i Waihi North Project Assessment of Groundwater Effects – Tunnel Elements 43 Moresby Ave Waihi 3610 New Zealand Report Prepared for: Oceana Gold NZ Ltd 14th June 2022 Document Reference: WAI-985-000-REP-LC-0040_ Final Rev 0 This document has been produced for New Zealand consenting purposes only. Information contained herein must not be relied on for investment purposes.
ii Contents 1. Introduction...............................................................................................................1 1.1 Background ........................................................................................................ 1 1.2 Project Description............................................................................................. 1 1.3 Scope of Report.................................................................................................. 3 2. Existing Environment .................................................................................................4 2.1 Regional Geology ............................................................................................... 4 2.2 Regional Hydrogeology ...................................................................................... 5 2.3 Regional Hydrology ............................................................................................ 5 3. Groundwater Effects Assessment – WUG Access Tunnel .........................................7 3.1 Tunnel Description............................................................................................. 7 3.2 Characterisation of Tunnel Alignment ............................................................... 7 3.3 Conceptual Groundwater Model ..................................................................... 11 3.4 Groundwater Effects Assessment.................................................................... 12 4. Groundwater Effects Assessment – Willows Farm Access Tunnel..........................15 4.1 Characterisation of Tunnel Alignment ............................................................. 15 4.2 Conceptual Groundwater Model ..................................................................... 21 4.3 Groundwater Effects Assessment.................................................................... 23 5. Groundwater Effects – WUG Dual Tunnels .............................................................27 5.1 Characterisation of Tunnel Alignment ............................................................. 27 5.2 Conceptual Groundwater Model ..................................................................... 30 5.3 Groundwater Effects Assessment.................................................................... 30 6. Recommendations...................................................................................................34 6.1 Discussion......................................................................................................... 34 6.2 Recommendations for Monitoring .................................................................. 34 7. References ...............................................................................................................36 8. Limitations ...............................................................................................................37
iii Tables Table 1 Aquifer Hydraulic Properties........................................................................ 11 Table 2 Waihi Basin Groundwater Availability.......................................................... 12 Table 3 Willows Farm Hydraulic Conductivity values ............................................... 18 Table 4 Waihi Basin Groundwater Availability.......................................................... 23 Table 5 Stream Depletion Model Results................................................................. 24 Table 6 Hydraulic Conductivity Values...................................................................... 30 Table 7 Otahu Catchment Groundwater Availability................................................ 30 Figures Figure 1 WNP Proposed Tunnel Sections..................................................................... 2 Figure 2 Regional Geologic Setting............................................................................... 4 Figure 3 Regional Surface Water Catchment Extents (from NIWA) ............................ 6 Figure 4 WUG Access Tunnel Alignment ...................................................................... 7 Figure 5 WUG Access Tunnel Profile ............................................................................ 8 Figure 6 Water Table Map in the Location of the Tunnel ............................................ 9 Figure 7 Andesite Piezometric Surface Map .............................................................. 10 Figure 8 WUG Access Tunnel Conceptual Hydrogeological Model............................ 11 Figure 9 Groundwater Users near the WUG access Tunnel....................................... 13 Figure 10 Willows Farm Site Topography................................................................. 15 Figure 11 Mataura Stream Catchment Area and Willows Farm .............................. 16 Figure 12 Willows Farm Exposure Showing Regolith Overlying Andesite Rock ....... 17 Figure 13 Soil Distribution Over the Willows Farm Site ........................................... 17 Figure 14 Monitoring Well Locations and Interpreted Water Table Surface........... 19 Figure 15 Access Tunnel Hydrogeologic Profile........................................................ 20 Figure 16 Willows Farm Hydrogeologic Section 1 .................................................... 20 Figure 17 Willows Farm Hydrogeologic Section 2 .................................................... 20 Figure 18 Conceptual Hydrogeologic Model Section at Willows Farm.................... 22 Figure 19 Catchment Scale Conceptual Hydrogeologic Model at Willows Farm..... 22 Figure 20 Distance Drawdown for Weathered Tuff ................................................. 25 Figure 21 Otahu Surface Water Catchment Extents and Dual Tunnel Alignment ... 28 Figure 22 Topography and Geology along the Dual Tunnel Alignment ................... 29 Figure 23 Conceptual Hydrogeologic Model Section Willows Farm to WUG .......... 31 Appendices Appendix A Ground Models Appendix B Appendix C Tunnel Inflow Assessment Willow Farm Hydrogeologic Field Data Appendix D Groundwater Modelling
