GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 89 Parameter Mass flux (kg/day) to Ohinemuri River Pb 0.000025 SO4 139.9 Sb 0.00020 Se 0.00023 Zn 0.12 4.6 NRS potential effects summary and discussion The predicted influence on key environmental aspects is summarised in the following sections. 4.6.1 Uphill diversion drain construction dewatering Placement of the NRS is predominantly anticipated to be above the existing range of groundwater level fluctuations at the site. The exception to this is in the south-eastern boundary, where excavation into the elevated rhyolite dome is proposed to allow installation of the uphill clean water diversion drain. The dewatering assessment to estimate the influence of this drainage was highly conservative, assuming that the groundwater would be encountered along the full 240 m length of the excavation section. The predicted inflow rates are also those expected at the commencement of dewatering (10 – 70 m3/day) and are likely to reduce over time as groundwater levels reduce in response to the change in conditions. The dewatering required for installation, and the ongoing influence of the uphill diversion drains throughout operation and closure, is predicted to have a zone of influence of up to approximately 10 m and is therefore not likely to have a significant impact on groundwater gradients and flow across the remainder of the NRS site. Whilst localised changes may occur in the immediate vicinity of the dewatering, the minor volume is not expected to significantly influence the wider groundwater and surface water flow regime. Abstraction of groundwater during construction dewatering is also not expected to adversely impact groundwater quality. While there may be some localised changes in soil moisture in close proximity to areas of earthworks and dewatering, no impacts are expected beyond the immediate vicinity of the works. Intermittent wetting by rainfall is considered to provide the greatest influence on soil moisture levels across the Waihi North Project area. 4.6.2 Rock stack assessment, groundwater levels and flow Final landform After rock stored at the NRS has been used to backfill Gladstone Open Pit and various underground mining stopes, the proposed final maximum elevation of the NRS is 1,148 mRL. This includes a rock capping layer (Zone F, G and H materials) to limit oxygen and water ingress. The capping layer is proposed to be covered with topsoil and contoured. Groundwater levels and flow Groundwater flow to the Ohinemuri River has been estimated at approximately 1,170 m3/day. This represents only a small proportion (< 1%) of Ohinemuri River flow as it passes the NRS site (averaging 63,200 m3/day), with this supported by unmeasurable changes in river flow recorded between up- and down-gradient flow gauging sites. The potential reduction of direct rainwater recharge to the groundwater system due to placement of the NRS is predicted to be offset or increased by seepage of leachate through the liner. The results of the assessment indicate that between 415 – 605 m3/day of leachate is predicted to seep through the liner and discharge into the groundwater beneath the rock stack during the NRS operation scenario, reducing to approximately 260 m3/day in the NRS closure scenario.
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