EGL Ref: 8983 23 June 2022 Page 19 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. Ponds (SRP). Once PAF is placed dirty water will be diverted to lined collection ponds as is currently done for the Storage 1A and 2 sites. Figure 3 shows the current layout of collection ponds. The ponds are sized to have sufficient capacity to contain runoff in rainfall events up to a 1 in 10 year 24 hour storm, when dilution is effective to ensure that any discharge will have less than minor effects on the receiving environment. Contained water is pumped back to the existing Water Treatment Plant before being discharged to the Ohinemuri River or for use in the Process Plant. The collection ponds are pumped down during and after a storm events. Collection Pond S5, which currently provides retention of surface water from the north east part of Storage 1A and East Stockpile, will be buried by Storage 3 works. This pond will be replaced by a new pond at the intersection of the downstream toe of Storage 1A and 3. It will be called Collection Pond S6. Collection Pond S5 currently spills excess water to the Storage 1A 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 S4. A new collection pond is required to manage the additional embankment runoff area from Storage 3. It will be called Collection Pond S7. Its proposed location is the low-lying area at the toe of the Storage 3 embankment which is the natural drainage path for water on the site, and it is immediately adjacent to Collection Pond S6. Collection Ponds S6 and S7 will both be fully lined with a 1.5mm HDPE on a 0.6m thick earthfill liner. 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. A surface water perimeter drain will direct dirty water runoff from the embankment to the collection ponds. These drains will be HDPE lined. Realignment of 310m of the Ruahorehore Stream is required to make room for Collection Pond S7 and temporary sediment retention ponds indicated on the Drawings 511, 512 and 513 in Appendix A. The total footprint of Storage 3 TSF, including the extent of the stockpile and uphill diversion drain, is approximately 115ha. Of this area 20ha is already part of the existing footprint of Storage 1A and East Stockpile. The additional footprint is therefore 95ha. Geotechnical investigations indicate the depth to bedrock on average is greater than that encountered at the Storage 1A and 2 sites. There are limitations on the practical downstream toe position due to the presence of weak and compressible ground that is up to 32m deep and extends beyond the site. The embankment has been positioned to limit the excavation required to rock, while not significantly compromising the capacity of Storage 3. Generally, the downstream toe excavation will be 7 m deep, apart from across the ‘Paleo Gully’ where it may be up to 20 m deep. Refer to Sections 11.4 and 11.5 and drawings in Appendix A. This allows any risk from weak or liquefiable soils to be mitigated by removal and placement of structural fill directly on the bedrock. Weak or liquefiable soils only need to be removed at the toe of the embankments to provide stability for the embankment slopes.
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