iv EXECUTIVE SUMMARY Oceana Gold (New Zealand) Limited is seeking resource consents for the Waihi North Project (WNP) to, amongst other objectives, enable access to the Wharekirauponga mineral resource. The project consists of a number of elements that expand on the existing mining facilities in Waihi, as well as proposing new infrastructure to service the Wharekirauponga Underground Mine (WUG) at Willows Farm north of the Waihi township. One of the key elements of the project is the tunnelling required to connect the Wharekirauponga mineral resource to the proposed Surface Facilities Area and to the existing Processing Plant at Waihi to enable ore extraction, transport and processing. This report considers the likely effects on groundwater that may be associated with development of the tunnelling system needed to enable the WNP to go ahead. This report considers three components of the project that include: a WUG access tunnel from Waihi to Willows Farm; an access drive from Willows Farm that connects to the WUG access tunnel; and dual tunnels from Willows Farm to WUG that also connects to the WUG access tunnel. One aspect that is important to consider in this groundwater effects assessment is the proposed tunnel design. In summary, a tunnelling methodology will be used that mitigates the potential for effects to materialize in groundwater. Experience in Waihi has shown that the andesite rockmass is of a low permeability and does not dewater extensively, rather groundwater is retained in storage within fractures. Dewatering is only noted to occur to any significant degree if younger volcanic rock sequences are penetrated or if a fault or fracture system is encountered. In such circumstances, cement grout is applied in these zones to reduce the permeability and prevent drainage of groundwater from taking place. These zones are identified in advance through drilling, and are grouted off, either in advance of the driven tunnel or, within a few days of it being exposed. This means effects, if any, are short lived and are not expected to affect surface waters. This methodology has been successfully used for underground tunnels at Waihi and is proposed for the WNP tunnels. The WUG access tunnel will be driven north from a new portal sited near the existing portal to the Favona underground workings and south from Willows Farm. The initial southern part of the tunnel decline is already dewatered from the existing underground mining operations and for that reason no further effects on the shallow groundwater system or surface waters beyond that which have already taken place are expected. Once the tunnel is driven into the andesite, minimal groundwater inflow will occur except where large scale faults or fracture systems are encountered. Drilling in advance of the tunnel drive will identify these locations and they will be grout sealed as discussed above. There are a number of domestic and stock bores within reasonable proximity to the WUG access tunnels, however, the water supplies are not considered to be at any risk from the proposed tunnel as the dewatering effects will not extend any significant distance laterally. Groundwater monitoring is proposed in the existing network of wells that surrounds the tunnel decline section near town to ensure nearsurface drawdown effects do not develop. Additional monitoring of groundwater levels
v adjacent to the tunnel is proposed near existing groundwater users to ensure their supply remains unaffected. The WUG access transport tunnel will connect to the Willows Farm site at some 300 m depth below ground level at the location of the first vent shaft and commencement of the dual decline. The Willow access tunnel commences from a portal at the surface of the property at an elevation above the groundwater level. The drive then declines and connects with the WUG access tunnel and dual tunnels. The initial part of the access portal and tunnel will be within the shallow groundwater system hosted by the andesite rocks. The andesite rockmass at Willows Farm has been demonstrated to be of low permeability and, therefore, is not expected to drain readily. In the worst case, our assessment indicates that up to 15 m3/d could potentially be lost due to flow paths being diverted from the Mataura Stream while the access tunnel remains dewatered. There are, however, two locations where the Willows access tunnel drives through inferred fault or fracture zones beneath the Mataura Stream. Given there is a potential short term hydraulic connection between the tunnel and stream bed, an assessment of potential surface water losses was undertaken. This assessment has indicated that the short-term losses from a potential fracture zone would be in the order of 35 m3/d and any surface water losses are considered to be small relative to stream flow and would be indiscernible. The vent shaft at Willows Farm is assumed to be sealed off from groundwater as it is advanced. Some groundwater inflow is expected during construction and these volumes have been incorporated into the predicted dewatering volumes needed for the project. No significant drawdown effects are likely to develop from construction of the vent shaft. Monitoring of shallow groundwater is recommended using the existing network of wells to ensure sustained lowering of groundwater levels does not occur and that there is no potential for long term stream loss. The dual tunnels will be driven from the connection at Willows Farm to WUG at depths ranging from 150 m to 480 m below ground level within andesite. The andesite is the same rockmass present elsewhere in Waihi and will have a similar response to dewatering in that it will be limited to areas immediately adjacent to the tunnels. No effects are expected in the near surface groundwater or on surface waters. There are some locations where inferred structural features will be driven through and these may need to be sealed to prevent groundwater ingress as per the same methodology already stated. There are a further four vent shafts at the end of the dual tunnels and the bottom-up construction methodology will limit groundwater inflows. No significant drawdown effects are likely to develop as a consequence of the vent shaft construction. Given the depth of the dual tunnels and mitigating construction methodology, no groundwater monitoring is deemed necessary, nor is proposed over the alignment.
vi In summary, this assessment of effects has shown there to be minimal risk to shallow groundwater; surface waters; other groundwater users; and to plant growth from the proposed tunnels. The proposed tunnelling methodology will avoid effects to groundwater because: 1. The rockmass is of sufficiently low permeability that it will not dewater 2. The tunnels are sufficiently deep that depressurisation effects do not reach the surface If inflows zones are encountered these will be sealed off to mitigate any effects developing.
