GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 36 . 3.3.2 Shallow groundwater system The shallow groundwater system at Gladstone comprises groundwater flow through shallow soils, reworked young volcanics and highly weathered rock on the flanks and surrounds of Gladstone and Winner Hills. The presence of the Gladstone wetland on the southern flanks of Gladstone Hill results from groundwater recharge into the shallow groundwater system, while the intermittent watercourses are also expected to be connected to the shallow groundwater system at times. On-going weathering, erosion and fracturing of andesite where it outcrops on Gladstone Hill has provided a material with more porous characteristics than the deeper, less-weathered andesite, and a surface through which rainwater infiltration, whilst low, is expected to occur. The change in properties and desaturation of deeper andesite limits the volume of recharge that can infiltrate, promoting flow of infiltrating water radially away from Gladstone Hill into the younger volcanics of the shallow groundwater system. Hydraulic Properties Hydraulic conductivity (K) testing of the dacite and rhyolitic tuff (RT) at the locations of the GLD bore series installed on the southern flank of Gladstone Hill indicates very low permeabilities, as summarised in Table 3.2 (detailed permeability results are provided in Appendix D). This is generally consistent with the observed residual weathering, which presents as clay-like materials. The low permeability of these materials results in high groundwater levels (shallow water table), supporting the wetland to the south of the proposed Gladstone Pit. In contrast, the higher permeability of the rhyolitic tuff (RT) to the north and west of Gladstone Hill provides greater drainage, lower groundwater levels and more rapid flow of groundwater in comparison to the conditions to the south. The typically dry conditions of the intermittent tributary at monitoring location TB5 reflect the higher permeability of the materials to the north and west, which can dissipate high water levels faster than in lower permeability areas following rainfall events. Table 3.2 Hydraulic Conductivity of geological units in shallow groundwater system Geological Unit Geomean hydraulic conductivity (m/s) Ash/regolith south of pit (GLD01) 3.3 x 10-8 Ash/regolith west of pit (P68s1) 6.0 x 10-6 RT (rhyolitic tuff) south of pit (GLD01a) <1 x 10-8 RT (rhyolitic tuff) north of pit (P61s1) 2.5 x 10-6 RT (rhyolitic tuff) west of pit (P79i) 2.7 x 10-7 Sandy Ignimbrite (P79s & P68d1) 1.2 x 10-5 Dacite (GLD03 & GLD03a) 4.8 x 10-8 1. URS, 2003. Shallow groundwater levels Groundwater levels in the GLD bore series installed on the southern flank of Gladstone Hill are presented in Figure 3.7. Groundwater levels within the shallow system appear to respond to infiltration during rainfall events, with downward hydraulic gradients typically evident. However, higher groundwater levels are recorded to occur immediately following rainfall events at GLD02a, where an increase in groundwater pressures in the hydrothermal breccia unit is likely to have occurred due to differing recharge pathways and hydraulic conductivity in comparison to the overlying shallow volcanics. This provides upwards vertical hydraulic gradients following rainfall events that
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