SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 18 4.1.4 Hydrogeology Groundwater Levels A total of 20 machine drilled boreholes were completed as shallow groundwater monitoring wells during the geotechnical site investigations of Willows Farm. In addition, a vibrating wire piezometer with 3 tips was installed at the location of the proposed ventilation shaft. Figure 14 shows the locations of the monitoring wells, the groundwater elevations and interpreted water table surface. Figure 15 shows the hydrogeology along the access tunnel profile. Figures 16 and 17 provides hydrogeologic sections 1 and 2 at the locations shown in Figure 14. In general terms, in those wells at higher elevations the water table is 10’s of metres below ground level. At lower elevations the depth to groundwater is between 1 to 5 m. At two locations (WFBH001 and WFBH0011) there is a water table in the upper pyroclastic materials and lower-level groundwater present in the volcanic rock. The water level difference in WFBH0011 is relatively small being 1.7 m while at WFBH001 this is 6.2 m. These observations suggest perching of groundwater occurs in the shallow materials overlying the volcanic rockmass. Hydraulic Gradients The interpreted water table surface shows the topography of the site is the primary feature driving groundwater heads that show a close relationship to site morphology. Hydraulic gradients vary over the site depending on the local land forms but is on average 0.05 to 0.06 over much of the property, flattening to 0.02 in the central area and with locally steep gradients up to 0.1 near the Mataura Stream. Vertical hydraulic gradients are observed to exist at the vent shaft location where WNDD007 indicates a vertically downward gradient in the range of 0.02 to 0.06. Aquifer Parameters Rising head tests were undertaken on all of the monitoring wells constructed on the site. In addition to the testing undertaken on the monitoring wells, falling head tests and packer tests were undertaken on WNDD007. A summary of the results of these testing is included in Table 3. Table 3 Willows Farm Hydraulic Conductivity values Monitoring Well Hydraulic Conductivity (m/s) Min Max Weathered Tuff 5.3 x 10-7 2.2 x 10-6 Terrace Gravel 3.3 x 10-8 1.1 x 10-4 Sandy Soils 1.1 x 10-6 1.2 x 10-6 Silt Soils 2.0 x 10-7 1.7 x 10-5 Silt/Clay Soils 1.1 x 10-7 2.3. x 10-7 Altered Tuff 5.7 x 10-8 8.8 x 10-8 Tuff 1.1 x 10-6 7.1 x 10-6 Andesite 1.3 x 10-8 5.0 x 10-7
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