96 17.0 Effects of Tailings Storage TSF3 on Freshwater Ecology 17.1 Proposed Activities A new Tailings Storage Facility (TSF3) is proposed to provide the tailings storage for the WNP in addition to that provided by the proposed GOP TSF. TSF3 is a downstream earth and rockfill embankment structure, like TSF1A and 2, and forms an impoundment to store the discharged tailings slurry pumped from the Processing Plant. The layout of TSF3 is shown in Figure 24. This shows the TSF3 embankment constructed between the existing TSF1A embankment and the rising hills to the east which wrap around to the north and behind TSF1A to form the impoundment. The impoundment partially covers the existing East Stockpile area. The construction and continuing operation of TSF3 will result in the diversion of a tributary of the Ruahorehore Stream. Some 2,111 m will be diverted to form a 3,880 m length of reformed watercourse. Geotechnical investigations covering the proposed TSF3 site undertaken between 1995 and 2020 indicate the depth to firm bedrock on average is greater than that encountered at the TSF1A and 2 sites and there are also limitations on the practical downstream toe position due to the presence of weak and compressible ground of notable depth (up to 32 m) and extent beyond the site. To maximise the TSF3 site and provide for a structure of similar geotechnical integrity as the existing TSF1A and 2 embankments, some excavation of areas of weak and compressible soils and backfilling with a structural fill will be undertaken. An undercut to 20 m depth is proposed in an area termed the Paleo Gully, which is the controlling geological feature determining the downstream toe position. The tailings impoundment is to be fully lined with an earth liner. Additionally, a 1.5 mm HDPE geomembrane liner is proposed within the tailing’s impoundment and up to the initial starter embankment height (RL135) to further minimise tailings seepage. Groundwater beneath the facility will be collected under the base liner of the impoundment and the embankment through a series of subsurface drains. Leachate from the material within the embankment will be collected via a series of leachate drains. Groundwater and leachate collected in the drains will be pumped back to the Water Treatment Plant or the Processing Plant via the perimeter ring main system around TSF1A and 2. Closure for the TSF3 includes: • A partial dry capping of the perimeter of the impoundment as has been done at TSF2; • A wet capping of the tailings in the centre of the impoundment not covered by the dry capping; • Spill of the uphill diversion drain into the impoundment; and • Spillway to discharge clean water from the impoundment to the Ruahorehore Stream.
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