This document has been produced for New Zealand consenting purposes only. Information contained herein must not be relied on for investment purposes. OceanaGold (New Zealand) Limited WAIHI NORTH PROJECT Resource Consent Application and Assessment of Environmental Effects June 2022
Waihi North Project – Overview of Application Structure i APPLICATION STRUCTURE This application document is in support of applications by OceanaGold (New Zealand) Limited (“OGNZL”) for resource consents from Waikato Regional Council (“WRC”) and Hauraki District Council (“HDC”) under the Resource Management Act 1991 (“RMA”) to authorise all necessary activities associated with the Waihi North Project (“WNP”). All matters required to be addressed in accordance with Schedule 4 of the RMA are contained within. This application comprises 11 parts as follows: Part A Is an overarching Assessment of Environmental Effects (“Overarching AEE”) document which provides an overview of: The WNP; OGNZL; The approvals required for the WNP; Tangata whenua of the area in which WNP would occur and how the various tangata whenua groups are feeding into the WNP and this resource consent application process; The consultation OGNZL has undertaken in respect of the WNP; An overview of the effects of allowing the WNP to proceed; An overview of the proposed conditions for the WNP; and An overview of the RMA context in which the WNP applications are to be considered. Part B Contains a more detailed site-specific assessment of environmental effects (“AEE”) for each of the geographically distinct specific project areas which comprise the WNP and includes: B1: Coromandel Forest Park and Area 1 Sub AEE B2: Area 2 Sub AEE B3: Area 3 Sub AEE B4: Area 4 Sub AEE B5: Area 5 Sub AEE B6: Area 6 Sub AEE
Waihi North Project – Overview of Application Structure ii B7: Area 7 Sub AEE Part C Contains the application forms for the RMA resource consents required from WRC and HDC for the WNP. Part D Is a graphic supplement of technical drawings for key project components. Part E Contains a suite of proposed consent conditions for the WNP. Part F Is a report on the existing mining activities at Waihi and how they are authorised under the RMA. Part G Contains a report on the consultation and engagement undertaken in respect of the WNP. Part H Contains 46 reports prepared by technical specialists on the WNP and its effects. Part I Contains a suite of draft management plans. Part J Contains a checklist of statutory provisions that have been considered in preparing this AEE. Part K Contains all relevant Records of Title.
H PART H OceanaGold (New Zealand) Limited Waihi North Project SUPPORTING TECHNICAL ASSESSMENTS
Part H – Technical Assessments 1 Part H: Supporting Technical Assessments Report # Author Topic Title Reference within AEE Engineering Design, Dam Breach and Natural Hazard Reports 1 Engineering Geology Limited Tailings Storage and Rock Disposal Waihi North Project - Tailings Storage and Rock Disposal Volume 1 - Natural Hazards and Options Assessment EGL (2022a) 2 GHD GOP TSF Design Gladstone Pit TSF Design Report GHD (2022a) 3 Engineering Geology Limited GOP TSF Design Peer Review of GOP TSF Resource Consent Technical Report EGL (2022b) 4 Engineering Geology Limited TSF 3 Design Tailings Storage and Rock Disposal Volume 3 - Proposed Tailings Storage Facility - Storage 3 RL155 EGL (2022c) 5 Engineering Geology Limited Northern Rock Stack Design Waihi North Project - Tailings Storage and Rock Disposal Volume 4 Northern Rock Stack RL173 Proposed Rock Disposal Facility Design Report EGL (2022d) 6 Engineering Geology Limited TSF 3 Dam Breach Storage 3 - Tailings Storage Facility - RL155 Dam Breach and Potential Impact Classification Assessment EGL (2022e) 7 Beca Stormwater Management Willows Farm Stormwater Management Report Beca (2022b)
Part H – Technical Assessments 2 Report # Author Topic Title Reference within AEE 8 OGNZL Functional Need Assessment Waihi North Project – Tailings and Rock Storage – Functional Need Assessment OGNZL (2022) Geotechnical 9 PSM GOP Geotechnical Waihi North Project - Gladstone Pit - Geotechnical Assessment PSM (2022) 10 WSP Willows Rock Stack Design and Options Assessment Waihi North Project - Wharekirauponga Underground Mine - Willows Rock Stack and Surface Facilities Geotechnical Assessment WSP (2022a) 11 WSP WUG Geotech Engineering and Development Waihi North Project - Geotechnical Assessment Underground Mine and Tunnels WSP (2022b) 12 Engineering Geology Limited Ground Settlement Waihi North Project - Ground Settlement Assessment EGL (2022) Geochemistry 13 AECOM Geochemistry Waihi North Project Geochemical Assessment - Geochemistry of Tailings and Overburden, Treatment and Mitigation AECOM (2022) Contaminated Land
Part H – Technical Assessments 3 Report # Author Topic Title Reference within AEE 14 Williamson Water and Land Advisory Ground Contamination Waihi North Project - Preliminary Site Investigation (Ground Contamination) WWLA (2022) Hazardous Substances 15 Tonkin & Taylor Hazardous Substances Waihi North Project - Waihi Processing Plant, Water Treatment Plant and Development Site Tonkin & Taylor (2022) 16 Tonkin & Taylor Hazardous Substances Waihi North Project – Storage and use of hazardous substances at Willows Road site and the Wharekirauponga Underground Mine Tonkin & Taylor (2022a) Air Quality 17 Beca Air Quality Waihi North Project - Air Discharge Assessment - Waihi Facilities Beca (2022) 18 Beca Air Quality Waihi North Project - Air Discharge Assessment - Wharekirauponga Underground Mine Beca (2022a) 19 Tonkin & Taylor Air Quality Waihi North Project – Technical Review of Air Quality Assessments Tonkin & Taylor (2022b) Water 20 Southern Skies Erosion and Sediment Control Waihi North Project - Erosion and Sediment Control Assessment Report Southern Skies (2022)
