GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 103 5.3.3 Deeper groundwater The deep groundwater system is located within the deeper weathered and fractured rhyolite rock and tuff units underlying the shallow system. This is considered to form the primary aquifer beneath TSF3. Groundwater levels vary at TSF3; groundwater levels and vertical gradients in the deeper system were measured as follows: – Maximum measured water level of 1,160 mRL in the hills, to 1,108 m RL in the lower valley near the Ruahorehore Stream (e.g. well AP04; Figure 5.9). – Strong vertical downward gradients were measured in the hills (wells DH10, DH23 and AP10). These conditions gradually change to upward vertical gradients toward the lower valley floor. Mild flowing artesian conditions are reported near the Ruahorehore Stream (wells AP01, AP04, AP05, DH14). The localised artesian conditions are inferred to be caused by a combination of the low permeability of the alluvium relative to the underlying tuff (Table 5.3). Welded tuff is locally present at some locations, underlying sensitive tuff within the paleo-gully (refer to EGL TSF3 GFR Figure 13). Anisotropy of the tuff units are also likely to contribute to occurrence of artesian conditions and separation of shallow and deeper groundwater. Deeper groundwater is inferred to be predominantly recharged via rainwater infiltration into competent, fractured rhyolite in the ridge areas, where strong downward gradients are present, and which is considered to form the local groundwater divide. The rates of recharge are expected to be higher than those of the shallow groundwater system as indicated by strong downward vertical hydraulic gradients in the ridges and higher permeability of the rhyolite lava units (Section 5.3.4). 5.3.4 Hydraulic conductivity Interpreted hydraulic conductivity (K) value based on site testing (Appendix A and Appendix D) are presented in Table 5.3. The K across the site is highly variable both spatially and vertically on small scales; refer to plans presented in Appendix D. This is attributed to multiple factors such as differing geological units, mode of deposition, anisotropy, reworking and weathering. Table 5.3 Summary of hydraulic conductivity Values for TSF3 Groundwater System Geological Unit Geomean K (m/s) K Range (m/s) Shallow Alluvium / Ash 4.5 x 10-8 3.2 x 10-9 to 8.5 x 10-7 Deeper (Rhyolite) Redeposited Tuff 2.8 x 10-6 3.2 x 10-9 to 1.0 x 10-4 Tuff 4.8 x 10-8 5.7 x 10-10 to 1.5 x 10-5 CW-MW Flow Rhyolite 1.3 x 10-7 1.2 x 10-9 to 1.0 x 10-4 MW-SW Flow Rhyolite 5.0 x 10-6 1.0 x 10-8 to 1.0 x 10-4 Froth Flow Rhyolite 2.8 x 10-7 3.2 x 10-9 to 2.4 x 10-5
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