GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 116 – Post-closure (1,155 m RL): The rate of infiltration for the long-term scenario is the same as for closure (270 m3/day). Assuming drain failure (with only the downstream toe operating), 30 m3/day (~10%) of this is predicted to discharge to the receiving environment. As indicated by the flow lines in Figure 5.15, and supported by the conceptual model (Section 5.3), none of the embankment infiltration is expected to discharge locally to the Ruahorehore Stream. Table 5.8 TSF3 rock embankment infiltration* Stage (Tailings height) Embankment length along model alignment (m) Infiltration rate (mm/year) Water flow through embankment (m3/day) Proportion captured by drains Discharged to the receiving environment Starter embankment (1,135 m RL) 140 600 (~30% of rainfall)** 230 100% 0% Operational to closure (1,155 m RL) 260 370 (~20% of rainfall)# 270 100% 0% Long-term / post-closure (1,155 m RL) ~90% (240 m3/day) ~10% (30 m3/day) * Values presented are rounded. ** Resulting infiltration after applying 35% rainfall as seepage face review. # From AECOM (2020) for TSF1a 5.5.4 Collection pond leakage The collection ponds (S6 and S7; Figure 5.16) will both be fully lined with a 1.5 mm HDPE on a 0.6 m thick earthfill liner (Zone A material). The collection ponds are sized to manage runoff from a 1 in 10-year 24-hour rainfall event, so that dilution is effective to ensure that any discharge will have less than minor effect on the receiving environment (EGL, 2021b). The operational requirement is that they are to be pumped dry as soon as practicable after rainfall events. As such, when dry, no pond leakage is expected to occur. The seepage rate from the ponds when full are estimated using the lower limit of liner leakage due to defects (42 L/ha/d; Section 5.5.2). This value is adopted due to the significantly lower pressure head on the pond liner when full compared with the tailings. It is considered to be appropriately conservative given the temporary nature of the full ponds, which will limit the driving head on the pond liners. Across the total lined pond area of 3.3 ha, this results in an estimated temporary leakage rate of 140 L/day. This leakage is expected to discharge to the deeper groundwater system, as indicated by the flow lines in Figure 5.15. For conservatism, this discharge is assumed to discharge to the realigned Ruahorehore Stream. The water quality in the pond, for conservatism, is assumed to be impacted by the embankment rock for the starter and operational phases. An assessment of the potential impact to the Ruahorehore stream is included in Section 5.6.2 below. From closure, the rock embankment will be capped. The pond water quality is expected to be minimally impacted by the underlying Zone A material (NAF). Figure 5.16 Collection ponds S6 and S7 (EGL 2021 DWG 0513)
RkJQdWJsaXNoZXIy MjE2NDg3