Part H – Technical Assessments 4 Report # Author Topic Title Reference within AEE 21 GHD Water Management Waihi North Project - Water Management Studies GHD (2022) 22 GWS Hydrogeology Waihi North Project - Assessment of Groundwater Effects – Wharekirauponga Deposit GWS (2022) 23 GWS Hydrogeology Waihi North Project - Assessment of Groundwater Effects - Tunnel Elements GWS (2022a) 24 GHD Hydrogeology Waihi North - Groundwater Assessment GHD (2022b) 25 Flo Solutions Hydrogeology Wharekirauponga Underground Project Updated Conceptual Model – EG Vein Flo Solutions (2022) 26 Valenza Hydrogeology Wharekirauponga Underground Mine - Wharekirauponga Conceptual Mitigation - Phase 1 Report Valenza (2022) 27 GWS Hydrogeology Waihi North Project Summary of Potential Groundwater Effects GWS (2022b) Ecology 28 The Ecology Company Biodiversity Project Waihi North Biodiversity Project Consultation Document The Ecology Company (2021) 29 Bioresearches Terrestrial Ecology (TSF3, NRS, GOP) Waihi North Project - Assessment of Terrestrial Ecological Values and Effects Bioresearches (2022)
Part H – Technical Assessments 5 Report # Author Topic Title Reference within AEE 30 Bioresearches Terrestrial Ecology Terrestrial Ecology Mitigation, Restoration and Offset Plan for Waihi North project (Waihi Area) Bioresearches (2022a) 31 Boffa Miskell Terrestrial Ecology (WUG) Waihi North Project - Terrestrial Ecology Values and Effects of the WUG Boffa Miskell (2022b) 32 RMA Ecology Limited Frogs Memorandum: OGNZL Wharekirauponga frogs: Potential adverse ecological effects RMA Ecology (2022) 33 Bioresearches Frogs Memorandum: Vibration effects on amphibians (Leiopelmatid frogs) Bioresearches (2022b) 34 Boffa Miskell Frogs Pest Animal Management Plan – Wharekirauponga Compensation Package Boffa Miskell (2022c) 35 Lloyds Ecological Consulting Frogs Estimating the Proportion of the Coromandel’s Archey’s Frog Population in the Area Affected by Vibrations from the Proposed Wharekirauponga Mine Lloyd (2022) 36 Boffa Miskell Aquatic Ecology Waihi North Project – Freshwater Ecological Assessment Boffa Miskell (2022a) Historic Heritage, Amenity, Social Impact and Climate Change 37 Clough & Assoc Historic Heritage Waihi North Project - Assessment of Heritage and Archaeological Effects Clough (2022) 38 Stantec Transportation Waihi North Project - Transportation Assessment Stantec (2022)
Part H – Technical Assessments 6 Report # Author Topic Title Reference within AEE 39 Sense Partners Economics Waihi North Project - Economic Impacts Sense Partners (2022) 40 Pederson Read Lighting Waihi North Project – Assessment of Environmental Effects - Lighting Assessment Pederson Read (2022) 41 Heilig & Partners Vibration Waihi North Project - Blasting and Vibration Assessment Heilig (2022) 42 Boffa Miskell Landscape and Visual Waihi North Project - Landscape, Natural Character and Visual Effects Assessment Boffa Miskell (2022) 43 Rob Greenaway & Associates Recreation Waihi North Project - Recreation and Tourism Assessment Greenaway (2022) 44 Marshall Day Noise Waihi North Project - Assessment of Noise Effects Marshall Day (2022) 45 WSP Social Impact Waihi North Project - Social Impact Assessment WSP (2022) 46 OGNZL Climate Change, Energy Use and Greenhouse Gas Management Waihi North Project - Climate Change, Energy Use and Greenhouse Gas Management OGNZL (2022a)
This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx OGNZL Document Reference: WAI-985-000-REP-LC-0002 Revision: 0 Prepared for: 23 June 2022 OceanaGold (New Zealand) Ltd P O Box 190 WAIHI 3641 OCEANA GOLD (NEW ZEALAND) LIMITED WAIHI OPERATION WAIHI NORTH PROJECT TAILINGS STORAGE AND ROCK DISPOSAL VOLUME 1 NATURAL HAZARDS AND OPTIONS ASSESSMENT TECHNICAL REPORT EGL Ref: 9215 This document has been produced for New Zealand consenting purposes only. Information contained herein must not be relied on for investment purposes.
EGL Ref: 9215 23 June 2022 Page ii This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. DOCUMENT CONTROL Document information Title OceanaGold (New Zealand) Limited – Waihi Operation – Waihi North Project – Tailings Storage and Rock Disposal – Volume 1 – Natural Hazards and Options Assessment Technical Report Revision 0 Date 23/06/2022 EGL Reference 9215 Client Reference WAI-985-000-REP-LC-0002 File Name WAI-985-000-REP-LC-0002_Rev0.docx Document roles and approvals Role Name Credentials Signature Doc. Rev. Date Author T. Matuschka BE (Hons) Civil, PhD, FEngNZ, CPEng 0 23/06/2022 Reviewer E. Torvelainen BE (Hons) Civil, MEngNZ 0 23/06/2022 EGL Approval T. Matuschka EGL Director 0 23/06/2022 Final copy issue requires signatures. Document revision and issue record Revision. Date Revision Description Issue by 0 23/06/2022 Resource Consent Issue E. Torvelainen Draft revisions are given alphabetic characters. Final copy issue and subsequent revision are given numeric characters. Document applicability and disclaimers This report has been prepared by EGL (Engineering Geology Limited) solely for the benefit of Oceana Gold (New Zealand) Limited as our client with respect to the particular brief given to us. The content of this report and any advice given in undertaking this work cannot to be relied on by any person or parties other than the client, or for any purpose other than the client’s particular brief, without EGL’s prior agreement. This report shall only be read in its entirety. Where this report is issued in draft the contents shall be for initial information and review only and are subject to change and shall not be relied upon.