SECTION 1 Introduction 1 1. Introduction 1.1 Background Oceana Gold (New Zealand) Limited (OGNZL) is applying for resource consents for the Waihi North Project (WNP). This project has a number of associated elements that are necessary to enable access to the Wharekirauponga Underground (WUG) mineral resource. The WUG resource is located approximately 11 km north of the township of Waihi. The resource lies within the Wharekirauponga Minerals Mining Permit (60541) area and is beneath Department of Conservation (DOC) land. 1.2 Project Description A full description of the Waihi North Project is provided in the Assessment of Environmental Effects prepared by Mitchell Daysh Limited. The elements relevant to this assessment are described below and locations shown in Figure 1. These will include the following. A tunnel to transport ore to Waihi that connects with the Willows Farm site (WUG access tunnel) that will include; • Portal and a single tunnel (WUG access tunnel) near the Processing Plant, that connects with the dual tunnel (4,700m length); • A link or bypass drive that connects the tunnel from the Willows Farm portal (Willows access tunnel) to WUG access tunnel from Waihi (270m length); and • Stockpiles at 150m spacing along the length of tunnel and sumps (every 640m). The Willows Farm access tunnel will include: • Portal and a single tunnel (Willows access tunnel) at Willows Farm to the edge of DOC land (1,300m length); • Vent shaft on the Willows Farm south of the DOC boundary (250m deep); • Stockpiles at 150m spacing along the length of tunnel and sumps (every 640m); and • Surface infrastructure including; waste rock stacks, silt ponds, etc. Dual decline tunnels to access the orebody from Willow Farm to WUG that will include: • Dual tunnel from the edge of DOC land to the footwall of the WUG orebody (5,500m length); • Multiple declines as the dual tunnel approaches the top of the WUG, for access to the lower portions of the orebody (500 – 1,200m length); • Cross cuts at 150m spacing along the length of the dual tunnel, providing a connection between the intake and exhaust tunnels (1,000m length); • Cuddies to cater for infrastructure requirements including ventilation, sumps, pumps, and electrical equipment (200m length); and • Four ventilation shafts of various depths along the paper road corridor at the tunnel approach to the WKP development works.
SECTION 1 Introduction 2 Figure 1 WNP Proposed Tunnel Sections When considering the potential for effects on groundwater due to the construction of the tunnels it is important to understand the mitigating design philosophy. As stated in the project description prepared by OGNZL, the following provides the proposed approach to groundwater management. “Incidental, minor quantities of water emanating from the ground and/or from normal tunnelling operations will be drained to sumps within the tunnels. Thereafter water will be
SECTION 1 Introduction 3 pumped by electric pumps through poly pipe installed as part of mine services to the surface holding tank before treatment through the water treatment plant. Where significant quantities of water are encountered in tunnelling, the ground in the immediate vicinity will be shotcreted and/or grouted to provide an effective seal and prevent any significant and/or sustained drainage of local aquifers.” Simply put, the tunnel will be designed to limit the potential for groundwater effects to develop as it is constructed and this premise sits behind this effects assessment. All level information in this report is based on a mine datum set at 1,000m below a pre-1949 geodetic datum. The current standard, New Zealand Vertical Datum 2016 (NZVD2016), is approximately 1,002m above the mine datum. That is, reduced levels stated in this report can be reduced by 1,002m to approximate the same level to NZGD2000/NZVD2016. 1.3 Scope of Report This document describes the groundwater conditions and potential effects on groundwater associated with the development of the proposed tunnels that will be driven from Waihi to Willows Farm and from Willows Farm to the WUG ore deposit. The groundwater effects assessment associated with the development of the WUG resource itself is included in a separate report prepared by GWS (WAI-985-000-REP-LC-0030A). We note that this report does not include any effects on groundwater associated with the Willows Farm surface infrastructure (e.g. waste rock stacks, silt ponds, etc.) other than that associated with the tunnel elements. Surface infrastructure effects are included in the GHD hydrogeology report (WAI-985-000-REP-LC-0012). This assessment of effects on groundwater relates to the development of the portals, shafts and the tunnels. The purposes of this assessment are to determine: • Groundwater inflows to the tunnel elements. • Drawdown effects related to the tunnel elements. • Potential for effects on aquifers. • Potential for effects on surface waters. • Potential for effects on other groundwater users. • Potential for effects on plant growth. This report has been prepared based on recent and historical information from adjacent areas that provides an understanding based on a long association with groundwater systems in the area. This understanding has been taken forward alongside the project scope and has included further technical analysis to enable potential associated effects to be quantified.
SECTION 2 Existing Environment 4 2. Existing Environment 2.1 Regional Geology The following provides a general description of the geology along the entire tunnel alignment. More detailed descriptions of the geological conditions for each tunnel section are provided in the ground models prepared by GHD (Aug, 2020) and Golder (Sept, 2021). These are included in Attachment A of this report. The proposed works are located towards the Southern part of the Coromandel Volcanic Zone, a Miocene to early Pliocene andesite-dacite-rhyolite, subaerial volcanic sequence. The Coromandel Ranges are flanked to the west by the Firth of Thames, a Northward continuation of the Hauraki Rift, and to the east by the Pacific Ocean (Braithwaite & Christie, 1996). Figure 2 Regional Geologic Setting (Braithwaite & Christie, 1996) The most extensive geological unit in the area is the Waiwawa sub-group (7.9-5.6 Ma) of the Coromandel Group. This unit comprises andesite and dacite lava flows and tuff breccias, and dacitic ignimbrite, tuff and siltstone. Hydrothermal alteration has been reported. A well-defined NNE structural alignment and subsequent erosion has exposed both younger Omahine subgroup (6.7-6.6 Ma) which will be intercepted partway along the dual tunnel alignment and Kaimai subgroup (5.6-3.9 Ma) rocks which lie to the east of the portal area. The Omahine subgroup comprises andesite and dacite, intrusive andesites and lava flows, with minor intercalated tuff and tuff breccia. The Kaimai subgroup comprises andesite and
