GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 104 The results of the sensitivity testing are summarised as follows: – Leakage rates of the liner: Estimated by EGL (2019b) to range from 42 to 170 L/ha/d. Both upper and lower bound values were assessed and applying the lower leakage rates provided the following general results (Table I.24): The leakage of tailings through the liner decreases by 48% (Scenario 5) to 58% (Scenario 3) with the lower leakage limit. Drains are the primary recipient of the upper leakage limit in Scenario 3. Tailings discharge is to ground in Scenario 5 for both the base model and sensitivity analysis. Neither analysis results in flow lines to the Ruahorehore Stream from beneath the TSF. The greatest effect of increasing the liner leakage is to increase groundwater flow the deep groundwater receptor, and inferred discharge to the Ohinemuri River. The upper leakage value was reported in the assessment for conservatism. Table I.24 Sens.1 results – liner leakage Scenario Water rate (m/d) Scenario 3 – Tailings discharge (m3/day) Scenario 5 – Tailings discharge (m3/day) Through Zone A To drains To receiving environment Through Zone A To drains To receiving environment Sens.1 4.3 x 10-6 2 2 0 7 0 7 Base model 1.7 x 10-5 5 5 0 13 0 13 Decrease under lower leakage: 58% 48% – Permeability of the tailings: The scenario testing analysed an order of magnitude higher and lower than the base model K. The results (Figure I.13) showed: The flow directions of the tailing pore water changes so that is more horizontal under the highest K applied, and more vertical under the lowest K applied (Figure I.14). No change in seepage volume from TSF3 (including the embankment) under Scenario 3, as the liner leakage is controlled by the applied liner rate and the drains are operational. Sensitivity to changes in K for Scenario 5 tailings, where: – An increase of tailings discharge to the environment 12 times that of the base model is reported for the increased K (Sens.2). – The total volume of embankment infiltration does not change (controlled by the applied infiltration rate through the capping layer; AECOM 2020), but the changes in discharge from the tailings influences the proportion of embankment infiltration that goes to the drains. Under the higher K scenario (Sens.2), all embankment infiltration reports to the one active drain (downstream toe drain); whereas under decreased tailings K conditions, the majority discharges to the receiving environment. Neither analysis results in flow lines to the Ruahorehore Stream from beneath the TSF.
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