iii This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. CONTENTS Page No. 1.0 INTRODUCTION 1 2.0 WAIHI OPERATION LOCATION AND SITE DESCRIPTION 2 2.1. Location 2 2.2. Waihi Mine site description 3 3.0 SITE CONDITIONS 5 3.1. Climate 5 3.1.1. Climate change 5 3.2. Geological setting 5 3.2.1. Tectonic setting 5 3.2.2. Regional Geology 8 3.2.3. Geology Around the Development Site 9 3.3. Waihi operation site specific seismic hazard 11 3.4. Hydrology 13 3.5. Hydrogeology 15 4.0 WAIHI OPERATION – EXISTING TAILINGS STORAGE AND ROCK DISPOSAL 15 4.1. Nature of the ore and overburden at Waihi 15 4.2. Nature of tailings at Waihi 16 4.2.1. General 16 4.2.2. Ore-bearing rock 17 4.2.3. Process and water treatment plant setup 17 4.2.4. Tailings delivery and discharge 18 4.2.5. Tailings impoundment lining and subsurface drains 19 4.2.6. Tailings profile within the impoundments 20 4.3. Existing zoned embankment design 22 4.4. Storage 2 design and construction 24 4.5. Storage 1A design and construction 27 4.6. Operation 30 4.7. Monitoring and Surveillance 30 4.8. Peer review 31 4.9. Performance 32 4.9.1. Seepage 32 4.9.2. Porewater Pressures 34
iv This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 4.9.3. Environmental monitoring 34 4.9.4. Deformation monitoring 34 4.9.5. Surface water diversion and collection ponds 35 5.0 FAILURE OF TAILINGS STORAGE FACILITIES COMPARED TOWAIHI TSFS 36 6.0 INPUTS FOR TECHNICAL DESIGN OF TAILINGS STORAGE AND ROCK DISPOSAL 37 6.1. Future Sources of Ore 37 6.2. Affected stakeholders 40 6.3. Geotechnical stability 40 6.4. Geochemical stability 40 6.5. Groundwater quality 40 6.6. Surface water quality 40 6.7. Rehabilitation 41 6.7.1. Rehabilitation of slopes 41 6.7.2. Tailings pond surface 41 6.8. Sterilisation of resource 42 7.0 REVIEW OF TECHNOLOGIES FOR TAILING DISPOSAL 42 7.1. Tailings Technologies 42 7.1.1. Conventional slurry tailings 42 7.1.2. Thickened tailings 42 7.1.3. Paste 43 7.1.4. Filtered tailings 43 8.0 POTENTIAL LOCATIONS FOR EXPANSION OF TAILINGS STORAGE AND ROCK DISPOSAL 44 8.1. Existing Storage 2 site 44 8.2. Existing Storage 1A site 44 8.3. North of Storage 2 45 8.4. Northeast of Storage 2 45 8.5. Northeast valley 45 8.6. Site east of Storage 1A 46 8.7. Beyond the current Development Site 46 8.8. Martha Open Pit 46 8.9. Gladstone Open Pit 46 8.10. Underground 47 9.0 OPTIONS FOR DISPOSAL OF TAILINGS AND MINE PIT OVERBURDEN 50
v This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 9.1. Options for tailings disposal 50 9.1.1. In-pit disposal 50 9.1.2. Underground paste backfill 51 9.1.3. Central thickened discharge 51 9.1.4. Dry stacks 52 9.1.5. Mixed rock stacks 53 9.1.6. Downstream and centreline embankment dams 53 9.2. Rock disposal options 55 9.2.1. Underground backfill 55 9.2.2. Pit backfill 55 9.2.3. TSF embankments 55 9.2.4. TSF capping 55 9.2.5. Existing stockpiles 55 9.2.6. New rock stacks 56 10.0 OPTIONS FOR ASSESSMENT 57 10.1. Category, Criteria and Weighting 57 10.2. Potential project options 63 10.3. Assessment of Options 72 11.0 PROPOSED WAIHI NORTH PROJECT TAILINGS STORAGE AND ROCK DISPOSAL STRATEGY 74 11.1. Mine Overburden Disposal 74 11.2. Tailings Disposal 74 11.3. Closure 74
This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx 1.0 INTRODUCTION Engineering Geology Ltd (EGL) has been appointed by Oceana Gold (New Zealand) Limited (OGNZL) to prepare a series of technical reports for resource consent for tailings storage and rock disposal for the Waihi North Project. This report is Volume 1 of a 4-part series of reports on tailings storage and rock disposal for the Waihi North Project. This report documents a review of natural hazards, and the locations and best available technologies for tailings storage and rock disposal for the Waihi Operation. Volume 2 is a technical report on the use of the proposed Gladstone Open Pit (GOP) as a Tailings Storage Facility (TSF). This will involve partial backfilling of GOP with rock, so the GOP TSF provides for both tailings storage and rock disposal for the Waihi North Project. The technical report for GOP TSF is prepared by GHD (Ref. 1). Volume 3 is a technical report on a new TSF named Storage 3, located east of Storage 1A. It provides for both tailings storage and rock disposal for the Waihi North Project. Volume 4 is a technical report on the Northern Rock Stack (NRS). It provides for disposal of rock and is located north of the existing TSF Storage 2. These preferred facilities are compared against a range of options. This report covers: • Waihi Operation location and site description. • An overview of the climate, geology, hydrology, hydrogeology, and seismicity of the Waihi area. • An overview of the Waihi Operation existing storage facilities which have performed well geotechnically and environmentally. • A comparison of the Waihi TSFs with practices seen globally. • Waihi North Project tailing storage and rock disposal requirements. • Locations for tailings storage and rock disposal. • Tailings storage best available technologies. • Rock disposal best available technologies. • Assessment of options. • Proposed Waihi North Project tailings and rock disposal strategy. The chosen options for tailings storage and rock disposal take into consideration a wide range of inputs to result in designs that provide: long term security for the disposal or rock and 23 June 2022 OCEANA GOLD (NEW ZEALAND) LTD WAIHI OPERATION WAIHI NORTH PROJECT TAILINGS STORAGE AND ROCK DISPOSAL VOLUME 1 NATURAL HAZARDS AND OPTIONS ASSESSMENT TECHNICAL REPORT EGL Ref: 9215
EGL Ref: 9215 23 June 2022 Page 2 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. containment of tailings;, minimise impacts on groundwater, receiving waters and landform; create rehabilitated landforms that will provide the opportunity for a net-gain in terms of biodiversity, minimise risk to people and property. To achieve these objectives requires a collaborative approach with input from a wide range of technical experts (geologists, hydrogeologists, hydrologists, geochemists, engineers), scientific experts (ecologists, biologists, dust and noise consultants), landscape architects and consultation with iwi. The aim is to meet obligations under the Resource Management Act, meet community and iwi expectations, and to design and construct structures that meet structural stability and durability requirements under the Building Act, and comply with company and international standards for tailings and rock storage. It is noted that early assessments of suitable options drove subsequent decisions around land acquisition and the Overseas Investment Act approvals processes and completion of land purchases that followed. These represent significant milestones in the project that would be difficult, highly uncertain and time-consuming to unwind and re-execute in the event of new land acquisition being required 2.0 WAIHI OPERATION LOCATION AND SITE DESCRIPTION 2.1. Location Waihi is a long-standing mining town in the Hauraki District. For over a century Waihi has been associated with gold mining. Waihi Township is located at the southern end of the Coromandel Peninsula and is within the area covered under the Hauraki District Council and Waikato Regional Council jurisdictions. By road it is 144 km southeast of Auckland, 68km northwest of Tauranga City and 21 km east of Paeroa Township. The existing Waihi gold operation is partly located in the township of Waihi around Martha Open Pit and partly to the southeast, accessed via SH2 and Baxter Road, where the two existing TSFs (Storage 1A and 2) are located. They are shown in Figure 1. FIGURE 1: LOCATION OF EXISTING TAILINGS STORAGE FACILITIES STORAGE 2 AND STORAGE 1A
