EGL Ref: 9018 22 June 2022 Page 4 WAI-985-000-REP-LC-0006_Rev0.docx This report shall only be read in its entirety. 2.4. Management of current stockpiles with zonation, liming and drainage The overburden rock and ore at Waihi is of volcanic origin and some of the material is potentially acid forming (PAF) if left exposed to air and water. Acid generation can result in the mobilisation of naturally-occurring metals from the overburden rock into the water. The existing Central and East Stockpiles embankments include specific fill zones to prevent and manage the long-term acid generation of PAF materials. Limitations on where PAF rock can be used are set and PAF materials are encapsulated in an outer layer of low permeability NAF fill as each facility is progressively built toward its final profile. This final surface is established in pasture or vegetation. The low permeability NAF fill restricts both oxygen and water entry in the long-term. Figure 4 shows crosssections of the East Stockpile which is designed to dispose of PAF overburden. Zone A fill is a low permeability zone that restricts seepage from overlying rock materials within the stockpiles into underlying ground. Further description of the zoned stockpile fills are described in Section 9.2 which are intended to be used for the design of the proposed NRS. Until the final surface is reached, and rehabilitation layers constructed exposed PAF surfaces in the stockpiles are managed through liming and testing. Liming provides acid neutralisation capacity to the exposed surface of the stockpile and testing confirms which materials are potentially acid generating. Subsurface drains are incorporated beneath the stockpile Zone A base pads and beneath the TSFs (subsoil drains). They function to intercept groundwater seepage. Leachate drains are installed on top of the Zone A base pad to collect leachate from the overburden material. The collected groundwater seepage and leachate are collected in sumps and pumped back to the Water Treatment Plant for treatment prior to discharge to the Ohinemuri River. The subsoil drains are located in low points in the original ground profile below the stockpile and include a downstream toe drain cut into rock at the downstream toe. For final rehabilitation a surface drainage system comprising berms/benches, sumps and pipes, perimeter collection drains, and ponds is operated. Currently, runoff collected in Collection Ponds S3, S4 and S5 is of a suitable quality that allows runoff to be directly discharged to the Ruahorehore Stream. If water does not meet the required water quality criteria for direct discharge, it is pumped back to the Water Treatment Plant where it is treated to meet the required receiving water standards and then discharged to the Ohinemuri River. The Northern and Southern Uphill Diversion Drain start above Storage 1A and divert clean surface runoff from upslope to tributaries of the Ohinemuri River and the Ruahorehore Stream. 2.5. Annual monitoring and surveillance A comprehensive monitoring and surveillance program is in place to enable the performance and condition of TSFs at Waihi. Management and performance of the TSFs encompasses the monitoring and surveillance of adjacent design elements such as the stockpiles. This includes:
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