SECTION 5 Groundwater Effects Assessment – WUG Dual Tunnels 27 5. Groundwater Effects – WUG Dual Tunnels 5.1 Characterisation of Tunnel Alignment 5.1.1 Physiography Figure 21 shows the topography above the proposed dual tunnel alignment based on the GHD ground model (August, 2020). Ridge elevations are shown to extend to over 1480 m with the deepest valley deepening to approximately 1150 m. Surface gradients along the main drive are expected to be similar to Willows Farm being up to 45 degrees in the upper slope reducing to 22 degrees in the mid slopes and flatter areas locally being less than 10 degrees. 5.1.2 Hydrology Figure 21 shows the location of the Otahu surface water catchment. The position of the Waiharakeke Stream where the tunnel passes beneath is shown in Figure 22. The tunnel passes beneath the Waiharakeke Stream at a depth of 1150 m and also crosses the headwaters of a second branch of the Waiharakeke Stream and Thompson Stream and stops short of the Wharekirauponga Stream. The upper reaches of the catchment are steep and high surface run-off is expected resulting in high stream flows during and after rainfall. Stream baseflow is expected to be mostly sourced from the shallow regolith soils, with low flows fed by rockmass discharge. 5.1.3 Soils and Geology Surface geology mapping has been undertaken by a number of parties in past years along and around the tunnel alignment for the purpose of mineral exploration. This information, along with the published mapped geologic units, is included in the ground model prepared by GHD (August 2020) and is included as Figure 20. The information contained within the geologic model summarises the present level of geological knowledge along the alignment and has been used as the basis for undertaking this effects assessment. 5.1.4 Hydrogeology There have been no intrusive groundwater investigations undertaken along the tunnel alignment prior to this assessment being prepared. This is considered justified based on the geology being similar to that at Waihi and the proposed tunnelling methodology that will ensure drainage effects are avoided or managed to be minimal. This includes sealing any high inflow zones and allowing only rockmass drainage to occur. This means any drainage effects will be localised to around the tunnel and not develop in the near surface due to the relative depth of the tunnel. Figure 23 shows a generalised hydrogeologic section along the exploration tunnel profile. Groundwater Levels For the purpose of calculating groundwater inflows the groundwater elevations have been calculated with an algorithm that uses the observed vertical hydraulic gradients at Willows Farm to determine heads based on surface elevation.
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