GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 40 4.3 Decant Water Treatment The quality of the decant water and its treatment requirements change depending on the active status of tailings deposition and length of time following this. An assessment of monitoring data from TSF2 has been compiled with key water quality parameters represented in Figure 23. The dataset is representative of the period immediately prior to the final tails deposition date and encompasses data from 2004 to current. This dataset therefore represents decant water quality during tailings deposition through to closure (i.e., decant direct discharge) and enables an assessment of the required treatment requirements and timeframes post deposition. Raw data includes reported pipe failure events in 2008 and 2009 which resulted in tails discharge to TSF2. This data has been excluded as it is not considered representative of decant water quality from the closed facility. Following cessation of tailings deposition in July 2005, the decant total cyanide concentration exhibits a rapid decrease (Figure 23). Based on this observed trend, cessation of treatment for cyanide removal can occur 1.5 years after the final deposition of tailings occurs. Likewise, the decant copper, chromium, antimony, selenium and zinc concentrations are shown in Figure 23. As per the cyanide trend, an obvious decrease in concentrations of trace elements is visible post final tails deposition. The cut-off for all treatment has been calculated as 2.5 years after the final deposition of tailings occurs. Once the requirement for treatment is past, the water in the TSF ponds is of sufficient quality to directly discharge to the Ohinemuri River. It is assumed that all TSFs utilised as part of the Waihi North Project (TSF1A, TSF2, TSF3 and GOP TSF) will require treatment for this observed duration following final tailings disposal. The actual water quality of the decant also differs depending on the ore being processed. Applied decant water quality in the WBM reflects current derived decant water quality as per AECOM, 2022 and assumes both Martha Ore, WUG Ore and Gladstone Ore are being processed simultaneously.
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