SECTION 2 Existing Environment 5 dacite intrusives, lava flows and domes, tuff and tuff breccias with intercalated volcaniclastic sediments and local welded dacitic ignimbrite. Older rocks of the Coromandel Group have been emplaced by faulting. These rocks comprise lithic and pumice-rich ignimbrites and local rhyolite and obsidian-rich pumice breccia deposits and tuff. Extensive hydrothermal alteration occurs locally. The rocks will be intercepted at the termination of the tunnel. Tauranga Group sediments infill faulted and erosional depressions. These materials comprise pumiceous alluvial gravelly sand, silty clay and peat; estuarine silt and mud interbedded with ignimbrite; and tephra from the Taupo Volcanic zone and are the host rocks of the Wharekirauponga deposit. A northeast trending fault is inferred in the Waiharakeke valley with a strong north to northeast trending fault block at the tunnel termination. Less prominent faulting may occur along the other valleys and, if present, may be penetrated by the proposed tunnel. 2.2 Regional Hydrogeology Groundwater distribution and movement in the area will be controlled by the topography, together with the stratigraphy and structural trends. Recharge would be expected to occur in the elevated areas with downward moving groundwater. In the deeply incised valleys, upward moving groundwater (discharge) would be expected. The quantum of groundwater movement would depend on the particular type of deposit present, modified by postdepositional structures and alteration and weathering. Where fracturing has developed, such as typically in lavas, groundwater movement may be greater. Fine grained tuffs would have lesser groundwater movement. Fault zones, along which valley systems have eroded lengthwise and downwards, are linear features and are expected to concentrate groundwater and can act as both conduits and/or perpendicular impediments to groundwater movement depending on whether faulting was extensional or compressional. Hydrothermal alteration can result in clay-rich fault zones which can impede groundwater flow. Underground mine development at Waihi and the Waitekauri Valley Golden Cross mine have encountered low groundwater inflows outside the vein systems in hydrothermally altered rocks. Such rocks are expected to be encountered towards the completion of the exploration drive and while zones of altered rock may be encountered along the exploration drive alignment, the majority of the rock units encountered are likely to be unaltered. Faults are expected as the alignment passes beneath valleys and possibly beneath defined stream locations. 2.3 Regional Hydrology The tunnels elements traverse two surface water catchments. The Willows Farm access tunnel and the WUG access tunnel fall within the Waihou surface water catchment area. The Waihou catchment is a large catchment (circa 1,990 km2) extending from Rotorua to the Firth
SECTION 2 Existing Environment 6 of Thames. The Mataura Stream and Walmsley Stream that bound the Willows Farm property to the north and south respectively, join the Ohinemuri River which then flows to the west through the Karangahake Gorge to ultimately discharge into the Waihou River. The majority of the WUG dual access tunnels traverses the Otahu surface water catchment. This is a smaller catchment by comparison being some 71 km2 in size. This catchment drains to the north east towards Whangamata and discharges via the Otahu River. Figure 3 shows the extents of the Waihou and Otahu surface water catchments. Figure 3 Regional Surface Water Catchment Extents (from NIWA)
SECTION 3 Groundwater Effects Assessment – WUG Access Tunnel 7 3. Groundwater Effects Assessment – WUG Access Tunnel 3.1 Tunnel Description The WUG access tunnel will be a single tunnel driven from a newly constructed portal at the water treatment plant (WTP) near the existing Favona portal in Waihi. The portal will start at an elevation of approximately 1125 mRL and will decline vertically by some 180 m over a 1,500 m distance. The remainder of the tunnel out to the Willows Farm connection is a gentle incline of around 50 m over 4,000 m. It is anticipated that the tunnel will be advanced at a rate of around 8-10 m/d and will be driven from both ends to meet in the middle to avoid the need for ventilation shafts. The tunnel alignment is shown on Figure 4. Figure 4 WUG Access Tunnel Alignment 3.2 Characterisation of Tunnel Alignment 3.2.1 Physiography Along the first 2,500 m of the tunnel from the portal to SH25 the topography is generally flat lying at an elevation of approximately 1120 mRL. North of SH25, the topography steepens into the Coromandel Ranges to a maximum of around 1250 mRL beneath the Willows Farm site. For the first part of the alignment through the Waihi township the tunnel is approximately 90 m below ground level. Beneath the Willows Farm site, it reaches a maximum of 275 m below ground level below the crest of a hill.
SECTION 3 Groundwater Effects Assessment – WUG Access Tunnel 8 3.2.2 Hydrology The WUG access tunnel is within the Waihi Basin surface water catchment which drains to the west. The main channel is the Ohinemuri River and this is east of the proposed tunnel alignment. The tunnel does not pass beneath the Ohinemuri River but will be driven below tributaries to the river. 3.2.3 Geology The geology of the WUG access tunnel is described in detail (Golder, Sept 2021) and shown in cross section in Figure 5. In summary, the tunnel will pass through Waipupu Formation Andesite (aw) which consists of andesitic flows, breccias, tuffs some of which is hydrothermally altered. The tunnel will then pass through the younger Whitiroa Andesite (ah), being andesitic flows, breccia and tuffs, before returning back into Waipupu Formation Andesite. The younger andesite is present in the mid-section of the alignment due to being in a down thrown block that is bounded by regional scale faults. It is expected that there will be fracture zones associated with these faults and that ground conditions will be weaker than the general andesite rockmass. These zones are expected to be permeable and will allow some groundwater inflow prior to grout sealing. Figure 5 WUG Access Tunnel Profile (Modified after Golder, Sept 2021) 3.2.4 Hydrogeology At the location of the WUG access tunnel the groundwater system consists of surficial deposits of alluvium and younger volcanic materials that host a shallow water table as shown in Figure 6. These deposits have formed in a paleo channel on the surface of the underlying andesite rocks. Groundwater flow is in a south east direction driven from heads in the Coromandel Ranges. The proposed WUG access tunnel does not intercept these materials.
