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
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