EGL Ref: 9215 23 June 2022 Page 3 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 2.2. Waihi Mine site description In 1988 open pit mining of Martha Hill commenced. Open pit mining realised parts of the resource that were previously uneconomic and importantly for the Waihi Operation created a source of earth and rock fill to construct downstream embankment dams for storage of tailings. Downstream embankments are typically the safest type of dam for tailings slurry storage. Other construction techniques such as upstream construction are higher risk if not properly designed, constructed and operated. Martha Open Pit mine is located in the township, centred on a small hill known as Martha Hill. It is shown in Figure 2. A series of underground mines and most of the old historical mine workings are located beneath the eastern side of the Waihi township, and within or immediately adjacent to Martha Open Pit. Further underground mines are located to the southeast of the township towards the Process Plant on the west side of the Ohinemuri River. Figure 2 indicates the locations and names of the underground mines and historic workings. Martha Underground Mine (under Project Martha) is currently being developed and is aligned beneath the south wall of Martha Open Pit. The two TSFs; Storage 1A and 2 and associated rock stockpiles are located to the east of the Ohinemuri River away from the underground working as shown in Figure 1. The tailings storage and rock disposal, and the Water Treatment Plant and Processing Plant area is called the Development Site. The two existing TSFs are shown in the oblique aerial photo in Figure 3, with Storage 2 to the left and Storage 1A to the right. This photo was taken in 2018. FIGURE 2: MARTHA OPEN PIT AND UNDERGROUND MINE LOCATIONS
EGL Ref: 9215 23 June 2022 Page 4 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. FIGURE 3: AERIAL PHOTO OF STORAGE 2 (LEFT) AND STORAGE 1A (RIGHT) WHICH CURRENTLY STORE THE TAILINGS AT THE WAIHI OPERATION The existing TSFs approximately 3 km south-east of the Waihi Township are connected to the Martha Open Pit by a conveyor indicated on Figure 1, which transports rock and soil between the two areas without truck movements. At Martha Open Pit a crusher breaks the rock down before being placed on the conveyor. Material can be offloaded at the Process Plant or at the Loadout which is located up at the level of the crest of Storage 2. This provides an effective location to then transport the earth and rock fill to the TSFs or stockpile via haul routes on or behind the TSFs. Both existing TSFs are formed by construction of ‘U shaped’ embankments which abut the naturally rising land to the northeast, to form the impoundments which store the tailings. The embankments provide the disposal location for rock from the pits. Storage 2 was constructed first, starting in 1987, followed by Storage 1A with preliminary works starting in 1998 and first tailings discharge in 2001. Since May 2001, virtually all tailings produced on site have been disposed into Storage 1A. Between 2001 and 2005 a small amount of tailings were deposited within Storage 2 and since July 2005 no tailings have been deposited in Storage 2. Storage 2 has been successfully rehabilitated and water in the pond of Storage 2 is now clean and able to be discharged direct to the Ohinemuri River. Raising of Storage 2 to RL160.7 is within the consented Life of Mine (LOM) works and will see the embankment crest raised up to 4.7m above its current level. Tailings deposition will resume into Storage 2 and supernatant water in the tailings pond will be pumped back to the Water Treatment Plant for processing before discharge to the Ohinemuri River. Following completion of tailings discharge Storage 2 will again be rehabilitated so the water is clean for discharge direct to the Ohinemuri River. Storage 1A has a crest level of RL173.6 as of March 2021 and it is consented for raising to RL182 to provide storage for the resource associated with the Martha Underground and MOP4.
EGL Ref: 9215 23 June 2022 Page 5 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 3.0 SITE CONDITIONS 3.1. Climate The climate within Waihi is temperate. Mean monthly temperatures range from 8.9 °C in July to 18.9 °C in January. Waihi township is approximately 100 m above sea level and receives on average between 1500 to 3000 mm of rainfall per annum, with approximately 31% of rainfall expected within the winter months between June and August and 22% of rain in the summer months between December and February. 3.1.1. Climate change NIWA has published information on possible effects of climate change. The effects will generally result in higher temperatures, lower annual rainfall but higher intensity rainfall in extreme events. Higher temperatures and lower annual rainfalls are considered unlikely to affect tailings or rock disposal. Higher intensity rainfall events can be considered in the design of drains, surface water collection ponds, and freeboard within the TSF impoundments. NIWA provides high intensity rainfall data for different climate change scenarios. 3.2. Geological setting 3.2.1. Tectonic setting Earthquake hazard at the Waihi operation was assessed by the Institute of Geological and Nuclear Sciences (GNS) in 2017 (Ref. 2). GNS is a New Zealand Crown Institute and is one of New Zealand’s leading authorities on seismicity in this country. The Waihi operation is in the northwestern region of the New Zealand tectonic setting, which is experiencing tectonic crustal extension as shown on Figure 4, as opposed to dextral (lateral) or contractional tectonic mechanisms present in other parts of New Zealand. This extensional region typically experiences lower seismicity compared to more central parts of New Zealand because of its distance from the interface between the Australian and Pacific Plates shown in Figure 4. The Waihi operation location is marked MH for Martha Hill. To the southeast and east of Waihi is the more active Extensional Havre Trough, Taupo Rift, North Island Fault Belt and the Northwest dipping Hikurangi Subduction Zone regions.