SECTION 3 Groundwater Effects Assessment – WUG Access Tunnel 9 Figure 6 Water Table Map in the Location of the Tunnel Beneath the shallow groundwater system, groundwater is present within the andesite rockmass as shown in Figure 7. The rockmass along the first section of the tunnel is already dewatered from mining of the Martha and Favona vein system.
SECTION 3 Groundwater Effects Assessment – WUG Access Tunnel 10 Figure 7 Andesite Piezometric Surface Map Groundwater Levels The water table surface shown generally reflects the topography, except to the east of Martha Pit where the water table has been affected by drainage due to the exposure of younger volcanic rocks in the pit. At the location of the WUG access tunnel, the inferred water table is relatively flat and lies between approximately 1110 m RL near Union Hill rising to 1120 m RL at Wharry Road. At the decline section of the tunnel, in the vicinity of the WTP, groundwater monitoring (P60, P61, P64, P75) indicates a lowered or absent water table in the near surface and depressurised conditions in the andesite due to existing mine dewatering. Hydraulic Gradients The groundwater flow direction in the area of the southern half of the WUG access tunnel is to the west and the tunnel will be perpendicular to the groundwater flow direction. In this area the hydraulic gradient is relatively flat being around 0.001. As the WUG access tunnel passes beneath the hill approaching Willows Farm, the hydraulic gradient steepens to around 0.04. A downward vertical gradient is expected throughout much of the tunnel alignment, with an upward gradient and discharge zone likely near the Ohinemuri River.
SECTION 3 Groundwater Effects Assessment – WUG Access Tunnel 11 Aquifer Parameters No site-specific testing has been undertaken to characterise the properties of the rock through which the WUG access tunnel will be driven. These geologic units are, however, the same as those mined in Waihi and have been previously characterised as shown in Table 1. Table 1 Aquifer Hydraulic Properties Hydraulic Conductivity Storativity Material Max (m/s) Min (m/s) Geomean (m/s) Max Min Shallow Aquifers Ash / Alluvium 1 x 10-4 1 x 10-7 0.3 0.1 Ignimbrite 1 x 10-5 1 x 10-8 0.01 0.001 Rhyolitic Tephra 1 x 10-6 1 x 10-7 0.1 0.05 Deep Aquifer Andesite Surface 3 x 10-5 2 x 10-6 5 x 10-6 0.3 0.1 Andesite to 50 m Depth 7 x 10-9 6 x 10-9 0.01 0.005 Andesite to 100 m Depth 6 x 10-7 6 x 10-9 3 x 10-8 0.01 0.005 Andesite 1 x 10-5 1 x 10-8 0.05 0.001 For the purpose of the groundwater effects assessment that follows, values within the identified ranges have been adopted. 3.3 Conceptual Groundwater Model A conceptual hydrogeologic model for the WUG access tunnel along the alignment is presented in Figure 8. In summary, based on previous studies and what we know about the area, the model assumes that the initial part of the decline is already dewatered from underground mining. Figure 8 WUG Access Tunnel Conceptual Hydrogeological Model
SECTION 3 Groundwater Effects Assessment – WUG Access Tunnel 12 At some point along the tunnel decline, fully saturated conditions will be encountered. As the tunnel is driven, groundwater will be intercepted and the adjacent rockmass will be depressurised. Dewatering to the ground surface will not take place due to the relatively low permeability of the andesite and the perched shallow groundwater system which has substantially greater storage and rainfall recharge. 3.4 Groundwater Effects Assessment 3.4.1 Groundwater Inflows Groundwater inflows for the tunnel have been adopted from the groundwater inflow assessment included in Attachment B. This assessment indicates up to 3,000 m3/d groundwater will be taken from the Waihi Basin catchment during construction and returned to that catchment after treatment. 3.4.2 Groundwater Availability The WUG access tunnel is located within the Waihi Basin aquifer management area as identified by the Waikato Regional Council (WRC, 2012). This catchment is further subdivided into the Waihi Basin shallow aquifer system (0.5 to 30 m depth) and the Waihi Basin deep aquifer system (>30 m depth), however the resources are managed as one. The availability of groundwater for the Waihi Basin is shown in Table 2. Table 2 Waihi Basin Groundwater Availability Management Limit a 6,000,000 m3/year Existing Allocated 4,155,000 m3/year Available b 1,845,000 m3/year Other WNP Takes (GOP, TSF3) c 521,950 m3/year WUG Access Tunnel d 1,095,000 m3/year Total WNP Takes 1,616,950 m3/year Remaining 228,050 m3/year a - Combined shallow and deep limits b – WRC advised 23/11/2021 c – Based on GOP take of 1,100 m3/d and TSF3 take of 330 m3/d for 365 days d- based on 3,000 m3/d for 365 days On the basis of this assessment, there is sufficient groundwater available for the proposed take. 3.4.3 Potential for Effects on Springs and Streams Groundwater modelling has been undertaken to assess the effects of the tunnel on the near surface environment. The modelling has indicated that once the tunnel is 20 to 30 m below the ground surface, depressurisation effects are limited to the rockmass surrounding the tunnel with no connection with the surface or shallow groundwater system expected. Given that the tunnel decline is already dewatered to a depth of approximately 70 m below the ground surface, and the tunnel will continue to be driven at a depth greater than that, no
SECTION 3 Groundwater Effects Assessment – WUG Access Tunnel 13 further drainage effects are expected in the near surface. Therefore, the potential for effects on streams and springs is considered to be negligible. 3.4.4 Potential for Effects on other Groundwater Users Figure 9 shows the locations of groundwater users adjacent to the proposed tunnel alignment. Two of these bores (72_5193 and 72_771) are 86 m deep and come to within 400 m proximity to the proposed WUG access tunnel. These bores are small diameter and do not have associated groundwater take consents and are assumed to be for domestic or stock purposes. Another bore (72_1223) is within closer proximity to the tunnel but there are no construction details. If this bore exists it too is expected to be a domestic or stock water supply. Figure 9 Groundwater Users near the WUG access Tunnel Experience with tunnelling in Waihi and groundwater modelling both indicate the lateral effects of depressurisation around the tunnel will be limited due to the andesites low rockmass permeability. For this reason, we do not consider it likely that groundwater users will be adversely affected by the proposed WUG access tunnel.
