SECTION 6 Groundwater Effects Assessment – Recommendations 34 6. Recommendations 6.1 Discussion This assessment of effects has shown there to be minimal risk to shallow groundwater, surface waters, other groundwater users, and plant growth from the proposed WNP tunnels. The depth of the tunnelling and low permeability of the surrounding rockmass means any surface expression will not be discernible. Where more permeable structures are dewatered that could result in short term connections back to the surface, tunnel inflows will be mitigated such that the effect is de minimis. This will be achieved through grouting or other appropriate methodologies to prevent groundwater ingress. These features would be identified in advance of tunnelling by probe drilling and would either be grouted in advance of the tunnel being driven or within a few days of the feature being exposed in the tunnel. This means that effects on groundwater associated with the tunnelling, if any, will be short lived. At locations where the tunnel alignment is shallow and effects on surface waters or other groundwater users are potentially possible, appropriate monitoring would be conducted to ensure any observed response is within the predictions made in this assessment. 6.2 Recommendations for Monitoring 6.2.1 WUG Access Tunnel There are two locations along the WUG access tunnel alignment where groundwater monitoring should be undertaken. The first is around the decline from the WTP where the near surface connection exists. Existing monitoring suggests the shallow groundwater system is already dewatered locally and conditions will not change significantly as a result of the tunnel construction. We therefore, recommend monitoring of groundwater levels using the existing network of wells to ensure no significant changes develop that are not expected. The monitoring wells P62, P63, P64 and P78 are sufficient to monitor for potential effects. There are some groundwater bores within proximity to the tunnel and these bores take groundwater from a similar depth to the WUG access tunnel. While it is unlikely these will be affected by the proposed tunnel, it would be prudent to monitor groundwater levels in the area as the tunnel is being driven. This could be done using the water bores as observation points, wells in the existing monitoring network or though purpose-built piezometers e.g. on SH25. 6.2.2 Willow Access Tunnel The Willows Farm access tunnel decline intercepts the shallow groundwater system and, because of this, there is some potential for effects on surface waters by temporarily reducing baseflow. For this reason, it is recommended that monitoring of shallow groundwater levels is undertaken to ensure no long-term lowering effects are observed. The existing monitoring network is considered suitable for this purpose, however, some additional wells may need to be installed to improve the adequacy of the network locally. The proposed monitoring will be specified in a groundwater monitoring and contingency plan that will also include ground settlement monitoring.
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