EGL Ref: 9215 23 June 2022 Page 6 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. FIGURE 4: WAIHI MINE (LABELLED MH) IS LOCATED IN THE EXTENSIONAL WESTERN NORTHEN FAULT ZONE, WHICH IS ADJACENT TO THE EXTENSIONAL HAVRE TROUGH AND TAUPO RIFT. AT DEPTH TO THE SOUTHEAST IS THE HIKURANGI SUBDUCTION ZONE (GNS, 2017) The closest known active faults to the Waihi operation are those of the Kerepehi Fault System, located in the Hauraki Rift beneath the Hauraki Plains and the Firth of Thames in Figure 5. This fault system runs up through the Hauraki Plains and the Firth of Thames. As is common to faulting in the region, the Kerepehi Fault System is characterised by extensional normal faulting (Persaud et al. 2016, Ref. 3). The Hikurangi Subduction Zone to the east of Waihi dips from east to west beneath the North Island and is capable of magnitude 9 (Mw) earthquakes. The distance from Waihi to the Hikurangi Subduction Zone is over 200 km. This distance provides some attenuation of the shaking. The Coromandel was once an active volcanic area approximately 2 to 12 million years ago (Ref. 4). The Waihi area was part of this activity, and the Waihi Basin is a past caldera volcano (similar to Lake Taupo) that is now filled in with sediments associated with being a lake and other volcanic activity. Calderas result in faults around their edges as large blocks of the earth crust drop downwards toward the centre of the Caldera as the volcanic process is occurring. Some fault traces around the Waihi area are associated with past tectonic or volcanic activity. However, they are understood not to be seismically active today. Earthquakes do occur on unknown faults. A record of historic earthquakes is documented on the Geonet New Zealand Earthquake Database. In Figure 6, a search of the database for historic earthquakes greater than magnitude (Mw) 5 and less than 100km deep shows isolated historic earthquakes in the region, with more activity to the southeast in the Bay of Plenty closer to the plate boundary. The historic earthquakes in the region are not necessarily on known active fault traces and the GNS (Ref.2) study considers earthquakes on smaller unknown faults in the region, including those
EGL Ref: 9215 23 June 2022 Page 7 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. potentially closer to the Waihi operation than the Kerepehi Fault, based on observed historic earthquake activity rates in the region from the Geonet Database. FIGURE 5: HAURAKI RIFT AND KEREPEHI FAULTS (GNS, 2017) FIGURE 6: GEONET NEW ZEALAND EARTHQUAKE DATABASE - EARTHQUAKES MAGNITUDE 5 AND GREATER, LESS THAN 100KM DEEP RECORDED IN HISTORY Waihi
EGL Ref: 9215 23 June 2022 Page 8 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 3.2.2. Regional Geology The Waihi operation is located on the eastern side of the Waihi Basin which comprises extrusive and intrusive volcanic rocks of various ages, and lacustrine (lake) deposits. These materials are mantled by tephras (volcanic airfall deposits) such as ignimbrites, tuff, volcanic ash, and colluvial and alluvial deposits. Brathwaite and Christie (Ref.5) interpret the geology of the Waihi Basin as part of the Coromandel Volcanic Zone, a sub-aerial 8-million-year-old (late miocene) to 1.5 million-year-old (early pleistocene) andesite-dacite-rhyolite sequence that forms the Coromandel-Kaimai ranges. The sedimentary or metamorphic basement rock is likely at considerable depth below the volcanic sequence. The Waihi Basin itself is a caldera (large volcanic centre) within the Coromandel Volcanic zone (Ref. 6, 7, and 8) as interpreted by Hayward (Ref. 2). The oldest volcanic formations in the Waihi area are andesites and dacites of the Late Miocene Waiwawa Subgroup of the Coromandel Group and includes the Waipupu Formation which are 7.9-6.3 million years old. Dating (K-Ar) indicates a geological erosional time break of about 1 Ma (million years) between the andesites and dacites of the Waiwawa Subgroup and the eruption of andesites and dacites of the Kaimai Subgroup, which contains dacites belonging to the Uretara Formation which are 5.6-4.3 million years old. Of similar age to the Kaimai Subgroup are rhyolites of the Minden Rhyolite Subgroup within the Whitianga Group, which includes domes of Homunga Rhyolite which is 5.5-5.2 million years old and forms the Ruahorehore Dome on and against which the existing TSFs Storage 1A and 2 are predominantly located (Ref. 5). The Waihi Basin caldera is infilled with pliocene to early pleistocene lacustrine (lake) sediments and ignimbrites of the Whitianga Group. At the base are lacustrine sediments of the Romanga Formation (4.5-3.0 million years old) part of the Coroglen Subgroup. Lacustrine sediments have also been found around the Martha Open Pit. The overlying ignimbrites are grouped into the Ohinemuri Subgroup consisting of Corbett ignimbrites at 2.9 million years old and Owharoa ignimbrites (late pliocene) and Waikino ignimbrite at 1.5 million years old (early pleistocene). Eruptions of ash and pumice blanketed the area. Typical ash soils found in the area include the Waihi Ash series, the Hauparu Ash and the Rotoehu Ash (Ref. 9).
EGL Ref: 9215 23 June 2022 Page 9 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. FIGURE 7: 1:50000 GEOLOGY OF THE WAIHI AREA MAP FROM BRATHWAITE AND CHRISTIE (1996) WITH MAIN UNITS MARKED 3.2.3. Geology Around the Development Site The Development Site area, where Storage 1A and 2 are situated, is underlain by a sequence of ignimbrite, lacustrine deposits, rhyolite, dacite, and finally andesite. The older dacite dips beneath the rhyolite. The dacite outcrops approximately along the Ohinemuri River to the west and is observed in borehole WG4 beneath Storage 1A’s southwest toe at 16.5 m down hole depth. Toward the northeast side of Storage 1A a deep borehole GTO20 indicates dacite at 156 m, beneath the rhyolite of the Ruahorehore Dome which is visible as the hills behind Storage 1A and 2. The west toe of Storage 2 is founded on ignimbrite over the dacite. The rest of Storage 2 is founded on rhyolite. Storage 1A is constructed over a series of Rhyolite ridges, knolls and gullies. Figure 8 shows the general geology of the Development Site.
