GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 111 Detailed model outputs are provided in tables within Appendix I. Table 5.6 Summary of dewatering results Excavation Stages (days) Expected inflow ranges* Predicted upper bound** Initial partial excavation (0-30 d) <10 – 25 m3/day 510 m3/day Full excavation (30-75 d) 30 – 330 m3/day 2,450 m3/day Partial backfilled (75-120 d) 0 – 10 m3/day 100 m3/day Sum for construction period 1,970 - 16,130 m3 total 130,000 m3 total Portion of Available Aquifer Management Level# 0.1% – 0.9% 7% Notes: * Ranges based on values provided by Cross-section 1 (including sensitivity testing) and Theis (1935). ** Predicted upper bound provided by Cross-Section 2. # WRC Waihi Basin (1,845,000 m3/year) Groundwater level drawdown The predicted groundwater level drawdowns are illustrated in Figure 5.13 and Figure 5.14, where: – The drawdown ZOI is predicted to extend up to 610 m. The ZOI reflects the distance from the deepest point of excavation to the distance at which predicted groundwater drawdown becomes less than 0.5 m. The ZOI is mapped and discussed further in Section 5.6. – The ZOI is not circular and is expected to be influenced by the shape of the excavation and subsurface features (e.g. changes in lithology, paleo-gullies, paleo-ridges). However, the limitations of the approaches adopted provide a conservative, circular ZOI. – EGL have considered the drawdown results for settlement effects to TSF1A. The drawdown in Figure 5.14 is expected to produce settlement of less than 50 mm (EGL, 2021b). To have a material effect, settlement would need to be greater than 100 mm, and EGL concluded that settlements in the range calculated will not have an effect of the TSF1A Zone A liner or Zone G capping. Following backfilling of the excavation, groundwater levels are expected to increase to levels reported in the future operational TSF3 scenarios. Changes to groundwater recharge and flow regime The potential effects associated with the TSF3 construction dewatering include changes to aquifer recharge, groundwater flow directions, and subsequent impacts on the Ruahorehore Stream where groundwater is in direct hydraulic connection: – Groundwater: The groundwater flow regime will be temporarily altered near the excavation during the construction period, with ground flows locally directed to the excavation. Recharge to the broader groundwater system will be reduced by the volume of dewatered groundwater during construction. The majority of groundwater removed during dewatering is expected to come from the deeper aquifer. This results in an approximate reduction in recharge of 0.2% of the Waihi Basin aquifer management level (Table 5.6). The upper limit of this reduction may potentially be up to 2.1% but is unlikely to occur. – Surface water: The Ruahorehore Stream will be diverted to approximately 40 m south of its current location at the toe of the TSF3 embankment (Figure 5.12). Without this diversion the Ruahorehore Stream would be significantly adversely affected by TSF3 construction dewatering. Whilst the stream is considered to be losing at this location (Figure 5.9), the rate of loss may increase due to dewatering of the shallow groundwater system, as well as underdrainage which may occur due to dewatering. Rates of stream depletion have not been quantified, as any effects to the Ruahorehore Stream are expected to be mitigated by diverting clean abstracted groundwater into the stream to maintain stream flows.
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