SECTION 5 Groundwater Effects Assessment – WUG Dual Tunnels 33 5.3.6 Potential for Effects on Groundwater or Surface Water 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 and backfilled wasterock, however this is not expected to change the overall quality in the aquifer due to the limited contact area relative to the system throughflow. This statement is similarly applicable to the vent shaft following construction. In summary, no adverse effects on groundwater quality are expected from the tunnel. Given the limited connections between groundwater and surface waters, and the lack of expected effects on groundwater, the effects on surface water quality is similarly expected also to be negligible. 5.3.7 Potential for Saline Intrusion The dual tunnels are 7.5 km from the ocean, which is too far for any effect to develop. For this reason, we consider the potential for saline intrusion to occur to be less than minor. 5.3.8 Potential for Ground Settlement Effects For the majority of the tunnel alignment the tunnel is constructed in relatively incompressible materials. There is the potential for the drive to intercept hydrothermally altered rock that has been reduced to clay. Further, there may be weathered zones within the volcanic rocks that have formed silty clay soils. Given the nature of these materials they would have properties that allow consolidation to occur if dewatered. It is, however, expected that these materials will be of low permeability and would not readily dewater, particularly in the timeframe within which mitigation would be put in place. Overall, we do not expect there to be long term drainage that could result in dewatering and therefore settlement. 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. 5.3.9 Effects on Plant Growth Any dewatering associated with the tunnel will be in the lower rockmass. Soil moisture conditions in the regolith in the near surface layers are not expected to change as a consequence of dewatering at depth. We therefore consider the effects of tunnel dewatering on plant growth to be less than minor.
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