B-7 – Area 7 - Assessment of Environmental Effects 41 3.2.5.3 TSF3 Perimeter Drain and Collection Ponds A HDPE lined surface water perimeter drain will direct dirty water runoff to the collection ponds. The existing Collection Pond S5, which holds surface water from the northeast part of TSF1A and East Stockpile, will be buried by TSF3. Collection Pond S5 will be replaced by a new Collection Pond S6 at the intersection of the downstream toe of TSF1A and TSF3. Collection Pond S5 currently spills excess water to the TSF1A Perimeter Drain which flows to Collection Pond S4. Collection Pond S4 has a spillway for excess flow to the Ruahorehore Stream. To separate the discharges to allow for more efficient management of surface water as rehabilitation is completed, Collection Pond S6 will have its own spillway to the Ruahorehore Stream, rather than spilling to Collection Pond S4. The discharge of water from ponds S4 and S5 to surface water will occur during high flow events. A new Collection Pond S7 will be constructed to manage the additional embankment runoff area from TSF3. Its proposed location is immediately adjacent to Collection Pond S6 at the low-lying area at the toe of TSF3 embankment which is the natural drainage path for water on the site. The realignment of a 310m-long section of Ruahorehore Stream is required to make room for Collection Pond S7 and temporary sediment retention ponds.4 Collection Ponds S6 and S7 will both be fully lined with a 1.5 mm HDPE geomembrane liner that will be anchored at the crest. A forebay and causeway into the main pond will be features of the ponds that will aid in maintenance, as is used in the existing collection ponds. The collection ponds will have sufficient capacity to limit discharge over a spillway to the Ruahorehore Stream only in high flow events (10 year ARI, 24 hour storm) when dilution is effective. In all other situations, the water will be pumped back to the WTP before being discharged to the Ohinemuri River or for use in the Processing Plant until the water quality improves sufficiently to allow direct discharge into adjacent waterways. 3.2.6 Erosion and Sediment Control Southern Skies (2022) describe the erosion and sediment control measures that will be implemented for these works. A copy of Southern Skies (2022) is provided in Part H of these application documents. 4 Refer to EGL (2022e), Plans 511-513.
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