www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 11 FloSolutions completed a robust re-evaluation of the packer test results to confirm the general distribution of permeability values across the test zones (Section 5.3.1). The VWP installations, consisting of 9 sensors in the three drillholes indicate a strong downward hydraulic gradient with a perched shallow system in the superficial geology. 3.3. POTENTIAL SURFACE WATER EFFECTS The access tunnel from the southern portal to the CFP boundary lies beneath the Waihi Basin surface water catchment with the main channel of the Ohinemuri River draining to the west and lying to the east of the proposed tunnel alignment. The tunnel does not pass beneath the Ohinemuri River but will be driven below tributaries to the river. Beyond the divide between the Otahu and Waihou catchments in the Coromandel Ranges, the tunnel alignment lies beneath the Wharekirauponga sub-catchment. 3.3.1. POTENTIAL TUNNEL EFFECTS GWS have completed both groundwater monitoring studies and assessments of effects from tunnelling and mining operations (ref 1). GWS conclude that the bulk of the formation through which the tunnel is constructed consists of low-permeability, low-storativity andesite with any groundwater being stored and transmitted in fractures. Dewatering is therefore largely limited to management of the groundwater contained in fault and vein systems. It is likely that some of these fractures connect with the surface water system where the impacts of uncontrolled dewatering and groundwater drainage within the tunnel could cause an impact. Control of underground groundwater inflow through an appropriate grouting method applied to these fractures as part of an engineered mitigation is therefore required, with dewatering at the surface needing to be avoided. The initial southern part of the WUG Access Tunnel decline is already dewatered from the existing Favona 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. Within the intact fresh andesite minimal tunnel inflows are expected, indicatively being 15m3/d (GWS). At two locations the tunnel alignment passes through fault or fracture zones (Section 3.1) which may be hydraulically connected to the Mataura Stream. Surface water losses in these situations (lost from diversion of flow paths in the andesite) amount to about 15m3/d (GWS) if uncontrolled from an underground position. According to GWS, this amount of stream water loss would be indiscernible in the context of the baseflow in the Mataura Stream. Assessment of the anticipated ground and groundwater conditions along the tunnel alignment is not achievable before construction commencement due to access restrictions. Geophysical investigations (CSAMT) are proposed as a non-intrusive method to aid in the assessment of tunnel conditions with the observational method (i.e., forward probing ahead of the advancing face) adding to the assessments presented in this report. The assessment of effects on groundwater includes the following: • Groundwater inflows to the tunnel elements. • Drawdown effects relate to the tunnel elements. • Potential for effects on aquifers. • Potential for effects on surface waters. • Potential for effects on other groundwater users.
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