SECTION 3 Groundwater Effects Assessment – WUG Access Tunnel 14 3.4.5 Potential for Effects on Aquifers The groundwater take will be from the deep rockmass and, as mentioned in report section 3.4.3, dewatering effects extending back to the near surface are expected to be negligible due to the low permeability andesite rockmass the tunnel will be driven through. The tunnel section will be perpendicular to the main direction of groundwater flow in the catchment and will intercept some flow paths locally, but will not affect the overall flow regime. The location where effects could have been expected in the near surface is the initial portal and first part of the decline, however, dewatering of the deep rockmass has already taken place due to underground mine dewatering. Taking groundwater from the deep aquifers is, therefore, not expected to affect water levels in the overlying aquifers and we, therefore, consider the potential for effects to be less than minor. 3.4.6 Potential for Effects on Groundwater Quality During tunnel dewatering there will be no consequential change in groundwater quality due to the water take. Groundwater will seep into the tunnel at a low rate, with cement grouting reducing localised inflows. The groundwater that flows into the tunnel will be pumped back to the treatment plant in Waihi and discharge to surface waters at a permissible standard. Once the tunnel is no longer required rewatering will occur and the groundwater system will return to its previous state. Some groundwater will come into contact with the cement grout, however this is not expected to change the overall quality in the aquifer due to the limited contact area relative to the system throughflow. In summary, no adverse effects on groundwater quality are expected from development of the tunnel. 3.4.7 Potential for Saline Intrusion The WUG access tunnel is 7 km from the ocean which is too far inland for any effect to develop given the low permeability of the andesite rockmass. For this reason, we consider the potential for saline intrusion to occur to be less than minor. 3.4.8 Potential for Ground Settlement Effects In the near surface, where compressible soils exist, no dewatering effects are expected beyond that which has already occurred due to existing mining activities. Where driven through the deep andesite rockmass, ground depressurisation will occur immediately around the tunnel, however the effects will not be laterally extensive and no significant settlement risk is considered likely. The primary rockmass being dewatered is the Rhyolite body and this is a hard, incompressible medium and is not expected to consolidate significantly as a result of dewatering. This has been assessed in detail in the EGL (WAI-985-000-REP-LC-0050) report. 3.4.9 Potential for Effects on Plant Growth Any dewatering associated with the WUG access tunnel will be in the deep rockmass. Soil moisture conditions in the regolith soils or terrace deposits in the near surface are not expected to change as a consequence of dewatering at depth. We, therefore, consider the effects of the WUG access tunnel dewatering on plant growth to be less than minor.
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 15 4. Groundwater Effects Assessment – Willows Farm Access Tunnel 4.1 Characterisation of Tunnel Alignment 4.1.1 Physiography Figure 10 shows the proposed access tunnel in relation to the site topography. The site is shown to slope north eastwards towards the Mataura Stream. The slope is cut by one prominent north east trending side gully and several smaller gullies. Slopes range from approximately 1v:1.5h (33 degrees) to 1v:7.7h (8 degrees). The steeper slopes occur in the gullies while the shallow slopes occur closer to the Mataura Stream. The portal would be initiated on slopes up to approximately 1v:2.7h (22 degrees); the shaft on slopes up to 1v:3.5h (16 degrees); and the infrastructure on slopes up to 1v:5.8h (10 degrees). Figure 10 Willows Farm Site Topography 4.1.2 Hydrology The location of the Mataura surface water catchment is shown in Figure 11. This catchment is 6.5 km2 in size and drains southeast to join the Ohinemuri River. The Willows Farm property occupies approximately one third of the lower end of the catchment. The upper reaches of the catchment are steep and high run-off resulting in high stream flows is observed during and after rainfall. Stream baseflow is expected to be mostly sourced from the shallow regolith soils, with low flows fed by rockmass discharge. The tunnel crosses beneath the Mataura Stream in andesite at a depth of approximately 225 m and the position of the Mataura Stream where the tunnel passes beneath it is shown in Figure 11.
