GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 18 2.5 Collection and Contingency Ponds 2.5.1 Background At the time the collection pond design criteria were devised in 1996, the potential for low pH and elevated trace elements content in the pond discharge water was a key consideration since there had been instances of pond water quality exhibiting these characteristics in earlier years of operation (1993/94). This was due to contact of runoff with PAF rock. Collection ponds were designed with large volumes to retain runoff from exposed PAF areas, the collected water pumped for treatment except under large rainstorm events when significant dilution flows occurred in the receiving water (Ohinemuri River and Ruahorehore Stream). In response to pond water quality, and in addition to the introduction of collection ponds, the site changed PAF rock management practises; in particular through the use of limestone addition during conveying of PAF rock and in placement. This change in practise in combination with progressive rehabilitation of catchment areas has resulted in gradual improvement in collection pond water quality. Over time a number of collection ponds have been reclassified as “silt ponds” and allowed to discharge directly to either the Ohinemuri River or Ruahorehore Stream when water quality and river flow parameters are met. All of these modifications have been subject to review of long-term water quality datasets, continuous monitoring of pH and turbidity, flow monitoring, and approval from Waikato Regional Council. Continuous monitoring of discharge water quality applies to check for any changes in water quality that would result in redirection of pond water to the WTP. A number of contingency ponds are located in the processing plant/WTP area including the Mill Contingency Pond (MCP), Tailings Contingency Ponds (TCP, TCP2, TCP1A) and WTP Contingency Pond (WTPCP). These ponds collect runoff from the ore stockpiles and conveyor, while also providing containment of any chemicals used for processing ore and water treatment in the event of spillage. These ponds will remain active until mine closure. Figure 9 shows the locations of existing collection and contingency ponds and associated catchment areas, and Table 6 summarises the current status of collection ponds in relation to direct overflow. Table 6 Collection Ponds and Status Collection Pond Direct Overflow Allowed From Period Northern Collection Pond (NCP) No NA Western Silt Pond (WSP) Yes Construction Collection Pond S3 Yes1 July 2014 Collection Pond S4 Yes1 July 2014 Collection Pond S5 Yes1 July 2014 Mill Contingency Pond (MCP) No NA WTP Contingency Pond (WTPCP) No NA Tailings Contingency Ponds (TCP, TCP2, TCP1A) No NA Favona Portal Contingency Ponds (FSPCP, FSPCP2) No NA 1. Reclassification of a number of collection ponds to silt ponds was approved on 17 July 2014. In accordance with condition 13 of Resource Consent 971312, direct discharges from S3, S4 and S5 to the Ohinemuri River and Ruahorehore Stream was approved; with discharge consent being transferred
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