EGL Ref: 9215 23 June 2022 Page 10 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx FIGURE 8: DEVELOPMENT SITE GEOLOGICAL MAP
EGL Ref: 9215 23 June 2022 Page 11 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx 3.3. Waihi operation site specific seismic hazard The design of all structures, including dams to store tailings, needs to be informed by the natural hazard posed by potential earthquakes. For that purpose, the New Zealand Dam Safety Guidelines (NZDSG) set out the approach that should be taken to the identification of seismic risk. This in turn informs the design of large dam structures so that appropriate safety features are incorporated in the design so structures perform appropriately under seismic loads. This is so even under the worst credible earthquake scenario (which would cause widespread damage in the area) there will not be a loss of contents from the TSFs. Seismic loads are also considered in the design of rock storage facilities. Estimates of seismic hazard for the site have been provided by GNS Science in 2007 and 2017 (Ref. 2). The 2017 update incorporated the latest knowledge of the Kerepehi Fault System (Ref. 3), the Hikurangi subduction zone and updated estimates of background seismicity. The Kerepehi Fault System is comprised of several faults and is 21 km from the site at its closest point. It is the closest known active fault to the site and is the Controlling Maximum Earthquake (CME) as defined by the NZDSG. The largest rupture scenario for this fault system is when all segments of the fault rupture together. This scenario represents a rupture length of about 81 km, a magnitude (Mw) of 7.3, resulting in normal (dip-slip) fault displacement of about 3.6 m. This scenario has a recurrence interval of approximately 10,000 years. Very large earthquakes, up to magnitude (Mw) 9, are associated with the Hikurangi subduction zone and contribute to the seismic hazard at the site. The subduction zone is approximately 200km distant from the site and so ground motions are attenuated and are not as significant as those from nearby smaller background earthquakes or the Kerepehi Fault. The GNS study 2017 considered new research recently published updating the understanding of the Kerepehi Fault (Ref. 3). A rigorous approach was adopted by GNS for determining estimates of seismic hazard that accounted for epistemic uncertainty (systematic uncertainty due to the method of assessment) as required by the NZDSG for High Potential Impact Classification (PIC) dams. As the Kerepehi Fault is comprised of several segments, GNS modelled different representations of the Kerepehi Fault to address uncertainty in the knowledge of how different segments could combine. There was negligible difference in results for the different representations that were considered. GNS consider the best estimate model is that representing the most up-to-date information regarding the Kerepehi Fault, and therefore, the recommended spectra are those produced using this best estimate model. The GNS 2017 study provided both probabilistic and deterministic (i.e., scenario based) estimates of horizontal peak ground accelerations (PGAs) and acceleration response spectra (weighted and unweighted). Probabilistic estimates were determined for return periods of 150, 2500 and 10,000 years. The 2017 probabilistic estimates of spectra are lower than the 2007 estimates. This is principally because there has been a reduction in the rate of seismicity associated with the local distributed earthquake source in the national seismic hazard model (Ref. 2). This source contributes the most to seismic hazard at the site.
EGL Ref: 9215 23 June 2022 Page 12 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. Deterministic estimates were calculated for the Kerepehi Fault worst-case scenario (Mw7.3 discussed previously) and the most likely scenario, which is considered to involve a rupture length of 65 km, with a magnitude of Mw7.2, resulting in normal (dip-slip) fault displacement of 2.9 m. There was insignificant difference between the two scenarios in terms of estimated shaking intensity at the Development Site. The 150-year return period spectrum is commonly adopted as the Operating Basis Earthquake (OBE) for dam design as defined by the NZDSG. For a High PIC dam, the NZDSG recommend that the Safety Evaluation Earthquake (SEE) be taken as the 84th percentile level ground motion associated with the CME if developed by a deterministic approach. Furthermore, this need not exceed the 10,000-year return period ground motion developed by a probabilistic approach. The 84th percentile level shaking from a CME on the Kerepehi Fault and 1 in 10,000-year return period ground motion has been adopted for the SEE design, along with an aftershock one magnitude less. The uniform hazard spectral accelerations from the probabilistic and deterministic estimates of seismic hazard are shown in Figure 9. The spectra are for a 5% damped oscillator, for the larger horizontal component ordinate for rock conditions. Horizontal Peak Ground Acceleration (PGA) values and corresponding average magnitudes at the base of the embankment at the rock surface are as follows: 150-year return period (OBE): PGA = 0.10g, Mw = 6.3 84th percentile level for CME (SEE): PGA = 0.23g Mw = 7.3 2,500-year return period: PGA = 0.27g Mw = 6.6 10,000-year return period: PGA = 0.39g, Mw = 6.9 PGA and spectral acceleration values are provided as values of gravitational force (g) i.e., 0.1g is 10% of the force of gravity. Amplification of ground accelerations do occur through soil profile as well as the embankment. This amplification is allowed for in specific design calculations as it varies with each application.
EGL Ref: 9215 23 June 2022 Page 13 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. FIGURE 9: WAIHI EARTHQUAKE HORIZONTAL SPECTRAL ACCELERATIONS FOR ROCK SITES 3.4. Hydrology Waihi receives on average 1500 to 3000 mm of rain per year making it one of the wetter regions in the North Island. NIWA provides statistical rainfall depths and intensities through their national high intensity rainfall database (HIRD by NIWA), based on monitoring sites across the region. For Waihi the latest information statistical estimates of rainfall depths and estimates for storm events based on historic data are included in Table 1 and Table 2 for different durations. Rainfall occurring in the Waihi area runs off into the Ohinemuri River and its tributaries. The Ohinemuri River flows to the west, past the Storage 2 and Waihi Township, along SH2 through the Karangahake Gorge, past Paeroa, into the Waihou River, which then flows out into the Firth of Thames near the Thames Township. The Ohinemuri River is without stopbanks in its natural channel as it passes the Waihi Operation Site. Flood estimates for a 100-year Average Recurrence Interval (ARI) event have been developed around the Waihi Operation Development Site and the TSFs are currently located above the 100-year flood extent. The flood extent is shown in Figure 10.