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 16 Figure 11 Mataura Stream Catchment Area and Willows Farm 4.1.3 Soils and Geology The majority of the site soils are indicated to be primarily residual soils as shown on Figure 12, with a weathered regolith overlying volcanic rock. Given indicated surface slopes, down slope movement would be expected to maintain reduced soil cover on the steeper slopes with an increased thickness of the soil profile on the lower slopes. On the flatter parts of the site near the Mataura Stream terrace deposits of alluvial material are measured to a depth of 7 m, with two levels of terraces apparent. Figure 13 shows the distribution of the soil types at the site. The primary soil mapped at the portal and infrastructure sites is Otorahanga orthic allophanic loam (well drained, moderate permeability), while at the proposed vent shaft site, Figure 8 shows Moehau 2 acidic orthic brown loam soils (well drained, moderate permeability). The geology of the site is included in the ground model prepared by GHD (August, 2020) and this has been complemented by an investigation program that has included test pits, boreholes and geotechnical testing. The data from the investigations relevant to this assessment are included in Attachment C. In general terms, the site is noted to consist of a depth of primary weathered rock and/or pyroclastic deposits that are weathered to form clay and silt soils. These materials are a few metres thick on the steeper slopes (Figure 8) and thicken in the topographic lows to some 7 to 15 m thick. Beneath these soils either lies relatively fresh andesite rock in the northern part of the site (Waipupu Andesite) or completely weathered tuff (Whiritoa Andesite). In the low-lying areas adjacent to the Mataura Stream alluvial terrace deposits exist consisting of silty gravel sands. These materials directly overly the completely weathered tuff.
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 17 Figure 12 Willows Farm Exposure Showing Regolith Soils Overlying Andesite Rock Figure 13 Soil Distribution Over the Willows Farm Site
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 18 4.1.4 Hydrogeology Groundwater Levels A total of 20 machine drilled boreholes were completed as shallow groundwater monitoring wells during the geotechnical site investigations of Willows Farm. In addition, a vibrating wire piezometer with 3 tips was installed at the location of the proposed ventilation shaft. Figure 14 shows the locations of the monitoring wells, the groundwater elevations and interpreted water table surface. Figure 15 shows the hydrogeology along the access tunnel profile. Figures 16 and 17 provides hydrogeologic sections 1 and 2 at the locations shown in Figure 14. In general terms, in those wells at higher elevations the water table is 10’s of metres below ground level. At lower elevations the depth to groundwater is between 1 to 5 m. At two locations (WFBH001 and WFBH0011) there is a water table in the upper pyroclastic materials and lower-level groundwater present in the volcanic rock. The water level difference in WFBH0011 is relatively small being 1.7 m while at WFBH001 this is 6.2 m. These observations suggest perching of groundwater occurs in the shallow materials overlying the volcanic rockmass. Hydraulic Gradients The interpreted water table surface shows the topography of the site is the primary feature driving groundwater heads that show a close relationship to site morphology. Hydraulic gradients vary over the site depending on the local land forms but is on average 0.05 to 0.06 over much of the property, flattening to 0.02 in the central area and with locally steep gradients up to 0.1 near the Mataura Stream. Vertical hydraulic gradients are observed to exist at the vent shaft location where WNDD007 indicates a vertically downward gradient in the range of 0.02 to 0.06. Aquifer Parameters Rising head tests were undertaken on all of the monitoring wells constructed on the site. In addition to the testing undertaken on the monitoring wells, falling head tests and packer tests were undertaken on WNDD007. A summary of the results of these testing is included in Table 3. Table 3 Willows Farm Hydraulic Conductivity values Monitoring Well Hydraulic Conductivity (m/s) Min Max Weathered Tuff 5.3 x 10-7 2.2 x 10-6 Terrace Gravel 3.3 x 10-8 1.1 x 10-4 Sandy Soils 1.1 x 10-6 1.2 x 10-6 Silt Soils 2.0 x 10-7 1.7 x 10-5 Silt/Clay Soils 1.1 x 10-7 2.3. x 10-7 Altered Tuff 5.7 x 10-8 8.8 x 10-8 Tuff 1.1 x 10-6 7.1 x 10-6 Andesite 1.3 x 10-8 5.0 x 10-7
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 19 Figure 14 Groundwater Monitoring Well Locations and Interpreted Water Table Surface (Note: Groundwater Level Elevations not Referenced to Mine Datum of +1,000 m RL)
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 20 Figure 15 Access Tunnel Hydrogeologic Profile Figure 16 Willows Farm Hydrogeologic Section 1 Figure 17 Willows Farm Hydrogeologic Section 2
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 21 Overall, the testing indicates that there are some permeable soils in the near surface, but that most of the materials beneath the Willows Farm site have relatively low hydraulic conductivity. The main geological unit the access tunnel is to be driven through is the andesite, which has a geometric mean of 3.0 x 10-8 m/s. This is similar to that measured elsewhere in Waihi. Given the rockmass being dewatered is of low permeability, any associated dewatering effects are expected to be limited. 4.2 Conceptual Groundwater Model The conceptual geologic model for the site underpins the groundwater effects assessment in that it identifies risk pathways associated with dewatering. It also forms the basis for the numerical modelling undertaken to quantify the drainage risks. The conceptual model for the Willows Farm site is described as follows and is illustrated in Figure 18 and 19. • Rainfall that does not run-off infiltrates the soil profile • High permeability shallow soils store recharge water with some of this water moving downslope (interflow) • Water moving down the slopes and direct rainfall infiltration results in a perched water table locally in the regolith and terrace deposits • Interflow water continues to move down slope to the Stream • Some rainfall infiltration percolates down into the deeper rockmass with saturation below the perched water table in the regolith • Flow paths then result in deep groundwater discharge to the Stream as baseflow • Deep groundwater flow moves down gradient though the catchment • Fracture zones that are orthogonal to the flow direction intercept some of this groundwater • Higher permeabilities in the fracture zones results in preferential groundwater flow down the length of the zone resulting in high discharge zones in the Mataura Stream. Based on this conceptual model, the key risk to understand is how much stream flow will be intercepted as the access tunnel passes though the fracture zones prior to these zones being sealed off. The risk is higher at these locations due to the assumed higher permeability values and given the tunnel is still relatively shallow as it continues on a descent. However, intercepted water is to be diverted to the water treatment plant before being discharged to the Ohinemuri River. This water is not lost from the greater catchment.
