11 Tonkin & Taylor Ltd Waihi North Project - Technical Review of Air Quality Assessments Oceana Gold (New Zealand) Limited 22 June 2022 Job No: 1017908.0000 study on pregnant rats and repeated exposure over the pregnancy of those rates, rather than short term acute exposure. Beca also notes that no other regulatory agency has published 1-hour average guidelines for mercury, even when having reviewed the same study regarding pregnant rats. Overall, we agree with Beca in this instance that the OEHHA REL acute guideline for mercury is not an appropriate guideline value to consider in this instance. 4.3 Section 5: Dispersion modelling methodology Beca has used the CALPUFF dispersion model to assess the metal discharges to air from the Processing Plant. We agree that CALPUFF is an appropriate model to use in this instance, given the topography of the area, but also the relatively high percentage of calm wind conditions (greater than 10% of winds less than 0.5 m/s). The latter parts of Section 5 of the Processing Plant report set out the meteorological inputs to CALPUFF, along with terrain data, receptor locations, building downwash effects, and various modelling parameters. We agree these inputs and settings are appropriate. One aspect of note is in relation to the preparation of the meteorological dataset that is used as input to the CALMET model. In this instance Beca has developed a data set that covers a single year (2016). Our preference, and understanding of industry good practice in New Zealand, is for a twoyear dataset to be used. However, we accept Beca’s arguments for using a single year of data, noting that site-specific monitoring data has been used and that there is relatively little inter-annual variation in the wind data for the monitoring site. Section 5.4 sets out two emission rates scenarios. The first being associated with the existing plant (uncontrolled emissions) (Scenario 1) and the second assuming a retort is used to control mercury emissions form the oven and furnace (Scenario 2). Discharge parameters for each discharge source (i.e., stack height, diameter, discharge velocity and temperature) are key parameters used to characterise the discharge sources in the CALPUFF model. The values for discharge velocity and temperature are in this instance derived from the single set of test results carried out in 2021. While this represents the best available information to inform the study, there is some uncertainty regarding the variability of those values. Accordingly, we recommend a programme of routine stack emission testing be carried out, should consents be granted, to confirm discharge parameters are consistent with those used by the Beca assessment. Section 5.9 discusses the effect that building downwash can have on stack discharges and presents a Figure 4-3 that illustrates the three discharge stacks that have been modelled relative to surrounding buildings. From that figure, it appears that the stacks are of a similar height or just below the height of surrounding buildings and structures. In our experience, this indicates that the discharges from the stacks are likely to experience building downwash effects, resulting in poorer dispersion and higher contaminant ground level concentrations than if the stacks were sufficiently tall to avoid downwash. 4.4 Section 6: Dispersion model predictions The dispersion modelling results are presented in Section 6 of the Processing Plant report. With regard to mercury, the report notes that predicted concentrations for Scenario 1 and 2 when processing the proposed ores with higher mercury content are 70% and 24.7% of the OEHHA annual average guideline for mercury (0.03 µg/m³). The corresponding concentration when processing the existing ore is significantly lower still. When compared to the New Zealand ambient air quality guideline of 0.33 µg/m³ (annual average), the predicted concentrations are considered to be negligible.
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