GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 100 I.3.2 TSF3 discharges The modelled discharge rates were extracted in m3/day per 1 m model width. The extracted results have been multiplied by 1,000 m, which is the approximate width of TSF3 perpendicular to the cross-section line, to provide results for the entire facility. Where appropriate, the results for the shear key model were considered for 500 m. Tailings As drains also capture groundwater, flow lines predicted by the SEEP/W models (refer to Figure 5.15 of the main report) were used to assist in estimating water sources and discharge rates. The modelled tailings discharge rates are provided in Table I.16. Where the discharge through the Zone A soil liner is: – Less than that of the HDPE liner, this is due to the some of the discharge through the HDPE liner being captured by the upstream cut-off drain. – Greater than the HDPE liner, this is where the tailings are emplaced above the HDPE liner (Scenarios 4 and 5), and where tailings discharge occurs over the top of the liner. More tailings discharge is captured in Scenario 4 where the drains are operational. Note: Discharge from the facility is captured during Scenario 5 (post-closure, where only the downstream toe drain is operational), but this is only embankment infiltration. Embankment The model embankment infiltration and discharge values are provided in Table I.17 The results indicate: – The total infiltration through the embankment is increased for Scenarios 4 and 5, which is expected due to full embankment construction (260 m), which is longer than Scenario 3 (140 m). – The discharge values (267 m3/day) are the same for Scenarios 4 and 5 (within rounding), as the same infiltration rate has been applied, across the same embankment construction. – The operational drains through Scenarios 3 and 4 are indicated to effectively capture the embankment rock infiltration, whereas only a small portion (~10%) of the discharge is captured by the downstream toe drain in Scenario 5. Collection pond Leakage from the ponds was calculated from the following inputs: – Total pond area = 33,000 m2 = 3.3 ha Pond S6 area = 11,500 m2 Pond S7 area = 21,500 m2 – Leakage rate = 42 L/ha/day – So that 42 L/ha/day x 3.3 ha = 139 L/day or 0.14 m3/day. Drain capture Drain widths across the embankment vary from 500 - 1200 m. For simplicity, and for direct comparison to the above seepage results, all values extracted from SEEP/W were multiplied by 1,000 m. The individual drain capture results are provided in Table I.19. The details of final drain capture composition is provided in Table I.20. Mass flux to the drains (Table 5.10 of the main report) was calculated from the following inputs: – Geochemistry of the discharges: Tailings pore water quality – predicted mean value from Table 24 of the AECOM (2021a) report. Embankment leachate water quality – predicted median values for the development stage (Scenarios 3 and 4) and closure stage (Scenario 5) from Table 8 of the AECOM (2021a) report. Groundwater quality – the median of water quality from AP01a was used. – Drain capture volumes for each component in Table I.20. So that: (predicted concentrations in g/m3 x drain volumes in m3/day) / 1000 = mass flux in kg/day to the WTP.
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