GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 131 The Mataura Wetland is located on a river terrace on the true right of the Mataura Stream, close to the confluence of Mataura Tributary 1. Further discussion of the Mataura Wetland is provided in the GHD (2022e) Mataura Wetland Assessment Technical Memorandum, which should be read in conjunction with this report. 6.2.3 Groundwater and surface water interactions A conceptual cross section developed by GWS (2022a) is presented in Figure 6.5. The conceptual section is an amalgamation of information from the two sections denoted on the map provided in Figure 6.6. Given the steep topography and low permeability of the surface materials at the proposed WRS, much of the rainfall that occurs directly across the proposed WRS footprint is expected to form run-off that is diverted through the base of the WRS gully, toward the Mataura Stream. Rainfall that does not run-off infiltrates the soil profile either to: – Deeper groundwater within the Whiritoa Andesite Rock: Recharge to deeper groundwater is expected to occur at higher elevations, at the ridges where surface topography is not as steep as at the WRS gully, and where large downward gradients are expected (Table 6.2). GWS (2022a) reports a deeper groundwater recharge rate of 10% of average annual rainfall. Groundwater in the andesite is interpreted from the piezometric contour plan shown in Figure 6.6 to flow toward the Mataura Stream, with a steep hydraulic gradient of approximately 0.1 beneath the WRS site. – Shallow perched water within the residual soils: Locally perched water is expected to occur within the regolith and terrace deposits, with interflow continuing downslope to the stream. Seeps present in the WRS gully were reported by WSP (2021b), where the vertical separation between the perched water level and andesite reduces gown the WRS gully: At the upper end of the WRS gully, seeps are present at a much higher elevation with respect to the groundwater level (>50 m) (Figure 6.6). This difference in elevation reduces to approximately 20 m above the andesite water level in the centre of the WRS gully. At the foot of WRS, the ground level is 180 m RL where the 170 m RL andesite groundwater level contour crosses the WRS gully. It is interpreted that the seeps in the WRS gully are from the perched shallow groundwater system, as represented in the conceptual cross-section (Figure 6.5). Trace elements in groundwater are naturally detectable due to the natural catchment conditions (e.g. aluminium, iron, manganese, zinc; refer to water quality data in Appendix E). Table 6.2 Vertical hydraulic gradients Drill hole ID Ground elevation (m RL) Geological units (piezometer depth bgl) Groundwater levels (m bgl) Vertical gradient WNDD007 1,238 HW Waipupu andesite (49 m) 38 -0.9 SW Waipupu andesite (140 m) 119 WFBH011 1,233 Residual soils (1-3 m) 5.8 -0.2 Whiritoa andesite (11.5-15.0 m) 7.4 WFBH006 1,151 Terrace alluvium (1-3 m) 1.8 0.0 Weathered tuff (5-6 m) 1.9 6.2.4 Hydraulic parameters Permeability of the various subsurface materials is reported by GWS (2022a) for the wider WUG site. The following values are adopted based on the information provided the GWS report, site logs and additional data specific to the WRS gully (test pitting and PSD analysis; Appendix D).
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