EGL Ref: 9215 23 June 2022 Page 30 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 4.6. Operation OGNZL is responsible for operation, maintenance, and surveillance of the tailings storage facilities (TSFs). Storage 1A is the currently active TSF (Figure 25), although water levels in Storage 2 need to be managed to maintain safe freeboard. Operation involves pumping tailings into the impoundment. Supernatant water on the TSFs is pumped back to Processing Plant or Water Treatment Plant. Water stored on the TSFs needs to be balanced with the demands for treatment of water from the subsurface and leachate drains and underground and pit dewatering. Monitoring of the freeboard (vertical distance between embankment crest and pond water level) is undertaken frequently to ensure that there is always sufficient volume under normal operation to store the runoff from a 72-hour probable maximum precipitation (PMP) rainfall event with 1 m freeboard. The 1 m freeboard is a condition of the Resource Consent issued and is recommended by NZDSG for TSFs. FIGURE 25: STORAGE 1A TAILINGS SURFACE WITH PONDED WATER MANAGED BY OGNZL THROUGH THE WATER TREATMENT PLANT 4.7. Monitoring and Surveillance A comprehensive monitoring and surveillance program is in place to enable the performance and condition of Storage 2 and 1A to be assessed. It is documented in the Operations, Maintenance and Surveillance Manual (Ref. 12). This manual was developed in accordance with the NZDSG. Monitoring and surveillance associated with the tailings embankment includes: • Visual inspection on a regular basis; • Measurement of pore pressures within the embankment fill and tailings by pneumatic, standpipe and vibrating wire piezometers; • Measurement of subsurface and leachate drain flows; • Deformation monitoring; and
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