GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 13 2.3 WTP Performance The consented operating regimes combined with the performance of the WTP provide OGNZL with flexibility to operate over a large range of both environmental and operational conditions. The recent (2019-2020) performance of the WTP has been summarised in Table 2 for reference (in both removal of trace elements and achieving the consented regime and in-stream concentrations). In summary, the WTP operates well within the consented criteria ensuring that there is minimal impact on the receiving surface waters within the Ohinemuri River. 2.3.1 Trace Element Removal Rates Trace element removal rates at the WTP have been calculated by assessing the inflow sources (both flow and concentrations) compared to the outflow rate and water quality. As not all parameters are measured daily, the median measured concentrations have been utilised together with the proportion of inflow water source to calculate the WTP inflow concentrations. Where the measured concentration is below the laboratory method detection limit (mdl), half the mdl has been applied to this representative sample in order to derive a representation of actual and assumed trace element removal rates. The calculated removal efficiencies show high removal rate for the majority of the trace elements (>90% in most instances). For mercury, all recent inflow and outflow water quality was measured at below the mdl. Water quality data spanning a period between 2005 and 2007 (when the RO plant was nonoperational and WTP inflow mercury concentrations were above the mdl) has been utilised to calculate the mercury removal efficiency. For some elements (e.g. mercury, chromium, antimony, cadmium and silver), the lower calculated removal rates are largely a function of low inflow and/or outflow concentrations at or near the mdl. Even iron, for which the calculated removal efficiency is high (0.990) is affected by concentrations in the outflow below the mdl. The actual removal efficiency for these elements is likely higher than that stated. It is noted that inflow and outflow concentrations are derived from dissolved trace element analysis where applicable. No specific process for ammoniacal nitrogen removal is present within the WTP and this is reflected in the lower (0.61) calculated removal rate (based on the measured inflow/outflow concentration data). Utilisation of the RO plant would result in increased removal rates for most of the trace elements presented. OGNZL also has the ability to increase the removal rate of antimony through an existing sulphuric acid dosing system. The utilisation of both the RO plant or the sulphuric acid dosing system to improve discharge water quality has not been considered in this assessment, however both are available contingency options that could be utilised if required.
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