GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 114 porewater leaving the facility along the northern edge of the HDPE liner is predicted to bypass the TSF3 drainage system for the operational/closure and long-term scenarios. This porewater is predicted to recharge deeper groundwater, and ultimately discharge some distance downgradient of the site via existing deeper groundwater flow paths to the Ohinemuri River (the receiving environment). This interpretation is supported by flow paths within the SEEP/W model and the conceptual understanding provided in Section 5.3.5. The following discharge rates are predicted for TSF3 and summarised in Table 5.7: – Starter embankment (1,135 m RL): Tailings are least consolidated at this stage and have the greatest hydraulic conductivity (KH = 1x10-7 m/s). The discharge rate from the TSF3 at this stage is 10 m3/day, all of which is through both the HDPE and Zone A soil liners. All of this discharge is predicted to be captured by the drains and prevented from downward infiltration, due to the upward hydraulic gradients at this location. – Closure (1,135 m RL): As TSF construction progresses through to full embankment, the tailings consolidate, becoming less permeable (KH = 5x10-8 m/s). The volume of tailings porewater predicted to discharge through the Zone A soil liner is 115 m3/day, with 70% captured by drains and the remainder (45 m3/day) entering the deeper groundwater system. – Long-term (post-closure): Over time, the tailings are expected to consolidate further to a final KH value of 1x10-8 m/s. All drains (except for the downstream toe drain) are assumed to have failed through sedimentation or collapse. At this stage of the TSF3 life, 60 m3/day discharge through the Zone A soil liner is predicted, with no capture by drains. It is therefore predicted that 60 m3/day of tailings porewater will enter the deeper groundwater and flow to the receiving environment in the long-term phase of the TSF3. The HDPE liner provides an effective reduction of tailings porewater discharge to the environment. Without this liner, the discharge to the receiving environment under the starter embankment stage (when tailings are least consolidated) is expected to be over 500 times (540 m3/day) the predicted volume without the liner (0 m3/day). Table 5.7 TSF3 tailings porewater discharge Stage (Tailings height) Tailings permeability (m/s) Tailings discharge through liners (m3/d) Proportion captured by drains Discharged to the receiving environment Starter embankment (1,135 m RL) KH = 1x10-7 KV = 2x10-8 10 100% 0% Operational to closure (1,155 m RL) KH = 5x10-8 KV = 1x10-8 115 70% (70 m3/day) 30% (45 m3/day) Long-term / post-closure (1,155 m RL) KH = 1x10-8 KV = 5x10-9 60 0% 100% Comparison scenario – no HDPE liner: Starter embankment KH = 1x10-7 KV = 2x10-8 700 25% (160 m3/day) 75% (540 m3/day)
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