GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 65 results presented are therefore reflective of the more conservative scenario. The average and maximum values presented are calculated across all model simulations and for the complete LOM. The model does not predict pH, dissolved oxygen or TSS within the discharge or receiving environment. These constituents are considered unsuitable to be accurately modelled within the mass balance model. pH is considered easily controlled within the discharge water by the current practice of using CO2 onsite to reduce the pH where necessary. The consented receiving water pH can therefore be maintained during high discharge events via operational controls. There is little recent data on dissolved oxygen concentrations in the WTP discharge. Data collected over the period between 2004 to 2010 shows a reasonably consistent discharge dissolved oxygen concentration of between 7 and 11 g/m3. These concentrations are in line with measured upstream concentrations over the same time period. TSS is largely removed through the treatment process and monitoring data shows WTP discharge TSS concentrations typically below the mdl. A greater predicted volume of discharged treated water (under Waihi North) is unlikely to have a significant impact on the either the TSS, pH or dissolved oxygen. The modelled results comply within the existing bounds of the relevant regime discharge consent conditions. Most of the modelled elements are expected to have concentrations well below the relevant compliance standards. The mass loads for individual elements are all within the implied mass load limits of the existing consent. In the event that discharge concentrations approach the consented concentrations for the operational regime, the RO plant can always be utilised to further improve the treatment process and is a contingency measure available to OGNZL. For hardness related trace elements, compliance values are likely to be higher than those stated in the existing discharge permit, because the treated water hardness is typically higher (typically above 1,000 g/m3) than the value used to derive these compliance values.
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