SECTION 5 Groundwater Effects Assessment – WUG Dual Tunnels 32 On the basis of this assessment, there is sufficient groundwater available for the proposed take. 5.3.3 Potential for Effects on Springs and Streams The effects of the tunnel on springs and stream flow have been undertaken using numerical modelling in SEEP/W (R2 2019). A long section was developed to enable a simulation of the tunnel passing beneath the Waiharakeke Stream to assess what stream losses might occur without mitigation being put in place. A second model section was also developed that simulates the plane of the fault to assess near surface effects. To provide a conservative assessment the models assume free draining conditions exist for 30 days before the tunnel is sealed. The model results indicate a maximum of up to 520 m3/d could be diverted before grout mitigation is put in place to seal off any inflows. In the context of the baseflow in the Waiharakeke Stream, this amount of stream water loss would likely be indiscernible. On this basis we consider the effects to surface water due to the construction of the tunnel to be less than minor. 5.3.4 Potential for Effects on other Groundwater Users There are three groundwater bores in the Otahu Catchment that are >100 m deep. These bores are least 6 km from the closest point of the tunnel. Given the separation distance between the bores and the tunnel, the bores being down gradient of the tunnel, and assessing the limited extent of dewatering the tunnel causes, the effects of constructing the tunnel would not be discernible in the bores. For these reasons we consider the potential effects on other users to be less than minor. 5.3.5 Potential for Effects on Aquifers The groundwater diversion will be from the Waipupu and Whiritoa volcanic rocks that will be intercepted along the length of the tunnel alignment. 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. Taking groundwater from these rocks is, therefore, not expected to affect other rocks nor the perched regolith aquifer and we, therefore, consider the potential effects on shallow aquifers to be less than minor. The vent shaft 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 and it is considered that the potential effects on other aquifers from construction of the vent shaft to be less than minor.
RkJQdWJsaXNoZXIy MjE2NDg3