16 Tonkin & Taylor Ltd Wharekirauponga Underground Mine – unplanned detonation air quality assessment Oceana Gold (New Zealand) Limited June 2022 Job No: 1015212.2000 vRev0 6 Assessment of effects The maximum 1-hour average GLC for CO in either an ANFO or booster detonation are both less than 5% of the applicable assessment criteria, which corresponds a negligible impact on air quality when considering the IAQM guidance. The maximum 1-hour average GLC for NH3 in an ANFO detonation is less than 0.1% of the applicable assessment criteria, while a detonation of boosters produces a maximum GLC of 5.4%. The latter corresponds to a ‘slight’ impact on air quality when considering the IAQM guidance. There is no anticipated background level of NH3 in the forest park environment. Given the total concentration in the maximum case is significantly lower than the assessment criteria, the effect relative to the human health assessment criteria from the release of NH3 from the WUG is considered negligible. Nitrogen oxide (NO) can react with atmospheric background ozone and oxidise to form NO2. This atmospheric conversion can be accounted for in air quality assessments. However, in this instance we have conservatively assumed all NOX emitted is in the form of NO2 and have added those predicted GLCs to background levels identified in Section 5.2. The peak predicted NOX GLC is at 30% of the 1-hour average assessment criteria for NO2 in an ANFO detonation (and 13% for a detonation of the boosters). This represents a moderate level of impact according to the IAQM criteria. However, when combined with the background concentrations, the maximum predicted cumulative 1-hour average NO2 GLC is less than half the assessment criteria for the protection of human health. This maximum impacted location is approximately 130 m northeast of the surface vent in the Coromandel Forest Park and is approximately 830 m from the nearest point of the public walkway (which is currently closed due to repairs required at the swing bridge). Given the above context, it is highly unlikely that a person would be located in the most impacted location during the worst-case meteorological conditions and during a failure of management controls resulting in an explosives detonation. However, even if they were present, the maximum level of NO2 that they could be exposed to is less than half the criteria for protection of human health, and therefore the impact on human health would be less than minor. Overall, given the level of conservatism inherent to modelling of the detonation scenario, the impact to human health from discharge of contaminants from the surface vent is very low.
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