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 22 Figure 18 Conceptual Hydrogeologic Model Section at Willows Farm Figure 19 Catchment Scale Conceptual Hydrogeologic Model at Willows Farm (red line is the tunnel alignment)
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 23 4.3 Groundwater Effects Assessment 4.3.1 Groundwater Inflows Groundwater inflows for the Willows Farm access tunnel have been adopted from the groundwater inflow assessment included in Attachment B. This assessment indicates that the decline would generate in the order of 500 m3/d groundwater from the rockmass during construction. 4.3.2 Groundwater Availability The Willows Farm access tunnel sits just outside of the Waihi Basin aquifer management area as identified by the Waikato Regional Council (WRC, 2012), but for the purpose of this assessment has been included in the availability calculations to remain conservative. The availability of groundwater has been determined as shown in Table 4. Table 4 Waihi Basin Groundwater Availability Management Limit a 6,000,000 m3/year Existing Allocated 4,155,000 m3/year Available b 1,845,000 m3/year Other WNP Takes (GOP, TSF3) c 521,950 m3/year WUG Access Tunnel d 1,095,000 m3/year Willows Farm Decline e 182,500 m3/year Total WNP Takes 1,799,450 m3/year Remaining 45,550 m3/year a - Combined shallow and deep limits b – WRC advised 23/11/2021 c – Based on GOP take of 1,100 m3/d and TSF3 take of 330 m3/d for 365 days d- based on 3,000 m3/d for 365 days e- based on 500 m3/d for 365 days On the basis of this assessment, there is sufficient groundwater available for the proposed take. 4.3.3 Potential for Effects on Springs and Streams The potential for effects of the tunnel construction on springs and stream flow have been undertaken using numerical modelling in SEEP/W (R2 2019). This entailed constructing a model section that replicates the hydrogeologic conditions perpendicular to the access tunnel across Willows Farm assuming three scenarios; • Assuming high permeability conditions replicating preferential flow along a fracture zone (K = 1 x 10-5 m/s) • Assuming typical rockmass being fresh andesite (K = 2.5 x 10-8 m/s) • Assuming typical rockmass being fresh weathered tuff (K = 1.0 x 10-7 m/s)
SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 24 The critical observation point in these models is the change in baseflow to the Mataura Stream, the results of which are provided in Table 5. Table 5 Stream Depletion Model Results Lithology Stream Loss (L/s) Weathered Tuff 0.64 Andesite Rock 0.17 Fracture Zones 0.39 The model calculations assume the Andesite and Tuff rockmass would be free draining and that the fracture zones (3 zones each 5 m wide) would be sealed after 14 days. So, while there could be a short-term drainage effect in the fracture zones, this would not result in long term baseflow loss. This being the case, the baseflow loss in the Mataura Stream due to the construction of the tunnel in the long term would be that lost from diversion of flow paths in the andesite being some 15 m3/d. In the context of the baseflow in the Mataura Stream this amount of stream water loss would be indiscernible. On this basis we consider the effects to surface water due to the construction of the tunnel to be less than minor. 4.3.4 Potential for Effects on other Groundwater Users There is only one registered bore (72_10311) that is within proximity to the tunnel. This bore is 1.2 km from the closest point to the tunnel and is 200 m deep. Given the bore diameter of 120 mm and the site location (33 Highland Road), the bore is likely used for domestic and stock purposes. Given the separation distance between the bore and the tunnel, it is down gradient of the tunnel, and assessing the limited extent of dewatering the tunnel causes, the effects of constructing the tunnel will not be discernible in the bore. For these reasons we consider the potential effects on other users to be less than minor. 4.3.5 Potential for Effects on Aquifers The groundwater take will be from the Waipupu and Whiritoa volcanic rocks that form the upper most aquifer along the length of the tunnel alignment. Taking groundwater from these aquifers is, therefore, not expected to affect other aquifers as the shallow system is perched and while recharge will move downwards, there is a disconnect between shallow saturation and deep saturation. The tunnel section will be perpendicular to the main direction of groundwater flow in the catchment and will intercept some flow paths locally, but will not affect the overall flow regime. On this basis we therefore consider the potential effects on other aquifers from construction of the access tunnel to be less than minor. The vent shaft at Willows Farm will be similar to a large diameter bore hole that will be continuously lined to prevent the ingress of groundwater. During construction there will be some localised drawdown of the groundwater system around the shaft. Following construction of the shaft the groundwater system will return to its previous state. The shaft will be constructed entirely within the Waipupu and Whiritoa volcanic rocks that constitutes one aquifer system. Construction of the shaft will not, therefore, result in the mixing of previously isolated aquifers.
RkJQdWJsaXNoZXIy MjE2NDg3