SECTION 4 Willows Farm Infrastructure C:\D Data\GWS C Drive\GWS\Client Files\1344 OGL - WKP\4 Deliverables\WUG Effects Summary\June 2022\WNP Groundwater Effects Summary_Final_Rev_0.docx 14 The Willows Rock Stack (WRS) will be up to 20 m above the natural ground level. It would be constructed from the valley bottom up in sequential lifts. There would be a rock drain underlying the WRS that will capture infiltrating water taking it to a settling pond at the toe of the structure where it will be pumped for treatment. 4.2 Characterisation The site has been characterised through intrusive site investigations including test pits and some 20 machine drilled bores focused where site infrastructure is to be located. Permeability testing, water level monitoring and groundwater chemistry were undertaken as well as surface water gaugings and water chemistry analysed. Figure 16 shows the site investigation locations and an interpreted piezometric surface map. Figure 16 Willows Farm Investigation Locations, Piezometric Surface and Access Tunnel 4.3 Hydrologic and Hydrogeologic Setting The Willows Farm is located within the Waihi basin in the Mataura Stream subcatchment which is around 7 km2. The geology of the site is generally monothithic being massive andesite with some dacite. The permeability of the rockmass has been measured as being low. The conceptual hydrogeologic model for the site is similar to WKP in that interflow from the regolith contributes the most to surface waters with some baseflow sourced from the rockmass. A conceptual hydrogeologic model for the site is included in Figure 17.
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