SECTION 3 Groundwater Effects Assessment – WUG Access Tunnel 14 3.4.5 Potential for Effects on Aquifers The groundwater take will be from the deep rockmass and, as mentioned in report section 3.4.3, dewatering effects extending back to the near surface are expected to be negligible due to the low permeability andesite rockmass the tunnel will be driven through. 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. The location where effects could have been expected in the near surface is the initial portal and first part of the decline, however, dewatering of the deep rockmass has already taken place due to underground mine dewatering. Taking groundwater from the deep aquifers is, therefore, not expected to affect water levels in the overlying aquifers and we, therefore, consider the potential for effects to be less than minor. 3.4.6 Potential for Effects on Groundwater Quality During tunnel dewatering there will be no consequential change in groundwater quality due to the water take. Groundwater will seep into the tunnel at a low rate, with cement grouting reducing localised inflows. The groundwater that flows into the tunnel will be pumped back to the treatment plant in Waihi and discharge to surface waters at a permissible standard. Once the tunnel is no longer required rewatering will occur and the groundwater system will return to its previous state. Some groundwater will come into contact with the cement grout, however this is not expected to change the overall quality in the aquifer due to the limited contact area relative to the system throughflow. In summary, no adverse effects on groundwater quality are expected from development of the tunnel. 3.4.7 Potential for Saline Intrusion The WUG access tunnel is 7 km from the ocean which is too far inland for any effect to develop given the low permeability of the andesite rockmass. For this reason, we consider the potential for saline intrusion to occur to be less than minor. 3.4.8 Potential for Ground Settlement Effects In the near surface, where compressible soils exist, no dewatering effects are expected beyond that which has already occurred due to existing mining activities. Where driven through the deep andesite rockmass, ground depressurisation will occur immediately around the tunnel, however the effects will not be laterally extensive and no significant settlement risk is considered likely. The primary rockmass being dewatered is the Rhyolite body and this is a hard, incompressible medium and is not expected to consolidate significantly as a result of dewatering. This has been assessed in detail in the EGL (WAI-985-000-REP-LC-0050) report. 3.4.9 Potential for Effects on Plant Growth Any dewatering associated with the WUG access tunnel will be in the deep rockmass. Soil moisture conditions in the regolith soils or terrace deposits in the near surface are not expected to change as a consequence of dewatering at depth. We, therefore, consider the effects of the WUG access tunnel dewatering on plant growth to be less than minor.
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