EGL Ref: 9215 23 June 2022 Page 14 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. FIGURE 10: 100 YEAR ARI FLOOD EXTENT PAST STORAGE 1A AND 2 TABLE 1: HIRDS (V4) RAINFALL DEPTHS FOR ANNUAL RECURRENCE INTERVAL ARI (yrs) AEP Duration (hr) 10min 20min 30min 1hr 2hr 6hr 12hr 24hr 48hr 72hr 1.58 0.633 9.38mm 14.7 19.1 29 43.1 75.1 101 131 161 177 2 0.5 10.4 16.3 21.1 32.1 47.5 82.8 112 144 177 194 5 0.2 13.9 21.9 28.2 42.8 63.2 110 148 190 232 255 10 0.1 16.7 26.1 33.7 51 75.2 130 175 224 274 300 20 0.05 19.6 30.6 39.4 59.6 87.8 151 203 260 317 347 30 0.033 21.4 33.4 43 64.9 95.5 164 220 282 343 376 40 0.025 22.7 35.4 45.6 68.8 101 174 233 298 362 396 50 0.02 23.8 37 47.6 71.8 106 181 243 310 377 412 60 0.017 24.6 38.4 49.3 74.4 109 188 251 321 390 426 80 0.012 26 40.5 52 78.4 115 197 264 337 409 447 100 0.01 27.1 42.2 54.2 81.6 120 205 274 350 425 464 250 0.004 31.7 49.3 63.2 94.9 139 237 316 403 488 532 1000 0.001 37.2 57.9 74.2 111.3 162.9 277.4 369.6 471.0 569.8 620.9 10000 0.0001 46.4 72.1 92.3 138.5 202.6 344.4 458.6 583.8 705.5 768.4 PMP* 1200^ * PMP stands for Probable Maximum Precipitation ^ PMP scenario is used to determine the water volume to for setting the operation to maintain the freeboard condition
EGL Ref: 9215 23 June 2022 Page 15 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. TABLE 2: HIRDS (V4) RAINFALL INTENSITY FOR ANNUAL RECURRENCE INTERVAL *PMP stands for Probable Maximum Precipitation 3.5. Hydrogeology The hydrogeology of the Waihi Mine area is currently controlled by the Ohinemuri River and dewatering of the current underground and open pit mines. Around the Mine Site to the west of the Ohinemuri River the current underground and open pit mines draw down the natural ground water table. Groundwater is drawn down to the west of the Ohinemuri River at the Development Site because the underground mines and pits are below the riverbed level. Once mining stops, water levels will return close to the surface and groundwater will flow back to the Ohinemuri River. To the east of the Ohinemuri River at the Development Site the ground water flows to the Ruahorehore Stream and the Ohinemuri River as there are no open pits or underground mines to draw the groundwater down and the surrounding ground and TSFs are higher. Subsoil drains beneath the TSFs intercept groundwater, which is flowing beneath the TSFs, driven by the head or water in the surrounding hills. 4.0 WAIHI OPERATION – EXISTING TAILINGS STORAGE AND ROCK DISPOSAL 4.1. Nature of the ore and overburden at Waihi Typically, mining operations remove both ore-bearing rock and non-ore-bearing rock. At Waihi ore bearing rock is crushed, ground and processed to extract the gold and silver and forms the by-product known as tailings (See Section 4.2). The non-ore bearing rock and overlying soils at Waihi associated with Martha Open Pit and the underground mines requires transfer and placement for long term storage. This material is currently placed in the TSF embankments and associated stockpiles. ARI (yrs) AEP Duration 10min 20min 30min 1hr 2hr 6hr 12hr 24hr 48hr 72hr 1.58 0.633 56.3mm/hr 44.2 38.1 29 21.5 12.5 8.46 5.46 3.35 2.45 2 0.5 62.3 48.9 42.2 32.1 23.8 13.8 9.31 6.01 3.68 2.7 5 0.2 83.7 65.6 56.4 42.8 31.6 18.3 12.3 7.91 4.84 3.54 10 0.1 100 78.4 67.3 51 37.6 21.7 14.6 9.35 5.71 4.17 20 0.05 118 91.9 78.9 59.6 43.9 25.2 16.9 10.8 6.61 4.82 30 0.033 129 100 86 64.9 47.7 27.4 18.4 11.8 7.16 5.22 40 0.025 136 106 91.1 68.8 50.5 29 19.4 12.4 7.55 5.5 50 0.02 143 111 95.2 71.8 52.8 30.2 20.2 12.9 7.86 5.73 60 0.017 148 115 98.6 74.4 54.6 31.3 20.9 13.4 8.12 5.91 80 0.012 156 122 104 78.4 57.5 32.9 22 14 8.53 6.21 100 0.01 163 127 108 81.6 59.8 34.2 22.8 14.6 8.84 6.44 250 0.004 190 148 126 94.9 69.4 39.5 26.4 16.8 10.2 7.39 1000 0.001 223.5 173.7 148.3 111.3 81.5 46.2 30.8 19.6 11.9 8.6 10000 0.0001 278.6 216.4 184.7 138.5 101.3 57.4 38.2 24.3 14.7 10.7 PMP* 16.7
EGL Ref: 9215 23 June 2022 Page 16 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. As described above in Section 3.2, the rock and soils at Waihi are associated with a past geological time of volcanism. Some of the rocks are Potential Acid Forming (PAF) when exposed to air and water, and some of the rocks are Non-Acid Forming (NAF). The potential for acid generation is related to the geochemistry of each rock source and testing is undertaken to determine which rocks are PAF. Acid generated from exposed PAF can mobilise heavy metals in the rock into solution which can be transported into the wider environment if not mitigated. This is called Acid Rock Drainage (ARD). The disposal of rocks at Waihi requires special mitigation measures to limit any ARD from PAF rock and tailings. This includes: • Testing of the source rock; • Controls over rock handling and placement; • Liming of the PAF rock, zoning of the embankments to limit oxygen ingress; • Placement of NAF material on the final external surface; • Leachate and subsoil seepage collection systems; and • Ground water monitoring. 4.2. Nature of tailings at Waihi 4.2.1. General Tailings is the common by-product of processing the ore-bearing rock to extract the valuable metals and minerals. Tailings at the end of the mineral extraction process typically take the form of a slurry consisting of fine particles and water. The fine particles are typically clay to sand sized and are created by the crushing and grinding of the ore bearing rock, and processing of the grind using methods such as leaching and adsorption to extract the valuable metals and mineral. Some processing operations also use methods to remove water from the tailings slurry for the purpose of water conservation or so the tailings can be transferred and placed at a higher density in their final storage location. Water is removed from tailings using plant called thickeners and filter presses. The nature of tailings leaving the process plant is, therefore, a function of the: 1. Ore-bearing rock processed; 2. The ore processing method; 3. The water management processes; Tailings which remain suspended in water as a slurry are pumped to the storage facility via a discharge delivery pipe. The amount of water in the slurry depends on the extent of thickening undertaken at the process plant. Thicker slurries are more difficult to pump. Filtered tailings are transported by truck. Cement can also be added to a thickened slurry to form a paste which when left to harden has improved strengths and is often used for filling underground workings. Tailings which have water removed using a filter press behave as a soil and are transferred by dump trucks or conveyors to their final storage location. Tailings slurries and filter pressed tailings consolidate to form soil-like deposits. These deposits can be tens to hundreds of metres thick. The profile,
