3 Tonkin & Taylor Ltd Assessment of mine vent air quality impacts to inform an assessment of ecological effects on Archey’s frog Oceana Gold (NZ) Limited June 2022 Job No: 1017908 The discharge and emission parameters assumed for each vent and used in the dispersion modelling assessment are summarised in Table 2.3. For the purposes of this assessment, it has been conservatively assumed all of the TSP discharge is in the PM10 size fraction. Table 2.3: Discharge and emission parameters for each vent used in the dispersion modelling Stack parameter Value and unit Comment Discharge height 8 m above ground level Provided by Oceana Gold Stack diameter 5.5 m Provided by Oceana Gold Air flow rate 380 m³/s Provided by Oceana Gold 338 Nm³/s Calculated, assuming 20°C discharge temperature Discharge velocity 16 m/s Calculated based on air flow rate of 380 m³/s Discharge temperature 20 °C Rounded down from 2007 emission testing TSP emission rate (assumed to be PM10) 0.34 g/s Calculated from normalised flow rate and 1 mg/Nm³ discharge concentration NOX emission rate 1.04 g/s Calculated from normalised flow rate and 3.1 mg/Nm³ discharge concentration 3 Approach 3.1 Overview Dispersion modelling has been used to predict ambient contaminant concentrations in the vicinity of likely vent locations. These contaminants can then be compared to measured contaminant typically associated with a metalled public road in Northland where Waka Kotahi undertook ambient monitoring3. That study focused on measured concentrations of PM10 at varying distances from an unsealed public road as part of a study into the benefits and costs of available dust mitigation measures. This comparison can then be used to infer concentrations that might be expected for a public unpaved road in the Coromandel to the north of the mine vents where it is understood that ecological surveys of the Archey’s frog have been undertaken. 3.2 Dispersion modelling The CALPUFF air dispersion model (version 7.2.1) was used to predict contaminant ground level concentrations of PM10 and NO2 from the location of two mine vents provided by OGNZL. The discharge and emission parameters used as input to the model have been summarised Table 2.3. The CALPUFF model is widely used in New Zealand, particularly for locations of complex terrain as is the case in this instance. CALPUFF was configured to predict contaminant ground level concentrations centred on the two mine vents for three grids of receptors as follows: • 450 m by 450 m at a 50 m resolution. • 800 m by 800 m at a 100 m resolution. • 2,000 m by 2,000 m at a 200 m resolution. 3 Bluett J, Aguiar M, Gimson N 2017. Impacts of exposure to dust from unsealed roads. Report prepared by Golder Associated (NZ) Limited for the NZ Transport Agency (Waka Kotahi). April 2017. NZTA Research Report 590.
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