentially stored underground and
disposed in the pit. If parts of the Boxcut waste rock are unsuitable both geochemically or geotechnically
for waste rock dump capping this material will be placed within the pit beneath the water blanket.
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8.2.3. ROM Pad / Ore
Contingency measures for managing / stockpiling ore is for it to be stored temporarily underground for
AMD control if there is insufficient capacity on the ROM pad. This scenario is not anticipated.
8.2.4. Water Management
In the event of an emergency situation where freeboard is not maintained, water would be pumped
between facilities taking into consideration water chemistry. At the same time water treatment would be
ramped up to treat the excess water.
Should there be an emergency situation with the ROM pad stormwater sump with all contained water not
being able to be reused in the process; the water would be pumped to the evaporation ponds.
In the event of an emergency situation where the evaporation ponds exceeding their design capacity,
water would be treated and pumped to the Water Supply Dam.
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9. Roles, Responsibilities and Training The roles and responsibilities for the implementation of the AMD Management Plan are outlined in Table
9 below. This will be communicated to all relevant personnel during operations.
Table 9: Roles and Responsibilities
Task Responsible Accountable Consulted Informed
Implementation of, and compliance to AMDMP
General Manager Operations
Mine Manager Environment Manager
Geology Manager Managing Director Superintendent Mine Geology Senior Mine Geologists Environment Personnel Senior Mining Engineers Mining Contractors
Ongoing mineral waste characterisation and management
Mine Manager Geology Manager Environment Manager
Senior Mine Geologists
General Manager Operations
Ongoing water monitoring
Environment Manager
Environment Personnel
Mine Manager
General Manager Operations
Review and refine Rehabilitation Completion Criteria
Environment Manager
Environment Personnel
Geology Manager Mine Manager
General Manager Operations Environment Personnel Senior Mining Engineers Mining Contractors
9.1. Awareness, Training and Competence All senior geology, mining, processing and environmental personnel will have an understanding of AMD
through a site induction. All operational staff entering site, including contractors, are to be made aware
of the AMD Management Plan and the Water Monitoring Plan through a site induction, which would
include PAF material management.
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9.2. Records, Reporting and Document Control Records that are required to be held for this AMD Management Plan include:
• laboratory geochemical analytical results and geochemical monitoring reports
• tailings operating manual
• research and development reports (e.g. for long term management of WRDs)
• An inventory of all mineral waste placement which includes the following:
o quantities and nature of mineral waste located in specific areas within the pit and/or
underground
o the nature of emplacement
o quality control data as applicable
o materials that may be re-used at a later date, such as topsoil, NAF, AC, etc.
Records shall be maintained in accordance with PGO corporate policies and procedures, with all records
maintained into perpetuity to inform future site risk management.
Reporting would be undertaken consistent with approval requirements, and would include:
• geochemical data on mineral waste: to be included in annual updates of the AMD Management Plan
as an appendix to the Mining Management Plan under the NT Mining Management Act
• water monitoring data: to be included in a specific annual water report as required under the NT
Mining Management Act.
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10. References Australian National Committee on Large Dams Incorporated (ANCOLD) 2012. Guidelines on Tailings Dam:
Planning, Design, Construction, Operation and Closure
AMIRA (2002). ARD Test Handbook. Project P387A Prediction and kinetic control of acid mine drainage.
ANZECC/ARMCANZ (2000). Australian and New Zealand Guidelines for Fresh and Marine Water Quality.
Australia and New Zealand Environment and Conservation Council and Agriculture and Resource
Management Council of Australia and New Zealand, Canberra.
ANZECC/ARMCANZ (2000a). Australian Guidelines for Water Quality Monitoring and Reporting. Australia
and New Zealand Environment and Conservation Council and Agriculture and Resource Management
Council of Australia and New Zealand, Canberra.
Bowen H. J. M. (1979). Environmental Chemistry of the Elements. Academic Press, New York.
Commonwealth Government - Department of Industry, Tourism and Resources (2006). Leading Practice
Sustainable Development Program for the Mining Industry: Mine Closure and Completion. Canberra
Commonwealth Government - Department of Industry, Tourism and Resources (2007). Leading Practice
Sustainable Development Program for the Mining Industry: Managing Acid and Metalliferous Drainage.
Canberra
Commonwealth Government - Department of Resources, Energy and Tourism (2008). Leading Practice
Sustainable Development Program for the Mining Industry: Water Management. Canberra
Department of Mines and Petroleum (2009). Acid Mine Drainage – Environmental Notes on Mining,
updated September 2009. Government of Western Australia Available from:
http://www.dmp.wa.gov.au/Documents/Environment/ENV-MEB-220.pdf
GHD (2015). Toms Gully Mine Preliminary AMD Assessment and Waste Rock Classification. May 2015.
GHD (2015a). Toms Gully Project: Site Specific Trigger Values. April 2015.
GHD (2015b). TGU Underground Water Management Plan.
GHD (2018). Primary Gold Tom’s Gully Gold Project: Geochemical baseline and conceptual site model.
Report for Primary Gold Ltd.
GHD (2018a). Flooding Memorandum. Prepared for Primary Gold. Report for Primary Gold Ltd.
GHD (2018b). Tom’s Gully EIS – Baseline Studies. Groundwater Assessment and Modelling. Report for
Primary Gold Ltd.
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GHD (2018c). Tom’s Gully EIS – Baseline Studies. Aquatic Ecology Monitoring 2017. Report for Primary
Gold Ltd.
GHD (2018d). Tom’s Gully Mine Site Water Balance. Report for Primary Gold Ltd.
INAP (The International Network for Acid Prevention) (2009). Global Acid Rock Drainage Guide. Available
at www.gardguide.com.
Minesite Environmental Neutral Drainage (MEND). Various reports available at: http://mendnedem.
org/default/
Miller S. D. (1996). Advances in acid drainage: Prediction and implications for risk management. In:
Proceedings of the Third International and the 21st annual Minerals Council of Australia Environmental
Workshop, Newcastle, NSW. pp. 149-157.
NT EPA (2014). Draft Toms Gully Underground EIS Terms of Reference.
Price, W.A., (1997). Draft Guidelines and Recommended Methods for the Prediction of Metal Leaching and
Acid Rock Drainage at Mine sites in British Columbia. BC Ministry of Employment and Investment.
Primary Gold (PGOO) (2013). Primary Gold increases Tom’s Gully resource by 96% to 275,000 oz. ASX
release dated 23 April 2013.
Primary Gold (2015). Draft Toms Gully Underground Project Description.
Primary Gold (2015a). Unpublished water quality data.
PGO (2018). Routine surface water quality monitoring data December 2016 to March 2018. Provided by
PGO in March 2018.
Sener A.K. (2004). Characteristics, distribution and timing of gold mineralisation in the Pine Creek Orogen, Northern Territory, Australia. Unpublished PhD thesis, UWA.
Stewart W.A., Miller S.D. and Smart R. (2006). Advances in acid rock drainage (ARD) characterisation of
mine wastes. Proceedings of the 7th International Conference on Acid Rock Drainage (ICARD). Barnhisel
R.I (ed.), St Louis, America.
Queensland Department of Environment and Heritage (2016). Manual for assessing consequence
categories and hydraulic performance of structures.
Queensland Department of Environment and Heritage (2017). Structures which are dams or levees
constructed as part of environmentally relevant activities.
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Appendix A: Site Geochemical Sampling Procedure
This has been placed in Appendix S of this EIS Addendum to the Supplement