EGL Ref: 9215 23 June 2022 Page 17 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. permeability and strength of the tailings deposit formed depends on the tailings properties, degree of dewatering, and placement method. Tailings which are transported as a slurry can be discharged: • Sub-aqueous (below a water surface); or • Sub-aerial (above a water surface). Discharge methods include: • Simple discharge from the end of a large pipe called end pipe discharge; • Discharge from a series of smaller pipes known as spigot pipe discharge; • Discharge from a tailings cyclone, a device which separates the coarse and fine fractions of the tailings and some of the water from the solid particles before final discharge. Pastes can be deposited in underground workings or above ground in impoundments. Slurries without binders are typically limited to tailing impoundments, either in-pit or above ground tailings dams. Tailings which are filter pressed to form a soil are tipped from trucks at their final location and spread out by a bulldozer and compacted to form soil stockpiles. 4.2.2. Ore-bearing rock The existing consented ore sources at the Waihi Operation comprise the existing Martha Open Pit (MOP) and Martha Underground (MUG) mines. Ore associated with the Waihi North Project will be sourced from the proposed new mines (Gladstone Open Pit (GOP) and Wharekirauponga Underground Mine (WUG)). The ore bearing rock is andesite at MOP, MUG and the GOP, and rhyolite at WUG. Some of the ore is PAF and therefore a proportion of the tailings are also PAF (estimates based on testing on MOP and MUG ore indicate about 60% tailings are NAF and 40% PAF). The predominant heavy metals in the ore from MOP and the existing underground workings that could leach from the tailings are manganese, copper, iron, antinomy, and selenium. Monitoring of tailings pond water and seepage indicate levels to date are very low. The closure design is for a perimeter capping and pond. This will ensure the tailings remain saturated which will prevent oxidation of the tailings and potential release of heavy metals. 4.2.3. Process and water treatment plant setup At the current Waihi Process Plant the processing operations consist of a twostage grinding process followed by a conventional carbon-in-pulp (CIP) circuit. The current circuit is designed to grind the ore to a target particle size of 80% passing 75 μm on Martha ore. Ore associated with the Waihi North Project is proposed to be ground to a target particle size of 80 % passing 53 μm. Chemicals used in the process include sodium cyanide, oxygen, lime, and flocculant. The tailings are currently pumped to the TSF as a slurry between 38 to 43% solids by weight, with up to 200 ppm of WAD cyanide and at a pH
EGL Ref: 9215 23 June 2022 Page 18 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. of 10. Ore associated with the Waihi North Project is proposed to be pumped at about 45% solids by weight. 4.2.4. Tailings delivery and discharge Tailings are pumped as a slurry from the Process Plant into Storage 1A TSF through rubber lined steel and polyethylene pipelines. Two 250 mm diameter pipelines are used to deliver the tailings slurry from the Process Plant to the TSF. The pipelines are contained within open trenches or bunds, which act as a containment device in case of a pipeline rupture or spill. The trenches divert any spill towards collection ponds or tailings ponds. Each pipeline is pressure tested annually to confirm its operability. There are two 90 kW variable speed Warman primary slurry pumps. Booster pumps are located along the pipelines. Currently the tailings are pumped into Storage 1A over a plan distance of 1,750 m and elevation change of +74 m. Historically tailings were pumped into Storage 2 over a plan distance of 1,000 m and elevation change of +56 m. Tailings have typically been discharged via spigots, but end pipe discharge has been used at some times during the operation of Storage 2 and 1A. Tailings are deposited over short sections on a rotational basis to allow resting and drying. The pond water level is maintained low during operation to expose as large an area of tailings as possible to air-drying. Air-drying has the benefit of achieving higher density and strength. The deposition of tailings onto a beach (subaerial deposition) via spigots promotes segregation of the tailings. The coarsest tailings generally settle out closer to the point of deposition, with the finer fraction (slimes) transported further. The deposition of tailings on a rotational basis result in local variations in tailings characteristics both in between spigots and transverse to the embankment crest. Changes in ore characteristics can also affect the characteristics of the tailings. FIGURE 11: TAILINGS BEACH IN STORAGE 1A
EGL Ref: 9215 23 June 2022 Page 19 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 4.2.5. Tailings impoundment lining and subsurface drains Storage 2 impoundment is earth lined against the embankment and is unlined on the base and against the hills. Storage 1A is earth lined against the embankment and is unlined on the base. Storage 1A is lined against the hills above approximately RL160. It is unlined below this level. Subsurface drains are installed beneath the tailings and provide some under drainage of the tailings and collection of seepage through the tailings profile. Storage 1A and 2 tailings densities achieved are shown in Figure 12 and Figure 13. The discharge rate of tailings into Storage 2 has varied from approximately 0.8 to 1.0 Mtpa (dry weight) up to 2001 when Storage 1A was commissioned. The discharge rate of tailings in Storage 1A has varied from approximately 1.1 to 1.5 Mtpa (dry weight) from 2002 to 2006. From 2006 to 2015 discharge varied between about 0.5 to 0.9 Mtpa. Since 2016 tailings discharge has been between about 0.4 to 0.45 Mtpa. For the Waihi North Project tailings production will start at 0.8 Mtpa and increase up to about 1.75 Mtpa. FIGURE 12: STORAGE 1A TAILINGS DENSITIES WITH TIME
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