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.
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