GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 75 . 4.3 Conceptual groundwater model (NRS CGM) 4.3.1 Overview The geology at the NRS is notably different to that at Gladstone and TSF3, with no clear separation of shallow and deeper groundwater systems. This is due to the presence of multiple alluvial paleo-channels within and overlying variable volcanics (ignimbrite, dacite and rhyolite). The deep andesite, which outcrops to the west of the Ohinemuri River, is present below 200 m depth at NRS due to the dip of the unit and faulting (Golden Valley Fault). All groundwater monitoring wells at NRS are installed in the shallow system, with some paired between the water table and deeper within the shallow system (up to 25 m bgl; referred to as “deeper” wells / groundwater in this section), to provide an understanding of vertical hydraulic gradients beneath the site. Overall, whilst slight differences in groundwater are recorded between the shallow and deeper monitoring wells, it is interpreted that the system is hydraulically connected and collectively discharges to the Ohinemuri River west of the NRS. The NRS CGM discussion is focused around the shallow system to 25 m depth; any reference to the deep regional andesite aquifer is clarified as such where discussed. 4.3.2 Shallow groundwater system Monitoring wells installed at the proposed NRS site are located between 100 m to 770 m east of the Ohinemuri River (Figure 4.1). The majority of monitoring wells at NRS have recorded groundwater levels at less than 5 m bgl, with some locations within 10 m bgl (1,095 – 1,106 mRL). The well pair located at the foot of the rhyolite dome (WRS03 and WRS03a) 770 m east of the river, record groundwater at much higher elevations than the other NRS monitoring wells (approximately 1,130 mRL) due to their elevated location. Groundwater recharge is expected to occur from rainfall infiltration over the elevated rhyolite dome, as well as across the alluvial river plain. Recharge to the elevated rhyolite drives upward vertical hydraulic gradients at the base of the hill, east of the proposed NRS footprint. Groundwater and surface water levels recorded in a number of locations across the site are presented in Figure 4.5 and Figure 4.6, alongside rainfall. The following is noted: – When comparing groundwater levels within shallow monitoring wells, those nearest to the Ohinemuri River and tributary show the greatest response to rainfall. The rapid response of groundwater is likely to be due to the higher permeability of the shallow geological units (completely weathered to residual volcanics in wells WRS04, WRS05, WRS08). – The deeper groundwater levels near tributary location TB1a demonstrate a greater response to rainfall when compared to the water table (DH05/DH05a/WRS06; Figure 4.5). This is considered to be a function of the close proximity to the ignimbrite paleo-channel and underlying boulder and cobble alluvium at depth, these providing a preferential groundwater flow pathway and allowing transmission of recharge from higher in the catchment. The water table and deeper groundwater piezometric contours (Appendix C) demonstrate the influence that this preferential pathway has on groundwater flow directions. – Downward vertical hydraulic gradients are recorded in close proximity to the Ohinemuri River and along the tributary, with deeper groundwater inferred to be recharged in these areas. The strongest downward vertical gradients during summer baseflow conditions are observed near the existing northern storage area. Milder downward gradients are observed closer towards the Ohinemuri River, where groundwater levels and vertical gradients suggests that both shallow and deeper groundwater is discharging to the river (WRS04 and WRS05). – A delayed and more muted response to rainfall occurs at the foot of the rhyolite dome in the east of the site (WRS07/DH09 and WRS03/WRS03a). Mild to moderate upward vertical hydraulic gradients occur at these locations, which indicates that the upper reaches of the tributary (near WRS TB) are likely spring fed. Groundwater levels and gradients typically follow topography, with a steeper gradient at the base of the rhyolite dome to the east (0.1), becoming gentler across the alluvial flats (0.03). The shallow groundwater flow direction is approximately west to northwest, following the site topography towards the perennial tributary (TB1f to TB1a) and the Ohinemuri River.
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