| Dispersion Modelling Methodology | WNP Processing Plant Air Discharge Assessment | 4397169-66885702-113 | 16/06/2022 | 13 Sensitivity: General 5 Dispersion Modelling Methodology 5.1 Overview of the Modelling Methodology The impact that emissions from the Processing Plant will have on ambient air quality has been assessed using dispersion modelling methods. The ambient air contaminant concentrations predicted by the modelling have been compared to the relevant ambient air quality standards and guidelines to assess the potential for adverse health effects. 5.2 Model Selection The dispersion model CALPUFF (v7.2.1) was used for this assessment. CALPUFF is a non-steady-state ‘puff’ dispersion model that simulates the three-dimensional effects of time and space-varying meteorological conditions on pollution transport. CALPUFF is recognised by the USEPA and the Good Practice Guide for Atmospheric Dispersion Modelling (MfE, 2004) as an appropriate model in cases where contaminant dispersion will be influenced by complex terrain features and meteorological conditions. CALPUFF is increasingly used in New Zealand for regulatory assessment purposes in such instances. The primary advantage of CALPUFF over simpler straight-line Gaussian dispersion models, such as AERMOD, is that meteorological conditions are modelled using a time-varying three-dimensional grid that simulates the effects that valleys, hills and land/sea interfaces can have on air flows. CALPUFF can also model the effects that changes in land use types have on atmospheric dispersion conditions. CALPUFF is generally considered to be a conservative dispersion model, tending to over-predict ground level contaminant concentrations. CALPUFF is an appropriate model for this assessment as the local Waihi topography is expected to affect air flows and the dispersion of contaminants. 5.3 Modelled Mercury Emission Scenarios Two different emission scenarios have been used to assess the potential impact that emissions from the Processing Plant will have on annual average mercury concentrations. The modelled scenarios vary with regards to the level of emission control which is assumed to have been implemented at the site. Scenario 1 is based on the plant’s existing configuration and assumes no emission control has been implemented. Scenario 2 assumes the proposed retort oven has been installed. Table 5-1 summarises the modelled emission scenarios. It is important to note that the retort oven would be installed before any processing of the GOP and WUG ore occurs. Scenario 1 has been included in the assessment only for comparisons purposes to show the relative effect that installing the retort oven would have on ambient air concentrations.
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