9 Tonkin & Taylor Ltd Wharekirauponga Underground Mine – unplanned detonation air quality assessment Oceana Gold New Zealand Limited June 2022 Job No: 1015212.2000 vRev0 D R A F T 4 Air dispersion modelling approach 4.1 Introduction Air dispersion modelling is the mathematical simulation of how air contaminants emitted from a source disperse in the ambient atmosphere. The outputs from the dispersion model are the predicted ground level concentrations (GLCs) of air contaminants from the modelled sources, which can be calculated for different averaging periods to align with the relevant assessment criteria. The objective of the dispersion modelling is to assess the potential effects of air discharges of contaminants from the site by comparing the predicted GLCs (and added to background concentrations where relevant) against assessment criteria. Dispersion modelling has been undertaken using the most recent (non-beta) version of the CALPUFF air dispersion model (version 7.2.1). CALPUFF is an advanced dispersion model that is widely used in New Zealand, especially in areas of complex terrain as presented at this location. In accordance with the Good Practice Guide for Atmospheric Dispersion Modelling3 (GPG Modelling), the 99.9th percentile is reported for 1 hour averaging periods. 4.2 Modelled receptor locations The CALPUFF model was configured to predict GLCs for four nested receptor grids of increasing spatial resolution (Table 4.1) centred on Site 5 (Rise #2). The nested receptor grid approach provides a high level of resolution close to the discharge source where the magnitude and spatial variation in impacts is typically greatest, with decreasing resolution in grid spacing further afield. The receptor grid is shown onFigure 4.1. Table 4.1: Nested receptor grids Distance from centre (m) Receptor spacing (m) 0 – 200 25 200 – 400 50 400 – 600 100 600 – 2200 200 The area around Site 5 (Rise #2) is within the State Forest land administered by DOC. It is part of a large area of secondary indigenous forest and scrub that forms the Coromandel Forest Park. As discussed in Section 2.2, a partially closed public walking track terminates at a swing bridge located approximately 1 km north of Site 5. This track is shown in green on Figure 4.1. 3 Ministry for the Environment. 2004. Good Practice Guide for Atmospheric Dispersion Modelling. Wellington: Ministry for the Environment.
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